Abstract:
The strain hardening behaviour of AISI 301 metastable austenite steel was analysed
by evaluating tensile data against the empirical mathematical equations of Hollomon, Ludwik
and Ludwigson. It was found that these equations were inadequate to model this TRIP steel
with low stacking fault energy (SFE). It was found that the fraction of strain-induced
martensite could be expressed as a sigmoidal function of the applied strain. The log-log plots
of true stress and true plastic strain from 5% to εUTS performed with uniaxial isothermal tests at
30 oC were thereafter adequately fitted with a sigmoidal curve. The instantaneous strain
hardening exponent was determined as the slope of the above-mentioned sigmoidal curve at a
specific strain value. The strain hardening exponent and the rate of strain hardening (dσ/dε)
increases with deformation due to formation of strain-induced martensite to a maximum and
thereafter decreases as the volume fraction of strain-induced martensite approximates
saturation. The variation of the instantaneous strain hardening exponent as a function of plastic
strain and the strength coefficient, K, at 30 oC was deduced. A high value of K, 1526MPa, was
determined. A correlation between the extent of martensitic transformation and the value of the
instantaneous strain hardening exponent was observed. This work is part of the project that
seeks to develop a constitutive model describing the flow stress during plastic deformation as a
function of both plastic strain and the resulting martensitic transformation at different
temperatures and strain rates and which accounts for the isotropic hardening process.
