Effect of boron additions on the toughness of heat-treated low-alloy steels

Abstract

Despite the widespread commercial use and acceptance of boron-containing heat treated steels for many engineering applications, some controversy persists about the toughness characteristics of these steels in comparison with boron-free grades. Therefore, the present investigation was conducted to compare the heat treating response and toughness properties of three boron-containing steels, namely 10B30, 50B35, and 94B30, with those of 8637 steel without boron. The boron additions in the range 0.0012% to 0.0018% significantly increased the hardenability of the boron steels, but did not influence their tempering behavior, as expected. Charpy V-notch impact transition curves were determined for the steels after quenching to fully martensitic structure and tempering to hardness levels of 35 and 40 HRC. Allowing for the slight differences in hardness among the steels, and a slightly coarser grain size in one of the steels, the toughness characteristics of the boron and boron-free steels were essentially similar.These results confirm those of earlier studies on fully quenched and tempered boron steels, indicating that boron additions in the optimum range from 0.001% to 0.003% for hardenability do not adversely affect the toughness characteristics of these steels. It is concluded that with proper control of the boron addition, boron steels, when properly heat treated to tempered martensitic structure, can be substituted for alloy steels in most engineering applications where toughness properties are specified.