EFFECT OF MODIFIED HEAT-TREATMENT ON MECHANICAL-PROPERTIES OF 300M STEEL

effects of modified heat treatment (MHT) on the mechanical properties of 300M steel have been studied to assess MHT steel for possible ultrahigh strength applications. The microstructure of MHT steel has variable amounts of martensite, carbide free upper bainite, and retained austenite. This is produced by partial isothermal transformation at 593, 623, or 673 K for the required times, followed by oil quenching and subsequent tempering at 473 K after 1173 K austenitisation. The optimum combination of plane strain fracture toughness with other relevant mechanical properties was obtained when 50 vol.-% bainite was associated with tempered martensite and retained austenite for MHT at 593 and 623 K. The MHT steel transformed at 593 K showed improved plane strain fracture toughness over conventional quenched and tempered steel at a similar tensile strength level, with little change of percentage elongation and Charpy 2 mm V-notch impact energy. Compared with conventional quenched and tempered steel, the MHT steel transformed at 593 K had a dramatically improved plane strain fracture toughness owing to an increase in percentage elongation and Charpy V-notch impact energy, while the strength decreased somewhat.