The boron hardenability effect in thermomechanically processed, direct-quenched 0.2 Pct C steels

Abstract

The segregation and precipitation of boron have been studied in thermomechanically processed 0.2C-0.6Mn-0.5Mo steels containing nominally 0, 10, 20, 50, and 100 ppm B. These steels were hot-rolled in the laboratory (in simulation of production multipass rolling), and their transformation behavior during subsequent water quenching was examined for different finish-rolling temperatures (980 °C and 870 °C) and quenching temperatures (730 °C to 950 °C). The results showed that in general, a “free” boron content of 10 to 20 ppm (which is similar to the levels used for conventional quenched-and-tempered steels) will provide a boron hardenability increment similar to that for conventional quenched-and-tempered steels. The delay time prior to quenching (over the range of 10 to 100 seconds) did not have a significant effect on hardenability except in the steels containing 50 or more ppm B. In these higher B steels, precipitation of borocarbides occurred along austenite grain boundaries with a resultant decrease in hardenability.