Optimization of Pre-Heat Treatment for Nitriding.

Matthias Steinbacher,Stefanie Hoja

doi:10.3390/ma14247766

Matthias Steinbacher, Stefanie Hoja

Open Access

https://doi.org/10.3390/ma14247766

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Abstract

To achieve a core strength that meets the requirements during service life, components to be nitrided are subjected to a pre-heat treatment. Since a higher strength prior to nitriding also has a positive effect on the achievable strength in the nitrided layer, an optimization of the pre-heat treatment may lead to better service characteristics of nitrided components. For this purpose, different optimizations of pre-heat treatment were investigated on the nitriding and quenching and tempering steels EN31CrMoV9 and EN42CrMo4 (AISI4140). One strategy was a change of the austenitization temperature for EN31CrMoV9 from 870 °C to 950 °C in order to solve the coarse carbides of the as-delivered state and realize a finer distribution of the carbides in the quenched and tempered structure. This special treatment lead to a higher hardness compared to the conventional treatment. The second investigated pre-heat treatment variant was a bainitic treatment instead of quenching and tempering. The bainitic initial microstructure increased the diffusion depth compared to conventionally quenched and tempered specimens. In addition the maximum hardness of the nitrided layer, the core hardness was significantly higher on the specimens with the bainitic microstructure. During subsequent nitriding, however, the bainite is tempered and loses some of its hardness.

Highlights

◦ C for the material EN42CrMo4 followed by oil quenching (Ipsen, Kleve, Germany). Both materials were tempered at 630 ◦ C for two h. This relatively high tempering temperature was selected because treatment temperatures of approx. 480–590 ◦ C are common for nitriding and nitrocarburizing, and the tempering temperature for quenching and tempering should be approx. 30–50 ◦ C above the subsequent nitriding or nitrocarburizing temperature to avoid further tempering during the thermochemical heat treatment
Nitriding only achieves an increase in strength in the surface layer
In order to achieve a core strength that meets the requirements, it is necessary to subject the components to a preheating treatment before nitriding

Summary

Introduction

In order to increase the strength and wear resistance in the surface layer of the components, machine parts are frequently nitrided [1]. In this process, the components are exposed to a nitrogen-containing atmosphere at temperatures of approx. The increase in hardness during nitriding is essentially due to precipitation hardening by iron and alloying element nitrides formed in the diffusion zone when the relatively low solubility of nitrogen in ferrite is exceeded and/or alloying elements with high nitrogen affinity react with the nitrogen. Below the surface, where the nitrogen concentration is the highest, the precipitated nitrides form a closed layer, the so-called compound layer ( white layer) composed of either gamma prime (γ’) or epsilon nitrides (ε), which give the component higher wear resistance due to their high hardness and improved corrosion resistance

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Conclusion