Effect of Electrode Types on the Solidification Cracking Susceptibility of Austenitic Stainless Steel Weld Metal

The effect of electrode types on the solidification cracking susceptibility of austenitic stainless steel weld metal was studied. Manual metal arc welding method was used to produce the joints with the tungsten inert gas welding serving as the control. Metallographic and chemical analyses of the fusion zones of the joints were conducted. Results indicate that weldments produced from E 308-16 (rutile coated), E 308-16(lime-titania coated) electrodes, and TIG welded joints fall within the range of 1.5 ≤ Creq./Nieq. ≤ 1.9 and solidified with a duplex mode and were found to be resistant to solidification cracking. The E 308-16 weld metal had the greatest resistance to solidification cracking. Joints produced from E 310-16 had Creq./Nieq. ratio < 1.5 and solidified with austenite mode. It was found to be susceptible to solidification cracking. E 312-16 produced joints having Creq./Nieq. ratio > 1.9 and solidified with ferrite mode. It had a low resistance to solidification cracking.

.7 : Tensile properties for E308-16/12(rutile) test specimen 72 Table 4.8 : Tensile properties for E308-16/10(rutile) test specimen 73 Table 4.9 : Section dimensions and area of each test specimen after fracture 84  Austenitic stainless steels have become the most widely used stainless steels and correspond to about 70 percent of all the stainless steel produced worldwide, as a result of their mechanical and metallurgical properties and their good weldability (Cui et al., 2006). The excellent properties of ASS which range from high tensile strength, good impact resistance, excellent ductility, corrosion and wear resistances have found various applications in domestic as well as in many engineering industries, some of which are cooking utensils, food processing equipment, equipment for chemical industry, truck trailers, kitchen sinks, exterior architecture, pressure boilers and vessels, fossil-fired power plant, fuel gas desulphurization equipment, evaporator tubing, super heater and reheating tubing, steam headers and pipes among others (Galal et al., 2005; www. Steel.org/learning/glossary).
In recent times, advancement has been made in such joining process as adhesives, mechanical fasteners, brazing and soldering. However, welding remains the most important metal joining process, even as arc welding is the most widely used fusion-welding process. In the fabrication of austenitic stainless steel components, welding is one of the most employed methods (Afolabi, 2008;Ovat et al., 2012).
However, many research findings have proved that improper technique employed in welding austenitic stainless steel may lead to serious consequences of the structure (Woollin, 1994;Korinko et al., 2001;Parijslann, 2002).
Despite the good weldability property exhibited by ASS, hot cracking has been the major metallurgical problem encountered during welding of Austenitic stainless steel components. It is caused by the formation of low melting eutectics at the grain boundaries during welding, which cause failure under the action of shrinkage stresses associated with solidification. Solidification cracking is a type of hot cracking which depends on mechanical restraint and metallurgical susceptibility (Baldev et al., 2006). It consists of fractures at the interdendritic and/or intergranular weld metal boundaries in the solidification process, during which the liquid phase of the mushy melt becomes rich in impurities, mainly sulphur (S) and phosphorus (P). This phenomenon reduces the mechanical strength at the grain and dendritic boundaries, rendering them susceptible to cracking and failure eventually (Arantes et al., 2007). One of such failures is the corrosion cracking of a grade 304 stainless steel pipe improperly seam welded and meant for the conveyance of glucose solution in Illinois USA (James, 2000).
In view of the problem of solidification cracking in ASS weldment, many works have been carried out in order to explain the phenomenon of solidification cracking and ways of preventing it. As early as 1941, Scherer et al., found that crack resistance in ASS weld metal may be improved by adjusting the composition to 5 -35 percent ferrite in the completed weld. Hull (1967) confirmed this by stating that when ferrite content in the completed weld increase beyond 35 weight percent, the weld metal would become susceptible to solidification cracking, but the mechanism by which crack resistance is achieved by the effect of retained ferrite in the weld metal is still not completely understood.
Good attempts, however, have been made to explain the effect. Borland and Younger (1960) suggested that the higher solubility for impurity elements in delta -ferrite leads to less interdendritic segregation and reduces cracking tendency. Their et al. (1987) found that the volume contraction associated with ferriteaustenite transformation reduces tensile stresses close to the crack tip, which decreases cracking tendency. Apart from the effect of retained delta-ferrite in the control of solidification cracking in ASS weldment, Baldev et al. (2006) and Borland (1960) suggested that solidification cracking in ASS weld metal could be minimized by the various practices which reduce mechanical restraint in the completed weld metal. These research works all yielded positive results.

PROBLEM STATEMENT
As can be seen in some of the research works cited above, solidification cracking in austenitic stainless steel weldment is a function of the weld metal composition.
The composition of ASS weldment can be affected by the composition of the electrode used in carrying out the welding process. The choice of electrode type is critical and has continued to pose a serious challenge in the weldability of ASS materials. A well designed product, for example, can fail by cracking if the weld rod selected results in the weld zone having lower alloy content than that of the parent metal. Therefore, there is need to determine how the electrode type affects the solidification cracking susceptibility of ASS weldments. This research work is supposed to address the issues raised above while investigating the possible mechanism responsible for crack sensitivity in the weld microstructure.

OBJECTIVE
The main aim of this research is therefore, to investigate the effect of electrode types on the microstructural susceptibility of the austenitic stainless steel weldment to solidification cracking. The specific objectives are: i.
To determine the electrode type that would impact crack resistant properties in the ASS weldment. ii.
To determine the solidification mode of ASS weldments produced from different stainless steel electrodes iii.
To evaluate the metallurgical susceptibility of such solidification microstructures to solidification cracking. iv.
To determine the effect of the different electrodes on the strain hardening exponent of an austenitic stainless steel weldment.

