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Effect of microstructure on mechanical properties of isothermally bainite-transformed 300M steel

1993; Elsevier BV; Volume: 172; Issue: 1-2 Linguagem: Inglês

10.1016/0921-5093(93)90434-g

1873-4936

Yoshiyuki Tomita, Takeyoshi Okawa,

Metallurgical Processes and Thermodynamics

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 KIc value, owing to an increase in the Charpy impact energy; however, no improvement in strength was observed. As aresult 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.