1990-02-01

Ferritic Stainless Steels for Magnetic Applications 900384

Often the question has been posed as to where are markets for P/M stainless steels. This question has been difficult to answer. Stainless steels are more expensive than alloy steels, because they contain chromium to provide corrosion resistance. Chromium poses a problem for P/M parts fabricators. Many sinter parts at 2050°F (1120°C). Furthermore, most insist on including at least 25% nitrogen in the sintering atmosphere. If 2050°F (1120°C) is selected for sintering, surface oxides are often only partially reduced. In addition, if nitrogen dilutes the sintering atmosphere, nitriding occurs, limiting corrosion resistance. Therefore a limited market exists for P/M stainless steels because corrosion resistance is limited.
However, there is a substantial market for stainless steel bar stock. One such market, totally undeveloped, is for parts for soft magnetic applications. This market is becoming of increasing importance to the automotive industries as warrenties extend to as long as ten years. Ferritic P/M stainless steel parts can satisfy this emerging market. If high temperature sintering (2300°F, 1260°C) and hydrogen is employed to reduce surface oxides then DC magnetic properties can equal those of wrought products. This paper will include data comparing DC magnetic properties of P/M 410L and 434 stainless steel with those of wrought products. Properties discussed will include magnetic induction, residual induction, relative maximum permeability and coercive force. The problem of high shrinkage rates resulting from high temperature sintering will also be discussed and expected tolerance from as sintered parts will be presented.
Some magnetic parts also require cold deformation hence ductility, tensile properties including ultimate tensile strength, yield strength, and percent elongation will be included. These properties will be related to wrought properties.
The paper will conclude with consideration of problems demanding solution and suggestions of applications that may be emerging for the 1990's.

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