JUSTIFICATION OF STUDY
Irrespective of the problems encountered during fabrication of austenitic stainless steel (ASS) via welding of which solidification cracking is the most deleterious, welding remains the principal joining method. It is therefore, important to control the metallurgy of the weld metal in the quest to realize a quality weld with crack resistant property necessary to overcome premature failures arising from solidification cracking in ASS fabricated components. A study on the microstructural effect of electrode types on the solidification cracking susceptibility of ASS weld joints, therefore, is supposed to provide an insight into the assessment of cracking propensity by a simple analysis and evaluation of the weld metal composition and microstructure respectively. crystal structure is the more stable phase and is given the name of delta (δ) and alpha (α) ferrite respectively. Rapid cooling with the attendant high solidification rates from the molten steel can 'quench in' the normal high temperature phase.
Alloying additions can also alter the temperature ranges where these phases are most stable. Nickel atoms are normally the same size as iron atoms and arrange themselves in FCC structure over a large temperature range. Therefore, substitution of nickel atoms for iron atoms has the effect of stabilizing the austenite phase down to low temperatures. Chromium atoms are BCC and therefore a large substitutional addition of chromium to the steel has the effect of stabilizing the ferrite phase. Carbon and nitrogen atoms are smaller and occupy interstitial sites between the primary atoms in a given crystal. The unit cell structure of the austenite phase accommodates these interstitial atoms more readily than the unit cell structure of the ferrite phase. Carbon and nitrogen are therefore, very strong stabilizers at relatively small volume fractions. Sulphur and phosphorus are considered trace impurities, remnant from primary and secondary processing (Korinko et al., 2001). Woollin (1994) reported that steels which contain a minimum of around 12% chromium are usually referred to as 'stainless steels' as a result of their resistance to corrosion due to the formation of a tenacious surface film which protects the underlying steel. Baldev et al. (2006) Woollin, 1994).
Austenitic stainless steels can be hardened by cold working, but not by heat treatment. In the annealed condition, they are non-magnetic. While resistance to corrosion is their principal attribute, they are also selected for their good ductility, toughness and excellent strength properties at high or extremely low temperatures.
They are considered to be the most weldable of the high alloy steels and can be welded by all fusion and resistance welding processes.

APPLICATIONS OF AUSTENITIC STAINLESS STEELS
Austenitic stainless steels constitute the largest stainless steel family in terms of alloy type and usage. Because of its excellent mechanical properties and corrosion performance, it is largely used in various corrosive conditions between cryogenic to elevated temperature range. Austenitic stainless steel is probably the most common choice in material selection for chemical and food processing equipment, cryogenic vessels, welding construction, low and high pressure boilers, vessels, fossil-fired power plant, flue gas desulphurization equipment, evaporator tubing, super heater, reheating tubing, steam headers and pipes to mention but a few (Cui et al., 2006;Afolabi, 2008;Callister et al., 2012). Austenitic stainless steels are preferred over many other materials because of their performance in even the most aggressive environments, and they are fabricated by methods common to most manufacturers. The product manual from Afrox limited suggested the following guidelines for fabricating austenitic stainless steel components: i. Thermal cutting should be done with appropriate process, that is, plasma arc, laser or arc air, not oxy-fuel cutting.
ii. If machining is performed, it should be done without overheating the base metal, which would cause oxidation.
iii. Grinding should be done with the correct grade of disc and with discs segregated for use only on austenitic stainless steel.
iv. All hand tools (files, deburring, knives etc.) should be segregated and used only on austenitic stainless steel.
v. All wire brushes should be made of stainless steel and used only on austenitic stainless steel.

WELD JOINT DESIGN
Welding is one of the most important and versatile means of fabrication available to industry. In a welded fabrication welds are used to join different parts. Such junctions of parts are called weld joints and are defined as the locations where two or more members are to be jointed. Joint design is a general term used for a group of variables, which include the geometry of the parts as prepared for welding, thickness of the parts, arrangement of the parts, weld joint and restraint of the weld joint. The weld joint is designed to meet a certain combination of properties required for satisfactory performance in service. The objectives of weld joint design are to provide an assembly that: i. Will perform its intended functions ii. Will have required reliability and safety iii. Is capable of being fabricated, inspected, transported and installed in service at minimum total cost.
The optimum choice of a weld design is one that meets all design and service requirement at a minimum cost. The selection of the weld type is particularly assigned to the product design engineer, who considers the strength of the required joint, joint type and geometry, nature of materials to be joined, joining method to be employed, weldability of the material to be joined, and other service conditions where the weldment is to be applied. In addition, the joint design should be selected primarily on the bases of load requirements while the type of joint is determined by the relative positions of the two members being joined. The type of joint required often determines the weld type (fillet weld, groove weld, backing weld, flange weld, plug or slot weld, spot or projection weld, seam weld, and surfacing weld) to be adopted. The five basic types of weld joints as illustrated in i. Lap joint produced by overlapping the members to be joined and then welding the edges.
ii. Butt joint obtained by placing the members to be joined edge -to -edge.
iii. Corner joint obtained by joining the edges of the two parts to be joined whose surfaces are at right angle to each other.
iv. Edge joint produced by joining two parallel members.
v. T -Joint obtained by joining two parts whose surfaces are approximately at right angles to each other. (Baldev et al., 2006;Parmar, 2010; http://www.spartanmechanics.net/images/types of welding joints.JPG)

WELDING PROCESSES FOR AUSTENITIC STAINLESS STEELS
One of the most employed methods of fabricating austenitic stainless steel components is welding. A weld is a union between pieces of metal at faces rendered plastic or liquid by heat or pressure or both. The prime function of the welding operation is therefore, to provide metallic bonds between atoms at the interfaces of the joint (Okorafor et al., 2011).

PREWELD PREPARATION
Austenitic stainless steel needs to be prepared without contamination as any sources of free iron, rust, carbon or hydrogen etc; can cause welding or corrosion problems. It is imperative that the base metal be properly cleaned before welding.
In most cases this involves: i. Wirebrush or grind to remove any oxidation.
ii. Chemically clean all surfaces that were machine-cut with cutting fluids.
iii. Remove all oil, grease, moisture etc.
iv. Wipe all surfaces to be welded with acetone or isopropyl alcohol.
v. Weld in an area segregated from the welding of other alloys and use holddown fixtures, vices, tools, and clamps etc; made of stainless steel or covered with protective material to prevent contamination (Afrox product manual).

