Contribution to the crystallography of the bainite transformation in a high strength low alloy steel 40 NSCD 7-7-3 (AISI 300M)

Abstract

During continuous cooling performed from 950°C at a rate between 1 and 10°C s −1, the high strength low alloyed steel 300M acquires a mixed structure composed of plate martensite, lower bainite and retained austenite. The formation of lower bainite has been interpreted in terms of phenomenological theory developed in the case of martensite formation. Indeed, the habit plane of lower bainite experimentally determined well agrees with the one calculated from matrix algebra or graphical method, when the shear elements of the austenite lattice are as follows: (1̄11) γ, [211] γ and (1̄11) γ, [121̄] γ. The PITSCH's orientation relationship was evidenced between cementite and ferrite of lower bainite, while between ferrite of lower bainite and austenite, an orientation relationship closer to Nishiyama-Wassermann's than Kurdjumov-Sach's was found. For appropriate thermal treatments, upper bainite and austenite under form of “S” where also revealed. Moreover, silicon does not seem to induce the formation of silicocarbides and would not play a role in the mechanism of the bainite transformation; it only contributes to delay this transformation.