The corrosion resistance of a new free-cutting soft-magnetic stainless steel containing Ti 4 C 2 S 2 instead of MnS was examined by immersion corrosion tests in pure water and anodic polarization measurements in 0.1 M Na 2 SO 4. The Ti 4 C 2 S 2 inclusions hardly dissolved in pure water at 353 K and did not suffer anodic dissolution in the passivity region of stainless steel in 0.1 M Na 2 SO 4. In contrast, conventional free-cutting softmagnetic stainless steels with (Mn,Cr)S inclusions caused the significant release of Mn species during immersion in pure water, and showed a marked increase in the dissolution current of MnS under anodic polarization in 0.1 M Na 2 SO 4. The thermodynamic stability of Ti, Mn and Cr oxide films on the inclusions makes the difference in the corrosion resistance of Ti 4 C 2 S 2 and (Mn,Cr)S, with no dissolution of Ti from Ti 4 C 2 S 2 and the selective dissolution of Mn from (Mn,Cr)S.

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We investigated the effect of alloying elements Ni, Al, and Mo on the mechanical properties of precipitation-hardened soft-magnetic stainless steels, whose aging condition and chemical composition were varied. Thermodynamic calculations suggested that the aging treatments led to precipitation of a β-NiAl compound with B2 structure and Fe 2 Mo-type Laves phases. Although coarse precipitates, which are less effective for precipitation hardening, were observed in bright-field images obtained by transmission electron microscopy (TEM), the B2-type precipitates were detected in TEM electron diffraction patterns. By changing the concentration of the alloying elements, the B2-type β-NiAl precipitates were mainly responsible for the precipitation hardening, and the Vickers hardness was hardly affected by the Mo content. Small-angle X-ray scattering analysis was used to determine the size and number density of the fine β-NiAl precipitates, which contributed to the hardening. The high-density β-NiAl precipitates grew to a few nanometers in radius after an adequate period of aging, suggesting that the β-NiAl precipitates were responsible for the precipitation-hardening characteristics. On the other hand, the size of the precipitates was less affected by the amounts of Ni and Al, and the number density decreased with decreasing Ni and Al content. The improvement in hardness resulting from the β-NiAl precipitates had a linear relationship with the square root of the product of the precipitate size and number density.