WELDING AUSTENITIC STAINLESS STEELS
An important part of successful welding of austenitic stainless steel requires proper selection of alloy (for both the base and the filler rod), and correct welding procedures. According to AISI handbook (1988) the two important objectives in making weld joints in austenitic stainless steels are: i. Preservation of corrosion resistance ii. Prevention of cracking In pursuit of these objectives stated above, it is therefore necessary, to exercise a reasonable degree of care during welding to minimize or prevent any deleterious effect that may occur and to preserve the same degree of corrosion resistance and strength in the weld zone that is an inherent part of the base metal.
The two basic methods for welding stainless steels are: i. Fusion welding ii. Resistance welding In electric arc fusion welding, heat is provided by an electric arc struck between a carbon or metal electrode (connected to one terminal of a power supply) and the metal to be welded (which is connected to the other terminal).
In resistance welding, bonding is the result of heat and pressure. Heat is produce by the resistance to flow of electric current through the parts to be welded, and pressure is applied by the electrodes (Srinivasan, 2008;Okorafor et al., 2011).
In the study of the effect of electrode types on the solidification cracking susceptibility of austenitic stainless steel weldment, electric arc fusion welding procedure was adopted.
There are four principal processes for fusion welding of stainless steels. They are: i. Shielded metal arc welding (SMAW) ii. Gas tungsten arc welding (GTAW) iii. Gas metal arc welding (GMAW) iv. Submerged arc welding (SAW) Other fusion welding methods for welding stainless steels include electron beam, laser and plasma arc welding. In all the cases, the weld zone is protected from the atmosphere by a gas, slag or vacuum, which is absolutely necessary to achieve and preserve optimum corrosion resistance and mechanical properties in the weld joint (AISI handbook 1988;Parmar, 2010 respectively. The electrode coating is beneficial to the weld joint in several ways as stated below: i. The primary function of the electrode coating is to initiate and maintain the arc column between the electrode and the workpiece.
ii. Impurities are removed from the molten metal by the flux.
iii. A gaseous envelope is formed by the decomposition of the ingredients in the electrode coating. This excludes the oxygen and nitrogen in the air from contact with the molten weld pool.
iv. The slag formed on top of the weld metal acts as a protective covering against contamination by the atmosphere as the weld cools, and serves to control the shape of the weld pool.
v. It provides alloy additions to the weld metal.
The handling and storage of coated stainless steel shielded metal arc welding electrodes is very important because coatings tend to absorb moisture, and the moisture in the weld zone during welding can lead to porosity which weakens the weld and becomes focal points for corrosion. For this reason, austenitic stainless steel electrodes must be stored in a warm, dry environment and preferably in the original sealed container (AISI handbook 1988;Baldev et al., 2006;Srinivasan, 2008;Parmar, 2010).

GAS TUNGSTEN ARC WELDING (GTAW)
Gas tungsten arc welding (GTAW) or tungsten inert gas (TIG) welding as it is sometimes called is a fusion arc welding process that uses an inert gas usually argon, to protect the weld zone from the atmosphere. The fact that gas tungsten arc welding lends itself to autogenous welding makes it very significant in this study.  (Kalpakjian, 2008) When deep penetration is not important, alternating current, AC, is preferred to direct current, DC. This is because alternating current combines the work of cleaning action of electrode positive (reverse polarity) with deep penetration characteristic of electrode negative (straight polarity) of direct current (AISI handbook 1988;Baldev et al., 2006;Srinivasan, 2008).

WELDING PROCEDURE
The two welding techniques employed in this study as discussed in preceding sections, are: i. Shielded metal arc welding ii. Gas tungsten arc welding In order to investigate the effect of electrode types on the solidification cracking susceptibility of austenitic stainless steel weldment, the base metal was welded in two different welding procedures namely: i. Welding with no filler addition or with filler metal of matching composition with the base metal. This welding technique is regarded as autogenous welding and is applied in this study during the gas tungsten arc welding.
ii. Welding with non-matching filler rod or with a consumable electrode. This welding technique is applied in this study during the shielded metal arc welding operation. The resulting weld metal composition would depend upon the filler material type, and the level of dilution which is controlled by the welding process and the variables employed.

WELDING ELECTRODES
Arc welding is a fusion welding process in which coalescence of the metals is achieved by the heat from an electric arc between an electrode and the workpiece.
The electrodes used in arc welding process are classified as: Consumable electrodes provide the source of the filler metal in arc welding. These electrodes are available in two principal forms: i. Electrode rods (also called sticks) ii. Electrode wire Welding rods are typically 255 -450mm long and 9.5mm or less in diameter. The problem with consumable welding rods, at least in production welding operations, is that they must be changed periodically, reducing arc time of the operator.
Consumable electrode weld wire has the advantage that it can be continuously fed into the weld pool from spools containing long lengths of wire, thus avoiding the frequent interruptions that occur when using welding sticks. In both rod and wire forms, the electrode is consumed by the arc during the welding process and added to the weld joint as filler metal.
Non -consumable welding electrodes are made of tungsten (or carbon) which resists melting by the arc. Despite its name, a non -consumable electrode is gradually depleted during welding process, analogous to the gradual wearing of a cutting tool in a machining operation by vaporization mechanism. For arc welding processes that utilize non -consumable electrodes, any filler metal used in the operation must be supplied by means of a separate wire that is fed into the weld pool (Parmar, 2010;www.wikipedia.org

