The effect of silicon, vanadium and nitrogen on the microstructure and hardness of air cooled medium carbon low alloy steels

This paper presents a comparison of structural investigations of four medium carbon low alloy steels: 38MnSi6, 38MnSiV5 and modifications with higher silicon (“SiV”) and nitrogen contents (“SiVN”). The main objective of this investigation was to find the influence of increasing the silicon concentration from 0.5 to 1.3% and vanadium from 0.005 to 0.16% on the microstructure and hardness. The role of nitrogen addition in high silicon steel was also considered. Steels containing higher levels of vanadium and silicon had a higher volume fraction of proeutectoid ferrite, which was more uniformly distributed in the structure. The higher volume fraction of proeutectoid ferrite was associated with a finer prior austenite grain size. It was found that higher concentrations of vanadium and nitrogen resulted in non-random dispersion of V(C,N) precipitates. Precipitates which were not dissolved during heat treatment act as pinning agents preventing austenite grain growth. Nitrogen did not show a significant effect on the structure of high silicon steel. The microhardness of the proeutectoid ferrite was between 213 and 334 HV0.2 and was increased with silicon and vanadium additions.