Composition and grain size dependencies of Md₃₀, the parameter proposed by Angel for indicating austenite stability during deformation in metastable austenitic Fe-Ni-Cr stainless steels, were examined.
The measurement of Md₃₀ by changing composition, C, N, Si, Mn, Ni, Cr, and others, give the following empirical equation: Md₃₀ (C) =551-462 (C%+N%) -9.2Si%-8.1Mn%-13.7Cr%-29.0 (Ni%+Cu%) -18.5Mo% -68.0Nb%. A great coefficient for Ni in this equation is quite different from that in the Angel's equation. With decreasing grain size of austenite, the amount of strain-induced martensite decreases, and Md₃₀ is lowered. Consequently, Md^GS₃₀ which is Md₃₀ modified by the grain size effect, can be expressed as Md^GS₃₀=Md₃₀ -1.42 (ν-8.0); where ν is ASTM grain size number.
The proof stress drops during deformation at lower temperatures because of the inducement of martensite at a small strain. This phenomenon easily occurs in the specimen with finer grain size. On the stress-strain curve the serrated region can be easily found in the deformation of the specimen with finer grain size. The amplitude of serration becomes too small to be observed in such cases as the specimen with coarser grain size, lower deformation temperature and stabilization of austenitic phase, even though a lot of martensite is induced.