Weldability of Structural Steels

STEEL CONSTRUCTION: APPLIED METALLURGY __________________________________________________________________________

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STEEL CONSTRUCTION:

APPLIED METALLURGY

Lecture 2.6: Weldability of Structural

Steels

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 structural steels. A steel is

considered to exhibit good weldability if joints in the steel possess adequate strength and

toughness in service.

Solidification cracking, heat affected zone - liquation cracking, hydrogen-induced

cracking, lamellar tearing, and re-heat cracking are described. These effects are

detrimental to the performance of welded joints. Measures required to avoid them are

examined.

1. INTRODUCTION

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 use of high


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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 in which 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 molten weld pool to bridge the gap between the surfaces

and to produce the required weld shape and dimensions on cooling. The most common

welding processes for 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 and brittleness in the

solidified weld metal. The techniques used to avoid gas absorption in the weld pool vary

according to the welding process. The main welding processes 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 between the 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 the workpiece. Electromagnetic forces created in

the arc help to throw drops of the molten electrode onto the molten area of the workpiece

where the two metals fuse 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 molten metal. 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 proportion of 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 calcium carbonate 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 their

coating.

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 by forming 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 SAW machine may feed

several wires, one behind the other, so that a multi-run weld can be made. Submerged arc


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welding produces more consistent joints than manual 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 and weld 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 actual shape 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 fabrication procedure. 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 easily accessible in terms of both the welding process and welding position.

The detailed weld shape is designed to distribute the available heat adequately and to

assist 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 be difficult 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 (the boundary 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 cast microstructure. In the

welding of structural steels, the weld filler metal does not usually have the same

composition as the parent metal. If the compositions were 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 using weld 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 the fusion boundary, peak

temperatures near the melting point are reached, whilst material only a few millimetres

away may only reach a few hundred degrees Celsius. The parent material close to the

fusion boundary is heated into the austenite phase field. On cooling, this region transforms

to a microstructure which is 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 low

temperature transformation structures, such as bainite and martensite, which are harder

and more brittle than the bulk of the parent metal. This region is known as the heat

affected zone (HAZ).

The microstructure of the HAZ is influenced by three factors:

1. The chemical composition of the parent metal.

2. The heat input rate during welding.

3. The cooling rate in the HAZ after welding.

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 directly affects the

grain size in the HAZ. The longer the time spent above the grain coarsening temperature

of the parent metal during welding, the coarser the structure 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 heat input 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 a function of the welding process parameters: arc voltage, arc current and

welding speed. In addition to heat input rate, the cooling rate in the HAZ is influenced by

two other factors. First, the joint design and thickness are important since they determine

the rate of heat flow away from the weld during cooling. Secondly, the temperature of the

parts being joined, i.e. any pre-heat, is significant since it determines the temperature

gradient which exists between the weld and parent metal.


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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 contraction is 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 stress tends to be predominantly tensile in

nature. This tensile residual stress is balanced by a compressive stress induced in the

parent metal [2]. A schematic view of 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 the weld metal and HAZ. It is important to note that the residual stresses

arise because the material undergoes local plastic strain. This strain may result in cracking

of the weld metal and HAZ during welding, distortion of the parts to be joined or

encouragement of brittle failure during service.

Transverse and longitudinal contractions resulting from welding can lead to distortion if

the hot weld metal is not symmetrical about the neutral axis of a fabrication [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 side of 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, as shown 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 the weld

to resist distortion. Examples of both these methods are shown in Figure 5. Distortion

problems are most easily avoided by using the correct weld preparation. The use of non-

symmetrical double sided welds such as those shown in Figure 2e and 2i accommodates

distortion. The distortion from the small side 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 welding should

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 the free 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 those already 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 to occur. Creep relieves the

residual stresses through plastic deformation. Steel welded components are usually heated

to a low red heat (600C) during stress relieving treatments. The heating and cooling rates

during this thermal stress relief must be carefully controlled otherwise further residual

stress patterns may be 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

required and 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 small ovens 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 technique can 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 be increased rather than decreased. Purely

mechanical stress relief can also be applied provided sufficient is available to

accommodate the necessary plastic deformation.

2. THE WELDABILITY OF STRUCTURAL STEELS

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 successfully welded.

