THE METALLURGY OF WELDING

1 INTRODUCTION Steels form the largest group of commercially important alloys for several reasons: The great abundance of iron in the earths crust The relative ease of extraction and low cost The wide range of properties that can be achieved as a result of solid state transformation such as alloying and heat treatment

1.1 ROLE OF CARBON IN STEEL Steels are alloys of iron with generally less than 1% carbon plus a wide range of other elements. Some of these elements are added deliberately to impart special properties and others are impurities not completely removed (sometimes deliberately) during the steel making process. Elements may be present in solid solution or combined as intermetallic compounds with iron, carbon or other elements. Some elements, namely carbon, nitrogen, boron and hydrogen, form interstitial solutions with iron whereas others such as manganese and silicon form substitutional solutions. Beyond the limit of solubility these elements may also form intermetallic compounds with iron or other elements. Carbon has a major role in a steels mechanical properties and its intended use as illustrated in Figure 1. As the carbon concentration is increases carbon steel, in general, becomes stronger, harder but less ductile. This is an important factor when a steel is required to be welded by joining or surfacing.

1.2 WELDABILITY OF STEELS When considering a weld, the engineer is concerned with many factors such as design, physical properties, restraint, welding process, fitness-for-purpose etc., which can conveniently be summarized as the base materials weldability. Weldability can be defined as the capacity of a metal to be welded under the fabrication conditions imposed into a specific, suitably designed structure, and to perform satisfactorily in the intended service. Welding is one of the most important and versatile means of fabrication and joining available to industry. Plain carbon steels, high strength low alloy (HSLA) steels, quench and tempered (Q&T) steels, stainless steels, cast irons, as well as a great many non-ferrous alloys such as aluminium, nickel and copper are welded extensively. Welding is of great economic importance, because it is one of the most important tools available to engineers in his efforts to reduce production, fabrication and maintenance costs. A sound knowledge of what is meant by the word weld is essential to an understanding of both welding and weldability. A weld can be defined as a union between pieces of metal at faces rendered plastic or liquid by heat, or pressure, or both, with or without the use of filler metal. Welds in which melting occurs are the most common. The great majority of steels welded today consist of low to medium carbon steel (less than 0.4%C).Practical experience over many years has proved that not all steels are welded with ease. For example, low carbon steels of less than 0.15%C can be easily welded by nearly all welding processes with generally high quality results. The welding of higher carbon steels or relatively thick sections may or may not require extra precaution. The degree of precaution necessary to obtain good quality welds in carbon and alloy steels varies considerably. The welding procedure has to take into consideration various factors so that the welding operation has minimal affect on the mechanical properties and microstructure of the base metal.

The application of heat, generally considered essential in a welding operation, produces a variety of structural, thermal and mechanical effects on the base metal being welded and on the filler metal being added in making the weld. Effects include: Expansion and contraction (thermal stresses etc.) Metallurgical changes (grain growth etc.) Compositional changes (diffusion effects etc.)

In the completed weld these effects may change the intended base metal characteristics such as strength, ductility, notch toughness and corrosion resistance. Additionally, the completed weld may include defects such as cracks, porosity, and inclusions in the base metal, heat affected zone (HAZ) and weld metal itself. These effects of welding on any given steel are minimized or eliminated through changes in the detailed welding techniques involved in producing the weld. It is important to realize that the suitability of a repair weld on a component or structure for a specific service condition depends upon several factors: Original design of the structure, including welded joints The properties and characteristics of the base metal near to and away from the intended welds The properties and characteristics of the weld material Post Weld Heat Treatment (PWHT) may not be possible

As discussed, a steels weldability will be dependent upon many factors but the amount of carbon will be a principal factor. A steels weldability can be categorized by its carbon content as shown in Table 1.

In order to understand the physical and chemical changes that occurs in steels when they are welded, a basic understanding of the metallurgy of steels is necessary.

2.WELDING STEELS CONSIDERED DIFFICULT Welding of HSLA and Q&T steels may pose several problems and a careful study of the steel and its intended application is necessary before specifying a welding procedure. Sometimes welding is required after the steel has been heat treated i.e. Q&T making it almost impossible to achieve uniform properties across the welded joint. In such cases it is preferable to carry out the welding prior to the Q&T operation. The weld metal in this case must be selected to have matching chemical properties so that as near as possible, uniform properties are achieved after the Q&T heat treatment. To the maintenance engineer this is sometimes not possible to carry out for several reasons such as size, in situ, economics etc. In these cases, the weld metal and repair procedure have to be carefully considered to ensure, as near as possible, matching mechanical properties are obtained (or superior) with HAZ hardness taken into account.

