Implications of carbon, nitrogen and porosity on the γ → α′ martensite phase transformation and resulting hardness in PM-steel Astaloy 85Mo

Abstract

This study aims at a thorough characterization of the relationship of interstitially solved carbon and nitrogen on the γ → α′ transformation in PM steels, the accompanied volume change and the resulting hardness. Furthermore, the investigations include multiple porosity levels of 6.9, 7.2 and 7.35 g/cm3 to characterize porosity effects. Dilatometric samples were carburized and carbonitrided to seven distinct compositions to account for common compositions in the process of thermochemical case hardening heat treatment. The dilatometric samples were rapidly austenitized and quenched and the dilatometric response was evaluated. To fully characterize the martensitic transformation of PM steels, X-ray diffractometry evaluated the amount of retained austenite after quenching. Conclusive results of iterative quenching procedures along with elemental analysis after heat treatment give distinct evidence that PM steels underlie rapid decarburization. This effect ultimately leads to an erroneous evaluation of the martensite transformation kinetics, especially the often proposed effect of porosity on MS. However, a distinct effect on the accompanied volume change from austenite to martensite is proposed. To account for an interplay of solved carbon and nitrogen, an effective nitrogen contribution of 25% based on carbon equivalent is proposed. Utilizing the effective content, the impact of nitrogen can be projected on carbon within the range of common carbon and nitrogen contents, and a good predictability of the martensite transformation can be achieved. Regarding the resulting hardness, a dependency solely on carbon is suggested. The overall hardness shows a typical maximum at approximately 0.6–0.7 wt%, irrespective of the solved amount of nitrogen.

Keywords

• PM steels
• Dilatometry
• Martensitic transformation
• Carbon nitrogen effects
• Porosity dependence