Interrelation between Microstructure and Residual Stresses for Low-Pressure Carburizing of Steel AISI 5120 under Defined Process Parameter Variation

Abstract

Low-pressure carburizing (LPC) is a recipe-controlled process for surface layer hardening. These recipes are mainly based on experience and contain the process parameters used to achieve the desired hardening result. The process parameters influence the chemical gradients which have set in the boundary layer, the local microstructure and the depth distribution of the process-induced residual stresses. Within the scope of this work, a systematic parameter study and advanced characterization was carried out to quantify the influence of these process parameters on the resulting material state. The varied parameters include the carburizing temperature, the hardening temperature, the quenching rate as well as the number of repetitions and durations of the carburizing cycles’ steps. The results obtained should help to extend the fundamental process understanding of the LPC process. The analyses showed that the retained austenite content and its depth profile change significantly for certain process parameter variations, reaching contents of up to 45 vol% in the near-surface region. The differences regarding the residual stress states of the case-hardened samples can first and foremost be related to the formation of varying depth distributions of the retained austenite.