2.1 PROCEDURAL CONSIDERATIONS To prevent martensite from forming during welding, sufficient preheat must be applied to the component to hold it above the Ms temperature until welding is complete. All deposited weld metal and the HAZ remain austenitic during the welding operation and transform together on cooling to produce a uniform structure. In applying this approach the TTT diagram for the steel can be studied to determine the preheat temperature, the maximum allowable time for completion of welding, and the cooling rate required. The preheat and interpass temperatures thus selected must also be below the tempering temperature of the base metal in order to maintain its mechanical properties. The weld metal selected must provide adequate strength and toughness and, if necessary, without the benefit of a subsequent PWHT. 2.2 POST WELD HEAT TREATMENT (PWHT)

It is common practice to apply a PWHT or stress relief to temper the welded joint and soften the HAZ. Additionally PWHT removes hydrogen and lowers residual stresses imposed by the service conditions and the welding operation. In the case of a Q&T steel the PWHT must not be higher than the original tempering temperature otherwise a loss of physical properties such as strength could occur dropping it below specification. To reduce the risk of cracking the PWHT may be carried out immediately after welding is completed without letting the component cool down or carried out several times during the weld repair which can be costly. As discussed previously, it may be impractical to carry out PWHT. The weld repair procedure needs to be carefully considered to minimize the possibility of cracking in service by ensuring that the welded component has fitness-for-purpose.

2.3 THE HEAT AFFECTED ZONE (HAZ)

The HAZ undergoes a complete thermal cycle which determines the microstructure. Grain growth is an important factor in the HAZ and the weld. In the HAZ of a coarse grained steel there is a wide region where grain growth has occurred but in a fine grained steel, grain growth is resisted except in the narrow region immediately adjacent to the weld fusion boundary where temperatures are very high. Fig.7.10 shows an example of grain growth in a welded joint.

The type of microstructure formed in the coarse-grained region of a steel depends upon: The carbon content The alloy content The time at elevated temperature The cooling rate

For any given steel, the greater the weld heat input the longer the time spent above the grain coarsening temperature of the steel, and the coarser the grain size. Steels containing grain refining additions such as titanium, vanadium, niobium, and aluminium are exceptions in that a fine HAZ grain size may be achieved right up to the fusion boundary. Titanium nitride is very stable and may not completely dissolve in the HAZ even at the temperatures immediately adjacent to the fusion boundary. This can be advantageous with high heat input welds such as submerged arc welding. Figure 44 illustrates four welds in a carbon steel that have been welded with different heat inputs.Alongside each weld the HAZ transforms to a microstructure dependent on the cooling rate of that weld. For higher heat input welds, the cooling rate will be slower. In Figure !!, for the small rapidly cooled welds, martensite is formed. For the large, slowly cooled welds the HAZ structure is pearlite. The hardness of the HAZ is much higher in those welds in which martensite is present as illustrated. Adjacent to the weld the base metal undergoes various changes according to peak temperature and cooling rate experienced at various locations away from the weld joint. Close to the fusion zone the peak temperature will be high enough to cause complete transformation to austenite and some grain growth.

At some distance away from the fusion zone the temperature is not sufficient to cause any microstructural changes although other effects such as strain aging (plastic deformation) may occur. In between a range of mixed structures may be observed as illustrated in Fig.38.

2.3.1 LOSS OF TOUGHNESS IN THE HAZ The microstructure itself may have an effect on the crack sensitivity or toughness in the HAZ. Certain factors are known to lower the toughness of the HAZ: Grain size An increasing austenite grain size in the HAZ is likely to result in lower toughness. Grain size is determined largely by heat input and base metal chemistry. Heat Input An increasing heat input caused by welding amperage; arc process; weaving etc. can result in lower toughness. Indeed some high strength low alloy (HSLA) steels specify heat input requirements for weld joining. Precipitation Hardening From the presence of micro-alloy elements. Again precipitation is encouraged by high heat input because of the longer times at high temperatures and the slower cooling rates. Plastic Deformation The contraction of a cooling weld may cause plastic deformation in certain parts of the HAZ, particularly around any residing defects (such as nitrogen, sulfides etc.), with consequent loss of toughness. Post weld heat treatment (stress relief) of micro-alloyed steel can cause a considerable amount of precipitation of fine carbides with a substantial decrease in toughness in the HAZ.

