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Kinetics of phase transformations of undercooled austenite in 18CrNiMo7-6 steel applied for toothed wheels

Krawczyk J., Bała P., Pawłowski B.

Archives of Foundry Engineering

2010

Vol. 10, iss. 3 spec.

29--34

Toothed wheels constitute essential part of the steel structural elements market. The most often hypereutectoid structural steels for carburizing are used for toothed wheels. The final producers of toothed wheels are not demanding regarding a microstructure, provided that the determined requirements will be fulfilled (e.g. metallurgical purity determined by ultrasounds). Therefore delivered forgings can be in an annealed or quenched state. This results from the situation that the final heat treatment or heat-chemical one is being done at one of the last stages of the toothed wheel production. An essential factor allowing to develop the proper heat treatment is the knowledge of the kinetics of phase transformations of undercooled austenite and its relating to technological conditions, being at the producer disposal, as well as to forging dimensions. Such investigations should be carried out on real melts used for forgings for toothed wheels production together with an analysis of microstructure changes on the forging cross-section. They should be based on calculation methods determining the distribution of cooling rates on its cross-section in dependence of an applied cooling medium. The mentioned above problems in relation to 18CrNiMo7-6 steel - are elucidated in this paper. The aim of the investigations was the description of the kinetics of phase transformations of undercooled austenite in this steel. The CCT diagram was constructed for the austenitizing temperature determined on the basis of phase transformations temperatures (the so-called critical points).