Study on Austempering of Structural Low-Alloy Steels (1st Report). The Isothermal Transformation at the Region of Bainite Knee

We constructed the isothermal transformation diagrams at the temperature range of 260∼610°C for structural Ni-Cr steel (0.26 C, 3.39 Ni, 1.22% Cr) and Ni-Cr-Mo steel (0.45 C, 1.43 Ni, 0.84 Cr, 0.26% Mo) by micrography and hardness test, and then inspected the process of isothermal decomposition of austenite at the region of “bainite knee”, using the measurement of dilatation in austempering and tempering. The obtained results are summarized as follows: These isothermal transformation diagrams have the lower or bainite knee in the beginning lines respectively below about 550°C. The reaction in this range exhibits complexities in the form of multiple-stage transformaiotn of austenite as follows: γ→αm+γ′, αm→α+θ, that is, accicular αm separates from austenite at first and, with time, the separation of αm and the breakdown of αm to α−θ aggregate proceed simultaneously, the amount of γ decreasing. With more time, at so-called “ending point” γ disappears under the microscope and the expansion finishes, neverthless he dilatometric results in tempering subsequent to austempering reflect that austenite (chiefly γ′) still exists at the right-hand field of so-called “ending line”.