Effect of Si on the Strength and the Toughness of Boron Treated Cr-Mo-Nb Steels

Abstract

Effect of Si content on the various mechanical properties and microstructures in B-treated Cr-Mo-Nb steels has been investigated.
1) By increasing Si content, Charpy transition temperature of shear area (_vT_rsraises due to solution hardening and coarsening of austenite grain size, when coarse boro-carbides are not formed along grain boundaries during tempering treatment at high temperature.
2) Si suppresses the coarsening of intergranular boro-carbides in case of high temperature tempering. As a result, in a steel containing excess dissolved B and so having boro-carbides along grain boundaries, it is considered that Si does not change _vT_rs spite of solution hardening and coarsening of austenite grain size due to increase of Si content.
3) In an as quenched condition, austenite grain boundaries in a low Si content steel are clearly revealed, but those in a high Si content steel are not clearly observed after conventional etching. Consequently, it is considered that Si affects the grain boundary segregation.