failure analysis of welded components.pdf

IIW International Congress in Central and East European Region Slovakia, High Tatras, Stará Lesná, 14 – 16 October 2009

Jeager lecture

FAILURE ANALYSIS OF WELDED COMPONENTS – IMPORTANCE FOR TECHNICAL PRACTICE

Peter Bernasovský, Welding Research Institute – Industrial Institute of SR – Slovakia, [email protected]

ABSTRACT The contribution deals with possibilities of metallographical identification of crack types in steel structures. The main features of cracks predominately in welded joints are illustrated on real cases of breakdowns. Keywords: Crack types, Welded joints, Steel structures, Case studies, Cold cracking, Fatigue cracks, LME, SSCC

1. INTRODUCTION

The welded joints are most critical places of steel structures due to high residual stresses and stress concentration ( constructional and structural notches) and due to possible cracking originated from welding process. In service, cracks can easily propagate, either suddenly (brittle fracture) or gradually (fatigue, creep or corrosion). It is very important to distinguish the crack types, as different measures are often taken to eliminate the various cracks, for instance to eliminate hot cracking usually lower welding heat input is required, but this is unfavorable for cold cracking. In such cases microfractographical analysis gives us a very useful tool.

2. CRACK TYPES IN WELDED JOINTS There are four basic crack types which occur in the welded joint of steels, namely: hot cracks, cold cracks, lamellar tearing and reheat cracks. The cracks can be found in the weld metal (WM) or in the heat affected zone (HAZ) from where they can propagate to parent metal or remain in the weld in dependence on metallurgical factors or stresses. Even though there are some differences in the shape of the various cracks on metallographical cross sections we must often resort to subsequent microfractographical analysis to identify them more precisely. If the crack leads to fracture, its surface can be analyzed immediately. In the case of a shorter or an internal crack the section with a crack must be broken. This chapter will point out the morphological features of the crack types and it does not deal with the mechanism of crack formation. 2.1. Hot cracks

Three types of hot cracks occur in welded joints, namely:

- solidification cracks, which are formed during solidification in the weld metal, and are most often orientated towards the weld axis, in the direction of columnar crystals, they are of typical interdendritic character (Fig. 1)

- liquation cracks, which are formed in the underbead zone of the base metal, or in multi-pass weld of the weld metal

- polygonization cracks, which are formed in the lower temperature zone (800 – 1100°C), in the heat affected zone and the weld metal


If hot cracks are open (i. e. on the surface) they can generally be distinguished from other types of cracks by their oxidized black and brown tinged surface. This is more difficult with closed cracks. Hot cracks are usually shorter and more laminated than other types, and they are always of intercrystalline character. The residues both of solidified liquid film in the form of eutectic secondary phases (Fig. 2) and of round grains with typical soliditied bridges between them in the case of soluble lower temperature elements (Fig. 3) can be detected on the surfaces of solidification and liquation cracks. The surfaces of polygonization cracks are pure, and without secondary phases. Closed hot cracks which are not opened to air are characterized by thermal faceting of their surface (Fig. 4), which is present at above 900°C as a result of metal ion evaporation into vacuum and this unambiguously differentiates hot cracks from lower temperature ones.

Figure 1 – Interdendritic solidification crack fracture surface

Figure 2 – Eutectic TiX on the liquation Figure 3 – Solidified bridges on the liquation crack fracture surface crack surface

Figure 4 – Thermal faceting of liquation Figure 5 – Intercrystalline cleavage

crack surface character of the cold crack


2.2 Cold cracks Cold cracks (also called hydrogen-induced cracks) are longer, less laminated and generally more open than hot cracks . This is due to higher contraction stresses in the time of their formation. Their open surface is metallic lustrous or has a blue tinge. The oxidation layer is comparatively thin. The initial fractured areas are predominantly of intercrystalline cleavage type (Fig. 5). Crack propagation is of transcrystalline cleavage or ductile character in dependence of microstructure, loading and temperature. 2.3 Lamellar cracks (tearing) Lamellar cracks are typical defects in rolled steels with clearly anisotropic properties (i. e. with lower through-thickness plasticity, due to their contamination by rolled planar impurities such as sulphides and aluminates or the presence of line structure). They are mainly formed in fillet welds and thick through-thickness loaded plates. When the crack propagates, the local decohesions at different plate levels are connected by the shear mechanism, which gives them a typical cascade character (Fig. 6), whereby they are easily recognizable on the etch. Crack propagation (joining) is thus also conditioned by the lower toughness of the metallic matrix in the heat affected zone. Fractographical analysis of the lamellar fractured area will qualitatively distinguish its characteristic zones with planar sulphides, as shown in Fig. 7.

