esdep lecture note [wg2_]

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ESDEP WG 2

APPLIED METALLURGY

OBJECTIVE/SCOPE

The lecture briefly discusses the basics of the welding process and then examines the factors governing the weldability of structural steels.

PREREQUISITES

None.

RELATED LECTURES

Lectures 2.3: Engineering Properties of Steels

Lecture 2.4: Steel Grades and Qualities

Lecture 2.5: Selection of Steel Quality

Lecture 3.3: Principles of Welding

Lecture 3.4: Welding Processes

Lectures 11.2: Welded Connections

SUMMARY

The fundamental aspects of the welding process are discussed. The lecture then focuses on the metallurgical parameters affecting the weldability of structuralsteels. A steel is considered to exhibit good weldability if joints in the steel possess adequate strength and toughness in service.

ESDEP LECTURE NOTE [WG2:] http://www.fgg.uni-lj.si/kmk/esdep/master/wg02/l0600.htm

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Solidification cracking, heat affected zone - liquation cracking, hydrogen-induced cracking, lamellar tearing, and re-heat cracking are described. These effectsare detrimental to the performance of welded joints. Measures required to avoid them are examined.

1.1 A Brief Description of the Welding Process

Welding is a joining process in which joint production can be achieved with the use of high temperatures, high pressures or both. In this lecture, only the useof high temperatures to produce a joint is discussed since this is, by far, the most common method of welding structural steels. It is essentially a process inwhich an intense heat source is applied to the surfaces to be joined to achieve local melting. It is common for further "filler metal" to be added to the moltenweld pool to bridge the gap between the surfaces and to produce the required weld shape and dimensions on cooling. The most common welding processesfor structural steelwork use an electric arc maintained between the filler metal rod and the workpiece to provide the intense heat source.

If unprotected, the molten metal in the weld pool can readily absorb oxygen and nitrogen from the atmosphere. This absorption would lead to porosity andbrittleness in the solidified weld metal. The techniques used to avoid gas absorption in the weld pool vary according to the welding process. The main weldingprocesses used to join structural steels are considered in more detail below.

1.2 The Main Welding Processes

a. Manual Metal Arc welding (MMA)

In this process, the welder uses a metal stick electrode with a fusible mineral coating, in a holder connected to an electrical supply. An arc is struck betweenthe electrode and the weld area which completes the return circuit to the electricity supply. The arc melts both the electrode and the surface region of theworkpiece. Electromagnetic forces created in the arc help to throw drops of the molten electrode onto the molten area of the workpiece where the two metalsfuse to form the weld pool.

The electrode coating of flux contributes to the content of the weld pool by direct addition of metal and by metallurgical reactions which refine the moltenmetal. The flux also provides a local gaseous atmosphere which prevents absorption of atmospheric gases by the weld metal.

There are many types of electrodes. The main differences between them are in the flux coating. The three main classes of electrode are shown below:

1. Rutile: General purpose electrodes for applications which do not require strict control of mechanical properties. These electrodes contain a high proportionof titanium oxide in the flux coating.

2. Basic: These electrodes produce welds with better strength and notch toughness than rutile. The electrodes have a coating which contains calciumcarbonate and other carbonates and fluorspar.

3. Cellulosic: The arc produced by this type of electrode is very penetrating. These electrodes have a high proportion of combustible organic materials in theircoating.


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b. Submerged Arc Welding (SAW)

This process uses a bare wire electrode and a flux added separately as granules or powder over the arc and weld pool. The flux protects the molten metal byforming a layer of slag and it also stabilises the arc.

The process is used mainly in a mechanical system feeding a continuous length of wire from a coil whilst the welding lead is moved along the joint. A SAWmachine may feed several wires, one behind the other, so that a multi-run weld can be made. Submerged arc welding produces more consistent joints thanmanual welding, but it is not suitable for areas of difficult access.

c. Gas shielded welding

In this process, a bare wire electrode is used and a shielding gas is fed around the arc and weld pool. This gas prevents contamination of the electrode andweld pool by air. There are three main variations of this process as shown below:

1. MIG (metal-inert gas) welding - Argon or helium gas is used for shielding. This process is generally used for non-ferrous metals.

2. MAG (metal-active gas) welding - Carbon dioxide (usually mixed with argon) is used for shielding. This process is generally used for carbon and carbon-manganese steels.

3. TIG (tungsten-inert gas) - Argon or helium gas is used for shielding and the arc struck between the workpiece and a non-consumable tungsten electrode.This process is generally used for thin sheet work and precision welding.

