Effect of Salt Bath Nitrocarburizing and Post-Oxidation on Static and Fatigue Behaviours of a Construction Steel

Alberto Campagnolo,Manuele Dabalà,Giovanni Meneghetti

doi:10.3390/met9121306

Abstract

Several surface modification technologies are typically applied to improve the mechanical properties of the material surface of structural components subjected to severe conditions of load, wear and chemical erosion of the surface. The nitrocarburizing and post-oxidation heat treatment, also known as quench-polish-quench (QPQ), improves the fatigue, wear and corrosion resistance properties of the material, since it increases the surface hardness and generates surface compressive residual stresses. In the present contribution, the effects of the salt bath nitrocarburizing and post-oxidation heat treatment on the static and fatigue behaviours of 39NiCrMo3 construction steel have been investigated by experimentally testing plain as well as notched specimens. For comparison purposes, 39NiCrMo3 construction steel, both untreated and treated, and X5CrNiCuNb 16-4 stainless steel have been tested. First, the microstructure of the untreated and treated steel has been identified by metallographic analysis; micro-hardness measurements have been collected and residual stresses profiles have been obtained by using the X-ray diffraction technique. Then, experimental static and fatigue tests have been performed. Finally, the fracture surfaces have been analysed to locate fatigue crack nucleation sites.

Highlights

In the last decades, several surface treatments and coatings [1,2,3,4] have been developed for mechanical components facing severe in-service conditions in terms of load, wear, and chemical corrosion, such as gears, crankshafts, extrusion and forging dies, valves and springs
Among the available surface treatments, the nitrocarburizing is widely applied to components made of ferrous alloys to enhance wear as well as corrosion resistance and fatigue strength
Dealing with plain and notched specimens made of untreated 39NiCrMo3 steel, the microstructure is typical of a tempered steel: Figure 4a,c and Figure 5a,c show very fine grains and small particles of carbides obtained after heat treatment performed on the quenched steel at a temperature close to 600 ◦ C

Summary

Introduction

Several surface treatments and coatings [1,2,3,4] have been developed for mechanical components facing severe in-service conditions in terms of load, wear, and chemical corrosion, such as gears, crankshafts, extrusion and forging dies, valves and springs. Among the available surface treatments, the nitrocarburizing is widely applied to components made of ferrous alloys to enhance wear as well as corrosion resistance and fatigue strength. This treatment increases the surface hardness and generates a compressive residual stress state on the surface of the component. It can be applied by means of three different media: gas, liquid and plasma [5,6,7,8,9,10]. In the 1980s, an advanced salt bath heat treatment, given by the combination of nitrocarburizing and post-oxidation, known as quench-polish-quench (QPQ)

Methods

Results

Conclusion