Figure 6 – Lamellar tearing Figure 7 – Planar sulphides on the lamellar crack surface 2.4 Reheat cracks Reheat cracks are formed mainly during stress-relief annealing of welded joints. They are a serious problem in the huge structures of low-alloyed Cr, Ni, Mo, V steels. These are mostly microcracks situated in the coarse-grained underbead zone of the heat affected zone normal to the fusion line. Reheat cracks are of intercrystalline character with smooth , or more frequently intercrystalline ductile facets with carbide particles in the dimples (Fig. 8). Underclad cracks, which are formed during clad surfacing with a strip electrode, are a special type of reheat cracks where an annealing cycle is replaced by heat effect of the following deposit.


3. BREAKDOWN IN LOWER GATE OF DANUBE LOCK CHAMBER [1] A serious breakdown occurred in the left gate lock of Gabčíkovo dam lock chamber resulted in shut down of ship transport through the lower section of Danube river for more than five weeks. Steel structure of the gate was of all-welded box design of considerable size (18,5 m width, 21,95 m height and 2 min thickness). It was evident on first sight that the gate breakdown took place in brittle mode (Fig. 9). The employed steel was a high strength (S530Q) low–alloy Cr-Mo-B type with unfavorably high carbon equivalent of Ce = 0.79 to 0.82, see Table 1. The breakdown fractures initiated from the cracks in welded joints (Fig. 10, 11). In this case the cold, hydrogen induced cracks are concerned which were formed due to insufficient preheat temperature applied in welding. This is proved also by the presence of martensitic structure in the HAZ (Fig. 12 bottom). These cold cracks with intercrystalline surface appearance (Fig. 13) at repeating cyclic loading of the gate propagated by the mechanism of low-cycle (high-strain) fatigue with typical striations and openings of the elongated inclusions (Fig. 14) till they attained the critical size, when they followed in a sudden brittle fracture of a limited length with the characteristic river morphology (Fig. 15). Thanks to a box design, brittle fractures were arrested in the zones of stress drop, but due to stress redistribution during service and to reduced rigidity of whole structure, it finally resulted to breakdown fracture.

Figure 9 – Broken gate of Gabčíkovo dam chamber

Figure 8 – Intercrystalline ductile facets on the reheat crack surface


Table 1 - Chemical composition of plates in S530Q steel and MMAW and SAW weld metals (wt%)

Specimen h

(mm)

C Mn Si Cr Mo Ni Cu B V Ti S P Se

PM 20

35

0,152

0,161

1,51

1,33

0,35

0,28

1,47

1,44

0,55

0,51

0,12

0,24

0,07

0,06

0,0028

0,0030

-

-

0,01

0,01

0,0054

0,0101

0,015

0,0135

0,819

0,791

MMAW

WM

SAW

WM

20

35

0,096

0,067

1,47

1,24

0,47

0,43

0,61

0,39

0,47

0,43

1,26

1,21 -

-

-

0,02

0,05

-

-

-

-

-

-

-

-

Figure 10 – Hydrogen induced crack Figure 11 – Fracture surface of the stiffener weld

Figure 13 – Intercrystalline fracture of cold crack initiation

Figure 12 – Martensitic structure in the HAZ


Figure 14 – Crack prolongation by Figure 15 – Transcrystalline cleavage low cycle fatigue fracture surface

4. GAS PIPELINE FAILURES

Slovakia belongs to the countries with the densest network of high pressure transmission gas pipelines. Four large diameter lines are already passing through its territory, whereas the 5th line is being completed at present. The oldest gas pipelines are in service for almost four decades and though they are inspected periodically, from time to time some failures of pipelines occur. First 2 cases occurred on the 1st international gas pipeline which was built in 1965. This line, made of an old Russian steel 15 G2S (type L380N) low alloyed with Si (see Table 2) is the most problematic one at present. In this line low ductility and toughness of steel have met together which poor workmanship and defective corrosion protection of pipeline. In the first case (spirally welded pipe OD 720 x 8 mm) 1.8 m long crack was running along the spiral weld (Fig. 16). The initiation point was in the place where the spiral weld meets the tie strip weld. The spiral weld which exhibits very high misalignment of both linear and opposite runs is shown in Fig. 17a. The crack was initiated by LME (liquid metal embrittlement) of remelted copper (Fig. 17b). Cu came from abrading of Cu-electric contact plates which were applied at manufacturing of spiral welds at that time.