1.3 Welded Joint Design and Preparation

There are two basic types of welded joints known as butt and fillet welds [1]. Schematic views of these two weld types are shown in Figure 1. The actualshape of a weld is determined by the preparation of the area to be joined. The type of weld preparation depends on the welding process and the fabricationprocedure. Examples of different weld preparations are shown in Figure 2. The weld joint has to be located and shaped in such a way that it is easilyaccessible in terms of both the welding process and welding position. The detailed weld shape is designed to distribute the available heat adequately and toassist the control of weld metal penetration and thus to produce a sound joint. Operator induced defects such as lack of penetration and lack of fusion can bedifficult to avoid if the joint preparation and design prevent good access for welding.


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1.4 The Effect of the Welding Thermal Cycle on the Microstructure

The intense heat involved in the welding process influences the microstructure of both the weld metal and the parent metal close to the fusion boundary (theboundary between solid and liquid metal). As such, the welding cycle influences the mechanical properties of the joint.

The molten weld pool is rapidly cooled since the metals being joined act as an efficient heat sink. This cooling results in the weld metal having a chill castmicrostructure. In the welding of structural steels, the weld filler metal does not usually have the same composition as the parent metal. If the compositionswere the same, the rapid cooling could result in hard and brittle phases, e.g. martensite, in the weld metal microstructure. This problem is avoided by usingweld filler metals with a lower carbon content than the parent steel.

The parent metal close to the molten weld pool is heated rapidly to a temperature which depends on the distance from the fusion boundary. Close to thefusion boundary, peak temperatures near the melting point are reached, whilst material only a few millimetres away may only reach a few hundred degreesCelsius. The parent material close to the fusion boundary is heated into the austenite phase field. On cooling, this region transforms to a microstructure whichis different from the rest of the parent material. In this region the cooling rate is usually rapid, and hence there is a tendency towards the formation of lowtemperature transformation structures, such as bainite and martensite, which are harder and more brittle than the bulk of the parent metal. This region isknown as the heat affected zone (HAZ).

The microstructure of the HAZ is influenced by three factors:

The chemical composition of the parent metal.1.The heat input rate during welding.2.The cooling rate in the HAZ after welding.3.

The chemical composition of the parent metal is important since it determines the hardenability of the HAZ. The heat input rate is significant since it directlyaffects the grain size in the HAZ. The longer the time spent above the grain coarsening temperature of the parent metal during welding, the coarser thestructure in the HAZ. Generally, a high heat input rate leads to a longer thermal cycle and thus a coarser HAZ microstructure. It should be noted that the heatinput rate also affects the cooling rate in the HAZ. As a general rule, the higher the heat input rate the lower the cooling rate. The value of heat input rate is afunction of the welding process parameters: arc voltage, arc current and welding speed. In addition to heat input rate, the cooling rate in the HAZ isinfluenced by two other factors. First, the joint design and thickness are important since they determine the rate of heat flow away from the weld duringcooling. Secondly, the temperature of the parts being joined, i.e. any pre-heat, is significant since it determines the temperature gradient which exists betweenthe weld and parent metal.

1.5 Residual Welding Stresses and Distortion

The intense heat associated with welding causes the region of the weld to expand. On cooling contraction occurs. This expansion and subsequent contractionis resisted by the surrounding cold material leading to a residual stress field being set up in the vicinity of the weld. Within the weld metal the residual stresstends to be predominantly tensile in nature. This tensile residual stress is balanced by a compressive stress induced in the parent metal [2]. A schematic viewof the residual stress field obtained for longitudinal weld shrinkage is shown in Figure 3. The tensile residual stresses are up to yield point in magnitude in theweld metal and HAZ. It is important to note that the residual stresses arise because the material undergoes local plastic strain. This strain may result incracking of the weld metal and HAZ during welding, distortion of the parts to be joined or encouragement of brittle failure during service.


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Transverse and longitudinal contractions resulting from welding can lead to distortion if the hot weld metal is not symmetrical about the neutral axis of afabrication [2]. A typical angular rotation in a single V butt weld is shown in Figure 4a. The rotation occurs because the major part of the weld is on one sideof the neutral axis of the plate, thus inducing greater contraction stresses on that side. This leads to a distortion known as cusping in a plate fabrication, asshown in Figure 4b. Weld distortion can be controlled by pre-setting or pre-bending a joint assembly to compensate for the distortion or by restraining theweld to resist distortion. Examples of both these methods are shown in Figure 5. Distortion problems are most easily avoided by using the correct weldpreparation. The use of non-symmetrical double sided welds such as those shown in Figure 2e and 2i accommodates distortion. The distortion from the smallside of the weld (produced first) is removed when the larger weld is put on the other side. This technique is known as balanced welding.


