razumevanje dinamike zaštitnog gasa i poboljšanje kvaliteta

NAUKAISTRAŽIVANJERAZVOJ SCIENCERESEARCHDEVELOPMENT

ZAVARIVANJE I ZAVARENE KONSTRUKCIJE 2/2013, str. 61-80 61

Sefton Mamo, James Foden prevod: Miloš Pavlović

RAZUMEVANJE DINAMIKE ZAŠTITNOG GASA I POBOLJŠANJE KVALITETA ZAVARIVANJA MIG/MAG POSTUPKOM UZIMAJUĆI U OBZIR UTICAJ PROMAJE

UNDERSTANDING THE SHIELDING GAS DYNAMICS AND IMPROVING THE WELD QUALITY IN MIG/MAG WELDING WITH RESPECT TO DRAUGHTS

Originalni naučni rad / Original scientific paper

UDK / UDC: 621.791.55

Rad primljen / Paper received: April 2013.

Adresa autora / Author's address: Sefton Mamo, James Foden

Institute of Mechanical Engineering, MCAST (Malta College of Arts, Science and Technology), Triq Kordin, Paola PLA 9032, Malta. e-mail: [email protected]

Ključne reči: MIG/MAG zavarivanje, zaštitni gas, simulator promaje, kvalitet spoja, poroznost.

Keywords: GMAW, shielding gas, draught simulation, welding quality, porosity.

Izvod Tokom godina, zavarivanje u atmosferi zaštitnog gasa (MIG/MAG) u znatnoj meri je zamenilo ostale postupke zavarivanja, kao što je ručno elektro-lučno zavarivanje (REL). Ipak, određeni nedostaci još uvek postoje, kao što je visoka osetljivost na strujanje vazduha (promaja), što ograničava funkcionalnost postupka. Ova studija razmatra u najvećoj meri navedeni nedostatak, sa ciljem da se ustanove efekti različitih brzina promaje na zavareni spoj, u kombinaciji sa različitim protocima zaštitnog gasa. Svi ostali parametri procesa zavarivanja su pažljivo procenjeni i izabrani i bili su nepromenjeni tokom eksperimenta. Kvalitet zavarivanja je ocenjivan standardnim i nestandardnim metodama. Za simulaciju uticaja različitih brzina promaje na zavareni spoj, osmišljena je i sačinjena aparatura sa sistemom za komprimovanje vazduha. Korišćen je matematički pristup, tako da je kontrolisani tok vazduha bez turbulencije usmeravan na celokupnu dužinu zavarenih spojeva. Utvrđeno je, da do rasipanja zaštitnog gasa i kontaminacije zavarenog spoja, dolazi čak i pri tako malim brzinama promaje od 0.9 m/s. Poroznost je bila glavni diskontinuitet nastao usled skretanja toka zaštitnog gasa sa rastopljenog metala i u direktnoj je vezi sa brzinom promaje. Na spojevima koji nisu izloženi promaji, nije došlo do nastanka poroznosti ili drugih grešaka na površini niti na preseku spoja, pri čemu povećanje protoka zaštitnog gasa nije imalo uticaja. Sa druge strane, rezultati ispitivanja potvrdili su da u opsegu protoka gasa do 30 l/min, konstrukciona čvrstoća i integritet spoja, zavise od odnosa brzine promaje i protoka zaštitnog gasa.

Abstract

Over the years Gas Metal Arc Welding (GMAW) has widely replaced other welding processes, such as Manual Metal Arc Welding (MMAW). Anyhow, some drawbacks still exist, e.g. high draught sensitivity, which limits its functionality.This study was concerned mainly to this drawback; to establish the effects of a variety of draughts velocities on the weld with a combination of different flows of shielding gas. All other welding parameters, were evaluated and chosen but kept constant throughout all the experiment. The welding quality was evaluated using standardised and non-standardised methods. As for simulating different draught velocities on the weld, a draught simulation apparatus using an air compression system was design and fabricated using a mathematical approach, so that a controlled and laminar flow of air is projected on the whole length of the welds. It was found that that dissipation of the shielding gas and contamination of the weldment occurred at draught velocities as low as 0.9 m/s. Porosity was the main weld discontinuity generated due to the displacement of the shielding gas from the weld pool which had direct relationship with the draught velocity. Despite an increase in the flow of the shielding gas, no porosity or other weld discontinuities were observed at the surface or section of the weldments, when there was no draught displacement on the welds. On the other hand, up till a flow of shielding gas of 30 l/min, it was found that there is a correlation on the structural strength and integrity of the weld between the draught velocity and the flow of the shielding gas.

INTRODUCTION GMA welding is one of the most common welding techniques used in the structural steel industry. GMAW is preferred from other similar welding techniques such as MMAW. The latter is because it has a number of benefits on MMAW, namely; higher deposition rate and welding speed; high degree of universality from the base material point of view; up to

UVOD MIG/MAG zavarivanje je jedan od najčešćih postupaka koji se koristi pri izradi industrijskih čeličnih konstrukcija. MIG/MAG postupak se češće bira u odnosu na REL i druge postupke zavarivanja, jer ima niz prednosti od kojih su najznačajnije: veća brzina zavarivanja i količina nanetog depozita; veća univerzalnost sa gledišta osnovnog materijala; veća


62 ZAVARIVANJE I ZAVARENE KONSTRUKCIJE 2/2013, str. 61-80

produktivnost i energetska efikasnost do 80%. Glavni nedostatak je visoka osetljivost na strujanje vazduha, koja ograničava funkcionalnost ovog postupka. U uslovima promaje, dolazi do rasipanja zaštitnog gasa koji izlazi iz mlaznice. Zaštitni gas biva skrenut od luka, ostavljajući nezaštićen rastopljeni metal delovanju kiseonika i azota. Ovo je naročito važno ako se zahteva spoj visokog kvaliteta sa dobrim mehaničkim karakteristikama. Druga bitna svojstva zavarenih spojeva na koja zaštitni gas utiče su: brzina zavarivanja, dubina i širina zavara, metalurške osobine spoja, kao i uticaj na uspostavljanje i stabilnost luka [1,2].

Prema EN 439, najčešće korišćeni zaštitni gasovi su: argon, helijum, ugljen-dioksid, kiseonik, vodonik i azot. Pošto su argon i helijum inertni gasovi, oni se za MIG postupak koriste pojedinačno ili u kombinaciji. Sa druge strane, kombinacija argona, ugljen-dioksida, kiseonika, vodonika i azota (poslednja tri u malim količinama) koriste se kao aktivni gas za MAG zavarivanje. Sastav zaštitnog gasa jako utiče na efikasnost, kvalitet kao i na celokupan proces zavarivanja u atmosferi zaštitnog gasa. Povećanje udela CO2 i O2 u mešavini zaštitnog gasa, dovodi do smanjenja poroznosti u spoju, ali istovremeno dovodi do povećane osetljivosti na promaju i dolazi do povećanog odgorevanja pojedinih legirajućih elemenata kao što su Si i Mg u osnovnom materijalu.

Još jedan važan faktor koji treba razmotriti pri zavarivanju u atmosferi zaštitnog gasa jeste sam protok zaštitnog gasa. Protok gasa utiče na kvalitet metala šava, kao i na izgled površine zavarenog spoja [3]. Vasco (2009) je objavio da velika brzina protoka zaštitnog gasa izaziva turbulenciju koja dovodi do kontaminacije rastopljenog metala vazduhom [1]. Dreher (2010) je uočio da i obrnuti slučaj sa malim protokom gasa takođe dovodi do kontaminacije rastopljenog metala vazduhom koji ga okružuje. Posledice ove vrste kontaminacije su: rasprskavanje, unutrašnja poroznost i oksidacija [4]. Postoji nekoliko faktora koji utiču na pravilan izbor protoka zaštitnog gasa: oblik mlaznice, jačina struje, položaj zavarivanja, brzina promaje i prečnik otvora za isticanje gasa na mlaznici [3,4].

Vasco (2009) je opisao zavisnost između veličine mlaznice i protoka gasa, gde je maksimalna postignuta brzina bez turbulencije bila 15 l/min za mlaznicu prečnika 12 mm i 23 l/min za prečnik od 16 mm [1]. Dreher (2009) je uočio da protok zaštitnog gasa pri kojem dolazi do turbulencije u mlaznici, varira u zavisnosti od prečnika otvora za isticanje gasa. Mlaznica koja ima manji prečnik otvora za protok gasa je osetljivija za nastanak turbulentnog protoka gasa, što za posledicu ima poroznost pri većem protoku.

Takođe, Dreher (2009) je objavio da brzina kojom zaštitni gas ističe iz mlaznice direktno zavisi od prečnika otvora za isticanje gasa. Što je manji prečnik otvora, veća je brzina zaštitnog gasa na izlazu iz mlaznice, što u uslovima promaje može značiti bolju

80% energy efficient and higher productivity. The main drawback related to GMAW is that it has a higher draught sensitivity when compared to MMAW; limiting its functionality. In draughty environments, the shielding gas exiting the nozzle is displaced from the arc, exposing the weld pool to oxygen and nitrogen.

The latter is important especially when a weld with high quality and good mechanical properties is desired. Other important features that the shielding gas has on the weldment, are that it affects the welding speed; depth and width of the weld bead; metallurgical properties of the weld and also helps in the arc plasma ignition and stability [1,2].

With reference to EN439, the most common used gases for shielding the arc in this welding process are; argon, helium, carbon dioxide, oxygen, hydrogen and nitrogen. Since argon and helium are both inert gases, they are used in the MIG welding process individually or in a combination. On the other hand, a combination of argon, carbon dioxide, oxygen, hydrogen and nitrogen (the last three are used in small percentages) are used in the MAG welding process. The shielding gas composition will strongly affect the efficiency, quality and overall operation of the GMAW. Increasing the CO2/O2 content in the shielding gas mixture will lead to decrease porosity in the weldment, but decrease the draught sensitivity and lead to the raising of the degree of burn-out of alloying elements such as silicon and manganese in the base material.

Another important factor that should be considered when welding with GMAW is the shielding gas flow rate. The latter is because it will influence the quality of the weld metal and the surface of the weld [3]. Vasco (2009) reported that high gas flow rate will cause turbulence which contaminate the weld pool with air [1]. On the contrary, Dreher (2010) observed that too low gas flow rate will also cause contamination of the weld pool with the surrounding air. The consequences of this contamination of the weld pool with air are; spatter, internal porosity, and oxidation [4]. There are several factors which will affect the selection of the right flow of shielding gas, namely; nozzle design, current intensity, welding position, draught velocity and borehole diameters [3,4].