Table 2 – Chemical composition [wt %] and mechanical properties of the 15 G2S steel

C Mn Si P S Cr Ni Cu Ti

0,14-0,15 1,37-1,45 1,07-1,08 0,014 0,029 0,06 0,05 0,08 0,032

Re [MPa] Rm [MPa] A5 [%] ChV FATT

(50J.cm-2

) Upper shelf

387 – 404 591 - 612 22 - 27 + 140 C 55 J.cm

-2

Figure 16 – A crack along the spirally weld


a) b)

Figure 17 – a) Linear and opposite runs misalignment – poor workmanship

b) Liquid metal embrittlement – Cu in the HAZ of repair weld

Next case of gas leakage was revealed after a small earthquake in East Slovakia. The 180 mm crack occurred in the parent metal close to the girth weld (Fig. 18). The crack was initiated by sulphide stress corrosion cracks (SSCC) on the outer pipe surface (Fig. 19) in place of pipe insulation damage. It should be stressed that this section of line is burried in slightly aggressive soil surrounded by a marsh. The occurrence of SSCC was alleviated by appearance of an irregular bainitic – martensitic microstructure of the parent metal at that place (Fig. 20). It was found out that the appropriate part of the pipe corresponds just to the strip end, which probable had been exposed to higher cooling rate after reeling of the coil at Russian producer. The rest of the pipe had regular polygonal ferritic – pearlitic microstructure.

Figure 18 – Crack close to a girth weld

Figure 19 – SCC cracks on the surface


Figure 20 – Irregular microstructure at the end of a strip

The last case of failure did not happen during pipeline service. It appeared at construction (laying) of 4th transmission line OD 1420 x 18.6 mm made of X-70 steel grade spirally welded pipes. During bending of pipes on site a few pipes cracked along the spiral welds. The cracks in length up to 1 m always appeared in the same distance from the pipe end (Fig. 21). The crack occurrence corresponded with appearance of a cold impression on the outer spiral weld reinforcement (Fig. 22 and 23). It was found that such impression was formed due to incorrectly installed supporting steel rollers in the furnace used for pipe insulation. In this furnace the pipes are flame heated prior to PE insulation up to 300 0C, whereas they rotate (about 70 revolutions) on the rollers. In certain period of manufacture some pairs of rollers were taken out for the repair, and therefore the pipe weight (about 12 tons) was supported by the remaining pair (in 11.5 m distance). Since all inspections of pipes were performed prior to pipe insulation, such impression could not be revealed. This finding resulted in repeated ultrasonic inspection of all bent pipes manufactured in that period (about 300 pipes). The problem was, that the concerned pipes were already distributed in the section about 150 km long, which was already buried under the soil.

Figure 21 – Sketch of the fractured pipe


Figure 22 – Fracture of the spiral weld Figure 23 – Overlaping due to impression

5. FAILURES IN PETROCHEMICAL INDUSTRY 5.1 Sulphide stress corrosion cracking of air coolers in hydrockrack plant

This chapter deals with a case study of cracking in the air cooler of a hydrocrack plant in Bratislava. The air cooler was designed with a plug header (see the scheme of the header in Figs. 24a, b) with dissimilar welded joints, because the tubeplate (h = 40 mm), as an internal part of the plug header is made of the 15Mo3 steel and the tubes φ 25/2 mm are made of duplex steel DIN 1.4462. The welds were fabricated by TIG process using a special automatic welding machine and the ER 309Mo wire φ 0,6 mm, which is generally recommended for welding the given combination of steels. The chemical composition of the materials used is given in Table 3.

Table 3 – Chemical composition (wt%)

Material C Mn Si S P Cr Ni Mo Nb N

15Mo3 0,130 0,77 0,22 0,010 0,009 - - 0,12 - 1.4462 0,025 1,69 0,31 0,003 0,033 22 5,45 3,04 0,53 0,135

ER 309Mo

0,034 1,49 0,40 0,012 - 24 14,2 2,74 0,13 -

The working medium of air cooler consisted of hydrogen (up to 70%), hydrocarbons (up to 28%)

and hydrogen sulphide, water and other admixtures. The working pressure was 13,48 MPa and working temperature 50 to 122oC.