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It is not possible to predict accurately the distortion in a geometrically complicated fabrication, but one basic rule should be followed. This rule is that weldingshould preferably be started at the centre of a fabrication and all succeeding welds be made from the centre out, thus encouraging contractions to occur in thefree condition.

If distortion is not controlled, there are two methods of correcting it; force and heat. The distortion of light sections can be eliminated simply by using force,e.g. the use of hydraulic jacks and presses. In the case of heavier sections, local heating and cooling is required to induce thermal stresses counteracting thosealready present.

1.6 Residual Stress Relief

The most common and efficient way of relieving residual stresses is by heating. Raising the temperature results in a lower yield stress and allows creep tooccur. Creep relieves the residual stresses through plastic deformation. Steel welded components are usually heated to a low red heat (600°C) during stressrelieving treatments. The heating and cooling rates during this thermal stress relief must be carefully controlled otherwise further residual stress patterns maybe set up in the welded component. There is a size limit to the structures which can be thermally stress relieved both because of the size of the ovens requiredand the possibility of a structure distorting under its own weight. It is possible, however, to heat treat individual joints in a large structure by placing smallovens around the joints or by using electric heating elements.

Other methods of stress relief rely on thermal expansion providing mechanical forces capable of counteracting the original residual stresses. This techniquecan be applied in-situ but a precise knowledge of the location of the compressive residual stresses is vital, otherwise the level of residual stress may beincreased rather than decreased. Purely mechanical stress relief can also be applied provided sufficient is available to accommodate the necessary plasticdeformation.

2.1 Introduction

If weld preparation is good and operator induced defects (e.g. lack of penetration or fusion) are avoided, all the common structural steels can be successfullywelded. However, a number of these steels may require special treatments to achieve a satisfactory joint. These treatments are not convenient in all cases.The difficulty in producing satisfactory welded joints in some steels arises from the extremes of heating, cooling and straining associated with the weldingprocess combined with microstructural changes and environmental interactions that occur during welding. It is not possible for some structural steels totolerate these effects without joint cracking occurring. The various types of cracking which can occur and the remedial measures which can be taken arediscussed below.

2.2 Weld Metal Solidification Cracking

Solidification of the molten weld pool occurs by the growth of crystals away from the fusion boundary and towards the centre of the weld pool, untileventually there is no remaining liquid. In the process of crystal growth, solute and impurity elements are pushed ahead of the growing interface. This processis not significant until the final stages of solidification when the growing crystals interlock at the centre of the weld. The high concentration of solute andimpurity elements can then result in the production of a low freezing point liquid at the centre of the weld. This acts as a line of weakness and can cause


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cracking to occur under the influence of transverse shrinkage strains. Impurity elements such as sulphur and phosphorus are particularly important in this typeof cracking since they cause low melting point silicides and phosphides to be present in the weld metal [3]. A schematic view of solidification cracking isshown in Figure 6.

Weld metals with a low susceptibility to solidification cracking (low sulphur and phosphorous) are available for most structural steels, but cracking may stillarise in the following circumstances:

a. If joint movement occurs during welding, e.g. as a result of distortion. A typical example of this is welding around a patch or nozzle. If the weld iscontinuous, the contraction of the first part of the weld imposes a strain during solidification of the rest of the weld.

b. If contamination of the weld metal with elements such a sulphur and phosphorus occur. A typical example of this is the welding of articles with a sulphurrich scale, such as a component in a sulphur containing environment.


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c. If the weld metal has to bridge a large gap, e.g. poor fit-up. In this case the depth to width ratio of the weld bead may be small. Contraction of the weldresults in a large strain being imposed on the centre of the weld.

d. If the parent steel is not suitable in the sense that the diffusion of impurity elements from the steel into the weld metal can make it susceptible to cracking.Cracking susceptibility depends on the content of alloying element with the parent metal and can be expressed in the following equation:

Hot cracking susceptibility =

Note: The higher the number, the greater the susceptibility.

Solidification cracking can be controlled by careful choice of parent metal composition, process parameters and joint design to avoid the circumstancespreviously outlined.

2.3 Heat Affected Zone (HAZ) Cracking

2.3.1 Liquation cracking (burning)

The parent material in the HAZ does not melt as a whole, but the temperature close to the fusion boundary may be so high that local melting can occur atgrain boundaries due to the presence of constituents having a lower melting point than the surrounding matrix. Fine cracks may be produced in this region ifthe residual stress is high. These cracks can be extended by fabrication stresses or during service [3]. A schematic view of liquation cracking is shown inFigure 7.