Vasco (2009) describe the relationship between the size of the nozzle and gas flow, where the maximum flow associated with laminar flow is 15 l/min for a 12 mm diameter nozzle and 23 l/min for a 16 mm diameter nozzle [1]. Dreher (2009) reported that the flow of shielding gas at which a turbulent flow is achieved in the nozzle may vary with the diameter of boreholes. A welding nozzle with smaller boreholes will be more susceptible to a turbulent flow of shielding gas which will eventually result in porosity in the weld at high flows of shielding gas. Moreover, Dreher (2009) reported that the velocity at which the shielding gas exits the nozzle is also a function of the boreholes diameter. Smaller boreholes, contributed to



zaštitu rastopljenog metala od vazduha [5,6]. Skretanje mlaza zaštitnog gasa usled promaje, dovodi do apsorpcije azota i kiseonika u metal šava. Kao posledica javlja se poroznost, koja snižava žilavost i zateznu čvrstoću zavarenog spoja [5,7].

Johnson (2000) je objavio da rasipanje zaštitnog gasa i kontaminacija zavarenog spoja može nastati pri strujanju vazduha od samo 0.73 m/s. Kada je brzina strujanja povećana iznad 0.89 m/s, vizuelnim pregledom je uočena fina poroznost na licu šava. Dodatno povećanje brzine promaje dovodi do odgovarajućeg porasta poroznosti, što se može uočiti vizuelnim pregledom i radiografijom. Kada brzina promaje pređe interval od 3.6 do 4.4 m/s, poroznost se ne uočava vizuelnim pregledom, iako su karakteristike luka promenjene a razbrizgavanje pojačano. Nakon radiografskog ispitivanja, uočena je pojava poroznosti unutar šava, dok na licu šava poroznost nije vidljiva [7].

Ramsey (2012) je sproveo sličan eksperiment kao Johnson (2000) i zaključio da pokrivenost zaštitnim gasom i kvalitet zavarivanja opadaju sa porastom brzine bočne promaje. Dodatno, smanjenje prečnika mlaznice dovodi do povećanja brzine protoka zaštitnog gasa sa porastom pokrivenosti zaštitnim gasom [5].

PROJEKTOVANJE OPREME I POSTUPAK EKSPERIMENTA

Metodologija eksperimenta je osmišljena tako da se može posmatrati kako različiti protoci zaštitnog gasa štite zavarivačku kupku od uticaja kiseonika i azota, pri različitim brzinama promaje.

Na osnovu eksperimenata koje je sproveo Ramsey (2012), protok zaštitnog gasa je glavni faktor koji doprinosi kvalitetu zavarenog spoja, uzimajući u obzir različite brzine promaje. Jedan od faktora koji ima direktan uticaj na način raspoređivanja zaštitnog gasa oko luka, je oblik mlaznice i njene dimenzije.

Na osnovu eksperimenata koje su sproveli Dreher (2010) i Ramsey (2012), optimalni oblik i dimenzije mlaznice odabrani su na početku i bili nepromenljivi tokom svih eksperimenata, da bi se ograničio broj eksperimenata. Kada su jednom izabrani stalni i promenljivi parametri zavarivanja, definisani su njihovi različiti nivoi, prikazani u tabeli 1 [4,5]. Nakon definisanja faktora i njihovih nivoa, sprovedena su ispitivanja u skladu sa dijagramom toka na slici 1.

a higher velocity of the shielding gas at the nozzle outlet, which eventually offers greater protection of the weld pool from air in draughty environments [5,6].This displacement of shielding gas due to side draughts will result in absorption of nitrogen and oxygen into the weld. The latter cause porosity which will result in decrease in toughness and tensile strength of the welded joint [5,7].

Johnson (2000) reported that dissipation in shielding gas and contamination of the weldment done by GMAW, can even occur at wind speeds as low as 0.73 m/s.When the wind speed was increased above 0.89 m/s, fine porosity was visually observed on the face of the weld bead. Additional increase in wind speed resulted in a corresponding increase in porosity, observed both visually and in the radiographs. When wind speeds exceeded 3.6 to 4.4 m/s, porosity was not evident when visually inspected, although the arc characteristic changed and spatter had increased. When radiographs of the weld were conducted, it was found that porosity occurred inside the bead but was not visible on the face of the weld [7]. Ramsey (2012) conducted similar experiment to does done by Johnson (2000), were he concluded that the shielding gas coverage and quality of the weld decreased as the side draught velocity increased. Moreover, a reduction in nozzle diameter was found to increase the shielding gas velocity with a consequent of increasing the shielding gas coverage [5].

DESIGN OF EQUIPMENT AND EXPERIMENTAL PROCEDURE

The experimental methodology was designed to observe how different shielding gas flows protect the weld pool from oxygen and nitrogen in different draught velocities.

Based on experiments conducted by Ramsey (2012), the shielding gas flow rate was the main factor that contributed to the quality of the weld with respect to different draught velocities. A factor that has direct influence on how the shielding gas is displaced on the arc, is the nozzle design and its dimensions.

Based on experiments conducted by Dreher (2010) and Ramsey (2012), the optimum nozzle design and dimensions were chosen and held constant throughout all the experiments, in order to reduce the number of experiments due to different constraints.

Tabela 1: Nivoi izabranih faktora

Table 1: Levels for the chosen factors

Brzina promaje (m/s) Draught Velocities (m/s)

0 0.3 0.6 0.9 1.2 1.5 1.8 2.1 2.4 2.7 3

Protok zaštitnog gasa, (l/min) Shielding Gas flow rate (litres/min) 8 12 15 20 25 30



Slika 1: Dijagram toka postupka ispitivanja

Figure 1: Methodology of Testing Flow Diagram

Sprovedena su sledeća ispitivanja:

Ispitivanje savijanjem, u skladu sa EN 910: Ispitivanje savijanjem zavarenih spojeva metalnih materijala. Primarno, test je sprovođen do pojave prvih znakova nepotpunog stapanja ili grešaka tipa poroznosti, u delu čeonog spoja koji je bio izložen simuliranim uslovima promaje.

Vizuelno ispitivanje, je korišćeno za ispitivanje uticaja različitih brzina promaje na zavareni spoj

Once the fixed and variable welding factors were chosen, their different levels were determined, presented in table 1 [4,5].

Once the factors and their levels were determined, the tests were conducted as presented in the flow diagram, Figure 1. These tests are:

Bending tests; were done with reference to EN 910 'Bend testing of welds in metallic material'. Primarily, this test was done since any signs of



pri različitim protocima zaštitnog gasa. Poroznost, zajedi, nepravilan oblik zavara i nakupljanje topitelja mogu biti utvrđeni ovom vrstom ispitivanja.

Ispitivanje tečnim penetrantima je korišćeno za ispitivanje uticaja različitih brzina promaje na zavareni spoj pri različitim protocima zaštitnog gasa. Ovim testom ispituje se lice spoja na pojavu površinskih grešaka.

Radiografsko ispitivanje je korišćeno za ispitivanje uticaja različitih brzina promaje na zavareni spoj pri različitim protocima zaštitnog gasa. Ispitivanje je sprovedeno radi otkrivanja zapreminskih diskontinuiteta u zavarenom spoju. Glavne greške, koje se mogu detektovati ovom vrstom ispitivanja su poroznost, neprovar, uključci i prsline unutar spoja.

Sve probe zavarivanja izvedene su na limovima od niskougljeničnih čelika, sa vrednošću ugljeničnog ekvivalenta od 0.27%. Niskougljenični čelik je najpogodniji za zavarivanje, uglavnom zbog niske vrednosti ugljeničnog ekvivalenta, čime se eliminiše potreba predgrevanja i praćenja temperature između prolaza.

Priprema uzoraka Uzorci za ispitivanje savijanjem pripremani su u skladu sa EN 9692-1: Preporuke za pripremu spoja i EN 910: Ispitivanje savijanjem zavarenih spojeva metalnih materijala. Sečenje limova obavljeno je na kružnoj testeri, na dimenziju 50x8x600 mm, kako je propisano navedenim standardom. Tokom sečenja primenjeno je kontinuirano hlađenje uljanom emulzijom, radi sprečavanja nastanka martenzita po ivicama lima.

U cilju sprečavanja kontaminacije, brušenjem su uklonjene sve nečistoće sa površine limova. Za vizuelnu kontrolu i ispitivanje penetrantima, uzorci su sečeni iz limova dimenzija 80x8x6000mm. Uzorci su gasno rezani na dimenzije 80x8x140mm. Kao i za ispitivanje savijanjem, sa limova su brušenjem uklonjeni svi ostaci troske nastali tokom rezanja, na način propisan u EN 9692-1.

Oprema za zavarivanje Za zavarivanje u svim eksperimentima je korišćen uređaj Telwin-SuperMIG 460, ulaznog napona 460V. Saglasno eksperimentima koje je sproveo Ramsey (2012), pištolj za zavarivanje je opremljen mlaznicom prečnika 14mm [5]. Takođe, pištolj za zavarivanje korišćen u eksperimentu imao je kontaktni vrh uvučen 3 mm u mlaznicu. Ovakav izbor mlaznice baziran je na rezultatima koje su dobili Dreher (2010) i Johnson (2006). Oni su zaključili da se najniži sadržaj kiseonika u zoni spoja ostvaruje sa uvučenim kontaktnim vrhom. Suprotno, kada je kontaktni vrh izvučen, dolazi do značajnog porasta sadržaja kiseonika [2,4].

Korišćena je dvostepena kontrola i regulacija protoka zaštitnog gasa do mlaznice, primenom dvostrukog

leak of fusion or weaknesses such as porosity due to the simulated draughts within the butt weld were exposed.

Direct visual examination; was used for inspecting the affect of different draught velocities on the weld with a variety of shielding gas flows. Porosity, undercut, weld bead shape and flux accumulation could be inspected using this method.

Dye penetrant examination; was used for inspecting the affect of different draught velocities on the weld with a variety of shielding gas flows. This test was performed to examine the surface of a section of the weld for any surface-breaking flaws.

