High-cycle fatigue and surface layer stability of case-hardened PM-steels with graded porosity

ABSTRACT

In order to increase the fatigue performance of mechanically loaded PM-components, case hardening procedures can be used to improve the surface layer states in terms of hardness and residual stress. In this work, the effects of carbonitriding treatment on two typical PM-steels with multiple densities are investigated in terms of their surface layer state (hardness, residual stress and retained austenite) before and after fatigue testing and their resulting pulsating fatigue performance. The results confirm fatigue strengths that can be formulated linearly as a function of porosity as well as a cyclic stability of the surface layer condition at loads below the fatigue strength. Further optimisation of the fatigue strength can be expected by tailoring the surface layer with respect to residual stresses. Comparisons to other studies are difficult due to many process parameters, different load cases and limited data, but indicate comparable fatigue strengths.

KEYWORDS:

• Fatigue performance
• powder metallurgy
• graded porosity
• carbonitriding
• surface layer stability
• residual stress
• retained austenite

Acknowledgements

This research was supported by the AiF (Grant Number 19887 N) within the IGF program from the Federal Ministry of Economic Affairs and Energy (BMWi). The authors want to thank Höganäs AB for the provision of material and the Leibniz-Institut für werkstofforientierte Technologien Bremen (IWT Bremen) for the carbonitriding procedure.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Notes on contributors

James M. Damon

James M. Damon: is a Master of Science (M.Sc.) and scientific assistant at the Institute for Applied Materials – Material Science (IAM-WK) at the Karlsruhe Institute for Technology (KIT) since 2016. His research includes the characterization and modelling of the influence of case hardening of powder metallurgical parts. The project aims on the predictability of surface layer states of case hardened PM-parts and their resulting properties regarding fatigue.

Erik Jung

Erik Jung: is a final-year undergraduate student from the Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany, where he is pursuing a Bachelor of Science at the department of Mechanical Engineering. He completed his Bachelor Thesis at the Institute for Applied Materials – Materials Science and Engineering (IAM-WK) at the KIT. His research includes the characterization of surface layer conditions of carbonitrided PM components and their resulting fatigue strength as a function of porosity.

Michael Zürn

Michael Zürn: is a Master of Science (M.Sc.) for Mechanical Engineering and scientific assistant at the Institute for Applied Materials – Material Science (IAM-WK) at the Karlsruhe Institute for Technology (KIT) since 2017. His research includes the x-ray in-situ investigations of the case hardening process involving the phase transformation and evolution of residual stress. He has expertise in the X-ray analysis of retained austenite in steels for research and, for the most part, industry.

Stefan Dietrich

Stefan Dietrich: Dr. Stefan Dietrich is head of the department “Product and Component Behavior” since 2014. He was scientific assistant from 2009 to 2013 at the Institute for Applied Materials (IAM-WK) at the Karlsruhe Institute for Technology (KIT) and wrote his PhD thesis in 2013. His research topics include mechanical surface treatment, the simulation of heat treatment applications and special emphasis regarding additive manufacturing processes and computer tomography.

Volker Schulze

Volker Schulze: Is full professor at the Institute for Applied Materials (IAM) since 2007 and full professor at the Institute for Production Engineering since 2008 at the Karlsruhe Institute of Technology (KIT). He received his PhD degree in Mechanical engineering in 1993 from the Universität Karlsruhe (TU). He worked between 1995 and 2003 as head of section “Product and component behavior”. Prof. Schulze is an expert for surface treatment and heat treatment applications with respect to process-microstructure-property relations.