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In steels the low melting point grain boundary films can be formed from impurities such as sulphur, phosphorus, boron, arsenic and tin. As with solidificationcracking, increased carbon, sulphur and phosphorous make the steel more prone to cracking.

There are two main ways of avoiding liquation cracking. First, care should be taken to make sure that the sulphur and phosphorus levels in the parent metalare low. Unfortunately, many steel specifications permit high enough levels of sulphur and phosphorus to introduce a risk of liquation cracking. Secondly, therisk of liquation cracking is affected by the welding process used. Processes incorporating a relatively high heat input rate, such as submerged arc orelectroslag welding, lead to a greater risk of liquation cracking than, for example, manual metal arc welding. This is the case since the HAZ spends longer atthe liquation temperature (allowing greater segregation of low melting point elements) and there is a greater amount of thermal strain accompanying welding.


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2.3.2 Hydrogen induced cracking

This form of cracking (also known as HAZ, underbead, cold or delayed cracking) occurs in the HAZ at temperatures less than 200°C. Cracks can form withinminutes of welding or be delayed for several days. Three factors must co-exist if cracking is to occur. These factors are:

a. The presence of hydrogen

Hydrogen is introduced into the molten weld pool during welding as a result of the decomposition of hydrogen containing compounds in the arc, e.g. moisture,grease paint and rust. Once the gas has dissolved in the weld metal, it can diffuse rapidly into the HAZ both during cooling and at ambient temperatures. Indue course, the hydrogen will diffuse out of the steel. The diffusion can take a period of weeks for a thick-walled vessel.

b. A susceptible weld metal or HAZ

The cooling rate following most fusion welding processes is relatively rapid. This cooling can lead to the formation of martensite or other hardened structuresin the HAZ and possibly the weld metal. These structures can be embrittled by the presence of only small quantities of hydrogen.

c. A high level of residual stress after welding.

Cracking develops under the action of the residual stresses from welding in the susceptible microstructure of the HAZ or weld metal, where embrittlement hasoccurred due to the presence of hydrogen in solution [3]. A schematic view of hydrogen cracking in the HAZ of different weld designs is illustrated in Figure8.


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The methods of avoiding hydrogen cracking involve removing or limiting one of the three factors which are necessary for it to occur. Hydrogen cracking canbe avoided by choosing a material which does not harden in the HAZ or weld metal with the particular welding process employed. The likelihood ofhardening in the HAZ is controlled by the cooling rate after welding and the hardenability of the parent steel. The hardenability of a steel is governed by itscomposition. A useful way of describing hardenability is to assess the total contribution to it of all the elements that are present in the steel. This assessment isdone by an empirical formula which defines a carbon equivalent value (CEV) and takes account of the important elements which affect hardenability. Atypical formula for the CEV (accepted in British Standards) is shown below:

CEV =

As a general rule, hardening in the HAZ can be avoided by using a steel with a CEV of less than 0,42 although it should be noted that the welding processparameters influence this value.


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Increasing the heat input rate of the welding process (where possible) is beneficial since it results in a slower cooling rate after welding and therefore a lowerlikelihood of hardening in the HAZ. For the same reason, there is a less risk of hydrogen cracking when welding thin plates and sections, since the coolingrate in the HAZ is less than in thick sections.

Limiting the presence of hydrogen by avoiding damp, rust and grease, by using controlled hydrogen electrodes (properly dried basic coated electrodes) andlow hydrogen welding processes (MIG or submerged arc welding) is another step towards avoiding cracking.

If these precautions are not sufficient, preheating is necessary. Preheating and the maintenance of a minimum interpass temperature during multi-pass weldinghas two effects. First, it results in softening of the HAZ because the cooling rate is reduced. Secondly, it accelerates the diffusion of hydrogen from the weldzone so that less remains after the weld has cooled. The minimum pre-heat temperature required to avoid hydrogen cracking depends on the chemicalcomposition of the steel, the heat input rate and the thicknesses being joined.

The minimum pre-heat temperature can be calculated by interrelating these facts in a welding procedure diagram [3]. An example of one of these diagramsfor carbon manganese steels is shown in Figure 9. This diagram is used in the following way:


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Select the appropriate heat input (arc energy) on the horizontal scale.1.Move vertically to intersect the appropriate combined thickness line for the joint design in question.2.Move horizontally from the intersection point to read off the pre-heat temperature for the CEV of the steel being welded.3.

