Material Property Characterization for Low Pressure Carburization Process Design using Computer Modeling

Low Pressure Carburization (LPC) is widely used in the aerospace industry for hardening components made of steels with high alloy content and high heat resistant properties. The traditional gas carburizing process often generates Intergranular Oxidation (IGO) near the surface due to the existence of oxygen in the furnace atmosphere, which needs to be removed by grinding after hardening to restore bending and contact fatigue performance. LPC processing is done in a low pressure chamber without the existence of oxygen, so the surface microstructure is improved by eliminating IGO. High temperature resistant steels require high alloy element contents, and some elements are strong carbide formers, such as Cr, Mn, Mo, and V, etc. During LPC processing, both iron and alloy carbides can be formed, which significantly affect the carburization time required to reach a specified case depth and surface carbon. The carbides formed during the LPC process may not decompose completely prior to quench hardening, and these primary carbides will end up in the final processed parts. If the size of these primary carbides is not controlled, both bending and contact fatigue performance may be decreased. In order to control carbide formation during LPC, the carbon diffusivity of a material must be characterized. This characterization was recently performed under a program between DANTE Solutions and the Combat Capabilities Development Command Aviation and Missile Center (CCDC AvMC). In this research, a specifically designed coupon was used to characterize the carbon diffusivity and carbide forming properties during LPC processes. Using the characterized material properties, LPC process recipes can be designed by using modeling to achieve specific case depth and surface carbon content. The work was demonstrated using Pyrowear 675 steel and DANTE commercial heat treatment modeling software.