Stainless Steel 2000

Thermochemical Surface Engineering of Stainless Steel

Tom Bell, Katsuya Akamatsu, Tom Bell, Katsuya Akamatsu

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9781000724936
• Edition: 1
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

Austenitic stainless steels lend themselves to a wide range of applications. However, they normally stiffer from poor wear resistance and do not respond well to traditional surface treatments. This volume. the fruit of a current status seminar, reflects the enormous strides which have been made in the last few years in the study of the expanded austenite phase (also called the S phase) and the development of new surface treatment techniques. As well as the papers presented at the seminar, the book contains selection from related papers and a comprehensive bibliography of the literature on the subject from 1979 to 2000.

Towards Quantifying the Composition of Expanded Austenite

M.P. FEWELL^a*, P. GARLICK^b, J.M. PRIEST^a†, P.T. BURKE^c, N. DYTLEWSKI^c, K.E. PRINCE^c, K.T. SHORT^c, R.G. ELLIMAN^d, H. TIMMERS^d,e, T.D.M. Weijers^d,e and B. GONG^f

aPhysics and Electronics, University of New England, Armidale NSW 2351, Australia

bElectron Microscope Unit, University of New England, Armidale, NSW 2351, Australia

cAustralian Nuclear Science and Technology Organisation, Menai, NSW2234, Australia

dDepartment of Electronic Materials Engineering, Australian National University, Canberra, ACT 0200, Australia

eDepartment of Nuclear Physics, Australian National University, Canberra, ACT 0200, Australia

fSurface Science and Technology, School of Chemistry, University of New South Wales, Sydney, NSW 2052, Australia

This paper is dedicated to the memory of Trevor Richard Ophel, Professor of Nuclear Physics in the Australian National University, who died suddenly on 17th June 2000.

ABSTRACT

Secondary-ion mass spectrometry (SIMS) is an attractive method for elemental depth profiling for nitrided stainless steel. However, if taken at face value, the results suggest migration of Mo and Cr from the nitrogen-rich layer. More plausibly, the results indicate the presence of significant matrix effects, and this conclusion casts doubt on the interpretation of all the SIMS depth profiles, including that of nitrogen. We explored this question by examining standard samples prepared by nitrogen-ion implantation and comparing assays and depth profiles from SIMS, energy-dispersed X-ray microanalysis (EDX), heavy-ion elastic-recoil detection analysis (HI-ERDA) and X-ray photoelectron spectroscopy (XPS). We conclude that there are negligible matrix effects in the SIMS sputter yields of nitrogen and carbon, but that the yields of MoCs⁺ and CrCs⁺ ions are significantly affected by the concentration of nitrogen. Thus, the shape of the nitrogen and carbon depth profiles from SIMS can be taken at face value. HI-ERDA gives depth profiles of the same quality as SIMS, but is limited to a relatively thin nitrogen-containing layer. Results from EDX, HI-ERDA and XPS show that the relative concentrations averaged over the treated layer of the main metals in the alloy are the same as in the bulk material. Detailed examination of the peak positions and widths in XPS shows a clear chemical association between the nitrogen in the treated layer and Cr and Mo, and probably also an N–Fe association, but none between nitrogen and Ni. These results support a model of the kinetics of nitriding in austenitic stainless steel.

1. INTRODUCTION

Stainless steel hardened by nitriding is potentially such a useful material, and plasma nitriding gives such promise of a commercially practicable method of producing it, that much effort world wide is now being devoted to realising this potential.^{1, 2, 3, 4, 5} As in many other fields, a significant issue for this research is the characterisation of treated samples, for the purpose of comparing the efficacy of different treatment processes. Many diagnostic methods are used; among them are techniques for determining the composition of the treated layer. These range from methods familiar in metallurgy, such as glow-discharge optical spectroscopy, to the most sophisticated techniques developed in modern surface science. A feature of all methods is the need for calibration: whether one is measuring the yield of photons, electrons or ions, there is always the question of relating the observed yield to the concentration of the various species in the material under study. The work reported in this paper was prompted by just such a question of calibration, in this case in the results of secondary-ion mass spectrometry (SIMS) on samples of nitrided AISI 316 stainless steel.

Figure 1 shows a typical SIMS elemental depth profile from a nitrided sample. (Experimental details are given in §§ 2.1 and 2.2.) There are many interesting features in these data, not least the shape of the profiles for carbon and nitrogen. However, the profiles for the four main alloy elements indicate that the interpretation may not be straightforward. All four metals show a lower ion-current ratio ℐ_MCs⁺ in the nitrogen-containing layer compared with the bulk. This is to be expected from sputtering theory, which indicates that the ion-current ratio of a species M should be proportional to its atomic concentration c_M (atom %), and c_M values for the metal elements are reduced by the presence of the nitrogen. But closer inspection shows that the details do not match this explanation. Table 1 gives the ratio of ion-current ratios in the treated layer to the bulk for the four main alloy elements. A straightforward interpretation would explain the value for iron as reflecting a ~25 at. % concentration of nitrogen in the treated layer. However, one would then expect the values to be the same for all the metals, and clearly they are not. Either there has been migration of Cr and Mo from the treated layer, or the ion-current ratios of these elements are influenced by the presence of nitrogen differently from those of Fe and Ni. Given that Fe is the main constituent of the alloy (~65 at. %), there may even be evidence that ℐ_NiCs⁺ is behaving in an opposite manner from ℐ_CrCs⁺ and ℐ_MoCs⁺.

Fig. 1 Relative ion-current ratios ℐ_MCs⁺ = ℐ_MCs⁺/I_Cs⁺ as a function of sputter time and hence depth for a sample of AISI-316 stainless steel nitrided in the UHV chamber for 3 h at 400°C in a N₂ + 25% H₂ gas mixture of total pressure 0.53 Pa without pretreatment. The vertical line shows the thickness of the treated layer determined from a cross-sectional electron micrograph of the same sample (Fig. 4a). The broken line shows the carbon ion-current ratio in the bulk, to emphasise the manner in which carbon is piled up at the interface between the treated layer and the bulk.

Influences of this sort are referred to as ‘matrix effects’; their presence clearly complicates the interpretation of depth profiles. One can no longer assume that, for example, a halving of ℐ_MCs⁺ implies a halving of c_M; rather there may be a change in the calibration factor relating these two. Of the elements profiled in Fig. 1, most interest perhaps lies in nitrogen and carbon, but there is no reason why the profiles of these should be less affected by matrix effects than the profiles of the meta...