Abstract
In order to investigate the martensitic transformation in Fe-Cr-Ni stainless steel, the Ni equivalent has been introduced thermodynamically as a universal quantity which indicates stability of austenite with regard to chemical composition. The relation between the Ni equivalent and the amount of strain-induced martensite or athermal martensite has been studied by means of saturation flux density measurement. Furthermore, the mechanism of the formation of strain-induced martensite has been considered thermodynamically.
The results obtained are summarized as follows:
(1) The Ni equivalent, (Ni), in Fe-Cr-Ni stainless steel is expressed by
\phantom(1)(Ni)=Ni+0.65Cr+0.98Mo+1.05Mn+0.35Si+12.6C
\
oindentwhere Ni, Cr, etc. represent weight % of these elements in the stainless steel considered.
(2) The relation between the Ni equivalent and the amount of strain-induced or athermal martensite has been obtained.
(3) About 75% cold rolling of Fe-Cr-Ni stainless steel has the same effect on martensitic transformation as decreasing the Ni content by 3∼5%. In this case, the difference in the free energy at room temperature between the ferrite phase and the austenite phase increases by about 113∼188 cal/mol.
(4) The deduced value of the work done by transformation strain induced by the applied stress due to cold rolling is about 120∼126 cal/mol or about 192 cal/mol by compressive or tensile stress, respectively. These values agree very well with the above mentioned change of the free energy difference calculated from the variation in Ni equivalent. Therefore, under applied stress, the stored free energy required for martensitic transformation can be lowered by this work value, 120∼192 cal/mol.
