Influence of grain boundary carbide density on impact behaviour of C–Mn–Nb–Al steels

Abstract

The influence of grain boundary carbide density on impact behaviour has been examined for C–Mn–Nb–Al steels by (i) normalizing at increasing temperatures above the Ac₃, and (ii) tempering for long times below the AC₁. Low normalizing temperatures (i) resulted in a large number of grain boundary carbides possibly because incomplete homogenization on austenitizing produces a high concentration of carbon at the boundaries. Raising the normalizing temperature reduced the number of grain boundary carbides as well as refining their size, but the expected improvement in impact behaviour was not realized because grain size also increased. Tempering at 680°C raised the grain boundary carbide density considerably and completely destroyed the pearlite colonies; tempering at 600°C (ii) gave a lower increase in carbide density and destroyed fewer pearlite colonies. Only small changes in grain size and grain boundary carbide thickness were noted so that the deterioration in impact behaviour obtained on tempering at 680°C could be ascribed mainly to this increase in grain boundary carbide density. Analysis of all the results suggests that an increase in grain boundary carbide density by 20 mm⁻¹ at constant grain size results in an increase in transition temperature of ∼30 K. This agrees with the 20–30 K rise in transition temperature reported in a previously published paper which relied on a linear regression approach.

MST/424