Low temperature mechanical properties of modified heat treated 300M steel

Abstract

The mechanical properties of modified heat treated 300M (MHT 300M) steel have been studied in the temperature range 183–293 K to examine the viability of using MHT 300M steel in possible low temperature, ultrahigh strength applications. The microstructure of the MHT 300M steel consisted of triple phases of martensite, bainite, and retained austenite. This microstructure is produced by partial isothermal transformations at either 593 or 623 Kfor a required time, followed by oil quenching and subsequent tempering at 473 K (after austenitisation at 1173 K). Compared with conventionally quenched and tempered (CQT 300M) steel, in the MHT 300M steel the plane strain fracture toughness K1C in the temperature range 203–293 K was significantly improved, producing a beneficial effect on the ultimate tensile strength and per cent elongation. In addition compared with the CQT 300M steel, in the MHT 300M steel the ductile to brittle transition temperature in the Charpy impact test was lower. However, there was a significant reduction in the K_IC value at 183 K; therefore, at this temperature there is not much difference between the properties of the MHT and CQT 300M steels. The results are described in the present paper and the microstructural factors controlling the low temperature K_IC values of the MHT 300M steel are discussed in terms ofthefractography, the mechanical stability of austenite, etc. MST/3045