The Effect of Step Quenching on the Kinetics of Bainite Formation in Ferrous Alloys
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THE EFFECT OF STEP QUENCHING ON THE KINETICS OF BAINITE FORMATION IN FERROUS ALLOYS
M. Umemoto*, M. Igarashi** and I. Tamura* *Department of Metal Science and Technology, Kyoto University Sakyo-ku, Kyoto 606, Japan; **Formerly Student of Kyoto Univ. Now at Nippon Seiko (NSK) Co. Ltd., Fujisawa Works, Japan.
ABSTRACT The bainite reactions in the direct and in the step quenched conditions were compared using Fe-7.6Ni-0.48C and Fe-3.6Ni-l.45Cr-0.50C alloys. It was found that in both alloys the bainite reactions were markedly accelerated by step quenching to temperatures even in the stable austenite range prior to the transformation to bainite. Optical microscope observation revealed that this acceleration was caused by the enhanced nucleation of bainite at grain boundaries. This enhanced nucleation by step quench treatment was considered to be due to either the formation of carbon poor areas or the formation of ferrite embryos at the grain boundaries during holding at intermediate temperatures.
INTRODUCTION The diffusional decomposition of austenite is in most cases initiated at austenite grain boundaries. Thus the grain size and the condition of grain boundary prior to transformation strongly affect the transformation kinetics. In spite of a number of previous studies about the effect of grain size [1-3] not much attention have been paid to the effect of the grain boundary condition on the subsequent diffusional transformations. It has been well recognized that impurities and alloying elements segregate to grain boundaries [41 and as such can significantly affect nucleation kinetics [5-7]. For instance boron segregate easily to austenite grain boundaries during cooling, which results in the precipitation of iron-carbides and these precipitates inhibit ferrite nucleation [5,6]. Alloying elements, such as Ni, Cr, Mn, etc., segregate to the grain boundary and may affect nucleation kinetics through the attendant change in grain boundary energy [7]. The purpose of the present study is to examine the effects of holding after austenitization at intermediate temperatures in a stable and a metastable austenite ranges (referred to as conditioning treatment) on the subsequent bainite reaction.
EXPERIMENTAL PROCEDURE The alloys used were Fe-7.6Ni-0.48C and Fe-3.6Ni-I.45Cr-0.50C prepared by vacuum induction melting, and their chemical compositions are listed in Table 1 Chemical compositions (wt%) Table 1. The progress of bainite transformation during
Alloy
C
Fe-76Ni-0.48C
Si I Mn
P
S
Ni
Cr
Al isothermal holding was measured
0.48 0011 Z/010003 0004 7.67 ZQ001C001
Sy an automatic quench dilato-
Fe-36Ni-145Cr-05C 0.50 0020 0.01 0004 0005 360 1.45 Z0.01 meter (Formaster-F) using specimens of rods 3 mint x 10 mm 0 which were machined after homogenizing at 1200 C for 20 h. For the optical microscope observation, specimens cut from the homogenized sheet were austenitized at 1100'C for 15 min, quenched into the first salt bath kept at various temperature above A3, held for various times (conditioning treatment) and quench
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