EFFECT OF MICROSTRUCTURE ON MECHANICAL-PROPERTIES OF ISOTHERMALLY BAINITE-TRANSFORMED 300M STEEL

A study has been made of the effect of the microstructure on the mechanical properties of commercial-aircraft quality 300M steel which has been isothermally transformed in various bainitic temperature regions (593-673 K) after austenitization at 1173 K. The results were compared with those obtained by conventional quenching and tempering. Isothermal transformation of the steel at and below 623 K produced a dispersion of retained austenite (12-18 vol.%) in a carbon-free upper bainite matrix. The treatment improved the K(Ic) value, owing to an increase in the Charpy impact energy; however, no improvement in strength was observed. As a result of the isothermal transformation at and above 648 K, the retention of a large amount of retained austenite (22-25 vol.%) was encouraged, in conjunction with carbon-free upper-bainite; however, this was found to have a very detrimental effect on the strength and toughness. The results are described and discussed in terms of the microstructure and fractography.