Abstract
Boron treatment for Cr-Mo steels has been performed to obtain the higher strength with heavy thickness. But in these steels, even under the quenched microstructure, microalloying elements affect the toughness on tempering treatment at high temperature. So, the effect of Ti, B and N on the toughness and microstructures has been investigated in B-treated Cr-Mo steels tempered at high temperature. The results are as follows:
(1) In the case of Ti/N ≥ 3.42, the excess dissolved B ([B]) causes the precipitation of coarse M23(C, B)6 type boro-carbides and the formation of denuded zone along grain boundaries on tempering treatment at the temperatures higher than 600°C, and therefore the toughness decreases. But proper [B] causes no coarse boro-carbides precipitates along grain boundaries.
(2) In the case of Ti/N<3.42, the precipitation of BN occurs after hot rolling. However such non-equilibrium BN precipitates partially transform to AIN at the austenitizing treatment and [B] occurs. If [B] content is excess, then M23(C, B)6 precipitates and coagulates on grain boundaries during tempering treatment at high temperature, and consequently the toughness decreases. If [B] content is proper, then microstructures are fine and BN precipitates take no effect on the toughness.