WELDING METALLURGY OF AUSTENITIC STAINLESS STEELS
When joining any material to be used under demanding conditions, it is important to ensure that the joint has adequate properties to perform successfully in service.
When a fusion welding process is employed, the original microstructure of the fusion zone is destroyed and a new structure develops, in a similar manner to that of a casting. The resulting room temperature structure depends upon the phases developed during solidification, and the extent of subsequent solid state transformations (Woollin, 1994). The physical properties and serviceability of the resulting weld metal is determined by the microstructure and thermal history (Korinko et al., 2001). According to Srinivasan (2008)  i. Weld metal zone also regarded as the mixed zone which is essentially a solidified structure of the base metal and filler metal. The degree of dilution is controlled by the welding process and the variables employed.
ii. Unmixed zone in the base metal adjacent to the fusion line where the base metal has melted but is not mixed with the filler material.
iii. Partially melted zone which has experienced thermal cycles with peak temperatures lying between the solidus and liquidus temperatures. iv.
Heat affected zone (HAZ) which has experienced thermal cycles (peak temperatures) high enough to cause microstructural changes but insufficient (being less than the solidus temperature) to effect melting.
Each zone because of its characteristic microstructural features has different properties. Baldev et al. (2006) found that a ternary eutectic transformation occurs during solidification of austenitic stainless steel weld metal, with the simultaneous formation of austenite and delta-ferrite (δ) from the liquid (weld pool). As shown in Figure 2.7, the relative proportions of austenite and delta-ferrite (δ) in the . .

MICROSTRUCTURE
The various solidification modes occurring in austenitic stainless steels as reported by Shankar et al. (2003) and Baldev et al. (2006) are: In any of the solidification modes stated above, the fraction of ferrite remaining in austenite weld metal after solidification has been presented graphically as a function of composition on the WRC -1992 constitution diagram for stainless steels weldment as shown in Figure 2.9.
The WRC -1992 constitution diagram was developed from manual metal arc weld metal and is applicable to common arc welding processes. However, the microstructure developed may be sensitive to cooling rate, which controls the extent solid state transformation. Consequently, Laser and Electron Beam welds do not necessarily conform to the predictions of the diagram (David et al., 1987;David et al., 1989). Predictions can be made for a particular composition by  the primary phase and a part of the remaining liquid solidifies as eutectic ferrite.

When
≈ 1.6, the solidification mode would most likely be ferrite -austenite.

. .
Fully ferritic solidification mode results under condition when > 1.9 . . (Woollin, 1994;Baldev et al., 2006).  Austenitic stainless steels show no other major change during cooling to room temperature, and therefore, no pre-heat is normally required when welding them.
However, some precipitation may occur during cooling, such as the formation of chromium carbides on the heat affected zone grain boundaries, leading to depletion of chromium and leaving the material in a sensitized state and vulnerable to intergranular attack also known as weld decay. The risk of weld decay has reduced significantly in recent years through the development of low carbon grades which contains less than or equal to 0.03 percent carbon, and stabilized grades containing titanium or niobium which form carbides preferentially to chromium (Woollin, 1994). Besides hot cracking, Parmar (2010) listed stress corrosion cracking, knife edge attack, carbide precipitation, and ferrite and sigma phase transformations as the specific problems associated with the welding of austenitic stainless steels.
Welding austenitic stainless steels therefore, requires special attention considering the weldability problems discussed above which are associated with the weld metal.

WELD DEFECTS AND QUALITY
When performing welding on metals, the base materials are typically subjected to rapid heating and cooling in localized areas. Such thermal stress conditions coupled with entrapment of gases or foreign materials within the weld can produce distortion and lead to weld metal cracking which result in detrimental mechanical properties and poor performance of the welded assembly if not controlled. Weld defects are inevitable as no weld is completely perfect in real situations. Weld defects are generally stress raisers and initiation sites for crack initiation and propagation leading to premature failure of the weld in service.
Fracture mechanics utilizes the knowledge of these defects (their sizes, shape and location) and stress level (including residual stresses and externally imposed stress) in the control of weld metal failure. According to Baldev et al. (2006) defects in welded joints are classified into three namely: i. Physical discontinuity (unacceptable contour, surface irregularities, undercut, underfill, lack of fusion, insufficient or excessive penetration etc.) ii. Microstructural defects (cracks, porosity, blow holes, grain coarsening, solid inclusions, slag inclusions etc.) iii. Defects related to residual stresses and distortion Cracks are fracture -type interruptions either in the weld itself or in the base metal adjacent to the weld. This type of defect is perhaps the most serious welding defect because it constitutes a discontinuity in the metal that causes significant reduction in the strength of the weld metal. For this reason, cracks in weldments are never acceptable whereas there are acceptable limits for slag inclusions and porosity in welds. Welding cracks are caused by embrittlement or low ductility of the weld metal and/or base metal combined with high restraint during contraction.
For instance, if the contraction occurring during solidification of the weld is restricted, the strains developed will induce residual stresses that cause cracking.
Hot (cracks that occur at elevated temperature during weld metal solidification) or cold (cracks that occur after the weld metal has cooled to room temperature) cracks may manifest as centerline cracking (segregation induced cracking, bead shape induced cracking, or surface profile induced cracking), heat affected zone cracking or as transverse cracking which were explained in the James F. Lincoln Arc Welding Foundation manual on weld cracking.
Porosity consists of small voids in the weld metal formed by gases entrapped during solidification. The shapes of the voids vary between spherical (blow holes) to elongated (worm holes). Shrinkage voids are cavities formed by shrinkage during solidification.
Lack of fusion also known as incomplete fusion consists of a weld bead in which fusion has not occurred throughout the entire cross section of the joint. A lack of penetration means that fusion has not penetrated deeply enough into the root of the joint, relative to specific standards.
Solid inclusions are any non-metallic solid material entrapped in the weld metal.
The most common type is slag inclusion generated during the various arc-welding processes that use flux. Instead of floating to the top of the weld pool, globules of slag become encased during solidification of the metal.
Unacceptable contour which refers to defects in form of undercut, underfill or overlap appearing on the weld profile. The weld should have a certain desired profile for maximum strength. Baldev et al. (2006) opined that the testing, measurement and control of welds should be optimized based on fitness -for -purpose. In this way the reliability of weld performance is evaluated by measurement and control of weld properties.
Effective testing programme is to detect defects and measure weld properties as specified by the design based on fitness -for -purpose philosophy.  (Brooks et al., 1991;Arantes et al., 2007). Gooch (1990) reported that during weld metal solidification, some segregation inevitably occurs. In materials which solidify as austenite for instance, silicon, niobium, and impurity elements such as sulphur and phosphorus segregate into the interdendritic regions, which therefore solidify at lower temperatures than the bulk of the steel structure. This may lead to the formation of interdendritic solidification cracks as a consequence of contraction during cooling. Shankar et al. (2003) opined that solidification cracking occurs predominantly by the segregation of solutes to form low-melting phases, which under the action of shrinkage stresses accompanying solidification cause cracking in the weld metal.

