Sintering of Stainless Steel

E. Klar and P.K. Samal, OMG AMERICAS

Sintering is the most critical step in processing stainless steel parts. It is also more complex than the sintering of most other materials. Correctly sintered stainless steel parts will approach the corrosion resistance of wrought stainless steel of the same composition. Incorrectly sintered parts will have inferior corrosion resistance, frequently inferior by one to two orders of magnitude.

The majority of the studies dealing with the corrosion resistance of sintered stainless steels are based on immersion and/or salt spray testing in 5% aqueous NaCl, using the length of time until the appearance of rust as a quantitative measure of corrosion resistance (Ref 6). Some are based on weight loss measurements in H2SO4, HNO3, and HCl of various concentrations. Metallography (Ref 7) and potentiostatic anodic polarization tests (Ref 8) have been used to elucidate the various mechanisms of corrosion, as well as for troubleshooting.

Microstructure-corrosion resistance correlations of sintered stainless steels have shown that for maximum corrosion resistance, it is important to minimize or avoid the occurrence of precipitates of chromium carbides, chromium nitrides, and of silicon and chromium oxides in the structure (Ref 6). Surface depletion of chromium during vacuum sintering can also be a cause for low-corrosion resistance (Ref 9). Furthermore, certain sintered densities can cause severe crevice corrosion in a neutral salt environment (Ref 10).

Requirements for optimum sintering of stainless steels are described in this section for the most commonly used sintering atmospheres. The typical problems present during processing, particularly during sintering, are discussed together with measures necessary to avoid the previous problems. The precautions necessary to avoid the previous problems differ for the various sintering atmospheres. Moreover, to guarantee maximum corrosion resistance, quality control must begin powder production.

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