Radiographic examination; was used for inspecting the affect of different draught velocities on the weld with a variety of shielding gas flows. This test was performed to detect any weld discontinuities within the internal structure of the weld. The main weld discontinuities which could be detected using this examination method are porosity, LOF, inclusions and cracks within the weld.

All welding tests were carried out on low carbon steel plates, with a carbon equivalent value of 0.27 %. Low carbon steel has the best weldability properties, mainly because of its low carbon equivalent value, eliminating the need of preheating and monitoring of the interpass temperature.

Specimen Preparation The bend test samples were prepared with reference to EN 9692-1 'Joint preparation for steel' and EN 910 'Bend testing of welds in metallic material.' A rotating power saw was used to cut the plates to 50x8x600mm as implied in the mentioned standard. A continuous flow of soluble oil was supplied to prevent martensiteformatous at the edges of the plates. The plates were and the mill scale removed to prevent any contamination during the welding process.

For the visual and dye penetrant samples, plates were supplied measuring 80mm x 8mm x 6000mm. Using an oxy acetylene profile cutter, plates were cut, measuring 80mm x 8mm x 140mm. As done for the bend test samples, the plates were cleaned using an angle grinder from the mill scale and any slag produced by the oxy acetylene cutter. The latter was done with reference to EN 9692-1 'Joint preparation for steel.'

Welding equipment A 'Telwin-SuperMIG 460' welding set supplied with an input voltage of 460V, was used to conduct all the experiments in this study. With reference to the experiment conducted by Ramset (2012), the welding torch used for the welding of the specimens, was equipped with a 14mm diameter nozzle [5]. Moreover, the torch used to conduct the experiments was equipped with the contact tip withdrawn 3mm from the



manometra. Ovakav način regulisanja obezbeđuje ravnomeran dotok zaštitnog gasa, čak i kada pritisak u bocama opadne.

Potrošni materijal

Nelegirana elektrodna žica tipa ER 70 S-6 izabrana je za sprovođenje svih eksperimenata zavarivanja, a inače se koristi pri zavarivanju konstrukcionih i niskougljeničnih čelika. Ovaj tip dodatnog materijala ima tanku, homogenu bakarnu prevlaku za zaštitu od oksidacije. U skladu sa EN 4063: Zavarivanje i srodni postupci, izabran je prečnik žice od 0,8 mm za sprovođenje svih proba zavarivanja. Izabrani prečnik obezbedio je dovoljan depozit dodatnog materijala uz mali unos toplote. Nisu pronađeni rezultati radova koji opisuju uticaj sastava dodatnog materijala na sposobnost zaštite od uticaja promaje.

Kao zaštitni gas korišćena je mešavina 85% Ar + 15% CO2. Izbor je izvršen na osnovu rezultata istraživanja koje je sproveo Menzel (2002). On je zaključio da porast udela CO2 u mešavini, dovodi do povećanog odgorevanja legirajućih elemenata. Odgorevanje dovodi do smanjenja žilavosti zavarenog spoja [8]. Takođe, porast sadržaja CO2 u mešavini dovodi do smanjenja razbrizgavanja [9].

Projektovanje i izrada uređaja za simulaciju promaje Na slici 2 je prikazana sklopljena aparatura za kontrolisanu simulaciju promaje koja je korišćena tokom eksperimenta. Niz od 4 regulaciona ventila je postavljen za kontrolu brzine protoka vazduha koji stvara uslove promaje. Prvi ventil je povezan direktno na izlaz iz kompresora vazduha, sa glavnom funkcijom sniženja pritiska sa 6 na 2 bar. Istovremeno vrši stabilizaciju pritiska na ulazu u drugi, treći i četvrti ventil, eliminišući uticaj cikličnog rada kompresora.

Slika 2: Uređaj za simulaciju promaje

Figure 2: Draught simulation apparatus

Preostala 3 ventila su međusobno identična, koriste zajednički dotok vazduha pod pritiskom od 2 bar sa prvog ventila. Rešavanjem Rejnoldsove jednačine, dobijen je unutrašnji prečnik izlazne cevi od 8mm.

Za određivanje brzine promaje na spoju, korišćen je ručni merač brzine vetra. Na svakoj cevi, merena je

nozzle. This choice of the nozzle design, was made based on the results obtained by Dreher (2010) and Johnson (2006), were it was concluded that the least oxygen levels in the weld zone were present when the contact tip was withdrawn from the nozzle. On the contrary, when the contact tip was extruding out from the nozzle the oxygen levels in the weld zone increased by a significant amount [2,4].

Furthermore, a two stage and two gauge pressure regulator was used to monitor and control the gas flowing to the nozzle. This type of regulator delivers a constant flow of gas even as the pressure in the cylinder decreases.

Welding consumables The type of wire filler metal chosen to conduct all welding experiments was 'ER 70 S-6', an unalloyed wire electrode for welding general structural steels and low carbon steels. This type of filler metal can be used with pure CO2 or a variety of mixed shielding gas. Moreover, this type of filler metal has a thin and homogenous copper coating for protection from oxidisation on the wire. With reference to EN 4063 'Welding and allied processes' an eight millimetre diameter wire filler metal was used to conduct all welding experiments. The chosen diameter ensured good material deposition rate with a low heat input.

No research was found regarding the filler metal composition with respect to the shielding capabilities from draughts. Ar-85%, CO2-15% was used as a shielding gas. This decision was based on the results obtained by Menzel (2002), were he concluded that increasing the carbon dioxide content in the shielding gas mixture will lead to the increase of burn out alloying elements. The latter would result in a decrease in toughness of the weldment [8]. Moreover, the increase of carbon dioxide content in the shielding gas mixture, will lead to an increase of spatter [9].

Design and Manufacturing of Apparatus Figure 2 shows the assembled draught simulation apparatus used during all welding experiments. Four variable valves were used to control the draught velocity. The first was connected directly to the output of an air compressor, were its main function was to reduce the pressure from 6 bar to 2 bar.

Another important function of this valve was to stabilise the pressure on the input of the second, third and fourth valves and eliminate the pressure cycle by the compressor. The second, third and fourth variable valves were identical to each other. These three valves had a common inlet pressure of two bar which was supplied from the first valve by a manifold. Reynolds equation was used to calculate the inner pipe diameters and was found to be 8mm.

As for measuring the draught velocity on the weld, a hand held anemometer was used. Each air pipe velocity was measured by the anemometer and set individually from their respective valve. This technique



brzina promaje i podešavana na pripadajućem ventilu. Ova tehnika je korišćena tokom celog eksperimenta i obezbedila je dobijanje iste brzine na sve tri cevi, dužinom celog spoja koji se zavaruje.

Postupak zavarivanja

Na slici 3 dat je šematski prikaz redosleda nanošenja slojeva sučeonog spoja. Korišćena je tehnika zavarivanja unapred, zbog činjenice da je tada distribucija zaštitnog gasa mnogo ravnomernija i pruža bolju zaštitu od uticaja promaje.

Slobodni kraj žice bio je približno 10 mm u svim ogledima zavarivanja. Ovo je bitno jer duži slobodni kraj utiče na slabiji učinak zaštitnog gasa, pri čemu raste osetljivost na promaju. Sa druge strane, suviše kratak slobodni kraj dovodi do povratnog pregrevanja.

Slika 3: Šematski prikaz redosleda nanošenja slojeva sučeonog spoja

Figure 3: Schematic diagram representing the welding layers for the butt joint

U tabeli 2 date su vrednosti napona luka, jačine struje zavarivanja i unosa toplote, za svaki naneti sloj sučeonog spoja. Za merenje napona luka i jačine struje zavarivanja, korišćeni su voltmetar i induktometar. Na osnovu radova koje je objavio Funderburk (1999) izračunata je količina unete toplote [10].

REZULTATI Ukoliko nije drugačije navedeno, testovi su izvedeni korišćenjem parametara prikazanih u tabeli 3.

Ispitivanje savijanjem Ispitivanje savijanjem je rađeno u skladu sa EN 910: Ispitivanje savijanjem zavarenih spojeva metalnih materijala. Za ovaj test, brzina promaje je povećevana za 0.3 m/s u intervalu od 0 m/s do 3 m/s. Takođe, brzina protoka zaštitnog gasa povećavana je sa 8 l/min na 35 l/min.

Rezultati ispitivanja savijanjem prikazani su u tabeli 4. Do rezultata se došlo vizuelnim pregledom uzoraka nakon savijanja u saglasnosti sa BS EN ISO 15614-1:2004 – Specifikacija i kvalifikacija tehnologije zavarivanja metalnih materijala, kvalifikacija tehnologije zavarivanja.

Rezultati ispitivanja savijanjem Odluku da ispitivanja koja nisu izvedena budu svrstana u uspešna ili neuspešna potpomogli su rezultati dobijeni pri odsustvu promaje, odnosno pri brzini

of setting the air velocity was used throughout all experiments, and ensured that all three pipes had an equal air velocity covering the hole length of the weld.

Welding Procedure Figure 3 is a schematic diagram, representing the welding layers of the butt joint. The forehand welding technique was used throughout. This was implied due to the fact that the shielding gas distribution is much more efficient and offers greater draught resistivity. An approximate value of 10mm wire stick-out was maintained throughout all the experiments. The later was an important factor, since the larger the wire stick-out is, the less affective is the shielding gas distribution on the weld, increasing the draught sensitivity. On the other hand, too small stick-out will increase the risk of burn-backs.

Table 2 show the values of the arc voltage, welding current and heat input, used to weld the four layers of welding for the butt joint. A volt meter and an inductance meter were used to measure the arc voltage and the welding current respectively. With reference to the literature presented by Funderburk (1999), the heat input value could be calculated [10].

TEST RESULTS Unless otherwise mentioned, the tests were carried out using experimental parameters presented in table 3, as follows.

Bending Test

The bend tests were done with reference to EN 910 'Bend testing of welds in metallic material'. For this test, the draught velocity on the weld was increased incrementally by 0.3 m/s from a 0 m/s draught up to a 3m/s draught. Moreover, the flow of the shielding gas was increased from 8 l/min up to 35 l/min.

The results obtain from the bend test experiments are summarised in table 4. These results were concluded based on the visual examination done on the specimens after bending and with reference to BS EN ISO 15614-1:2004 'Specification and qualification of welding procedure for metallic material-welding procure test'.

Bend Test Results

The decision of marking these tests as a success or a failure, was based on the results achieved at a 0 m/s draughts. As one can note, when the draught velocity was at 0 m/s, all the bend tests were successful which donated that the flow of shielding gas up to 35 L/min did not have any influence on the quality of the weld.