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 welding process combined with microstructural changes and

environmental interactions that occur during welding. It is not possible for some structural

steels to tolerate these effects without joint cracking occurring. The various types of

cracking which can occur and the remedial measures which can be taken are discussed

below.


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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, until eventually there is no

remaining liquid. In the process of crystal growth, solute and impurity elements are

pushed ahead of the growing interface. This process is not significant until the final stages

of solidification when the growing crystals interlock at the centre of the weld. The high

concentration of solute and impurity 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 cracking to occur under the influence of transverse shrinkage strains. Impurity

elements such as sulphur and phosphorus are particularly important in this type of

cracking since they cause low melting point silicides and phosphides to be present in the

weld metal [3]. A schematic view of solidification cracking is shown 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 still arise 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 is continuous, the

contraction of the first part of the weld imposes a strain during solidification of the rest

of the weld.


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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 sulphur rich scale, such as a

component in a sulphur containing environment.

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 weld results 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 circumstances previously 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 at grain boundaries due to the


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presence of constituents having a lower melting point than the surrounding matrix. Fine

cracks may be produced in this region if the residual stress is high. These cracks can be

extended by fabrication stresses or during service [3]. A schematic view of liquation

cracking is shown in Figure 7.

In steels the low melting point grain boundary films can be formed from impurities such

as sulphur, phosphorus, boron, arsenic and tin. As with solidification cracking, 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 metal are low.

Unfortunately, many steel specifications permit high enough levels of sulphur and

phosphorus to introduce a risk of liquation cracking. Secondly, the risk of liquation

cracking is affected by the welding process used. Processes incorporating a relatively high

heat input rate, such as submerged arc or electroslag 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 at the liquation temperature (allowing greater segregation of low

melting point elements) and there is a greater amount of thermal strain accompanying

welding.

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 200C. Cracks can form within minutes 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. In due 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 structures in 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 has occurred due to the


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presence of hydrogen in solution [3]. A schematic view of hydrogen cracking in the HAZ

of different weld designs is illustrated in Figure 8.

The methods of avoiding hydrogen cracking involve removing or limiting one of the three

factors which are necessary for it to occur. Hydrogen cracking can be avoided by choosing

a material which does not harden in the HAZ or weld metal with the particular welding

process employed. The likelihood of hardening 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 its composition. 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 is done

by an empirical formula which defines a carbon equivalent value (CEV) and takes account

of the important elements which affect hardenability. A typical 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 process parameters influence

this value.

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 lower likelihood 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 cooling rate in the HAZ is less than in thick sections.


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Limiting the presence of hydrogen by avoiding damp, rust and grease, by using controlled

hydrogen electrodes (properly dried basic coated electrodes) and low 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 welding has 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 weld zone so that less remains

after the weld has cooled. The minimum pre-heat temperature required to avoid hydrogen

cracking depends on the chemical composition 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 diagrams for carbon

manganese steels is shown in Figure 9. This diagram is used in the following way:


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1. Select the appropriate heat input (arc energy) on the horizontal scale.

2. Move vertically to intersect the appropriate combined thickness line for the joint

design in question.

3. Move horizontally from the intersection point to read off the pre-heat temperature

for the CEV of the steel being welded.


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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 if the 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. sulphates and 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 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 and break up during rolling. The form, distribution and density of

inclusions in a rolled plate determine the through-thickness ductility. Only a small

proportion of steel plates have a sufficiently low through-thickness ductility to be

susceptible to lamellar tearing.


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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 is not 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 a temperature range


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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 plastic deformation. Cracking can occur during

this process if the ductility of the weld or HAZ is not sufficient to accommodate the strain

accompanying the residual stress relief [3]. A schematic view of re-heat cracking is shown

in Figure 12.

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. These stresses arise from

rapid non-uniform heating up to the stress relieving temperature. The presence of

geometric stress raisers, e.g. toes of fillet welds, and pre-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 is expected 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. The presence 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 transient thermal stresses.

3. CONCLUDING SUMMARY

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 integrity can only be

achieved if the welded joint microstructure possesses sufficient ductility to resist

residual stresses, which arise from the welding thermal cycle, without cracking.

The chemical compositions of both the weld and parent metals (carbon equivalent

value), together with the parameters of the welding process (heat input 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 formation will occur

during welding.

4. REFERENCES

[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.

Documents

Weldability of Structural Steels