Swing back in kinetics near M s in hypereutectoid steels
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INTRODUCTION
THE onset of the austenite decomposition is generally retarded with decreasing temperatures below the pearlite nose, but is significantly accelerated just above the Ms in some steels. This reversal in kinetics, which is called "swing back", has been reported in the kinetic studies by electrical resistivity, L2 dilatometric, 3 and microscopic measurements. 4's'6 The swing back occurred above the Ms in hypereutectoid steels, but below the Ms in hypoeutectoid steels.~ From this behavior in the latter it can be realized that the athermal martensite/austenite boundaries or stress fields around martensite plates play a role in the nucleation sites of bainites. Meanwhile, the acceleration of austenite decomposition occurring above the Ms cannot be attributed to the above explanation, because o f the absence of athermal martensites, and has been regarded as quite anomalous. Several investigations on the isothermal transformations of austenite at temperatures near the Ms have been reported in high carbon steels. Howard and Cohen4 observed a noticeable acceleration in the initial reaction rate at temperatures just above the M, in a 1.35 wt pet* C steel and they attributed it to the forma*Pct means weight percent hereafter, unless otherwise stated.
tion of a thin plate isothermal product not belonging in the bainite sequence. Schaaber3 found two-stage reactions occurring in the vicinity of Ms in some high carbon steels by means of dilatometric and magnetic measurements, and he suggested that the first stage might be the isothermal formation of martensite, preceding the isothermal formation of bainite in the second stage. Smith et al. s concluded from the hot stage metallography in a 1.16 pct C steel that the acceleration of the isothermal transformation just above the Ms (448 K) seemed to stem from the formation of martensite being activated by the start of the bainite transformation. Kennon and Edwards 6 reported that austenite in a 1.44 pct C steel transformed isothermally at temperatures near the M. OKA, Professor, and H. OKAMOTO, Research Associate, are with the Department of Mechanical Engineering, Tottori University, Koyama, Tottori 680, Japan. Manuscript submitted March 17, 1987.
METALLURGICALTRANSACTIONS A
Ms of 366 K to the two products, isothermal martensite and lower bainite. As described above, these results in high carbon steels have demonstrated the uncertain identity of the products of austenite decomposition at temperatures near Ms. The present authors have been investigating the isothermal decomposition of austenite in hypereutectoid steels (0.85 to 1.80 pct C)at temperatures below 623 K. Recently, they7 made it clear that "black line product" formed during austempering near theM, (346 K) in a 1.80 pct C steel was a thin plate isothermal martensite (TIM). Furthermore, theys have clarified that lower bainite with midrib (LBm) is a composite of TIM and lower bainite, which is formed at temperatures above ]14,. The purposes of this paper are to examine, by means of dilatometry and metallogra
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