Modeling Growth and Dissolution Kinetics of Grain-Boundary Cementite in Cyclic Carburizing
- PDF / 1,211,813 Bytes
- 10 Pages / 593.972 x 792 pts Page_size
- 90 Downloads / 231 Views
UUM carburization has recently attracted increasing intention as a rapid and energy-saving method for the industrial heat treatment of steel parts. Increasing the carburization temperatures allows for the operation time to be shortened. Furthermore, the carbon (C) content at the surface is kept higher than during conventional gas carburizing processes due to radical reactions with decomposed species of hydrocarbon gases. On the other hand, depending on the operating conditions and geometries of the workpiece, the C content in austenite (c) can easily exceed the solubility of cementite (h) and form a filmy h along c grain boundaries (excess carburizing), resulting in embrittlement of the workpiece. The degree of excess carburizing, i.e., the growth rate of h, is heavily reliant on the chemistry of case-hardening steels. For this reason, it is often a practical problem to adjust cyclic carburization-diffusion operation conditions to obtain the intended C concentration profiles without residual grain boundary h (hGB). In previous work,[1–3] the dependencies of both the surface C content and amount of hGB on the steel HIDEAKI IKEHATA, Senior Researcher, and KOUJI TANAKA, Research Manager, are with the Iron & Steel Process Engineering Laboratory, Toyota Central R&D Labs., Inc., Aichi 480-1192, Japan. Contact e-mail: [email protected] HIROYUKI TAKAMIYA, Researcher, is with the Electrical & Magnetic Materials and Processing Laboratory, Toyota Central R&D Labs., Inc. HIROYUKI MIZUNO, Senior Chief of Staff and TAKEYUKI SHIMADA, Engineer, are with the Aichi Steel Corporation, Development Div. No. 1. 1, Aichi 476-8666, Japan. Manuscript submitted June 27, 2012. Article published online April 17, 2013 3484—VOLUME 44A, AUGUST 2013
composition were interpreted based on the equilibrium with deposited graphite (Gr) from decomposed hydrocarbon gas. More specifically, the surface C content can be determined by thermodynamic calculation for steel consisting of c and h that is in equilibrium with Gr (three-phase equilibrium). For the example of AISI 5120 steel (SCr420 in JIS), this is approximately 4.2 mass pct at 1223 K (950 °C), and the solute C in c is around 1.3 mass pct. Therefore, it is possible to predict the C concentration profile by employing the diffusion equation with a three-phase equilibrium boundary condition and the equilibrium fraction of h (hereafter, this method is referred to as the conventional model). Engstro¨m et al.[4] implemented an analogous methodology in the framework of a multicomponent diffusion solver and successfully predicted the distribution of two types of carbide precipitating in the bond of dissimilar steels. However, the conventional model has not been successfully applied to comparatively verify the amount of hGB formed during short-time carburization. Ando and Krauss[5,6] reported that the fraction of hGB for 1.5Cr1.0C (mass pct) steels aged at temperatures lower than Acm increased initially, but became saturated at around 1/4 to 1/3 of the equilibrium fraction. They concluded that the retardati
Data Loading...