CONTROL OF SOLIDIFICATION CRACKING
Solidification cracking can be controlled by adopting the correct welding procedure to obtain a favorable solidification mode and structure. The most common means of avoiding solidification and liquation cracking is to choose a filler material type which gives some residual ferrite in the weld metal. The association of delta-ferrite with cracking resistance in austenitic stainless steel weld metal is quite old. As early as 1938, Scherer et al. (1941) filed a patent, which claimed that crack resistant austenitic stainless steel weld deposits could be produced if the composition is adjusted to result in 5 -35 percent ferrite in the completed weld. This has proved extremely effective, although the mechanism by which the effect is achieved is still the subject of debate. Hull (1967) however, found that when ferrite content in the completed weld increase beyond 35 weight percent, the weld metal would again become susceptible or sensitive to solidification cracking.
A number of factors have been proposed to explain the beneficial effects of deltaferrite on the cracking behavior of austenitic stainless steel weldment as compiled by Shankar et al. (2003): i. The higher solubility for impurity elements in delta-ferrite leads to less interdendritic segregation and reduces cracking tendency (Borland and Younger, 1960).
ii. Cracks are arrested by the irregular path offered by a duplex austeniteferrite structure. The peritectic / eutectic reaction interface arrests remaining pockets of liquid and thus crack propagation (Matsuda, 1979).
iii. The lower surface energy of the austenite -δ-ferrite boundary and its reduced wettability by eutectic films compared to austenite -austenite or δferrite -δ-ferrite interfaces is an important factor (Hull, 1967).
iv. The presence of δ-ferrite results in a large interface area due to the solid state transformation to austenite that begins soon after solidification. The increased area disperses the concentration of impurity elements at the grain boundaries.
v. The ductility of ferrite at high temperatures is greater than that of austenite, allowing relaxation of thermal stresses.
vi. The lower the thermal expansion coefficient of ferrite as compared to austenite results in less contraction stresses and fissuring tendency.
vii. The solidification temperature range of primary ferrite welds is less than that of primary austenite solidified welds, providing a smaller critical temperature range for crack formation (Pellini, 1952).
viii. The presence of ferrite refines the grain size of the solidified weld metal, which results in better mechanical properties and cracking resistance.
ix. The higher coefficients for impurity diffusion in ferrite as compared with austenite allow faster homogenization in ferrite and less tendency for cracking.
x. Coarse grain formation in the heat affected zone (HAZ) occurring by recrystallisation and grain growth in fully austenitic weld metals increases susceptibility to liquation cracking (Kujanpaa et al., 1985), while ferrite forming compositions are not susceptible to such cracking.
xi. The volume contraction associated with the ferrite -austenite transformation reduces tensile stresses close to the crack tip, which decreases cracking tendency .
Nevertheless, solidification cracking continues to cause concern in fully austenitic stainless steels, when ferrite is restricted and when composition adjustments during welding are difficult. Several theories have been postulated by Pellini (1952), ii. Microfissuring in the interdendritic regions which are revealed only by application of strain to the cracked region or at high magnifications.

EFFECT OF COMPOSITION ON CRACKING TENDENCY OF AUSTENITIC STAINLESS STEEL WELDMENT
Composition affects the tendency of austenitic stainless steel weld metal towards cracking in the following ways: i. Solidification mode is dependent on weld metal composition. Shankar et al. (2003) and Baldev et al. (2006) found that ferrite -austenite / ferrite solidification modes reduce the tendency for solidification cracking in austenitic stainless steel weld metal.
ii. Segregation pattern in the weld metal is also a function of the composition.
Segregation determines the wetting characteristics and constitutional supercooling in the interdentritic regions.

EFFECT OF SULPHUR AND PHOSPHORUS ON THE CRACKING PROPENSITY OF AUSTENITIC STAINLESS STEEL WELD METAL
Sulphur is known to be an undesirable impurity in welding of austenitic stainless steels due to the formation of low-melting sulphide films (Fe -FeS eutectics) along the interdendritic and grain boundary regions. Sulphur is almost insoluble in iron, chromium and nickel which are the three major constituents of stainless steels. The phase diagrams for sulphur binaries with all the three elements (iron, chromium, and nickel) as reported by Shankar et al. (2003) showed that a wide and deep solid -liquid regions with low partition coefficients for sulphur in austenite.
Consequently, sulphur is strongly rejected into the liquid during solidification of austenite, forming low-melting eutectics and lowering the melting point of the interdendritic liquid. This phenomenon of forming low-melting point eutectics along the interdendritic and grain boundary regions causes cracking in austenitic stainless steel weld metal during solidification. On the other hand, delta -ferrite shows higher solubility for sulphur, phosphorus, silicon, and nobium; and thus beneficial in reducing solidification cracking. This is true as sulphur, phosphorus, silicon and nobium have the ability of forming low-melting eutectic phases which promote cracking in the weld metal. Solidification in the ferrite -austenite mode is known to increase tolerance for sulphur content to as high as 0.05 weight percent. Arantes et al. (2007) found that found that austenitic stainless steel weldments with sulphur and phosphorus contents summing up to 0.042 weight percent showed good weldability. Lundin et al. (1988) investigated weldability of free -machining stainless steels, and found that, provided a ferritic solidification mode is maintained, austenitic stainless steel weldments with sulphur contents up to 0.35 weight percent could show satisfactory weldability.
Phosphorus ranks next to sulphur in the list of elements which are detrimental to good weldability in austenitic stainless steels. Like sulphur, phosphorus forms lowmelting eutectics with iron, chromium, and nickel. Phosphide eutectics at the interdendritic regions have been found to extend the brittleness temperature range in the varestraint test, to as much as 250K lower than the solidus solidus in fully austenitic type 310 steel (Matsuda et al., 1981). Accordingly, sulphur, phosphorus, boron, niobium, titanium and silicon were identified as most harmful, as they possess low solubility in the solidified weld metal and form low-melting eutectics with iron, chromium, and nickel which promote solidification cracking. Among the elements whose effects on solidification cracking were investigated, Hull (1960) found molybdenum, manganese, and nitrogen to be somewhat beneficial.

