Stainless steels are essentially Iron-Chromium or Iron-Chromium-Nickel system with other alloying additions such as Mo, N, Ti, Nb, etc. Stainless steels are generally named after the microstructure of the material. Accordingly, stainless steels are classified as austenitic, ferritic, martensitic and duplex stainless steels (austenitic + ferritic). The other grade, i.e., precipitation hardened stainless steels have basically austenitic or martensitic microstructures [1]. Different grades of stainless steels behave differently during thermal processing due to the inherent difference in thermo physical and metallurgical properties.  Hence, the material specific fabricability issues have to be addressed to achieve optimum properties in the final component assembly. 

processing different grades of stainless steels

2.1. Austenitic stainless steels

2.1.1. Welding - All austenitic stainless steels of grade 3xx and super austenitic stainless steels are readily weldable by both laser beam (LB) welding and electron beam (EB) welding. Power beam welding of various grades of austenitic stainless steels viz., 304, 316, 316LN, 904L, etc. were attempted and the properties of the welds obtained were superior compared to conventional welding processes.  Full penetration welds in single pass welds are obtained with optimized parameters. 

2.1.2. Surface Treatment -Austenitic stainless steels are very good against corrosive environments, impact wear, etc. However, the surface properties especially pitting resistance, erosion resistance can be further enhanced by employing surface modification techniques. Surface remelting or cladding the surface with different materials can be employed to improve specific properties can be employed. The low heat input of the processes can be of great advantage especially, in processing the near finished component. Pulsed Nd:YAG laser surface melting of SS 304 has improved the corrosion resistance of the material [13]. The improvement in the properties was attributed to the formation of thick protective oxide layer making the material to shift more towards the noble side. Similarly, the cavitation erosion resistance and pitting corrosion resistance of SS 31603 has increased by surface melting using laser [14]. The surface melting was also found to be beneficial in reducing the intergranular corrosion (IGC) in various grades of aged austenitic stainless steels 

2.2. Ferritic Stainless Steel (FSS) -Ferritic stainless steel is a direct Fe-Cr system with very little additions of other alloying elements. In AISI classification FSS is classified under 4xx series.In welding of FSS, grain coarsening in both weld and HAZ is one of the major concerns. Power beam welding also results in large grain sizes; however, the extent of coarsening can be reduced and also the width of both FZ & HAZ can be reduced.

The metallographic analysis has shown that the material is readily weldable by laser welding. However, even with rapid cooling rates, the grain coarsening could not be suppressed. The microstructural analyses revealed that after welding, the microstructure is no longer fully ferritic as in the base material and it has martensite decorating the grain boundaries. Also, the microstructure comprises of other polymorphic ferrite like widmannstatten ferrite. The presence of such composite structures improved the hardness of the welds. The hardness of laser welded SS 409 has increased by almost two times that of the base material. The elevation in hardness can be attributed to the presence of hard structures such as martensite and widmannstatten ferrite. In transverse tensile testing, the failure invariably occurred in the base material away from the weld & HAZ indicating that weld is stronger than the base material. The increase in strength is due to the increased hardness in the FZ and a very narrow HAZ in laser welds. EBW has also been attempted for welding of SS 409M and the properties such as hardness, residual stress, tensile strength, and fatigue strength were compared with GTAW welds of the same material [18].  The results have shown that EB welds of 409M were superior in all aspects.

2.3. Martensitic Stainless Steels (MSS) - MSS belong to the category of air hardenable stainless steels with higher weight percentage of carbon compared to ASS and FSS. MSS are classified under 4xx series in AISI classification. The processing of MSS are discussed below.

2.3.1. Welding - In general, the weldability of MSS is very poor due to high air hardenability. The welding is even more difficult with power beam processes as the cooling rates are very high, which can lead to high hardness in the FZ and HAZ resulting in cracking. Also, the weldments are prone for hydrogen induced cracking (HIC) due to hard martensitic microstructures in the weld and HAZ. Continuous wave laser welding has been attempted on 1.5 mm thick 410 MSS using two different laser beam modes viz., Gaussian and donut modes. 

2.3.2. Surface Modification -Laser surface modifications have been attempted on MSS. Laser surface melting of high sulphur SS 416 [23] has reduced the size of the inclusions from 2µm to 0.5µm and it has also changed the morphology from elliptical to spherical and has dispersed it uniformly throughout the parent alloy. This has shown to increase the pitting corrosion resistance of the material. Laser cladding of brass on SS 410 was also attempted to improve the thermal conductivity of the material [24]. The cladding could be achieved without any defect and the resultant clads have significantly improved the thermal conductivity of the surface in 410 MSS.  

2.4. Duplex Stainless Steels - Duplex stainless steels have almost equal composition of austenite and ferrite. The material can be readily welded by almost all the fusion welding process. The welding however will disturb the phase balance. The disturbance will be even more prominent in power beam processes due to low heat input and rapid cooling rates. The weld metal will solidify in predominantly ferritic mode and hence, the fusion zone will have more than 80% ferrite and 1020% austenite especially in autogenous welding. The presence of large ferrite in the FZ will reduce the corrosion resistance and impact toughness of the weld. The achievement of proper phase balance will be of prime concern as the properties of the final weld depend on the phase balance. Autogenous CO₂ laser welding has been attempted on 6mm thick UNS 31803 duplex stainless steel. The welding has been carried out by varying the laser power, welding speed and shielding gas.