Superplasticity in eutectoid steel

A eutectoid steel exhibited abnormally large tensile extensibility in three quite dissimilar circumstances: i) micrograin plasticity, defined by a strain-rate sensitivity exponent m = 0.42, was found at 716°C for straining the ferrite-cementite aggregate at the rate è = 4.4 x 10^-3 min^-1, giving 133 pct elongation; ii) at a much greater strain rate, ε = 25 min^-1, superplasticity appeared in austenite strained at 927°C, giving 142 pct elongation; iii) a new type of transformation plasticity was predicted and experimentally verified: 490 pct elongation resulted from thermal cycling 21 times across Ae₁. Plastic stability analysis distinguishes it from micrograin plasticity by showing that it owes to strain-hard ening during plastically stable flow; hence, there are no restrictions ε or m. Furthermore, it is not necessary to perform the straining during the transformation, since the strain-hardening capacity can be regenerated by thermal cycling through the phase transformation if the transformation serves to recover the flow stress. Additional work showed that straining during austenitizing fails to increase m above pretransformation levels. Formerly Graduate Student, Syracuse University, Syracuse, N. Y.