In practical terms, restrictions on heat input may mean some welding processes such as electroslag, submerged arc, and flux-cored arc cannot be used. Other restrictiorestrictions such as preheat; interpass temperature; and width of weave would need to be considered also.

2.4 PREHEAT & CARBON EQUIVALENT The preheat temperature required depends on the susceptibility of the HAZ to hydrogen cracking, and much research has been done to find compositional formulae to indicate this. One formula for calculating the preheat for welding of structural low alloy steels is given below:

SEFERIAN GRAPH The Seferian graph shown in Fig. 43 takes into account CE, and restraint in calculating preheat.

3.WELDING PROCEDURAL GUIDELINES Choose - Surfacing consumable - Welding process WHAT NEXT? BASE METAL CONSIDERATIONS

Recognize potential problems such as: HAZ embrittlement Loss of strength hardness in Q&T steels Reduction in corrosion resistance Porosity generation from base metal chemistry Contraction cracking Consequence of fracture Locate specification Spark analysis Component function Weldability Factors

Other factors for consideration: Cost Weldability of base metal Preheat Postheat Base metal properties

Correlation of CCT and TTT Diagrams With Jominy Hardenability Test Data for an 8630 Type Steel

4. HARDENABILITY / WELDABILITY OF STEELS The hardenability of steels can be determined by performing a Jominy end quench test. The alloy steel test specimen is a cylinder one inch diameter and four inches long, which is heated to the austenitic region (above 910C) then placed in a fixture where it is quenched by water or brine impinging on one end. The fastest cooling rate occurs at the bar surface in contact with the water jet with progressively slower cooling rates being experienced away from the end. Thus the microstructure formed in the surface region could be martensitic with high hardness and the interior could be pearlitic with no hardening at all. The depth to which a steel hardens is a measure of its hardenability. If we add alloying elements that allows deeper hardening, then that steel is said to have higher hardenability. This is important, for example, when considering mechanical properties and weldability of such a steel. Hardness tests are commonly used on Jominy samples to determine that steels hardenability. Figure 33 illustrates the TTT diagram for a common chrome-molybdenum steel (4137) with a Jominy end quench test superimposed. Thus the microstructure and hardness can be correlated on the one diagram. The cooling rate curves represent the same cooling rate conditions located along the Jominy end-quench test bar. At the top of Figure 33, the measured hardness curve has been superimposed over a schematic of the end-quenched bar. Four representative locations (A, B, C, D) along the bar have been related to the representative cooling curves(CCT) and isothermal transformation (TTT) curves. Thus location A on the bar experienced a fast cooling rate resulting in austenite transforming to martensite producing the high hardness indicated. Similar cooling rate effects need to be considered from a weldability viewpoint.The addition of alloying elements (for example Mo, Cr, Mn) to steel increases the hardenability by slowing down the rate of austenite transformation. The data is plotted as shown in Figure 34 for a 0.45%C steel with different alloying additions.

Figure 33 Typical End-Quench Curves for Several 0.45%C Low Alloy Steels

Several formulae have been developed which assign a contributing factor to each element addition and its effect on hardenability and conversely weldability. The maximum hardness attainable (and therefore its weldability characteristics) in carbon and low-alloy steels, however, is still almost exclusively dependent upon the carbon content.

4.1 CARBON EQUIVALENT (CE) & WELDABILITY Depth of hardening is not a relevant concept in a welding situation, but we are interested in the hardness produced at a given cooling rate or the critical cooling rate to produce a given hardness in the HAZ of a weld. There are several models that have been developed to calculate hardenability from a welding process. The simplest model is one in which the effects of individual alloying elements are added together (a linear model) to produce a carbon equivalent (CE) which in turn relates to a critical cooling rate to produce a given hardness. Figure 35 shows a reasonable correlation between the CE plotted against critical cooling rate from 540C to give a hardness of 350Hv in the HAZ.

Another linear model has been used to predict the hardness of the HAZ for different cooling rates in low alloy steels and is illustrated in Figure 36.

