Influence of carbon content on superplastic behavior in Ti- and B-added Cr-Mo steels
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I. INTRODUCTION
RECENT advances in grain-refinement technology have led to an increase in the superplasticity behavior of many alloys, allowing their application to many types of steel. Figure 1 shows the superplasticity of various ferrous alloys. Superplastic elongation in the austenite range increases considerably as the carbon content increases in alloys 1 through 3. However, superplastic elongation does not increase in the (a 1 g) phase region of alloy 6, as compared to alloys 4 and 5, suggesting that superplastic elongation may be independent of carbon content. Thus, the relationship between carbon content and superplastic elongation remains unclear. Previous research in the a 1 Fe3C temperature range has been conducted only for cases in which the carbon content of the material was 0.8 pct or higher; however, the superplasticity of hypoeutectoid steel at or below the A1 temperature has not yet been investigated. The present study investigates the effects of carbon content on the superplasticity, as a function of temperature and strain rate, of various steels, which differed only in carbon content and microstructure (grain size and carbide precipitation) at temperatures below A1. Chromium-molybdenum (Cr-Mo) steels, of varying carbon contents ranging from 0.24 to 0.83 mass pct, were used in this study. II. EXPERIMENTAL METHOD A. Materials A titanium-boron compound found to be effective in enhancing superplastic elongation[7] was added to Cr-Mo steel in order to enhance the superplasticity of the test specimens. The test specimens were produced using vacuum
M. ARAMAKI, Research Associate, K. HIGASHIDA, Associate Professor, and R. ONODERA, Professor, are with the Department of Materials Science and Engineering, Kyushu University, Fukuoka, 812 Japan. Manuscript submitted December 30, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
fusion, during which a fixed amount of the titanium-boron compound and varying amounts of carbon were added to a Cr-Mo steel (Japanese Industrial Standards: JIS SCM415, C 5 0.15 Mass pct). Table I lists the chemical compositions of the test specimens. Specimens with a carbon content of 0.24, 0.37, 0.58, 0.66, and 0.83 mass pct are referred to as C24, C37, C58, C66, and C83, respectively. Thermomechanical processing, as shown in Figure 2(a), was used to produce test specimens for the tensile test (gage length of 16 mm). The temperature (T ) for thermomechanical processing was high enough to cause austenization in 60 seconds to the fact owing that the temperatures of A3 or Acm are 758 8C to 764 8C. The austenitized specimens were quenched in water, leading to a martensite structure. Because several specimens with a carbon content were unsuccessfully performed at the cold-rolling stage due to cracking, the specimens were subjected to a heat treatment shown in Figure 2(b). The heat treatment consists of an annealing at 740 8C for 1.8 ks followed by furnace cooling (cooling rate of 4.1 3 1023 8C s21), resulting in a structure with spheroidal carbides. A transformation temperature (A1) was
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