Magnetic and 57Fe hyperfine structural features of nitrided austenitic stainless steel

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Highlights

  • A submicrometric layer is formed on the outermost surface.

  • MFM and Mössbauer correlation identifies formation of submicron layer.

  • Mössbauer fitting used a new approach.

  • Mössbauer hyperfine field distribution done from very low hyperfine field.

  • Formation of iron nitrides on austenitic stainless steel.

Abstract

Samples of an austenitic stainless steel were plasma-nitrided at 673 K, for 4 h, in 80% H220% N2 gas mixture, at different pressures: 4; 6 and 10 Torr. The samples were characterized by four different techniques: X-ray diffraction, Mössbauer spectroscopy, magnetic force microscopy and energy dispersive X-ray. The nitrogen concentration and thickness of the nitrided layer was found to be 7 at.% and 2 μm for the 4 Torr-sample; for those at 6 and 10 Torr, the concentration was 16 and 24 at.%, with nearly the same thickness, namely 7 μm. These values are well correlated with the framework of an expanded austenite, as pointed by the X-ray diffraction data. The magnetic force microscopy data are well correlated with the observed nitrogen concentration, evidencing the absence of any magnetic domain for the 4 Torr-sample; the magnetic patterns for the 6 Torr and 10 Torr-samples are quite similar, despite of the great difference in their nitrogen concentrations. The 57Fe hyperfine parameters at two different sample depths were assessed by Mössbauer spectroscopy, by collecting data with two distinct backscattering setup geometries. Fitting the corresponding spectra with model-independent hyperfine field distributions, starting from very low hyperfine field values, allowed correlating the magnetic force microscopy data to find out that the transition from paramagnetic to magnetic for the expanded austenite occurs at smaller hyperfine magnetic fields than it is usually reported in the scientific literature. Such correlations point to the formation of a submicrometer layer occurring on the outermost surface of the sample. Despite the complexity and difficulties in differentiating the nitrides and the martensitic structure on the surface of the samples obtained at 6 Torr and 10 Torr pressures, the results obtained in this work strongly indicate the existence of them in the surface layer.

Introduction

Nitriding is a surface treatment that, if applied to austenitic stainless steel (ASS), tends to increase its corrosion resistance, hardness and wear. Many different techniques are used for nitriding steels [1,2], each of them being very sensitive to the nitriding conditions [2]. Depending on those parameters, a myriad of physically and chemically distinguishable layers on the ASS matrix may be produced. The physical characteristics of each layer depend on the nitrogen spatial distribution all along the matrix and on the ASS matrix composition. Thus, a careful characterization, including its magnetic properties, is mandatory to better understand the atomic framework of the system.

At temperatures below 673 K, the substitutional chromium atoms (as similarly, for other chemical elements of the alloy) are thought to be spatially fixed in the steel matrix. Differently, the diffused nitrogen atoms are incorporated into the pristine ASS interstitial sites, forming a non-stoichiometric [3] metastable phase [4], called expanded austenite (EA) (also known as γN or S phase), with compressive residual stress [5]. The crystallographic structure of the EA, despite of being relatively/well studied, is not a matter of consensus [6]. The hyperfine structure for EA with low nitrogen content (less than 10 at.%) have somewhat detailed interpretations of the hyperfine parameters of their sites [7,8]. However, for higher nitrogen concentrations, the possible coexistence, with EA, of nitrides [9,10] and a martensitic structure [4,11] (which can originate from polymorphic transitions, due to its metastability) bring less straightforward (and often contradictory) interpretations of its hyperfine structure. Most of the nitriding processes forms a layer with a gradient of nitrogen distribution, which leads to a gradient of compressive residual stress too. Few works are based on EA with homogeneous nitrogen distribution [3], that allows a more careful interpretation of the crystallographic and hyperfine structures. Thus, a structural model for higher nitrogen contents is hardly a matter of agreements to date.

