Case Hardening Using Nitrogen-Based Atmospheres

Due to the absence of oxygen-containing gas components, nitrogen-based atmospheres offer a low-cost alternative to the case hardening treatments typically carried out using vacuum furnaces and, in several local economies, may cost less than the traditional atmospheres. When activated at the furnace inlet with electric plasma, nitrogen mixed with just a few percent of hydrocarbon, e.g. methane or propane, is effectively carburizing, and nitrogen mixed with ammonia and some hydrocarbon can be used to carbonitride lean steels within the high, austenitic temperature range, as well as nitride stainless steels in the low-temperature ranges. While offering quality advantages such as intergranular oxide-free cases, critical for non-machined surfaces or diverse near net shape products, the nitrogen atmospheres are non-equilibrium, i.e. require different sensing and process control techniques than the endothermic or methanol atmospheres. This paper provides an update on recent developments concerning the use and the process control for nitrogen atmospheres.