After about three weeks of service a leakage of the working medium through the welds was observed.

The cross section of a specimen extracted from the defective weld is shown in Fig.25. It is evident that a single pass weld is concerned. The crack propagated to weld metal in normal direction to surface (Fig.26). The arrows indicate the initiation points of short crack. The weld metal exhibited martensitic structure (Fig.27). Chemical microanalysis (EDX) of the weld metal revealed its considerable dilution by the tubeplate material:

element Cr Ni Mo wt% 10,03 6,42 1,55


As it is evident also from Fig.25 about 60% of the tubeplate material and only about 15% of the tube material fused into the weld metal. Based upon the calculated chromium and nickel equivalents (the content of non-analyzed elements was assessed approximately by the dilution degree) ECr = 11,9 and ENi = 10,2 plotted in the Schaeffler diagram, the weld metal structure was expected to be martensitic with a small proportion of austenite which well agrees with the morphological determination of the structure as well as with hardness measurements.

a) b)

Figure 24 – A scheme of the plug header in the aircooler

Figure 25 – Macrosection of the defected welded joint.

Weld metal hardness attained even the values of HV 435. The results of hardness measurement obtained from all zones of the welded joint are given in Table 4.

Table 4 – Hardness of the tube to tubeplate weld joint

Location

HV5

weld metal 412 – 435 tube – BM 248 – 262 tube – HAZ 257 – 262 tubeplate - BM 152 – 165 tubeplate - HAZ 322 – 381


From the viewpoint of SSCC the hardness should not exceed the values of HV 248 according to

NACE standard for low – alloy steels (for duplex steels, the hardness limit is higher, namely HV 285). Unacceptable hardness was observed also in the heat affected zone of the tubeplate which was

mostly bainitic with some martensitic islands. The performed analyses have shown, that the specific joint configuration and restricted welding

condition (access though a hole 25 mm in diameter on the opposite wall) resulted in undesired weld metal dilution by the tubeplate material leading to weld metal with a martensitic structure which is usually extremely susceptible to sulphide stress corrosion cracking. It was actually the main reason for necessity to repair all tube and tubeplate welded joints, including the joints where no corrosion cracking was observed yet.

Figure 26 – Sulphide stress corrosion crack in the WM.

Figure 27 – Martensitic microstructure of

the WM.

During the repair of air cooler, besides the martensitic weld metal also the hardened heat

affected zones of the tubeplate and tube ends (made of a duplex steel which is incapable to withstand the repeated heating) had to be removed. Thus the repair of all 3240 welded joints in tube tubeplate connections of the air cooler was a really demanding job. The repair procedure of air cooler was carried out in the following steps [2]:

a) removal of all original welds by use of a special tool, b) removal of all tubes from the air cooler plug headers, c) dehydrogenization of the plug headers by tempering, d) machining of grooves in the tubeplate bores, serving for tube end expanding by rolling with

a special roller, e) buttering beads of Ni-base alloy were deposited on the outs around the bore ends by use

of a special welding procedure, f) all butters were machined to the required shape, g) HAZ on the tubes were cut off (shortened by 10 mm), h) the buttering welds were tempered, i) the air cooler plugs were assembled with the tubes into the sections, j) tube to tubeplate welds were fabricated by use of Inconel electrodes and the leakage test

was performed, k) rolling expansions of tube to tubeplate by a special roller, l) pressure test of tube sections was performed


5.2 Welded joint failure of austenitic creep resisting Cr-Mn steel [3] In the hydrogenation fuel refining process, the hydrogen and hydrogen sulphide mixture attacks the material

of the radiation pipes (∅219 x 14 mm, seamless) which are subjected to temperatures ranging between 350 and 400°C. The newly developed chromium manganese austenitic steel (08Mn18Cr11V0,6) is concerned see Table 5. It belonged to the so-called economical austenitic steels in which nickel, which was lacking at the time, was replaced by manganese as a cheaper alternative.

The pipe failed in the heat affected zone (HAZ) of the circumferential welded joint near the fusion line. Cracks propagated from the inner surface, through the whole cross section, to the outer pipe surface (Fig. 28 – cross section). A more detailed picture shows that intercrystalline stress corrosion cracking was involved (Fig. 29).

