Dislocation creep controlled superplasticity in the high carbon steel 140NiCr16‐6

Superplasticity in the alloyed high carbon‐steel 140NiCr16‐6 with phosphorus additions and a fine grained microdupiex structure – containing cementite in volume fractions of 22 % (Fe,Cr,Ni) 3 C, particle size of about 1 μm and with a medium ferrite grain size of about 2 μm – has been investigated in the temperature regime of 550 to 675°C and in the strain rate range of 10 −5 to 5 · 10 −2 s −1 . Maximum strain rate exponents of m = 0,45 at 675°C with strain rates of the order of 10 −4 s −1 have been determined. Maximum superplastic elongations of about 700 % were detected. At higher strain rates of 10 −3 s −1 superplastic elongations of about 570 % were achieved. At relatively low test temperatures of 550°C elongations up to 230 % were recorded. The activation analysis in the temperature regime of 550 to 650°C show an activation energy for superplastic flow of 250 ± 20 kJ/mol. This is in agreement with the activation energy for lattice self diffusion of iron in α‐iron. Above 650°C the activation energy decreases to 70 kJ/mol. This is due to a stress induced decrease in the eutectoid α‐γ‐transformation temperature from 685°C to somewhat lower temperatures during superplastic deformation. The superplastic deformability ( m > 0.3) of this steel in a wide strain rate range at relatively low temperatures above 550°C allows near net shape forming of complex parts applying low flow stresses.