4 main groups of stainless steel are used in the fabrication industry - namely:
1. Ferritic/Martensitic Stainless       
2. Austenitic Stainless
3. Duplex Stainless
4. Super-modified Austenitics            
                    
Ferritic/Martensitic stainless have chromium levels of 12% - 18% as the main alloying element and have carbon levels to enable air hardening to a predominantly martensitic structure. Accordingly, detailed attention must be paid to both pre-heat and PWHT during and after welding.
Duplex stainless steels for general fabricational work can be considered non-heat treatable and it is imperative that the ratio of austenite to ferrite in the weld metal matches the microstructure of the parent plate. 
In order to achieve matching microstructure, the weld metal, particularly with reference to austenite formers, has a higher alloy content than the material being welded. 
The exception may occur with large repairs to castings when PWHT involves full solution heat treatment, eg: water quenching from 1100 ºC, then matching analysis weld metals should be considered.
Standard austenitics are those in the ASTM 3XX series and should, in the main, be welded with matching electrodes and for all fabricational purposes should be considered non-hardenable, non-heat treatable and thus show no PWHT requirement.