A whole network of cracks of similar character, which propagated from the fusion line into the parent metal, could be observed on the inner side of the circumferential joint (Fig. 30). Weld metal fabricated by ERNiCr-3 electrode did not exhibit any corrosion attack traces .

Table 5 – Chemical composition of the Cr-Mn Stainless steel (wt.%)

C Mn Si Cr V P S

0,05 – 0,08 17,0 – 20,0 0,25 – 1,0 9,5 – 11,5 0,45 – 0,75 max. 0,045 max. 0,035

Figure 28 – Breakdown crack, unetched (x 10) Figure 29 – Intercrystalline character of crack in pipe wall (x 50) Figure 30 – Crack in fusion line, 10% Cr2O3 Figure 31 – Intercrystalline character of crack electrolyte etching (x 100) (x 100) EDX of corrosion products (wt %) S – 23.856,Cl – 4.187, Cr – 2.456, Mn – 7.049,Fe – 62.452


EDX energy dispersion analysis of the corrosion products on the intercrystalline surface of the fractured crack (Fig. 31) showed increased S and partially also Cl contents, which proves the corroding effect of polythionic acids and chloride ions. Sulphur and chlorine can originate from the raw material (MONA gas and H2 circulation gas) but also from the technological process of catalyst regeneration. It can be assumed that the intercrystalline corrosion attack was caused by a condensate of polythionic acids which were formed by the conjoined effect of sulphides, humidity and air oxygen during equipment shutdowns. The condensate itself was a consequence of the improper N2-filling up process of the furnace at beginning of the shutdown. According to the operator, 11 major overhauls and about 20 catalyst regenerations have been performed on the furnace during its 14 years of service.

As this Cr-Mn steel has Cr content just on the limit of passive state and it easily sensitive to intercrystalline corrosion, it was recommended that the entire Cr-Mn steel piping system to be replaced by Cr-Ni (AISI 321) steel which has a proven track record in similar applications. In addition, it was recommended that more careful inspections be carried out while in shutdown mode (e. g. blowing of the furnace by nitrogen). 5.3 Cracking of longitudinally welded thermowell tubes [4]

The small diameter tubes (∅11 x 2 mm) made of AISI 316 Ti steel serving as thermowells in the crude oil distillation chamber were concerned. Three weeks after changing of a crude oil supplier an extensive appearance of cracks was detected. The cracks had a feature of sulphide stress corrosion cracking (SSCC), see Fig. 32, 33 and a fracture surface with detection of S and Cl on Fig. 34. After etching we could recognize visible traces of cold plastic deformation as they are deformation twins and needles of the strain induces martensite (Fig. 35). It means the tubes were delivered in the cold rolling state, in which a steel becomes susceptible to SSCC. A possible remedy is to apply a solution annealing (1050°C/10 mins/water), after what an austenitic microstructure of favourable hardness was fully restored, see Fig. 36 and Tab. 6.

Figure 32 – Cross section of the tube Figure 33 – SCC in the pipe wall

Figure 34 – SSCC fracture surface, Figure 35 – Traces of cold plastic EDX (wt%), S – 3,79, Cl – 8,96 deformation


Figure 36 – BM austenitic structure after solution annealing

Table 6 – Hardness HV5 results

Location As delivered After solution annealing BM 306, 289, 306 125, 130, 126 WM 321, 332, 336 132, 133,133 The easiest way how to distinguish proper state of the thermowells is detection by a magnet (ferromagnetic martensite or paramagnetic austenite)

6. CONCLUSIONS The paper dealt with main metallurgical features of cracks in the welded joints, which where illustrated on selected real cases of breakdowns. To distinguish the crack types and to know their causes is very important for adoption of proper measures at their remedy.

REFERENCES

[1] Bernasovský, P. – Bošanský, J.: Welding and technological causes of breakdown of the lock gate of waterwork chamber on Danube river, JOM-10 Conference , Helsingor, May 2001 [2] Bernasovský, P. – Šinál, J. et al.: Welding procedure of tube – tubeplate joints shop repair of air coolers in Hydrocracking Unit, Invention proposal No. 55/90 [3] Bernasovský, P. – Országhová, J.: Welded joint failure of austenitic creep resisting Cr-Mn steel, Welding in the World, No. 7/8, 2009 [4] Bernasovský, P. – Britanová, A. – Paľo, M.: Case study of cracking in the thin wall thermowell tubes. Technical report ME 098, VUZ – PI SR, Bratislava 18.06.2009

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