2.4 Lamellar Tearing

This problem can arise if the residual stresses from welding are applied across the thickness of at least one of the plates being joined [3]. Cracking occurs ifthe through-thickness ductility of the plate is very low. A schematic view of this mode of cracking is shown in Figure 10.

Cracking normally occurs in the parent metal close to the outer boundary of the HAZ. The cracks have a characteristic stepped appearance with the 'threads'of the steps being parallel to the rolling direction of the steel plate. In contrast to hydrogen cracking, lamellar tears are not necessarily confined to the HAZ.In some cases, cracking can occur at the mid-thickness of a plate if it is restrained by a weld on both sides.

Lamellar tearing arises because the through-thickness ductility of the plate is reduced by the presence of planar inclusions lying parallel to the plate surface.All common structural steels contain large numbers of inclusions which consist of non-metallic substances produced in the steelmaking process, e.g. sulphatesand silicates. These inclusions are formed as spheres, grain boundary films, or small angular particles in the steel ingot as it cools down after casting. When


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the ingot is rolled to make steel plate the inclusions deform into discs parallel to the plate surface. Different types of inclusions deform in different ways andbreak up during rolling. The form, distribution and density of inclusions in a rolled plate determine the through-thickness ductility. Only a small proportion ofsteel plates have a sufficiently low through-thickness ductility to be susceptible to lamellar tearing.

Lamellar tearing can be avoided in four main ways:

a. Improved joint design

The design of a fabrication can be altered to avoid residual stresses in the through-thickness direction of a plate. Examples are shown in Figure 11.


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b. The use of forged products

The lamellar distribution of inclusions in a plate is a result of the plastic deformation occurring during rolling. The inclusion distribution in forged products isnot so detrimental.

c. Plate selection

The use of steel plates with a relatively low population of planar inclusions and thus adequate through-thickness ductility.

d. Using a layer of low strength weld metal

This reduces the strain transmitted through the thickness of the welded steel plates since the soft weld metal can deform plastically. This technique, known as'buttering' is relatively expensive but can be used when susceptible joints cannot be avoided.

2.5 Re-Heat Cracking

The removal or reduction of residual stresses after welding by thermal stress relief is recommended for many fabrications. In this process, the joint reaches atemperature range where rapid creep can occur (about a third to a half of the melting point). As a result, the welding residual stresses are relieved by plasticdeformation. Cracking can occur during this process if the ductility of the weld or HAZ is not sufficient to accommodate the strain accompanying the residualstress relief [3]. A schematic view of re-heat cracking is shown in Figure 12.


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The residual tensile stress which acts as the driving force for the cracking process may be supplemented by transient thermal stresses in the weld zone. Thesestresses arise from rapid non-uniform heating up to the stress relieving temperature. The presence of geometric stress raisers, e.g. toes of fillet welds, andpre-existing cracks, e.g. liquation and hydrogen cracks, accentuate the problem.

The cracking problem is most prevalent during stress relieving operations, but it can also occur in service situations. In such cases the onset of cracking isexpected to take much longer since the service temperature is generally significantly below the stress relieving temperature.

Re-heat cracking is mainly confined in practice to alloy steels containing substantial amounts of strong carbide forming elements, e.g. Cr, Mo and V. Thepresence of the alloy carbides inhibits grain boundary sliding and thus reduces high temperature ductility. Cracking can usually be avoided by weld profiling,e.g. grinding away any geometric stress raisers such as the toes of fillet welds, before heat treatment and by control of the heating rate to avoid high transientthermal stresses.

A structural steel can only be considered to be weldable if joints in the steel behave satisfactorily in service.In order to achieve adequate levels of performance in structural applications, the integrity of the welded joint must be good. A high level of integritycan only be achieved if the welded joint microstructure possesses sufficient ductility to resist residual stresses, which arise from the welding thermalcycle, without cracking.The chemical compositions of both the weld and parent metals (carbon equivalent value), together with the parameters of the welding process (heatinput and cooling rates), are influential in determining joint ductility.The level of impurity elements, such as sulphur, phosphorous and hydrogen, is a particularly significant factor in determining whether crack formationwill occur during welding.

[1] Hicks, J. G., "Welded Joint Design", BSP Professional Books, 1979.

[2] Pratt, J. L., "Introduction to the Welding of Structural Steelwork", Steel Construction Institute, 3rd rev. ed. 1989.

[3] Baker, R. G., "The Welding of Pressure Vessel Steels", The Welding Institute, 1973.

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