Surface nanocrystallization by surface mechanical attrition treatment and its effect on structure and properties of plasma nitrided AISI 321 stainless steel

Abstract

A plastic deformation surface layer with nanocrystalline grains was produced on AISI 321 austenitic stainless steel by means of surface mechanical attrition treatment (SMAT). Low-temperature nitriding of SMAT and un-SMAT AISI 321 stainless steel was carried out in pulsed-DC glow discharge. The effect of SMAT pretreatment on the microstructure and properties of the stainless steel were investigated using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Vickers hardness tester and UMT-2MT tribometer. The results show that the plasma nitriding of AISI 321 steel can be enhanced considerably by means of SMAT process before nitriding, and a much thicker nitrogen diffusion layer with higher hardness was obtained for the SMAT samples when compared with un-SMAT samples. In addition, the wear resistance and load capacity of the nitrided layers on the SMAT samples was much higher than that of the un-SMAT samples due to the thicker S phase case and the gradient nitrogen diffusion layer.

Introduction

Austenitic stainless steels are extensively used in the chemical and food industries due to their excellent corrosion resistance. However, the low surface hardness and poor wear resistance has restricted their applications in engineering fields. Plasma nitriding is an effective process for improving the surface hardness and anti-wear properties of stainless steel. Generally, the treatment temperature for traditional plasma nitriding of AISI 321 steel is above 500 °C in order to obtain a sufficiently thick nitrided layer. However, at the same time, the corrosion resistance would deteriorate due to the precipitation of CrN from the stainless steel substrate when the treatment temperature was increased above 475 °C [1]. To avoid the precipitation of CrN, plasma nitriding can be carried out at a low temperature, normally not high than 400 °C, and then a so-called S phase with high hardness and better corrosion resistance (especially in chlorine ion solution) is obtained on the surface of the austenitic stainless steel [2]. Unfortunately, the diffusion coefficient of nitrogen in austenite stainless steel is decreased remarkably with the reduction of treatment temperature, which results in a very small depth of the S phase and usually a very long nitriding duration. Moreover, the plastic deformation is concentrated mainly in the substrate when the substrate is relatively soft or the hard coating is relatively thin [3]. It is anticipated that under more intensive loading conditions, premature failure of the modified layer will occur via subsurface plastic deformation [4]. Therefore, the load capacity of thin modified layer with single S phase is not satisfactory due to its intrinsically brittle characteristic and the plastic deformation of the substrate. On the other hand, a higher load capacity could be achieved for the hardened surface layer with a diffusion-type hardness profile that could eliminate the danger of stress concentration at the layer–core interface [4].

Intensive investigations of the S phase have been carried out in the past two decades, and a great variety of techniques have been used to attempt to improve the diffusion treatment of the austenite stainless steel and thus obtain a thick S phase. Generally, plasma ion immerse implantation (PIII) [5], [6], Rf-plasma [7], [8], [9], microwave-induced electron cyclotron resonance (ECR) [10], [11], [12] and intensified plasma-assisted processing (IPAP) [13], [14] were developed to treat austenite stainless steel through the improvement in the ion energy or flow density of the active species in glow discharge. However, the above techniques conventionally required a high vacuum condition and higher cost, which therefore limits its use in the industrial treatment of stainless steels. In addition, the modified layer produced by these techniques retains the brittle property and sharply decreased hardness profile which may restrict the applications of such steels in anti-wear fields in an aggressive environment, such as under conditions of high speed and heavy load, in which significant cracking within modified layers is observed.

A practical approach for solving such a difficulty is to introduce the new concept of surface mechanical attrition treatment (SMAT) as a pre-treatment method on stainless steel before the nitriding process, which was originally used to treat metallic materials because of the formation of a nanostructured layer on the surface [15]. However, nanocrystalline steel induced by means of SMAT possesses high chemical activity due to the large number of grain boundaries and abundant defects. The grain boundaries and other defects such as dislocations and mechanical twins could facilitate the rapid diffusion of nitrogen atoms in steels [16], [17]. Consequently, SMAT was widely used to pretreat metal substrates with a nanostructured surface layer, which leads to the significant reduction in nitriding duration and the increase in the thickness of nitrided layer for pure Fe and other alloys [16], [17], [18], [19].

In this paper, the nanostructured layer in the surface of AISI 321 austenite stainless steel was induced by means of SMAT before the plasma nitrating process, and the effects of SMAT on the microstructure, hardness and tribological properties of plasma nitrided AISI 321 stainless steel were investigated.