CEs are used widely in industry as measures of weldability. Several different formulae have been developed and some are even incorporated into national codes and specifications. In general terms, other factors being equal, as the carbon content increases, so does the difficulty in weldability. In practice, this means generally using higher preheats until cracking and restraint problems are overcome. Using an engineering/analytical approach becomes very useful when confronted with unknown material compositions, and weld repairs can become challenging where reverse engineering must be utilized to develop a repair procedure. The engineering approach may involve evaluating composition, hardenability, service conditions, size, restraint conditions, and PWHT feasibility. One of the popular methods for determining weldability is to review the hardenability of the base material. As discussed earlier the CE formula(s) have been developed as a convenient method of normalizing the chemical composition of a material into a single number to indicate its hardenability. Review of the literature indicates no less than a dozen different formulas have been developed. One of the most commonly used formulas for calculating the CE is the IIW formula:

It must be stated that low carbon steel and carbon manganese steels generally behave in a predictable manner and are successfully welded with preheat and PWHT criteria outlined in codes such as AWS D1.1, Structural Welding Code Steel. The CE is not usually evaluated on these materials. Medium carbon, HSLA, and Q&T Steels, however, present different challenges where consideration of CE, restraint, hydrogen control, PWHT not practicable, weld filler chemistry mismatch, weld heat input etc. can be critical to successful repair welding. These factors can be summed up as a materials weldability, and it is these factors that will be considered in Section 8.

4.2 TEMPERING EFFECTS OF REHEATING As discussed earlier martensite produced in a quenched steel is hard and brittle and in most cases the steel is unusable in that form. The toughness may be improved by a process of tempering. This involves reheating the steel to below the transformation temperature (723C), holding for a period of time, then cooling to ambient temperature as illustrated in Figure 37. During tempering the carbon trapped as an interstitial in the martensitic tetragonal structure is released. Carbon atoms diffuse and precipitate as small carbides. With enough time and at sufficiently high temperatures cementite (Fe3C) forms, not as plates as in pearlite, but as spherical particles. This microstructure is known as bainiteImprovement in toughness is accompanied by a loss of hardness which is a function of both temperature and time (however temperature is more effective the higher the temperature the faster the tempering transformation as illustrated in Figure 37). The temperatures typically selected for post weld heat treating or stress relieving welded steel are generally high enough to cause rapid tempering of the HAZ.

4.3 SECONDARY HARDENING In some steels containing specific alloy elements tempering may actually cause an increase in hardness as the tempering temperature is raised as shown in Figure 38. This is known as secondary hardening and is caused by strong carbide forming elements such as molybdenum, chromium, and tungsten combining with carbon to form alloy carbide precipitates in certain temperature ranges. This behavior of secondary hardening is put to good use in the tempering of tool steels such as high speed tool steels. When considering a weld repair on such steels, the preheat and interpass temperatures is normally selected at a temperature below the secondary(or tempering) temperature, particularly if PWHT is not practical.

5.HYDROGEN CRACKING RELATED TO WELDABILITY Hydrogen can embrittle a steel at both elevated and ambient temperatures. The term hot cracking is used to signify that cracking has occurred at elevated temperature while cold cracking is used to generally signify cracking in low alloy steel at ambient temperature. It was during World War 2 that it was realized that hydrogen dissolved in weld metal was one of the causes of cold cracking in low alloy steel welded joints (i.e. the catastrophic failure of the welded Liberty ships). These failures led to the development of low hydrogen electrodes which made possible successful welding of the alloy steels used today. Hydrogen pickup is derived from hydrogen containing chemical compounds that are dissociated in the arc column. They can originate, for example, from contamination on the workpiece or from moisture in the welding flux. It is the hydrogen sourced from electrode coatings or fluxes which is the most important. Electrode coatings consist of minerals, organic matter, ferro-alloys, and iron powder bonded with, for example, bentonite (a clay) and sodium silicate. The electrodes are baked after coating, and the higher the baking temperature the lower the final moisture content of the coating. Some electrode coatings may pick up moisture if exposed at ambient conditions (basic coated electrodes). Where hydrogen cracking is a risk, special flux coatings are used to maintain low hydrogen content. In practice, welding specifications stipulate the allowable moisture content. It is, however, important to note that the method or welding procedure adopted as well as the type of electrode flux used can affect the hydrogen content in a weld or HAZ. With hot cracking, embrittlement occurs in carbon and low alloy steels by a chemical reaction occurring between hydrogen and carbides which causes irreversible damage either decarburization or cracking or both. Of much greater importance in welding is hydrogen entrapped in the weld or HAZ causing embrittlement. Hydrogen cracking can subsequently occur at some later time (sometimes days) once a weld repair is complete, generally at service temperatures between 100C and 200C. This embrittlement is due to physical interactions between hydrogen and the crystal lattice structure of the steel and is reversible by removal of hydrogen by stress relieving allowing the ductility of the steel to revert back to normal. Hydrogen cracking can occur in either the weld metal, HAZ, or base metal and be either transverse or longitudinal to the weld axis. The level of preheat or other precautions necessary to avoid cracking will depend on which region is the more sensitive. In carbon - manganese medium strength steels the HAZ is usually the more critical region and weld metal rarely causes a problem. Cracking due to dissolved hydrogen is now thought to occur by decohesion. Where there is a defect, discontinuity or pre - existing crack and a tensile stress applied, hydrogen is considered to diffuse preferentially to the region of greatest strain i.e. near to the stress concentration such as near a crack tip. The presence of a relatively large concentration of hydrogen reduces the cohesive energy of the crystal lattice structure to the extent that fracture occurs at or near the stress concentrator. This view is consistent with observations that cracking can occur slowly (the crack velocity being dependent on the diffusion rate of hydrogen) and is quite often discontinuous. In welding, the region most susceptible to hydrogen cracking is that which is hardened to the highest degree (areas where the welding residual stresses is greatest) although regions of coarse grain growth can be a contributing factor. The most crack-sensitive microstructure is high carbon martensite.Hot or cold cracking in the weld metal or HAZ depends on the same fundamental factors as in the base metal, i.e. hydrogen content, microstructure and residual stress. In practice the controlling variables are usually strength, hydrogen content, restraint, stress concentrations, and heat input.

