|Cr-(Mn)-Ni Austenitics - Fabrication|
Thermal processing and fabricationANNEALING
Annealing of U-202, U-304 and U-304L type steel is achieved by heating to between 1 010°C and 1 120°C for 60 minutes per 25mm thickness (2.5min/mm) followed by water or air quenching. The best corrosion resistance is achieved when the final annealing temperature is above 1 070°C. U-321 should not be annealed above 1 060°C. Controlled atmospheres are recommended in order to avoid excessive oxidation of the surface.
The lower carbon grade (U-304L types) can be stress relieved at 450°C to 600°C for 45 minutes per 25mm thickness (1.5min/mm) with little danger 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 serious threat of grain boundary sensitisation occurring with a concomitant loss in corrosion resistance. In this instance, U-321 (titanium stabilised) should be used.
The Cr-(Mn)-Ni austenitics can be readily forged, upset and hot headed. Uniform heating of the steel in the range of 1 150°C to 1 250°C is required. The finishing temperature should not be below 900°C. Upsetting operations and forgings require a finishing temperature between 930°C and 980°C. Forgings should be air cooled. All hot working operations should be followed by annealing and pickling and passivation to restore the mechanical properties and corrosion resistance.
The Cr-(Mn)-Ni austenitics are extremely tough and ductile and can thus be readily deep drawn, stamped, headed and upset without difficulty. Since austenitic stainless steels work harden, severe cold forming operations should be followed by annealing.
Like all the austenitic stainless steels, this alloy group machines with a rough and stringy swarf. Rigidly supported tools with as heavy a cut as possible should be used to prevent glazing.
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. When welding U-202, nitrogen containing shielding gases are recommended. 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.