Moreover, the decision of marking these tests as a success or a failure, was based on the results achieved by visual examination. When comparing the results from the visual examinations to the bend test results, it was concluded that as porosity in the weldment due to draughts exceeded a medium level, the bend test would have resulted in a failure.



Tabela 2: Vrednosti napona luka, jačine struje zavarivanja, brzine zavarivanja i količine unete toplote za sučeoni spoj

Table 2: Values of the arc voltage, welding current, welding speed and heat input used for the butt joint

Oznaka sloja

Welding layer number 1 2 3 4

Napon luka Arc voltage, (V)

20.8 20.8 20.8 20.8

Jačina struje zavarivanja Welding current, (A)

110 110 110 110

Brzina zavarivanja Welding speed,

(mm/min) 400 330 280 230

Količina unete toplote Heat input, (KJ/mm)

0.34 0.42 0.49 0.6

Tabela 3: Prikaz osnovnih parametara korišćenih tokom eksperimenta

Table 3: Summary of the main parameters applied during experiments

Parametri zavarivanja

Welding parameters Vrsta Type Standard

Osnovni materijal Base material composition

Niskougljenični čelik Low carbon steel

EN 10204 3.1

Dodatni materijal Filler metal composition

Nelegiran Unalloyed

ER 70 S-6

Dimenzije dodatne žice Filler metal size, (mm)

0.8

Sastav zaštitnog gasa Shielding gas composition

Ar 85%; CO2 15%

Prečnik mlaznice Nozzle diameter, (mm)

14

Vrsta mlaznice Nozzle design

3 mm uvučen kontaktni vrh 3 mm withdrawn contact tip

Slobodni kraj žice Wire stick-out, (mm)

10

Položaj pištolja Torch position

Zavarivanje unapred Forehand welding technique

Napon luka Arc voltage, (V)

20.8

Jačina struje Arc current, (A)

110

Srednja brzina zavarivanja Average welding speed, (mmmin)

310

Srednji unos toplote Average heat input, (KJmm)

0.4625

Predgrevanje OM Base metal pre-heating, (°C)

45



Tabela 4: Pregled rezultata ispitivanja savijanjem

Table 4: Summary of bend test results

Brzina promaje / Draught velocity, (m/s)

0 0,6 0,9 1,2 1,5 1,8 2,1 2,4 2,7 3,0

Ppr

otok

gas

a / G

as F

low

(l/m

in)

8 Pass Pass Fail Fail

12 Pass Pass Pass Pass Fail

15 Pass Pass Pass Pass Pass Fail Fail Fail

20 Pass Pass Pass Pass Pass Fail

25 Pass Pass Pass Pass Pass Fail

30 Pass Pass Pass

35 Pass Pass

Napomena: Svetlo siva polja sa oznakom “Pass” predstavljaju ispitivanja koja su zadovoljila na testu savijanja. Tamno siva polja sa oznakom “Faill” predstavljaju ispitivanja savijanjem koja nisu zadovoljila. Svetlo siva i tamno siva polja bez oznake, znače da ispitivanje savijanjem nije izvedeno, ali bi očekivano bilo uspešno odnosno neuspešno.

Note: A cell highlighted in a light grey colour and marked as Pass represent that the bend test was done and was a success. A cell highlighted in dark grey colour and marked as Fail represent that the bend test was done and was a failure. A cell which is shaded and highlighted with a light or dark grey colour represent that the bend test was not done but would have been a success or a failure respectively.

0 m/s. Kao što se može primetiti, u uslovima bez promaje sva ispitivanja savijanjem tako zavarenih uzoraka su uspešna, tako da povećanje protoka zaštitnog gasa do 35 l/min nije imalo nikakvog uticaja na kvalitet zavarenih spojeva. Takođe, odluka da ova ispitivanja budu proglašena uspešnim ili neuspešnim, bazirana je na rezultatima vizuelnog pregleda. Pri poređenju rezultata dobijenih vizuelnim pregledom i savijanjem, zaključeno je da su rezultati ispitivanja savijanjem neuspešna u slučajevima kada stepen poroznosti usled promaje pređe srednji nivo.

Na slici 4 dat je grafički prikaz rezultata ispitivanja savijanjem. Rezultati daju linearnu zavisnostmaksimalno dopuštene promaje u odnosu na protok zaštitnog gasa pri kojima su rezultati ispitivanja savijanjem uspešni.

Vizuelni pregled, površinske greške

Za ovu metodu ispitivanja, pripremci limova su sečeni i zavareni pri različitim brzinama promaje i protoka zaštitnog gasa. Svaki lim je izložen određenoj vrednosti promaje,a zatim je naneto 6 navara pri različitim protocima zaštitnog gasa. Za svaki lim, brzina promaje je podizana za 0.3 m/s, počevši od 0 m/s do 3.0 m/s. Slika 5 prikazuje navare koji niisu bili izloženi promaji, a slika 6, navare izložene promaji brzine 3.0 m/s.

Pregled rezultata dobijenih vizuelnim ispitivanjem (površinski diskontinuiteti) dat je u tabeli 5. Težište vizuelnog pregleda usmereno je na otkrivanje sadržaja poroznosti na površini navara u zavisnosti od brzine

Graph on figure 4, gives the results obtained from the bend test experiments in a fitted line plot. It represents the maximum allowable draught velocity with respect to the gas flow for a successful bend test experiment.

Visual Examination Surface Discontinuities

For this examination method, plates were pre-cut and welded with different draught velocities and gas flows. Each plate was subjected to a specific draught velocity and six separate welds were done on each plate, each with a different supply of shielding gas flows. The draught velocity for each plate was increased incrementally by a velocity of 0.3 m/s, starting from a draught of 0 m/s up to 3.0 m/s. Figure 5 and 6, are photo image results of welds that were not exposed to any draught and welds that were exposed to a 3m/s draught velocity.

The results obtained from the visual examination (surface discontinuities) are summarised in table 5. The main focus of this visual examination was to identify the amount of porosity at the surface of the weld with respect to the draught velocity and the gas flow. The results were concluded based on the visual examination done on the specimens after welding. Moreover, the amount of spatter around the weld could also be observed.

Visual Examination Results, Surface Discontinuities

Referring to table 5, cells highlighted in a light grey colour, shaded and marked as low are representing



Slika 4: Linearna zavisnost maksimalno dopuštene promaje u odnosu na protok zaštitnog gasa za ispitivanje savijanjem

Figure 4: Fitted line plot- Maximum allowable draught velocity versus shielding gas flow (bend test)

Slika 5: Ploča sa navarima pri uslovima bez promaje, pri protoku zaštitnog gasa od 8, 12, 15, 20, 25 i 30 l/min

Figure 5: Flat welds which were not subjected to any draughts and supplied to with a shielding gas flow of 8, 12, 15, 20, 25 and 30 l/min

Slika 6: Ploča sa navarima koji su pri navarivanju izloženi promaji od 3.0 m/s pri protoku zaštitnog gasa od 8, 12, 15, 20, 25 i 30 l/min

Figure 6: Flat welds which were subjected to a 3.0 m/s draught and supplied with a shielding gas flow of 8, 12, 15, 20, 25 and 30 l/min



promaje i protoka zaštitnog gasa. Rezultati su bazirani na vizuelnom pregledu uzoraka nakon navarivanja. Takođe, posmatran je i obim razbrizgavanja oko navara.

Rezultati vizuelnog pregleda, površinske greške

U tabeli 5, svetlo siva polja sa oznakom Low predstavljaju navare sa malim sadržajem površinske poroznosti na početku navara. Na početku navara,poroznost u maloj meri zavisi od brzine promaje i protoka gasa. Ova poroznost nastaje usled uspostavljanja pritiska zaštitnog gasa od boce do izlaznog ventila. Uspostavljanje pritiska izaziva nalet gasa pri svakom otpočinjanju novog navara, što dovodi do turbulencije gasa na izlazu iz mlaznice u prvih nekoliko sekundi. Krajnja posledica je kontaminacija rastopljenog metala vazduhom (kiseonik, azot) pri čemu nastaje mali sadržaj poroznosti [5]. Zbog toga se poroznost na početku navara neće smatrati značajnim nedostatkom zavarivanja.

Grafik na slici 7 daje rezultate dobijene vizuelnim pregledom (površinska poroznost). Dobijena je linearna zavisnost maksimalne brzine promaje u odnosu na protok gasa, za slučaj male poroznosti ili bez poroznosti na površini zavara.

Vizuelni pregled, zapreminske greške Svaki lim je izložen određenoj vrednosti promaje, a zatim je naneto 6 navara na svaki lim pri različitim protocima zaštitnog gasa. Za svaki lim, brzina promaje je podizana za 0.3 m/s, počevši od 0 m/s do 3.0 m/s. Sa limova su uklonjene sve nečistoće, a zatim je

Tabela 5: Rezultati vizuelnog pregleda, površinski diskontinuiteti

Table 5: Summary of visual examination, surface discontinuities


0 0,6 0,9 1,2 1,5 1,8 2,1 2,4 2,7 3,0

Ppr

otok

gas

a / G

as F

low

(l/m

in)

8 None Low Low High High High High High High High

12 None None None Low High High High High High High

15 None None Low Low Low Low High High High High

20 None None None None None None None Medium Medium High

25 None None None None None None None Low None Medium

30 None None None None None None None None None None

Napomena: Svetlo siva polja označena sa None predstavljaju navar bez ikakve poroznosti. Svetlo siva polja sa naznakom Low predstavljaju navar sa malim sadržajem poroznosti koja se uočava na samom početku navara. Tamno siva polja sa naznakom Medium, predstavljaju navare sa značajnim sadržajem poroznosti, dok tamno siva polja sa naznakom High predstavljaju navare sa visokim stepenom vidljive poroznosti.

