The role of the divorced eutectoid transformation in the spheroidization of 52100 steel
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I. INTRODUCTION
HYPEREUTECTOID steels, such as tool steels and the popular 52100 bearing steel, are supplied from the mill in the spheroidized condition to promote machinability. General heat treatment rules and recipes have evolved over the years for successful spheroidization,[1,2] but there has been little scientific study of the nature of the spheroidization process. Hewit[3] presented a study of the spheroidization of 52100 steel in which he pointed out that the nature of the process is not well understood. He presented two processes for spheroidization of 52100 steel based on the work of Heron.[4] (a) Continuous-cool spheroidal anneal: Austenitize for a minimum of 1 hour at about 830 8C. Cool to 750 8C at not greater than 25 8C/h. Cool to 690 8C at not greater than 10 8C/h. Air cool from 680 8C. (b) Isothermal spheroidal anneal: Austenitize for a minimum of 1 hour at about 830 8C. Rapid cool to 700 8C and hold for a minimum of 2 hours. Cool to 690 8C in 1 hour and air cool from 680 8C. More recently, two similar processes have been recommended by the ASM,[5] in which the austenitization temperature is lowered to 795 8C. Also, the cooling rate from 750 8C in the A process is lowered to a maximum of 6 8C/h, and the hold time in the B process is extended to 16 hours and the temperature lowered to 690 8C. Note that most of these processes require quite long times, 10 to 16 hours. Normally when austenite is cooled through the A1 temperature at air cooling rates or lower, it transforms to a mixture of ferrite and cementite by the famous pearlite reaction, and the pearlite is difficult to spheroidize because of the low energy interfaces of this microstructure. However, it has been known in the literature for many years[6–9] that if the austenite contains a distribution of fine cementite particles the transformation may occur by either the pearlite reaction or the divorced eutectoid transformation (DET) reaction. A recent article has discussed the DET[10] and pointed out that there is a general lack of awareness of this transformation
J.D. VERHOEVEN, Emeritus Professor, is with the Department of Materials Science and Engineering, Iowa State University, Ames, IA 50011. Manuscript submitted November 2, 1999. METALLURGICAL AND MATERIALS TRANSACTIONS A
in metallurgy literature and textbooks in spite of the old literature. One would expect this transformation to occur in the spheroidization anneal processes presented previously, because it has been established that when 52100 steel is austenitized at 850 8C[11] or at 800 8C,[12] the resulting matrix contains a bimodal distribution of carbides. The larger carbides form on the austenite grain boundaries present at the austenitizing temperature and have diameters in the range of 1 to 2 mm, while the smaller and much more numerous carbides form intergranularly and have diameters of around 0.1 to 0.2 mm, below the resolution limit of the optical microscope.[11] Previous studies of microstructural development in the heat treatment of 52100 steel have either neglected any con
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