Even though the ASS is paramagnetic, it may become magnetically ordered if the nitrogen concentration is above a threshold value (the origin and the physical nature of this transition is not consensual either). The EA can thus behave either paramagnetic or ferromagnetically, depending on the nitrogen concentration [[12], [13], [14]]. The possibility of obtaining thin magnetic layers on a non-magnetic substrate opens perspectives for some potential technological applications, for instance, media recording, magnetic sensors or magnetic separators.

Magnetic force microscopy (MFM) is an interesting exploring technique, as it is sensitive to long-range magnetic forces in the material. The ASS magnetic domains (meaning domains with nitrogen concentration over the critical threshold) and the magnetic anisotropy, relatively to the crystallographic orientation, is depicted by MFM images of the surface [6,15]. The profile image thus reveals that the homogeneous magnetic domains swept all along the material face dimension, from its surface down to the border of the magnetic layer [13]. This is not a straightforward task, mainly due to variations of the nitrogen concentration gradient profile [13]. Another interesting exploring technique is 57Fe Mössbauer spectroscopy, which allows probing the short-range neighborhood of the iron nucleus. Usually, the backscattering experimental arrangement probes a more superficial layer (~0.1 μm deep) [8,14] of the sample. It may be even possible to extend the spectroscopic analysis throughout a thicker region by selecting the resonant radiation source energy, allowing a deeper probing into the sample [16].

The main magnetic and hyperfine structural data assessed by magnetic force microscopy and Mössbauer spectroscopy for four samples with different nitrogen contents are discussed in this report. The Mössbauer data were collected with two different backscattering setups: conversion X-ray (CXMS) and conversion electron (CEMS) Mössbauer spectroscopy.

Section snippets

Materials and methods

Mirror-polished disk-shaped samples (diameter ~19.7 mm; thickness ~2.0 mm) of austenitic stainless steel (AISI 316L) with a chemical composition of 16.1 mass% Cr; 10.1 mass% Ni, 2.1 mass% Mo, 1.8 mass% Mn and balance Fe, were plasma nitrided for 4 h at 673 K in an 80% H2–20% N2 atmosphere. The values adjusted for the voltage and current to maintain the cathode temperature (or nitriding temperature) are shown in Table 1. Three different pressures were used, producing three different nitrided

SEM

Scanning electron micrographs (images not shown) evidence that the average grain size in samples are in the order of tens of microns. Also, the nitrided layer thickness is variable for different grains. Despite of being of somewhat different in sizes, this variation does not affect appreciably the thickness of the observable layer, varying from ~2 μm, for the 4 Torr-sample, to ~7 μm for the other two samples.

EDX

To assess the nitrogen concentration profile, the EDX measurements allow probing a

Discussion

This discussion is divided into two parts. In the first, which is the main one, the magnetic domains are correlated with the crystallographic and hyperfine structure. In the second part, based on the findings of the first, the Mössbauer spectra are interpreted.

Conclusions

The characterization and interpretation of the results from nitrided ASS is a challengeable experimental task. However, the analysis of four samples by several techniques provides some interestingly new insights:

(i) The model-independent hyperfine field distribution of the Mössbauer data from very low hyperfine field values (instead of a quadrupole splitting distribution), as supported by the MFM results, suggest that the transition from paramagnetic to magnetic EA occurs at smaller hyperfine

CRediT authorship contribution statement

Danilo Olzon-Dionysio:Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft, Writing - review & editing.José Domingos Fabris:Methodology, Formal analysis, Writing - review & editing.Maximiliano D. Martins:Investigation, Resources.Mariana Andrade Boense Tavares:Investigation.José Domingos Ardisson:Conceptualization, Methodology, Formal analysis, Resources, Supervision.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was partly supported by the Brazilian research funding agencies: Foundation for Research of the State of Minas Gerais (FAPEMIG) (APQ-02345-14), São Paulo Research Foundation (FAPESP), National Council for Scientific and Technological Development (CNPq). The authors also acknowledge the Brazilian Synchrotron Light Laboratory (LNLS) (XDR1/9101) and Brazilian Nanotechnology National Laboratory (LNNano) (ME-22532).

D. Olzon-Dionysio is grateful to the Federal University of Jequitinhonha

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