Note: A cell highlighted in a light grey colour and marked as None, representa weld without any visible porosity. A cell highlighted in a light grey colour shaded and marked as Low, represent a weld with a small amount of visible porosity at its start. A cell highlighted in a dark grey colour and marked as Medium, represent a weld that has a considerable amount of a visible porosity along its length. A cell highlight in a dark grey colour and marked as High, represent a weld which a high degree of visible porosity.

weld that have low amount of visible porosity on the surface at the beginning of the weld. This porosity at the beginning of the weld has little correlation to the draught velocity and the shielding gas flow. This is because, this porosity is generated due to the fact that a pressure build up in the pipe between the shielding gas cylinder and the valve in the welding set that allow shielding gas to flow occur. This pressure build up causes an uncontrolled burst of shielding gas exiting the nozzle at each start of a new weld. This uncontrolled burst of shielding gas causes a turbulent flow of shielding gas to exit the nozzle for the first one to two seconds at each start of a new weld. The latter causes air contaminants (such as oxygen and nitrogen) to be drawn into the weld pool generating low amount of porosity [5]. For this reason, porosity at the start of the welds will not be considered as a major welding flaw.

Graph on figure 7, gives the results obtained from the visual examination (surface porosity) in a fitted line plot. It represents the maximum allowable draught velocity with respect to the gas flow for no or low visible pores on the surface of the welds.

Visual Examination, Section Discontinuities Each plate was subjected to a specific draught velocity and six separate welds were done on each plate, each with a different supply of shielding gas flows. The draught velocity for each plate was increased incrementally by a velocity of 0.3 m/s, starting from a draught of 0 m/s up to 3.0 m/s. The plates were cleaned from any flux, and using a surface grinding machine, the welds on the plates were grinded down to 1.5 mm, so that a section of the



brušenjem uklonjen deo šava do dubine od 1.5 mm, radi ispitivanja na preseku navara. Nakon toga urađeno je ispitivanje penetrantima. Slika 8 prikazuje navare izvedene pri promaji brzine 3.0 m/s.

Pregled rezultata vizuelnog ispitivanja zapreminskih grešaka (poprečni presek) dat je u tabeli 6. Cilj vizuelnog ispitivanja bio je određivanje sadržaja poroznosti u poprečnom preseku navara, u zavisnosti od brzine promaje i protoka zaštitnog gasa. Vizuelni pregled sproveden je nakon navarivanja, mašinske obrade i nanošenja tečnih penetranata.

U tabeli 6, svetlo siva polja sa oznakom Low predstavljaju navare sa malim sadržajem površinske poroznosti na početku navara. Na početku navara,poroznost u maloj meri zavisi od brzine promaje i protoka gasa. Ova poroznost nastaje usled uspostavljanja pritiska zaštitnog gasa od boce do izlaznog ventila. Uspostavljanje pritiska izaziva nalet gasa pri svakom otpočinjanju novog navara, što dovodi do turbulencije gasa na izlazu iz mlaznice u prvih nekoliko sekundi. Krajnja posledica je kontaminacija

Slika 7: Linearna zavisnost maksimalne brzine promaje u odnosu na protok gasa,

vizuelni pregled površinske poroznosti

Figure 7: Flat Fitted line plot- Maximum allowable draught velocity versus shielding gas flow visual examination-surface porosity

Slika 8: Lim sa navarima koji su izloženi promaji pri navarivanju od 3,0 m/s

i protoku zaštitnog gasa od 8, 12, 15, 20, 25 i 30 l/min.

Figure 8: Sectioned flat welds which were subjected to a 3.0 m/s draught and supplied with a shielding gas flow of 8, 12, 15, 20, 25 and 30 l/min.

weld could be examined. At this stage, a dye penetrate process was applied to the plates for further examination. Figure 8 is photo image result of welds that were exposed to a 3m/s draught velocity.

The results obtained from the visual examination (section discontinuities) are summarised in table 6. The main focus of this visual examination was to identify the amount of porosity at a section of the welds with respect to the draught velocity and the gas flow. The results in table 6, were concluded based on the visual examination done on the specimens after welding, machining and treating the specimens with a dye penetrate substance.

In table 6, cells highlighted in a light grey colour, shaded and marked as low are representing welds that have low amount of visible porosity at the section at the beginning of the weld. This porosity has no correlation to the draught velocity on the weld and/or the flow of the shielding gas. This is because, this porosity is generated due to a turbulent flow of gas at the nozzle for the first one to two second at the



Tabela 6: Rezultati vizuelnog pregleda, zapreminski diskontinuiteti

Table 6: Summary of visual examination, section discontinuities


0 0,6 0,9 1,2 1,5 1,8 2,1 2,4 2,7 3,0

Ppr

otok

gas

a / G

as F

low

(l/m

in)

8 None Low Low Medium High High High High High High

12 None None None Low Medium Medium High High High High

15 None None None None Low None High High High High

20 None None None None None None None None Medium High

25 None None None None None None None None None Medium

30 None None None None None None None None Low Low

Napomena: Svetlo siva polja označena sa None predstavljaju navar bez ikakve poroznosti. Svetlo siva polja sa naznakom Low predstavljaju navar sa malim sadržajem poroznosti vidljive na početku navara. Tamno siva polja sa naznakom Medium, predstavljaju navare sa značajnim sadržajem poroznosti, dok tamno siva polja sa naznakom High predstavljaju navare sa visokim stepenom vidljive poroznosti.

Note: A cell highlighted in a light grey colour and marked as None, representa weld without any visible porosity. A cell highlighted in a light grey colour shaded and marked as None, represent a weld without any visible porosity. A cell highlighted in a light grey colour shaded and marked as Low, represent a weld with a small amount of a visible porosity at its start. A cell highlight in an dark grey colour and marked as Medium, represent a weld that has a considerable amount of visible porosity along its lenght. A cell highlighted in a dark grey colour and marked as High, represent a weld which has a high degree of visible porosity.

rastopljenog metala vazduhom (kiseonik, azot) pri čemu nastaje mali sadržaj poroznosti [5]. Zbog toga se poroznost na početku navara neće smatrati značajnim nedostatkom zavarivanja.

Grafik na slici 9 prikazuje rezultate vizuelnog ispitivanja zapreminske poroznosti. Dobijena je linearna zavisnost maksimalne brzine promaje u odnosu na protok gasa, za slučaj male poroznosti ili bez poroznosti po preseku navara.

Radiografsko ispitivanje Za radiografsko ispitivanje, isečeni su pripremci iz ploča i zavareni pri različitim brzinama promaje. Uzorci sa različitim parametrima su izabrani na bazi rezultata dobijenih testom savijanja. Na slici 10 je radiogram uzorka koji nije bio izložen promaji, a na slici 11 je radiogram spoja zavarenog pri brzini promaje od 2.7 m/s.

Rezultati radiografskog ispitivanja su objedinjeni u tabeli 7. Težište ispitivanja je usmereno na određivanje maksimalnog sadržaja poroznosti, pri kom je rezultat ispitivanja savijanjem još uvek uspešan. Rezultati prikazani u tabeli 7 dobijeni su ocenom radiograma.

DISKUSIJA REZULTATA

Ispitivanje savijanjem

Diskusija rezultata ispitivanja savijanjem

Na osnovu rezultata ispitivanja savijanjem prikazanih u tabeli 4, uočava se da pri malim brzinama promaje nije došlo do loma, a da istovremeno povećanje brzine protoka zaštitnog gasa nije imalo uticaja. Sa druge strane, kada je brzina promaje dostigla 0.6 m/s

beginning of each weld [5]. This porosity at the start of the welds will not be considered as major welding flaw.

Graph on figure 9, gives the results obtained from the visual examination (section porosity) in a fitted line plot. It represents the maximum allowable draught velocity with respect to the gas flow for no or low visible pores on a section of the welds.

X-Ray Examination For the X-ray examination, plates were pre-cut and welded with different draught velocities. Specimens with different parameters were chosen with reference to the results obtained in the bending tests. Figure 10and 11, are photo image results of the X-ray films of a weld that was not exposed to any draught and a weld that was exposed to a 2.7m/s draught velocity.

The results obtained from the X-ray examination are summarised in table 7. The main focus of this examination was to identify the maximum number of pores in the weld which would still result in a success in the bend test experiment. The results in table 7, were concluded based on the visual examination done on the X-ray films.

DISCUSSION OF RESULTS

Bend Test

Discussion of Bend Test Results

From the bend test results, presented in table 4, it can be observed that there were no failures in the bending tests at low draught velocities, despite the increase of



pojedini testovi savijanja su bili neuspešni, u zavisnosti od protoka zaštitnog gasa. Kada je brzina promaje podignuta sa 0.9 na 3.0 m/s, broj neuspešnih testova savijanjem je značajno porastao.

Analizom grafika na slici 3 može se zaključiti da do protoka gasa od 35 l/min postoji korelacija konstrukcione čvrstoće spoja u zavisnosti od odnosa brzine promaje i protoka zaštitnog gasa. Kritični odnos brzine promaje i protoka zaštitnog gasa može biti određen na granici uspešnih i neuspešnih testova savijanja i iznosi približno 0.1.

Procena i obrazloženje razloga loma pri ispitivanjusavijanjem

Postoji nekoliko grešaka koje utiču na konstrukcionučvrstoću zavarenog spoja, uključujući nedovoljnouvarivanje, nepotpuno stapanje, zajede i poroznost.Brzina zavarivanja i jačina struje usvojeni u ovoj studiji bili su optimalni u odnosu na količinu unete toplote [10]. Prema tome, pojava nedovoljnog uvarivanja semože isključiti kao greška tokom zavarivanja uzoraka za test savijanja. Sledeći nedostatak koji je mogao imati uticaj na rezultate testa savijanja je nedovoljno stapanje.

Slika 9: Linearna zavisnost maksimalne brzine promaje u odnosu na protok gasa,

vizuelni pregled zapreminske poroznosti Figure 9: Fitted line graph- Maximum allowable draught velocity versus shielding gas flow

visual examination-section porosity

Slika 10: Sučeoni spoj zavaren u uslovima bez promaje i pri protoku zaštitnog gasa od 8 l/min.

Figure 10: Butt weld subjected to 0 m/s side draught and with a shielding gas flow of 8 l/min.

Slika 11: Sučeoni spoj zavaren pri brzini promaje od 2.7 m/s i pri protoku zaštitnog gasa od 20 l/min.

Figure 11: Butt weld subjected to 2.7 m/s side draught and with a shielding gas flow of 20 l/min.

the flow of the shielding gas. On the other hand, once the draught velocity exceeded 0.6 m/s, the bending tests started to fail with respect to the flow of the shielding gas. As the draught velocity increased from 0.9 m/s to 3 m/s, failures in the bend tests increased by a significant amount.

