Continuous Cooling Transformation Characteristics of Some Low Alloy Steels

Abstract

Continuous cooling transformation characteristics have been determined for four grades of steels which are being used as rotor shaft materials. Data on continuous cooling transformation characteristics of these low alloy steels were collected from literatures and were summarized together with the present result.
Critical cooling rates for the formation of pearlite and bainite were obtained respectively for twenty kinds of low alloy steels including four steels in the present paper, and steels were classified according to the critical cooling rate in consideration of actual cooling rates for larger mass.These steels have a so-called bainitic hardenability.Remarkably larger differences in critical cooling rate among steels were observed in the formation of pearlite rather than in the formation of bainite.The prediction of the critical cooling rate for the pearlite formation from the ideal critical diameter calculated by the conventional empirical formula was difficult for these steels with relatively large alloy contents.The effect of Ni on the critical cooling rate for the pearlite formation was remarkable. In mainly bainitic structures, the appearance of bainite was different among steels and this seemed to have a profound influence on hardness.The structural appearance of bainite was significantly dependent on the temperature of formation.Addition of Cr lowered this temperature, resulting in a fine distribution of carbides in bainite.