THEORY OF HOT CRACKING
The initial theory of hot cracking as proposed by Medovar (1954) states that the wider the liquid -solid range of the alloy, the greater the susceptibility to hot cracking. This theory provided a good foundation for the explanation of hot cracking phenomenon but had a serious short coming in that the theory neglected other factors that could be responsible for hot cracking, stressing that segregation resulting from the wide freezing range appeared to be the sole cause of hot cracking. A more complete theory was developed by Borland (1960). Borland stated that for high cracking susceptibility, in addition to a wide freezing range, the liquid must be distributed in a way that allows high stresses to be built up between the grains. Where are the interfacial energies of the solid -liquid and solidsolid boundaries respectively. And θ is the wetting angle. When θ is nearly zero, the wetting of solid -solid boundary is enhanced and cracking tendency of the weld metal would be promoted under this condition (Shankar et al., 2003;Baldev et al., 2006). Borland's theory revealed the importance of wetting in relation to solidification cracking tendency of the weld metal but the effect of wetting angle of the liquid and solid phases on cracking propensity is very difficult to quantify. In attempt to modify Borland's theory, Matsuda (1990) suggested that hot cracking is constituted by crack initiation and crack propagation acting distinctly at the critical solidification and is a function of the weld metal composition. The overall idea according to Shankar et al. (2003) is that, irrespective of the stress field experienced during welding, the temperature range of cracking susceptibility (or sensitivity) known as the brittleness temperature range (BTR) can be considered as a function of the weld metal composition. Matsuda et al. (1989 a, b)  Hot cracking is believed to occur due to the inability of the solidifying weld metal to support strain in the critical temperature range during solidification (Borland, 1960   A minimum ferrite number is necessary to avoid hot cracking in austenitic stainless steel welds (Vasudevan et al., 2004;Vasudevan, et al., 2005; http://www.diversetechnologies.net).

STRAIN EVALUATION CRITERIA
The amount of strain developed in the weld metal may be used to evaluate the solidification cracking susceptibility of an austenitic stainless steel weldment. In actual welds, the amount of strain experienced by the weld metal is difficult to estimate in view of complex geometric and thermal conditions. Hence controlled strain applied on a geometrically simple specimen is preferred for evaluation of cracking tendency. Several tests that satisfy the above condition exist for the determination of austenitic stainless steel weld metal propensity to solidification cracking (Shankar et al., 2003). Recent work on solidification cracking in austenitic stainless steel welds by Shankar et al. (2003) has shown that opinion is divided on the choice of strain criteria for cracking assessment of materials due to difficulties in applying the results of variable restraint tests to actual welding situations.

PHYSICAL METALLURGICAL ASPECT OF A WELD JOINT
During welding, a small volume of metal is molten by a heat source which is moved along the line in which a joint is sought. In comparism with casting, welding processes involve cooling rates of several orders of magnitude higher than that in conventional casting and the growth rates are correspondingly higher. Since the base metal must necessarily melt back for good fusion to occur, there is considerable dilution into the weld puddle. As the heat source moves away, the heat loss mainly by conduction starts the solidification process and the weld metal starts solidifying epitaxially at the fusion line, and nucleation is not required to initiate the growth of the solid. In pure metals, the melting and solidification is heat flow controlled. Alloys such as austenitic stainless steel, solidify over a range of temperature. The temperature above which the alloy is completely molten is called the liquidus while the temperature below which the alloy is completely solid is  (Kurz et al., 1989;Baldev et al., 2006;Parmar, 2010).

SUMMARY OF LITERATURE REVIEW
The literature review presented in this chapter reported extensively on the weldability of austenitic stainless steel. Many researchers believed that the major problem of austenitic stainless steel weld metal is hot cracking or solidification cracking (AISI handbook 1988;Woollin, 1994;Shankar et al., 2003;Baldev et al., 2006;Arantes et al., 2007). While maintaining that the standard welding practices for austenitic stainless steels focus on the selection of appropriate filler metal to control the weld metal microstructure. Building on the foundation of many researchers whose findings were cited in this work, the author investigated the effect of electrode types on the solidification cracking susceptibility of austenitic stainless steel weld metal. The composition evaluation criteria was employed in the assessment of the weld metal propensity for cracking as there are difficulties in applying the results of variable restraint tests (obtained through the strain evaluation criteria) to actual welding situation (Shankar et al., 2003). Literature on the physical metallurgy of weld joint, theories of hot cracking and development of weld metal microstructure were reviewed in attempt to establish the sensitivity of solidification cracking on weld metal composition.

MATERIALS
The base metal of the test specimens used for this study is type 304H austenitic stainless steel (18.55%Cr, 8.72%Ni, and 0.057% C). The bulk material was procured from Sabatex Stainless Steel Products, Onitsha. The nominal chemical composition of the material is shown in Table 3.1.   Necessary preweld preparations were made by wire brushing and grinding the surfaces to be joined in order to remove oxide films, scales, and dust particles. The smooth finished surfaces were cleaned with acetone to remove moisture, grease or oil. These cleaning operations were carried out in order to achieve metallurgical cleanliness of the surfaces to be joined as well as atomic closeness of the intended weld.

