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The relative effects of different post-weld heat treatments (PWHTs) on microstructure and mechanical properties of friction welded martensitic stainless steel type AISI 431 were studied. The weld microstructure consists of acicular martensite in equiaxed prior austenite grains and with heat treatment, the martensitic microstructure experiences coarsening. It was observed that retained austenite content decreases with an increase in the PWHT tempering temperature. The tensile properties of welds under different PWHTs were comparable to those of parent metal in the respective treatments. Welds exhibited higher notch tensile strengths than the parent metal in the respective heat treatment condition. Double tempering (670°C + 600°C) led to maximum reduction in hardness. Welds exhibited poor impact toughness in the as welded condition. The improvement in impact toughness was found satisfactory in welds subjected to double tempering, although did not match to the level of the parent metal at respective conditions. The mechanism and reasons for the observed behaviour have been discussed, correlating the microstructure, fracture features and mechanical properties.

The relative effects of single austenitization (SA) and double austenitization (DA) temperatures on the microstructure, mechanical properties, and pitting corrosion of electron beam welds of AISI 431 martensitic stainless steel were studied. The post weld heat treatments consist of austenitizing the weld samples for 1 h at 1000 and 1050 • C and double austenitizing at 1050 and 1000 • C and air cooling followed by double tempering, i.e. tempering at both 670 and 600 • C. The prior austenite grain size increased with an increase in austenitizing temperature from 1000 to 1050 • C. Grain refinement resulted after DA. Parent metal grain size was coarser as compared to grain size in the weld zone in the respective conditions. Retained austenite content increased after DA. Undissolved carbides were observed in welds and parent metal austenitized up to 1000 • C and they dissolved at austenitizing temperature 1050 • C. DA treatment resulted in the dissolution of most of the carbides. Coarsening of martensite laths was observed after tempering. DA after double tempering resulted in optimum mechanical properties, i.e. strength, hardness, and toughness. The pitting resistance of the DA-treated samples showed a considerable improvement over those obtained by either of the two SA treatments followed by double tempered conditions.

Many critical applications in chemical equipment, aircraft and ordinance demand a material of construction with high strength and good corrosion resistance. Frequently the strength requirement exceeds that obtainable with austenitic or ferritic stainless steel and it is necessary to use one of the martensitic stainless steels. Since martensitic stainless steels are structural materials, weldability has been an important consideration in their development. AISI 431 is one of the most potentially attractive steels in this class used extensively for parts requiring a combination of high tensile strength, good toughness and corrosion resistance. Although this material has been used for many years, little information is available on the welding behavior of these steels. Further, data on electron beam (EB) welding and solid state welding process like friction welding are scarce. The lack of knowledge constitutes a potential drawback to the more widespread use of these steels. Hence, a study has been taken up to develop an understanding on the electron beam welding and friction welding aspects of martensitic stainless steel type AISI 431. Various kinds of post weld heat treatments (PWHT) were investigated to determine their influence on microstructure and mechanical properties. Weld center in EB welding resulted a cast structure consists of dendritic structure with ferrite network in a matrix of un-tempered martensite. In friction welding, the weld center exhibited thermo-mechanical effected structure consists of fine intragranular acicular martensite in equiaxed prior austenite grains. In both the welding processes, post weld tempering treatment resulted in coarsening of the martensite which increases with increase in tempering temperature. In the as-weld condition, welds exhibited high strength and hardness and poor impact toughness. Increase in impact toughness and decrease in strength and hardness is observed with an increase in tempering temperature. The hardness of EB welds increased with increase in the austenitizing temperature up to 1100 °C and a marginal decrease thereafter was observed. Double austenitization after double tempering resulted in optical mechanical properties i.e., strength, hardness and toughness.

After development of tungsten alloys and polycrystalline cubic boron nitride (PCBN) tools, lot of progress is made in FSW of stainless steel. Although some issues remain to be solved, satisfactory welds are produced and weld properties are found suitable for the intended applications. This paper summarizes the progress of research work on Friction stir welding of different types of stainless steels. It covers the research made in the selection of suitable tool materials, optimizing the process parameters such as tool travel speed, rotational speed, tilt angle etc. The influence of Friction stir welding on microstructure and mechanical properties of welds has also been reviewed. Fusion welding involves use of filler materials, shielding gases, and development of high energy density which results wider heat affected zone. The weldments show appreciable modification in the microstructure and properties of weld and heat affected zones, which may result solidification defects like distortion, , lack of penetration, poor fusion, cracks etc. Use of plasma arc and laser beam welding techniques can produce sound welds of thicker materials with narrow heat affected zone [2], however these techniques are not suitable for certain materials such as aluminium, magnesium etc.The drawbacks in the fusion welding techniques can some extent addressed by solid state welding techniques (e.g. resistance welding, friction welding) in which welding takes place at a temperature lower than the melting point of base metals and also no filler material and / or shielding gases are required. In resistance welding coalescence occurs due to heat generated by contact resistance and applied pressure and hence, it is not suitable for materials having high electrical conductivity (e.g. aluminium, copper). Friction welding employs frictional heat generated when a moving workpiece and a fixed component are forced together in order to obtain the required heat and temperature for weld. However, the application of friction welding is limited by the geometry of the workpieces to be joined.The above difficulties can successfully overcome by friction stir welding (FSW), a solid state welding process which was developed by the welding institute (TWI) [3] primarily for welding of aluminium and magnesium based alloys. The major advantages of FSW over other welding processes are lower distortion, good dimensional stability, absence of cracking etc.