Effect of Microalloying Elements on the Hardenability and the Properties after Tempering at High Temperature in Boron Treated Cr-Mo-Nb Steels

Abstract

The hardenability and the properties after tempering at high temperature in boron treated Cr-Mo-Nb steels have been investigated with regard to the effect of microalloying elements. The results are as follows:
(1) The hardenability and the toughness are divided roughly into three regions according to B and N contents under the condition that Ti and sol. Al contents are fixed. In the first region, dissolved B ([B]) content is poor, so steels have low hardenability. In the second region, [B] content is excess and steels have inferior toughness because of coarse M₂₃(C, B)₆ boro-carbides precipitation and denuded zones formation along grain boundaries on tempering treatment at high temperatures. In the third region, [B] content is optimum and steels have good hardenability and high toughness. The upper shelf energy, however, decreases when B is more than 30 ppm.
(2) Ti and sol. Al contents affect [B] content, and therefore the boundary lines above mentioned shift with the addition and reduction of Ti and/or sol. Al.
(3) Grain boundary carbides are M₃C type carbides in B-free steels, and M₂₃(C, B)₆ type borocarbides in B-treated steels. In the N stabilized steels, M₂₃(C, B)₆ precipitates are fine, if [B] is less than about 10 ppm.