U-202, U-304 and U-304L - Steel annealing is achieved by heating to 1010°C - 1 120°C for 60 minutes per 25 millimeter thickness (2.5 minutes per mm) followed by air quenching or water. Corrosion resistance is best achieved when the final annealing temperature is higher than 1070°C. (U-321 should not be annealed above 1060°C). Atmospheres should be controlled in order to avoid excessive oxidation of the surface.
STRESS RELIEVING AUSTENITICS
Stress relieving of the lower carbon grade (304L) can be achieved at 450°C - 600°C for 45 minutes per 25 millimeter thickness (1.5 minutes per mm) with little problem of sensitisation. A lower stress relieving temperature of 400°C maximum must be used with U-202 and U-304 types with longer soaking times. If, however, stress relieving is to be carried out above 600°C, there is a large threat of grain boundary sensitisation occurring with a concomitant loss in corrosion resistance. In this instance, U321 should be used.
HOT WORKING AUSTENITICS
The Cr(Mn)Ni austenitics can be readily forged, upset and hot headed. Uniform heating of the steel in the range of between 1150°C to 1250°C is required. The finishing temperature should not be lower than 900°C. Upsetting operations and forgings require a finishing temperature of between 930°C and 980°C. Forgings should be air cooled and not water cooled. All hot working operations should be followed by annealing and pickling and passivation to restore the corrosion resistance and mechanical properties.
COLD WORKING AUSTENITICS
The Cr(Mn)Ni austenitics are extremely tough and ductile and can thus be readily stamped, deep drawn, headed and upset without much difficulty. Since austenitic stainless steels work harden, severe cold forming operations should be followed by annealing.
The Cr(Mn)Ni austenitics have good welding characteristics and are suited to all standard welding methods. Either matching or slightly over-alloyed filler wires should be used, such as 308L. Nitrogen containing shielding gases are recommended for U-202. For maximum corrosion resistance, the higher carbon type U-304 should be annealed after welding to dissolve any chromium carbides which may have precipitated. The weld discolouration should be removed by pickling and passivation to restore maximum corrosion resistance.