By analysing graph on figure 3, it can be concluded that till a flow of shielding gas of 35 l/min, there is a correlation on the structural strength of the weld between the draught velocity and the flow of the shielding gas. A critical ratio of shielding gas to side draught velocity can be determined, in doing so presenting the pass-fail boundary which is approximately of 0.1.

Evaluating and Justifying Reasons of Failure (Bend Tests)

There are several weld discontinuities that could affect the structural strength of the weld, including lack of penetration, lack of fusion, undercuts and porosity. The welding speed and the current adopted in this study were optimal based on the heat input value [10].

Thus it can be concluded that lack of penetration in the welding of the specimens is excluded.



Tabela 7: Prikaz rezultata radiografskog ispitivanja

Table 7: Summary of x-ray examination


0 0,6 0,9 1,2 1,5 1,8 2,1 2,4 2,7 3,0

Ppr

otok

gas

a / G

as F

low

(l/m

in)

8 3 6 17 Cluster

12 4 11 12 19

15 12 24 Cluster Cluster

20 7 10 4 Cluster

25 9 Cluster

30 2 11

Napomena: Vrednost u svakom polju predstavlja broj pora po spoju, u zavisnosti od brzine promaje i protoka zaštitnog gasa. Za polja bez unete vrednosti, radiografsko ispitavanje nije rađeno. Svetlo siva polja predstavljaju uzorke sa uspešnim testom savijanja, a tamno siva polja uzorke kod kojih je došlo do loma pri testu savijanja.

Note: The Value in each cell represent the number of pores in the weld respective to the gas flow and the draught velocity. Cells which have no value represent that the x-ray examination was not done . A cell which is highlighted with a light or dark grey colour represent a success or a failure in the bend tests results.

Ovaj nedostatak ubraja se u ozbiljne greške koje direktno utiču na konstrukcionu čvrstoću zavarenih spojeva. Nedostatak stapanja najčešće nastaje zbog nedovoljnog unosa toplote, kao i u slučaju da je metalna kupka prevelika, zbog loše tehnike zavarivanja [3]. Primenjeni unos toplote za testove savijanja bio je optimalan. Radiografskoim ispitivanjem nije potvrđeno prisustvo nedovoljnog stapanja u zavarenim uzorcima. Na osnovu ovakvog koncepa, može se zaključiti da do loma uzoraka pri ispitivanju savijanjem nije dolazilo zbog nedovoljnog stapanja. Sledeći mogući nedostatak koji može uticati na test savijanjem su zajedi. Iako ovaj nedostatak nije svrstan u značajne, ipak može uticati na konstrukcionu čvrstoću zavarenog spoja. Zajedi nastaju pri visokom naponu luka, velikoj brzini zavarivanja ili nepogodnom položaju zavarivanja.

Na osnovu radova Funderburka (1999) brzina zavarivanja i napon luka su imali optimalne vrednosti, s obzirom na izračunati unos toplote [10]. Vizuelnim pregledom pre testa savijanjem nije uočena pojava zajeda, pa se može zaključiti da zajedi nisu uzrokovalilom pri testu savijanja. Jedan od mogućih nedostataka koji je mogao imati uticaj na test savijanja je poroznost. Iako je poroznost u velikoj meri prihvatljiva greška, ipak može uticati na konstrukcionu čvrstoću zavarenog spoja. Postoji nekoliko uzroka pojave poroznosti: nedovoljna jačina struje zavarivanja, velika brzina zavarivanja, oksidacija komada koji se zavaruje, neodgovarajući zaštitni gas i jaka promaja koja oduva zaštitni gas ostavljajući rastopljeni metal izložen uticaju vazduha [12].

Kao što je prethodno rečeno, jačina struje i brzina zavarivanja primenjeni za zavarivanje uzoraka za test savijanja, imali su optimalne vrednosti [10]. Prema tome, lom uzoraka usled neodgovarajuće jačine struje

Another possible welding flaw that could have affected the bend test results is lack of fusion. This welding flaw is considered to be a serious defect, which has direct effect on the structural strength of the weld. This welding flaw commonly occur when heat input is to small and also when the welding puddle is too large due to poor welding technique [3].

The welding heat input adopted to weld the bend test specimens were of optimum value based on the heat input values. Furthermore, the radiographic results validates that lack of fusion in the weldment is not present. With this concept, it could be concluded that failure specimen due to lack of fusion in the weldments is excluded.

Another possible welding flaw that could have affected the bend test results is undercut. Although this type of welding flaw is not considered to be a serious defect,it can still effect the structural strength of the weld. This type of flaw occurs when welding with high voltage, high welding speed and/or unfavorableposition of the welding torch. Based on literature presented by Funderburk (1999) the welding speed and the arc voltage adopted in the welding of the specimens were of an optimum value based on the calculated heat input value [10]. Moreover, a visual examination of the test specimens before bending verified that there was no undercut present in the weld area. With this concept, it could be concluded that failure of the specimens due to undercut in the weldment is excluded.

Another possible welding flaw that could have affected the bend test results is porosity. Although this type of flaw is highly acceptable, it can still effect the structural strength of the weld. There are several causes that could develop this type of welding flaw, namely: low welding current, high welding speed,



i/ili brzine zavarivanja može biti isključen. Prema EN 9692-1: Priprema zavarenih spojeva čeličnih materijala, svi uzorci su pre zavarivanja očišćeni od nečistoća i neželjenih oksida na površini. Znači, lom uzoraka usled poroznosti uzrokovane oksidacijom može biti isključen.

Simulacije i testovi koje je sproveo Remsey (2012), doveli su do zaključka da pokrivenost zaštitnim gasom i kvalitet zavarenog spoja opadaju sa porastom brzine promaje [5]. Slični eksperimenti koje su sproveli Johnson (2000) i Bonesewski potvrdili su da povećanje bočne promaje dovodi do povećane poroznosti, što kao krajnji rezultat ima sniženje izduženja i apsorbovanu energiju zavarenih uzoraka [5,7].

Na osnovu tri slična eksperimenta koje su sproveli Ramsey, Johnson i Boneszewski može se zaključiti da do loma uzoraka pri ispitivanju savijanjem dolazi zbog neodgovarajućeg zaštitnog gasa i/ili velike brzine promaje. Nedovoljna zaštita zavarivačkog kupatila doprinosi porastu poroznosti, sa direktnim uticajem na konstrukcionu čvrstoću zavarenog spoja.

Radiografsko i vizuelno ispitivanje, površinske i zapreminske greške

Diskusija rezultata radiografskog i vizuelnog ispitivanja, površinske i zapreminske greške

Iz tabela 5 i 6 može se videti da rezultati koji se odnose na površinsku i zapreminsku poroznost imaju slične vrednosti. Uzorci koji su zavareni u uslovima bez promaje ili pri slaboj promaji, bez obzira na protok zaštitnog gasa, nisu imali vidljivu površinsku i zapreminsku poroznost. Sa druge strane, kada brzina promaje dostigne 0.9 m/s i više, pojavljuje se površinska i zapreminska poroznost, čiji udeo zavisi od protoka zaštitnog gasa.

Pri brzinama promaje od 1.2 do 3.0 m/s, poroznost raste u zavisnosti od protoka zaštitnog gasa. Kritični odnos brzine promaje i protoka zaštitnog gasa može biti određen na granici uspešnih i neuspešnih testova savijanja i iznosi približno 0.11.

Rezultati radiografskog ispitivanja prikazani u tabeli 7 govore da minimalni broj pora nastaje pri zavarivanju u uslovima bez promaje, bez obzira koliki je protok zaštitnog gasa. Takođe, uočava se da pri određenom protoku zaštitnog gasa, broj pora raste sa porastom brzine promaje. Pri daljem povećanju brzine promaje, pojedinačne pore prelaze u nakupine. Nakupine pokazuju da je došlo do potpunog skretanja mlaza zaštitnog gasa sa rastopljenog metala, koji je ostao izložen delovanju kiseonika i azota.

Na osnovu rezultata dobijenih u tri slična eksperimenta, može se zaključiti da postoji direktna zavisnost između pojave pora na površini i unutar zavarenog spoja. Zavisnost je iskazana kroz odnos brzine promaje i protoka zaštitnog gasa, sve do protoka od 35 l/min.

Tokom opisanog eksperimenta zavarivanja, uočeno je da razbrizgavanja gotovo nema u slučajevima kada je

oxidized work piece, inadequate shielding gas and/or high velocity draughts that blow away the shielding gas and exposing the weld pool to air contaminants [12]. As discussed earlier in this section, the welding current and the welding speed adopted in the welding of the bend test specimens were of optimum values [10]. Therefore, failure of the specimens due to porosity in the weldment due to inadequate welding current and/or welding speed can be excluded. With reference to EN 9692-1 'Joint preparation for steel', all the specimens were cleaned from any mill scale and other unwanted surface oxides before welding. Therefore, failure of the specimens due to porosity in the weldment due to an oxidized work piece can be excluded.

Simulations and tests done by Ramsey (2012), concluded that the shielding gas coverage and quality of the weld decreased as the side draught velocity increased [5]. Other similar experiment done by Johnson (2000) and Bonesezwski concluded that as side draught increased, porosity in the weldment increased that lead to the reduction of both elongation and energy absorption of the welded specimens [5, 7].

With these three similar experiments done by Ramsey (2012), Johnson (2000) and Boneszewski, it can be concluded that the failure of the bend tests specimen occurred due to inadequate shielding gas and/or high velocity draughts. This lack of shielding of the weld pool, contributed to an increase of porosity in the weldment of the specimens which had direct effect on the structural strength of the weld.

X-Ray and Visual Examination (Surface and Section Discontinuities

Discussion on X-Ray and Visual Examination Results (Surface and Section Discontinuities)

From tables 5 and 6, it can be observed that results concerning surface and section porosity are very similar to each other. It can be observed that welds which were subjected to no, or light draughts regardless the flow of the shielding gas, have no visible porosity on the surface or section of the weld. On the other hand, once the draught velocity increased above 0.9 m/s, porosity on the surface and section of the welds appeared with respect to the flow of the shielding gas. As the draught velocity increased from 1.2 m/s to 3 m/s, porosity at the welds increased respectively to the flow of the shielding gas. A critical ration of shielding gas to side draught velocity can be determined, in doing so presenting the pass-fail boundary which is approximately of 0.11.

