Crystallographic Study of the Tempering of Martensitic Carbon Steel by Electron Microscopy and Diffraction
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I.
INTRODUCTION
THEtempering of martensitic carbon steel is well studied, and previous studies have indicated that the structural changes taking place during tempering can be divided into three stages, l The first stage takes place at temperatures around 370 K, the second at around 550 K, and the third between 550 and 800 K. However, an additional stage has been found to occur at temperatures below 370 K as shown in Figure 1, where the specific heat vs temperature curve of a 1.39 wt pct C martensitic steel quenched into ice water followed by immersion in liquid nitrogen shows a heat evolution to start from a temperature below 170 K. This stage was called the preliminary or initial stage. 2Thus, we can say that there are four stages in the structural changes taking place during tempering of martensitic carbon steel. Structural changes taking place during tempering have essentially been studied by X-ray diffraction. However, because of the inability to obtain single crystalline diffraction patterns and of the changes taking place on a very fine scale, detailed information is difficult to obtain. In order to remove these difficulties, high resolution electron microscopy and selected area electron diffraction were used in the present investigation. The results obtained are described in this paper.
II.
EXPERIMENTAL
Zone-refined iron rods (99.99 pct purity) were coldrolled to plates of about 0.1 mm thick and carburized by a S. NAGAKURA, Professor, Department of Metallurgy, M. KUSUNOKI (n6e Toyoshima), Research Associate, and Y. NAKAMURA, Graduate Student, are all with Tokyo Institute of Technology, Oh-okayama, Meguroku, Tokyo 152, Japan. Y. HIROTSU, formerly Research Associate of Tokyo Institute of Technology, is now Associate Professor, Nagaoka Technological University, Nagaoka-shi 949-54, Japan. T. SUZUKI, formerly Graduate Student of Tokyo Institute of Technology, is now with Wakayama-Works, Sumitomo Metal Industries Company Ltd., Wakayama-shi 640, Japan. This paper is based on a presentation made at the "Peter G. Winchell Symposium on Tempering of Steel" held at the Louisville Meeting of The Metallurgical Society of AIME, October 12-13, 1981, under the sponsorship of the TMS-AIME Ferrous Metallurgy and Heat Treatment Committees. METALLURGICALTRANSACTIONS A
methane and hydrogen gas mixture at 1170 to 1320 K for about 6 ks. After austenitization in a vacuum of 1 x 10-3 Pa, the plates were quenched into ice water followed by immersion in liquid nitrogen, except in cases where a large amount of retained austenite was desired. The carbon content of the martensite was determined from its axial ratio by using the relation c/a = 1 + 0.045 (wt pct C). 3 Tempering was done in a vacuum of 1 x 10 -3 Pa for various periods of time. Specimen thinning for electron microscopy and diffraction was done using either the H3PO4-CrO3 electrolyte kept at 293 to 353 K or the HNO3-CH3COOH electrolyte kept at 243 K. The thinning time was 2 to 4 ks. For the preliminary stage, these electropolishings also served as the specimen tempering.
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