WELDING PROCEDURE
Two methods of welding were adopted namely, Shielded Metal Arc Welding (SMAW) and Tungsten Inert Gas (TIG) Welding. The welding operations were conducted under constant condition as displayed in Table 3.3 below. The variable parameter in this study was the welding electrodes while the weld joints produced from TIG autogeneous welding served as the control or standard of comparism. The specimens used in this research were the welded joints produced in six distinct categories, viz: i. CATEGORY A: Six specimens were produced by using E 308 -16, gauge 10 (rutile) welding electrode in a shielded metal arc welding process. ii.
CATEGORY B: Six specimens were produced by using E 308 -16, gauge 12 (rutile) welding electrode in a shielded metal arc welding process.
iii. CATEGORY C: Six specimens were produced by using E 308 -16, guage 12 (lime-titania) welding electrode in a shielded metal arc welding process.
iv. CATEGORY D: Six specimens were produced by using E 310 -16, gauge 10 welding electrode in a shielded metal arc welding process.
v. CATEGORY E: Six specimens were produced by using E 312 -16, gauge 10 welding electrode in a shielded metal arc welding process.
vi. CATEGORY F: Six specimens were produced from Tungsten Inert Gas (TIG) autogeneous welding process using the base metal as the filler rod.

WELD CLEANING AND INSPECTION
After the welding operation, the welded joints were visually examined or inspected. Faulty and poor welded joints were excluded. Satisfactory joints were cleaned by machining and grinding to a fine surface finish.

TEST SPECIMENS
Each specimen (welded joint) has a dimension of 110mm x 35mm x 3mm. Two test specimens from each category A -F, were prepared for tensile test, chemical analysis and micrographic test respectively.

PREPARATION OF SPECIMEN FOR TENSILE TEST
The test specimen for tensile test was machined to fit into the grip of the tensile testing machine, to a gauge length of 25mm. The tensile test specimen was 3mm thick and 4mm wide as shown in Fig. 3.2a.

.2 PREPARATION OF SPECIMEN FOR CHEMICAL ANALYSIS
The weld joints produced in this work were subjected to chemical analysis, with the aid of a spectrometer, to determine their respective fusion zone chemistry.
Although the weld metal had three zones (unaffected parent metal, heat affected zone and the fusion zone), the zone of interest for chemical analysis was the fusion zone since it consist of a solidified structure of the welding electrode (or filler rod) and the base metal -that is, the region of dilution. The sample for chemical analysis was prepared by cutting out the fusion zone from the weld metal and grinding the surface to the requirement of the spectrometer.

PREPARATION OF SPECIMEN FOR METALLOGRAPHIC
TEST Metallographic test of the weld joints were conducted using standard test techniques. The test was conducted distinctly on the parent metal, heat affected zone as well as the fusion zone for each test specimen. The samples for metallographic test were prepared in the following manner: i. Wet grinding with water was carried out for all the specimens by using silicon carbide (SiC) emery papers of grades 220, 400, 800, and 1100. ii.
Polishing process was carried out by using special polishing cloth with aluminum oxide (Al2O3) solution 5µm grain size.
iii. Etching process was done by immersing each specimen in etching solution (Nital solution) which consists of 98% methyl alcohol and 2% Nitric acid, for 30sec. After which the specimens were washed with water and alcohol and then dried. iv.
Microstructures of the parent metal, heat affected zone and fusion zone were examined for each test specimen, with optical microscope provided with computer and digital camera.
Results obtained from the tensile test, chemical analysis and metallographic tests were tabulated and discussed in the next chapter.

EXPERIMENTAL RESULTS
The experimental results in this work were obtained from tensile test, weldment analysis and metallographic test.

EXPERIMENTAL RESULTS FROM TENSILE TEST
The tensile flat specimen of gauge length 25mm, width 4mm and thickness 3mm was subjected to tensile load applied at a uniform rate until the specimen fractured.
The corresponding increase in length for each test load was measured and recorded. The experiment was repeated for each test specimen (welded joints produced from different electrodes, TIG autogeneous welded joint and the unwelded parent material). The following results were obtained.
From the results in Table 4.1, the following properties of the material were determined and tabulated in Table 4.2 -4.8.  where W= Cross sectional width of the test specimen =4mm, and T = Thickness of the specimen = 3mm.  Fracture mode is ductile and the location is slightly away from the centre line.

Tensile strength (T.S) = = = 425Nmm
Fracture mode is ductile and the location is at the heat affected zone. Fracture mode is ductile and the location is at the heat affected zone.

Tensile strength (T.S) = = = 425Nmm
Fracture mode is ductile and the location is at the heat affected zone.

Tensile strength (T.S) = = = 475Nmm
Fracture mode is ductile and the location is at the heat affected zone.

