Low pressure carburizing cycle determination for high alloy steels

High hardenability steels with high alloy content typically contain strong carbide forming elements such as chromium and molybdenum. During carburization, alloys with high amounts of strong carbide formers may form stable carbides on or near the surface during carburization that can effectively block carbon diffusion and retard the carburization process. This is especially true for low pressure carburization where the surface carbon content can rise quickly as the carbon source gas dissociates on the hot part surface. To get carbon penetration into the part and achieve the desired case depth, the low pressure carburization process must consist of a series of boost and diffusion steps in order to control the surface carbon content and the amount of carbides that are present. At issue is how to determine an acceptable carburization schedule in terms of boost and diffusion step times. This presentation will discuss a methodology used to develop a proper low pressure carburizing schedule for these high alloy steels. This methodology involves experiments to determine carbon diffusion rates, carbide formation kinetics, and carbide dissolution kinetics, and computer simulation of the process.