X-ray examination results presented from table 7, it can be observed that the minimum number of pores in the butt weld occurred when there was no draught velocity on the weld, despite the increased of the flow of the shielding gas. Moreover, it can be observed that the number of pores in the weld for individual shielding gas flows, increased with an increase of the draught velocity. As the draught velocity kept on



poroznost mala, a da razbrizgavanje raste proporcionalno porastu poroznosti. Pojava razbrizgavanja najizraženija je oko spojeva sa visokim sadržajem poroznosti.

Procena i obrazloženje rezultata, površinske i zapreminske greške

Nivo prihvatljivosti poroznosti u zavarenim spojevima je dosta visok, mada poroznost može uticati na konstrukcionu čvrstoću spoja. Nekoliko faktora utiče na nastanak poroznosti u zavarenom spoju: slaba struja zavarivanja, velika brzina zavarivanja, oksidi na radnom komadu, neodgovarajući ili suvišan protok zaštitnog gasa i/ili velika brzina promaje tokom zavarivanja [12]. Na osovu podataka koje je objavio Funderburk (1999) i proračuna količine unete toplote, struja zavarivanja i brzina zavarivanja ploča su izabrani na optimalnom nivou [10].

Stoga se nastanak poroznosti kao posledica nedovoljne jačine struje zavarivanja ili male brzine zavarivanja može isključiti. Takođe, mogućnost nastanka pora usled prisustva oksida i troske na radnom komadu je isključena, zbog detaljne pripreme i čišćenja sprovedenih pre zavarivanja. Ispitivanja koja su sproveli Vasco (2009) i Dreher (2010), pokazuju da slab ili prekomeran protok zaštitnog gasa mogu dovesti do nastanka poroznosti [1,4]. Pore nastaju usled nedovoljne zaštite ili turbulencija pri protoku zaštitnog gasa. Na osnovu rezultata dobijenih primenom ove tri metode ispitivanja, može se zaključiti da poroznost nije uzrokovana neodgovarajućim ili prevelikim protokom zaštitnog gasa. Ovaj zaključak je zasnovan na činjenici da pri odsustvu promaje ne dolazi do nastanka pora u spoju, bez obzira na protok zaštitnog gasa. Simulacije i ispitivanja koja je sproveo Ramsey (2012) pokazali su da pojava poroznosti raste sa porastom brzine promaje [5].

Takođe, kada se uporede rezultati vizuelnog, radiografskog i ispitivanja savijanjem, vidljivo je da su oni slični po pitanju poroznosti. Sa porastom brzine promaje, raste poroznost u zavisnosti od protoka zaštitnog gasa. Uzimajući u obzir istraživanje koje je sproveo Ramsey i sličnost rezultata tri istraživanja, može se zaključiti da do poroznosti u zavarenim spojevima dolazi pri velikim brzinama promaje u zavisnosti od protoka zaštitnog gasa.

Razbrizgavanje se ne smatra defektom već samo nedostatkom pri zavarivanju. Razbrizgavanje utiče na izgled površine komada, ali ne utiče na konstrukcionu čvrstoću spoja. Sato (2001) je objavio da količina razbrizgavanja koja nastaje tokom zavarivanja zavisi od mešavine zaštitnog gasa. Razbrizgavanje se pojačava sa povećanjem sadržaja CO2 u mešavini zaštitnog gasa [9].

Za eksperimente sprovedene u ovoj studiji, zavisnost razbizgavanja od sadržaja CO2 je nebitna, zbog toga što je sastav gasne mešavine bio konstantan tokom ispitivanja. Na pojavu razbrizgavanja može uticati površinska oksidacija osnovnog materijala. U ovoj

increasing, individual pores resulted in a weld with a cluster of pores for a specific gas flow. This cluster of pores indicate that the shielding gas was completely blow away from the weld pool exposing the weld to oxygen and nitrogen. Based on the results obtained from these three examination methods, it can be concluded that there is a direct relationship on the porosity on the surface, section and internal structure of the welds. This relationship is between the draught velocity and the flow of the shielding gas; up till a flow of 35 l/min.

During the welding experiment, it was observed that spatter levels were to the minimum as there was no porosity visible on the surface of the weld. On the other hand, as the amount of porosity increased, the amount of spatter increased gradually. Moreover, the amount of spatter around the weld was found to be the highest for those welds that have high amount of porosity.

Evaluating and Justifying Results (Surface and Section Discontinuities)

Porosity in the weldment is highly acceptable although it can still affect the structural strength of the weld. There are several factors that will contribute to the generation of pores in the weldment, namely: low welding current, high welding speed, oxidized work piece, inadequate or excessive flow of shielding gas and/or high draught velocities on the weldment during the welding process [12].

Based on the literature done by Funderburk (1999), and on the calculations made on the heat input, the welding current and the welding speed adopted in the welding of these plates were of optimum value [10].

Therefore, the possibility that porosity in the welds due to low welding current and/or high welding speed can be excluded. Moreover, the possibility that the formation of porosity in the weld due to an oxidized work piece can also be eliminated since all plates were cleaned from any mill scale and oxides. Therefore, it can be concluded that the formation of porosity due to oxides in the base material can be eliminated.

Tests done by Vasco (2009) and Dreher (2010), concluded that too low or high shielding gas flows could induce porosity in the weld [1, 4]. This formation of porosity is produced due to insufficient shielding of the weld pool or a turbulent flow of the shielding gas.

Based on the test results obtained from these three examination methods, it can be concluded that porosity was not the result of inadequate or too high shielding gas. This conclusion was based on the fact that when there was no side draught on the weld, no pores were visible in the weld, regardless the flow of the shielding gas. Simulations and tests conducted by Ramsey (2012), show that as the draught velocity on the weldment increased, porosity in the weldment increased [5].



studiji, svi uzorci su očišćeni od kovarine i oksida, pa jezavisnost razbrizgavanja od prisustva oksida nebitna. Nadalje, može se zaključiti da pojavi razbrizgavanja oprinosi jedino sadržaj poroznosti u zavarenom spoju.

Poređenje rezultata Primetna je sličnost između prikazanih rezultata ispitivanja. Prva sličnost je da bez obzira na povećanje protoka zaštitnog gasa do 30 l/min, pri uslovima bez promaje integritet spoja ima optimalne vrednosti. Ovo je moguće zaključiti na osnovu činjenice da nije bilo površinskih i/ili zapreminskih diskontinuiteta u zavarenom spoju, što su potvrdili rezultati ispitivanja savijanjem.

Još jedna uočena sličnost je da povećanje delovanja bočne promaje dovodi do porasta poroznosti i pada konstrukcione čvrstoće zavarenog spoja. Takođe, porast protoka zaštitnog gasa dovodi do bolje zaštite rastopljenog metala tokom procesa zavarivanja, uz smanjenje ili eliminaciju poroznosti i poboljšanje konstrukcione čvrstoće zavarenog spoja.

Može se zakljućiti da postoji direktna zavisnost integriteta spoja od brzine promaje i protoka zaštitnog gasa. Kada poredimo rezultate dobijene vizuelnim pregledom površine i poprečnog preseka, može se uočiti direktna zavisnost između površinske i zapreminske poroznosti.

Količina pora na površini srazmerna je količini pora u preseku spoja. Porast površinske poroznosti značio je i porast zapreminske poroznosti, bez obzira na brzinu promaje i protok zaštitnog gasa. Dalje, poređenjem rezultata testa savijanjem sa rezultatima radiografije, uočava se da je savijanje uspešno jedino kada broj pora ne pređe maksimum od 17 pora. Kada broj pora pređe ovu vrednost, dolazilo je do loma pri testu savijanjem.

Faktori i parametri koji mogu imati uticaj na dobijene rezultate Ovaj odeljak će dati osvrt na to kako promena činilaca i parametara zavarivanja može uticati na prikazane rezultate. Sa gledišta pojave poroznosti, porast udela CO2 u mešavini zaštitnog gasa utiče na smanjenje osetljivosti na uticaj promaje. Sa druge strane, Menzel (2002) je naveo da porast udela CO2 dovodi do povećanog razbrizgavanja uz istovremeno smanjenje žilavosti spoja [9]. Sledeći faktor je oblik mlaznice.

Za sve eksperimente u ovoj studiji, korišćena je mlaznica prečnika 14mm sa 3mm uvučenim kontaktnim vrhom i sa 6 otvora prečnika 2mm. Primenom mlaznice sa izvučenim kontaktnim vrhom, došlo bi do porasta sadržaja kiseonika u rastopljenom metalu [2].

Prema Dreheru (2009) smanjenje prečnika otvora na mlaznici dovelo bi do promene dinamike zaštitnog gasa, povećavajući kinetičku energiju turbulencije i brzinu kretanja zaštitnog gasa. Ovo povećanje kinetičke energije dovelo bi do povećane poroznosti u

Moreover, when comparing results obtained from this visual examination, the bend test and radiographic examination, similar results were obtained with respect to porosity. As draught velocity on the weldment increased, porosity in the weldment increased with respect to the shielding gas flow. Taking into account the experiment done by Ramsey and the similarities of the results between the three experiments; it can be concluded that porosity in the welds was obtained because of the high draught velocities on the weld respectively to the flow of the shielding gas.

Spatter is not considered to be a welding defect but only a welding flaw. Spatter will only reduce the surface finish of the weld but does not affect the structural strength of the weld. Sato (2001) reported that the amount of spatter generated during welding is dependent on the shielding gas mixture. The amount of spatter generated increased with the increase of CO2 levels in the shielding gas mixture [9]. For the experiments done in this dissertation, this increase of spatter due to increase of CO2 levels in the shielding gas mixtures is irrelevant. This is because the shielding gas mixture was kept constant throughout all the experiments.

Another factor that contributes to the amount of spatter generated during welding, is the amount of surface oxides present on the base material. With reference to the experiment done in this study, this increase in spatter levels with the increase of oxides on the surface of the base material is irrelevant. This is because the plates were cleaned from any mill scale and oxides before the welding process. Therefore, it can be concluded that the only factor that contributed to the amount of the spatter generated is the amount of porosity in the weldment.

Comparing Results It can be observed that similarities between the test results are present. The first similarity is that all tests demonstrated that despite the increase in the shielding gas flow till a flow of 30 l/min, in no draught velocity conditions, the integrity of the weld was of optimum value. This could be concluded based on the fact that there were no surface, section and/or internal discontinuities in the weldment, with the bend test results validating this conclusion. Another similarity that could be observed was that an increase in the side draught velocity on the weld, lead to an increase in the porosity and the decline in the structural strength of the weld.