ANALYSIS
The evaluation of the susceptibility of 304H stainless steel to solidification cracking when welded with different electrodes was based on the chemical composition and microstructural analysis of the weld metal. The chemical composition of the weldments produced from different stainless steel electrodes was determined by the spectrometer and the results are given in Table 4.11. From the results of the chemical analysis of the weld joints displayed in Table   4.11, the values of the chromium and nickel equivalence, Cr eq. / Ni eq. ratio, and (P+S) wt. % were computed and tabulated in Table 4.12. The Cr eq. and Ni eq. values were calculated for each weld joint using Welding Research Council (1992), Hammar and Svenson (1979) and Schaeffler (1949) equations -see details in appendix B. The Cr eq. /Ni eq. ratio was calculated for each of the weldment using the Welding Research Council (WRC) 1992 model equation given as (Kotecki et al., 1992) . . = + + 0.7 Results obtained in Table 4.12 above was compared to 300 series stainless steel WRC 1992 constitution diagram shown in Figure 2.8, to determine the solidification mode and ferrite number of the respective welded joints, as shown in Table 4.13 below. The WRC constitution diagram in terms of chromium and nickel equivalence showing the solidification mode of the weld joints were displayed in Appendix C.  Figure 4.22 shows the effect of (P+S) wt. % and Cr eq. / Ni eq. on the cracking propensity of the respective weld joints. Figure 4.22: plot of (P+S) wt. % versus Cr eq. / Ni eq. from readings obtained in Table 4.12  On the other hand, in the interdendritic region of primary ferrite at the final stage of solidification, because nickel is rejected into the liquid, the stability of austenite increases and causes the formation of austenite. Austenite, however, grows more easily from the austenite that has already solidified than the nucleation of austenite on the preceding ferrite or in the liquid, and therefore, the formation of the austenite at the dendrite boundaries is invariably epitaxially growth and fills the interdendritic region of the primary ferrite (Inoue et al., 2007).  (Inoue et al., 2007;Fu et al.,2009). The fusion zone micrograph of E 308 -16/12 (lime-titania) joint shown in Plate 4.3a revealed a plenty of fine colonies of lathy ferrite (dark) embedded in austenite (white) matrix. The result was a duplex microstructure consisting of thin lathy ferrite and austenite.
The micrograph of the HAZ of E 308 -16/12 (lime-titania) welded joint shown in Plate 4.3b revealed growth of austenite grain accompanied with much solute segregation and carbide precipitation along the grain boundaries.  The results displayed in Table 4.10 suggested that the type of welding electrode used in the fabrication of austenitic stainless steel components has effect on the strain hardening exponent of the material. In metal forming processes, the product shapes are produced by plastic deformation. In many products the mechanical properties depend on the control of strain hardening during processing, while in the other instances, precise control of deformation, temperature, and strain rate during processing is required to develop the optimum structure and properties (Dieter et al., 1988).
In order to optimize the processes involved in metal forming, it is important to know how the types of welding electrode used during fabrication affects the plastic flow properties of the material. Strain hardening coefficient is one of the most important property considered during metal forming. Callister et al. (2010) opined that strain hardening coefficient (or exponent) is a measure of the ability of a metal to strain harden, which is a phenomenon whereby a ductile material becomes harder and stronger upon plastic deformation. A number of authors have reported that materials with a high value of strain hardening exponent allow more tensile deformation before a localized neck develops and hence enjoy the benefits of cold working (Dieter, et al.,1988 ;Callister, et al., 2010;and Campbell, 2008).
Consequently, materials with low strain hardening coefficient tends be less useful in cold forming.
In the present work carried out at a constant strain rate of 108rev/min., it was found Consequently, more dislocations were created which increased the strain hardening exponent of the material. This has been confirmed by many research works (Talyan et al., 1998;Neff et al.,1969;Bressanelli and Moskowitz 1966;Angel, 1954).
Welding with E 308 -16/12 (lime-titania) electrode produced the least value of strain hardening exponent of about 0.35, and is supposed to exhibit a relatively lesser tensile deformation in the plastic region before necking, even though it has a relatively high ductility of about 36% in terms of percentage elongation.
The effect of electrode size on the strain hardening exponent was also investigated.
E 308 -16 (rutile) stainless steel electrode of gauges 10 and 12 were used on the same material and found that the one of E 308 -16/12 ( solidified with a primary ferrite solidification mode. The compromise reached between the parent material composition and filler rod or electrode dilution was found to be the major factor which determined the weld metal final microstructure and solidification mode. The findings of this research were found to be in line with the results of many researchers (Schino et al., 2000;Fu et al., 2009;Leone, et al., 1982;Udomphol et al., 2007;Arantes et al., 2007;and Baldev et al., 2006).

EFFECT OF ELECTRODE TYPES ON WELD METAL COMPOSITION AND CRCKING PROPENSITY
The results of chemical analysis (presented in Table 4.11) carried out on the weldments showed that electrode types have effect on the weld metal composition.
The TIG autogeneous weldment had nearly the same composition and Chromium -Nickel equivalence as that of the unwelded parent metal. However, remarkable difference in weld metal constitution was observed in the joints produced from the various electrodes relative to the Chromium -Nickel equivalence of the parent material. The results of Chromium -Nickel equivalence for the respective joints were found to be slightly more or less, and yet in agreement with the Welding Research Council (1992), Hammar and Svenson (1979), and Schaeffler's (1949) equations. The results showed that solidification cracking of the weld joints were sensitive to Cr eq. / Ni eq. ratio and solidification mode of the welds. E 308 - These results were compared with the cracking susceptibility of 300 series stainless steel based on Cr -Ni equivalence according to Hammar and Svenson (1979), and found to be consistent and also in line with the findings of Arantes et al. (2007); Baldev et al. (2006); Korinko et al.(2001); and Brooks et al. (1991)  1.01, which is less than 1.5 the value suggested by (Arantes, et al., 2007;Shankar, et al., 2003;Brooks, et al., 1991;and Scherer, et al., 1941)  HAZ: Heat Affected Zone -the region of the weld joint which has experienced peak temperatures high enough to cause microstructural changes but insufficient to cause melting.
Microfissuring: Cracks of microscopic dimensions which occur in the weld metal during cooling.
Microstructure: Totality of the phases, grain size and shape, their distribution and volume fraction of the phases present in the weld or cast structure.
SMAW: Shielded Metal Arc Welding also known as Manual Metal Arc Welding -A welding process in which heat required for melting is generated from an arc struck between a consumable coated ( with flux ) electrode and the workpiece.
Solidification: Phase transformation from liquid to solid state.
Solidification Cracking: Fractures at the interdendritic and/or intergranular weld metal boundaries in the solidification process, during which the liquid phase of the mushy melt becomes rich in impurities.
Susceptibility: The tendency to occur or the propensity for something to happen.
TIG Welding: Tungsten Inert Gas Welding -A welding process in which the heat necessary to melt the metal is provided by a very intense electric arc which is struck between a non -consumable Tungsten electrode and the workpiece.
Weld: A union between pieces of metal at faces rendered plastic or liquid by heat or pressure or both.
Weld Cracks: Fracture -type interruptions either in the weld itself or in the base metal adjacent to the weld.
Weld Decay: Carbide precipitation occurring during cooling at the HAZ, leading to the depletion of chromium and intergranular attack.
Weld Dilution: Loss of specific materials at the fusion zone as the parent material mixes with the filler metal (or electrode).
Weldability: The capacity of a metal or a combination of metals to be welded into a suitable design structure and for the resulting weld joint to possess the required metallurgical properties necessary to perform satisfactorily in service.
Weldment: The assemblage of parts joined by welding.