In Situ Observation for Abnormal Grain Coarsening in Vacuum-Carburizing Process
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NTRODUCTION
IN
the vacuum-carburizing process,[1–8] steel is treated at higher temperatures than conventional carburizing processes, and hydrocarbon gases react directly with the steel surface. These characteristics lead to faster diffusion of carbon atoms. The vacuum-carburizing process offers shorter processing time, higher quality parts without surface oxidization, less energy consumption, and lower CO2 gas emission than conventional gas carburizing processes. Therefore, it has been a topic of active research and industrial interest in the last several years. There are two problems that occur in the vacuumcarburizing process for surface hardening of steel. One is over-carburizing. Carbides are made on the grain boundary around the surface, which can lead to low ductility and brittleness of the carburized parts. In order to overcome this problem, optimization of both process conditions and alloying elements,[1,4,8–11] as well as modeling of microstructural changes[11–16] have been conducted. The other problem is abnormal grain coarsening occurring around the surface.[17] The abnormally coarsened grains decrease impact absorption values, fatigue strength, and wear resistance. Niobium carbide (NbC) has been discussed as the focus of prevention of abnormal grain coarsening during vacuum carburizing,[17] because it is well known that NbC can suppress the abnormal grain coarsening due to its pinning effect. It is reported that the solubility of Nb is strongly influenced by the carbon content and temperature, which also affect the precipitation behavior
YASUHIRO YOGO and KOUJI TANAKA, Researchers, are with the Toyota Central R&D Labs., Inc., 41-1, Yokomichi, Nagakute, Aichi 480-1192, Japan. Contact e-mail: [email protected] Manuscript submitted July 1, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS A
of NbC. However, in Reference,[17] the criteria for the temperature and carburizing time, in which abnormal grain coarsening can occur, were not well investigated. If an experimental method, which enables us to observe grain coarsening directly and identify critical carburizing conditions that result in abnormal grain coarsening, is available, then understanding of these phenomena would be improved. The purpose of this study is to develop an in situ observation method to observe grain coarsening in a simulated vacuum-carburizing process. With this method, critical temperature and carburizing time that cause abnormal grain coarsening are investigated. Through this investigation, viability of an in situ observation method is shown, and validity of observed results is confirmed.
II.
DEVELOPMENT OF IN SITU OBSERVATION METHOD
In order to conduct in situ observation of abnormal grain coarsening while vacuum carburizing, a confocal scanning laser microscope (CSLM) was applied in this study. In many papers, in situ observation results with CSLM are reported, such as for grain coarsening,[18] grain boundary behavior during hot deformation,[19,20] and phase transformation.[21–28] There are two difficulties which come from hyd
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