Moreover, the increase in the shielding gas flow, resulted in better shielding on the weld pool during the welding process, reducing or eliminating porosity and restoring the structural strength of the weld.

It could be concluded that there is a direct relationship on the integrity of the weld between the side draught velocity and the flow of the shielding gas. When comparing the results obtained from the surface and



zavarenom spoju pri povećanju protoka gasa, bez obzira na brzinu promaje [6]. Nadalje, smanjenje ili povećanje prečnika mlaznice dovodi do smanjenja ili povećanja osetljivosti na promaju, uz smanjenje ili povećanje poroznosti, respektivno [5].

Povećanje nivoa jačine struje povećava osetljivost na promaju. Ova pretpostavka je bazirana na rezultatima koje je objavio Dreher (2010). Poslednja dva činioca koji su doprineli različitim rezultatima su položaj pištolja i tehnika zavarivanja. Povećanje rastojanja između mlaznice i radnog komada, dovodi do povećanja struje luka uz povećanje osetljivosti na uticaj promaje.

Tehnika zavarivanjem unapred korišćena je u svim eksperimentima. Pri zavarivanju unapred, raspodela zaštitnog gasa je mnogo efikasnija. Ako bi koristili tehniku zavarivanja unazad, dobili bi znatno veću osetljivost na promaju zbog lošije distribucije zaštitnog gasa.

ZAKLJUČCI

1. Poroznost je glavna zavarivačka greška čiji je nastanak u direktnoj vezi sa brzinom promaje na mestu zavarivanja. Količina razbrizgavanja oko spoja raste progresivno sa porastom poroznosti. Nivo razbrizgavanja je najveći za spojeve kod kojih se poroznost javlja u obliku nakupina.

2. Vizuelni pregled i radiografija su pokazali da na spojevima koji nisu izloženi promaji, nema poroznosti na površini niti na preseku spoja, pri čemu povećanje protoka zaštitnog gasa nije imalo uticaja. Ovaj nalaz nije se uklapao u ranija istraživanja, gde je objavljeno da poroznost nastaje ako se prekorači protok gasa od 18 l/min [1]. Ova razlika je nastala uglavnom zbog toga što turbulentno kretanje zaštitnog gasa ne zavisi samo od oblika mlaznice, već i od prečnika izlaznih otvora na mlaznici [6].

3. Iz dobijenih rezultata došlo se do podatka da rasipanje zaštitnog gasa i kontaminacija zavarenog spoja nastaju već pri brzini promaje od 0.9 m/s.

4. Svi rezultati ispitivanja pokazali su da za određen protok gasa, povećanje brzine promaje dovodi do povećanja poroznosti, što na kraju dovodi do obrazovanja nakupina pora. Te pore su nastale nakon što je zaštitni gas oduvan sa rastopljenog metala, a spoj je ostao izložen štetnom dejstvu vazduha (kiseonik, azot). Povećanje poroznosti dovelo je do pada konstrukcione čvrstoće spoja.

5. Svi rezultati ispitivanja potvrdili su da u opsegu protoka gasa do 30 l/min, konstrukciona čvrstoća i integritet spoja zavise od odnosa brzine promaje i protoka zaštitnog gasa.

6. Na osnovu ispitivanja površine i preseka spoja, došlo se do zaključka da su površinska i zapreminska poroznost direktno proporcionalne.

section visual examination, it can be observed that there is a direct relationship between surface and section porosity.

The amount of pores at the surface of the weld correlated with the amount of pores at the section of the weld. An increase of surface pores denoted to an increase of section pores, despite the side draught velocity and the flow of the shielding gas. Furthermore, when comparing the bend test results with the results obtained from the x-ray examination, it can be observed that a success in the bend test experiment was obtained only when the number of pores did not exceed a maximum of seventeen pores. Once this value was surpassed, a failure in the bend test results was imminent.

Factors and parameters that may have an effect on the attained results

This section will provide a reflection on how changing the welding factors and parameters may effected the results presented. With respect to porosity, increase in CO2 levels in the shielding gas mixture would have led to a decrease of draught sensitivity. On the other hand, as discussed by Menzel (2002), this increase in CO2 levels in the shielding gas composition would have led to an increase of spatter levels and decrease in the toughness of the weld [9].

Another factor is the nozzle design. For the experiment in this study, a 14 millimeter diameter nozzle with a contact tip withdraw by 3 millimeter and with 6 borehole of 2 millimeter diameter each was used to conduct all welding experiment. An increase in oxygen levels in the weld pool would have been obtained if the contact tip would have been extruded out from the nozzle [2].

Moreover, as reported by Dreher (2009), decreasing the borehole diameter, would have changed the shielding gas dynamics in the nozzle, increasing the kinetic turbulent energy and velocity of the shielding gas. This increase in kinetic energy would have resulted in porosity in the weldment with the increase of the flow of the shielding gas, despite the side draught velocity [6].

Furthermore, reducing or increasing the nozzle diameter, would have reduced or increased the draught sensitivity therefore reducing or increasing porosity in the weld respectively [5]. Moreover, increasing the arc current levels would have increased the draught sensitivity. This assumption was made based on the results obtained by Dreher (2010).

The final two factors which would have contributed to different results are the torch position and welding technique. Increasing the nozzle to work piece distance would have led to an increase in the arc current hence the increase in draught sensitivity. The forehand welding technique was used for all welding experiments. If the backhand welding technique was



7. Svi rezultati ispitivanja pokazali su da kritični odnos brzine promaje i protoka gasa iznosi približno 0.1. Pri poređenju ovih rezultata sa ranije objavljenim istraživanjima, osetljivost na promaju je veća nego što je predviđeno [5]. Ostali činioci koji su mogli doprineti ovim razlikama rezultata su položaj pištolja pri zavarivanju, jačina struje, sastav mešavine zaštitnog gasa i prečnik na otvorima mlaznice.

LITERATURA / REFERENCES

[1] T. Vasco, Advances in Gas Metal Arc Welding and Application to Corrosion Resistant Alloy Pipes, vol. 1, NA: Cranfield University, 2009-2010.

[2] D. A. Johnson, “Experimental examination of welding nozzle jet flow at cold flow conditions,” Science technology of welding and joining, vol. 11, no. 6, pp. 681-687, 2006.

[3] K. Weman and G. Linden, MIG welding guide, 1st ed., London: Woodhead Publishing, 2006.

[4] M. Dreher, “Numerical optimization of gas metal arc welding torches using ANSYS CFX,” Annual assembly & international conference of the international institute of welding, vol. 3, no. 63, pp. 11-17, 2010.

[5] G. M. Ramsey, “A computational fluid dynamic analysis of the effect of side draughts and nozzle diameter on shielding gas coverage during gas metal arc welding,” Journal of material processing technology, vol. 1, no. 2, pp. 1694-1699, 2012.

[6] M. Dreher, “Simulation of shielding gas flow inside the torch and in the process region of GMAW,” 2009. [Online]. Available: http://tudresden.de/die_tu_dresden/fakultaeten/fakultaet_maschinenwesen/if/fue/forschung/arbeitsgruppelichtbogenschweissen/veroeffentlichungen_Dokumente/simulationofshieldinggasflowinsidethetorchandintheprocessregionofwelding.pdf. [Accessed 20 December 2012].

[7] M. Johnson, “State of the art report on welding and inspection,” California, 2000.

[8] A. F. Moreira, J. Gallego, R. C. Tokimatsu and V. A. Ventrella, “The effects of shielding gas mixture on inclusion distribution for MIG welding porcess,” 2007. [Online]. Available: http://www.dem.feis.unesp.br/maprotec/publica/publicado_2007/csbmm2007_mig.pdf. [Accessed 7 Augest 2012].

[9] T. Sato, “Influence of shielding gases on quality and efficiency in gas shielded arc welding,” Welding international, vol. 15, no. 8, pp. 616-619, 2009.

[10] R. Funderburk, “A look at heat input,” Welding innovation, vol. 16, no. 1, pp. 1-4, 1999.

[11] K. Kishore, “Analysis of defects in gas shielding arc welding of AISI 040 steel using taguchi method,” ARPN journal of engineering and applied science, pp. 37-41, 2010.

[12] ISF, “Mercury,” 2007. [Online]. Available: http://mercury.kau.ac.kr/welding/Welding Technology II - Welding Metallurgy/Chapter 9 – Welding Defects.pdf. [Accessed 18 September 2012].

adopted for the experiments, the draught sensitivity would have been much higher. The latter is due to the fact that gas distribution is much more efficient when welding with the forehand welding technique.

CONCLUSIONS

1. Porosity is the main welding discontinuity which has direct relationship with the draught velocity on the weld. The amount of spatter around the weld increased gradually, as the amount of porosity increased. Spatter levels were the highest for those welds which have a cluster of pores.

2. Visual and x-ray examinations have shown that when the welds were not subjected to any draughts, despite an increase in the flow of shielding gas, no pores were generated on and at the section of the welds. This finding was not conformable to past literature, were it was reported that porosity would be generated when exceeding a flow of shielding gas of 18 l/min [1]. The main reason for this difference in the results was due to the fact that the turbulent kinetic energy of the shielding gas is not only dependent on the nozzle diameter, but also on the borehole diameter [6].

3. From the test results it was observed, that dissipation in shielding gas and contamination of the weldment occurred at draught velocities as low as 0.9 m/s.

4. All test results have shown that for a specific flow of gas, increasing the draught velocity resulted in an increase of the amount of pores, which finally resulted in a cluster of pores. These pores, were the result of the shielding gas being blown away from the weld pool, exposing the weld to the harmful effects of air (oxygen and nitrogen). This increase in porosity, led to the decline in the structural strength of the weld.

5. All test results have verified that till a flow of shielding gas of 30 l/min, there is a correlation on the structural strength and integrity of the weld between the draught velocity and the flow of the shielding gas.

6. From the surface and section examinations, it was concluded that the amount of porosity on the surface and at the section of the weld are directly proportional to each other.

7. All test results have shown that the critical ratio of the shielding gas to the side draught velocity was found to be of an approximant value of 0.1. When comparing the results obtained with tests results from past literature, the draught sensitivity was higher than anticipated [5]. Other factors might have contributed to this difference in the results, namely; torch position, arc current, shielding gas composition and boreholes diameter.

razumevanje dinamike zaštitnog gasa i poboljšanje kvaliteta

Documents