In single pass welds and root runs of multiple pass welds the root pass may provide a stress concentration which can lead to longitudinal cracks in the weld metal. High dilution of the root run (high heat input) can often result in a harder weld bead more likely to crack (this is commonly seen in such applications as pipeline welding). Figure 40 illustrates the physical appearance of hydrogen cracking in welds.

In Figure 41 the crack has initiated at the root of the weld where a lack of fusion can be seen. The crack has then traveled through the HAZ mainly in the coarse grained region. In heavy multiple pass welds cracking will generally be transverse to the weld direction, sometimes running through the weld itself since the maximum cooling rate is along the weld axis. Many HSLA steels in critical repair situations where PWHT is impracticable are welded using a filler metal of good toughness and ductility and in such cases the HAZ may be more crack sensitive.The risk of hydrogen-induced cold cracking in the weld can be minimized by: Reducing hydrogen pick-up (low hydrogen flux chemistries) Maintaining a low carbon content Avoiding excessive restraint Control of welding procedures (preheat; heat input; PWHT etc.) Developing a non-sensitive weld microstructure

In carbon or carbon-manganese steels (i.e., those with steep hardening curves as shown in Figure 35!!) welding conditions can be selected to avoid the cooling rates at which martensite is produced. This could include preheat; high heat input welding; slow cooling etc. In low alloy steels or those where a hard HAZ cannot be avoided, other steps must be taken to prevent cracks. These often involve applying preheat and interpass temperatures to allow the diffusion of hydrogen out of the weld metal. Figure 43 shows that quite moderate temperatures are highly effective in removing hydrogen.

The freedom of selecting a suitable welding solution is sometimes limited. The solution must be practicable and economic. Further constraints may be applied by the job such as base metal condition, size, location, PWHT not practicable, equipment availability etc. In such cases, the welding engineer may need to consider the steels CE and Ms temperature by referring to its TTT and CCT curves in providing a weld procedure.

5.1 LAMELLAR TEARING Lamellar tearing is a form of cracking that occurs in the base metal of a weldment due to the combination of high localized stress and low ductility of the base metal. It is associated with regions under severe restraint, for example, tee and corner joints; heavy sections etc.

The cracks appear close to or a few millimetres away from the HAZ at right angles to the weld interface as shown in Figure 44. In HSLA steels that form martensite in the HAZ, hydrogen - induced cold cracking will generally form preferentially, but in plain carbon steels of low hardenability, hydrogen increases the susceptibility to lamellar tearing quite markedly due to HAZ stresses. There is no correlation between heat input and the incidence of lamellar tearing, but in the presence of hydrogen a low heat input might tip the balance towards hydrogen cracking because of a lack of time for hydrogen to dissipate away from the weld area. Lamellar tearing may, in principle, be avoided by: Design modification Buttering weld runs and temper bead welding Control of welding procedures (preheat; heat input; PWHT etc.)

6. CONCLUSIONModern structural steels with their demands for strength, toughness, and good welding behavior have evolved to depend less on carbon content as a strengthening agent and more on fine grain size and precipitation hardening. This has meant that welding (specifically weld repair) procedures may now have to utilize consumables which meet stringent property requirements as well as avoiding cracking and other defects during welding. 11