In situ x-ray observation of bainitic transformation of austempered silicon alloyed steel
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Esa Vuorinen Division of Engineering Materials, Department of Applied Physics and Mechanical Engineering, Lulea˚ University of Technology, SE-97187 Lulea˚, Sweden (Received 1 October 2008; accepted 7 January 2009)
The in situ x-ray diffraction observations of the bainitic transformation were conducted by using the high-temperature x-ray diffraction technique. The volume fraction and carbon content of austenite depend on the transformation temperature. The d{110} value of bainitic ferrite decreases with increasing austempering temperature, which is related to the decrease of carbon concentration in bainitic ferrite. Asymmetry diffraction peaks are obtained for samples at the early stage of transformation at any austempering temperatures. This asymmetry diffraction peak after the formation of bainitic ferrite could be attributed to a heterogeneous distribution of carbon in different regions of austenite and show that two types of austenite with different carbon contents, low-carbon austenite (gLC) and the high-carbon austenite (gHC), exist during the transformation. The microstructure after cooling down to room temperature is presented to show the effectiveness of the x-ray diffraction analysis.
I. INTRODUCTION 1–11
Silicon alloyed steel that has a fine duplex microstructure of bainitic ferrite and retained austenite (ausferrite) is a material of high interest, which demonstrates a combination of mechanical properties such as high strength, large ductility, high fatigue strength, high impact and fracture toughness, and wear resistance. No carbides will be precipitated in the structure because the silicon does not dissolve in the cementite and hence delays the austempering transformation of austenite. Silicon alloyed steels have great potential to be a low-cost and high-reliability structural material.4,9 Many research efforts are at present concentrated on the study of the effects of heat treatment process on the microstructure and mechanical properties. It has been shown that two types of austenite with different carbon contents exist until carbon enrichment finishes: (i) thin film-shaped retained austenite gHC with high carbon content and (ii) blocky-retained austenite gLC with low carbon content.12–17 Blocky-retained austenite is less thermally and mechanically stable. This instability can lead to the formation of brittle plate shape a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2009.0178 J. Mater. Res., Vol. 24, No. 4, Apr 2009
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martensite to occur in the microstructure, which degrades the toughness.9 The behavior of carbon during the austempering transformation of austenite to bainitic ferrite is a hot spot from a fundamental point of view. There are two competing hypotheses regarding the formation of bainitic ferrite from austenite. According to the “displacive” mechanism, it is supposed that the bainitic ferrite forms without diffusion and carbon redistributes and precipitates as carbide from the super
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