Characterization of the surface of Fe–19Mn–18Cr–C–N during heat treatment in a high vacuum — An XPS study
Highlights
► Surface characterization by means of XPS, SEM, and EDX analyses. ► Heat treatment of a high CrMn powder. ► Transfer of oxygen from the iron oxide layer to manganese-based particulate oxides. ► Progressive reduction of Mn oxides. ► Transformation of the Mn oxides into stable Si-containing oxides.
Introduction
The alloying concept of high-CrMn corrosion-resistant austenitic steels is based on the substitution of expensive nickel by more competitively priced manganese. Corrosion-resistant CrMn austenitic steels feature high toughness, good resistance to erosive corrosion as well as high temperature and creep strengths [1], [2]. The addition of nitrogen in combination with carbon increases the strength by solid-solution strengthening without a significant decrease in ductility (C + N concept) [1], [2], [3], [4]. Applying powder metallurgy as a production route to CrMn austenitic steels has several benefits compared to casting. PM production avoids macroscopic segregation and thus improves the homogeneity of the resulting product. Additionally, PM enables the production of near-net-shaped parts, which minimizes machining and thus saves costs and increases utilization of the material by reducing machining allowances [5]. A significant increase in the wear resistance can be achieved by the addition of hard particles to produce a corrosion-resistant metal-matrix composite [6]. However, sintering of high-manganese austenitic stainless C + N steel also raises challenges. The high affinity of chromium and manganese for oxygen can create a layer of oxides on the powder surface. Elaborated studies of powder particle surfaces using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) reveal that the surface of iron-base powders is covered by a heterogeneous oxide layer. This layer consists of a homogenous iron oxide layer that is discontinued by larger particles of highly thermodynamically stable oxides with Cr, Mn, and Si [7], [8], [9]. This issue is very important because the key parameter that determines the mechanical and chemical characteristics of a powder is the composition of the surface [10], [11]. The formation of sintering necks requires contact of oxide‐free metallic surface zones. Oxides act as a diffusion barrier and retard bridging and neck formation between adjacent powder particles [12]. For low-alloyed steels, this matter is of minor importance because the less stable iron oxide layer that forms on this type of steel is reduced well below the sintering temperature [7], [11]. Cr, Mn, and Si, however, form very stable oxide films or particulates during atomization and annealing [7], [11]. These oxides may grow and/or transform to more stable oxides by consuming oxygen from less stable oxides. After sintering, the surface oxides may remain in the product, which impairs the performance of the steel [13]. Research activities at the Department of Materials and Manufacturing Technology at Chalmers University of Technology during the last decades [10], [12], [14], [15], [16], [17], [18], [19] have shown that an increasing oxide thickness of a high-alloyed powder decreases the mechanical properties such as impact strength and tensile strength of the produced steel [18], [20]. The usefulness and strict processing requirements of a powder are determined not only by the amount of oxides but also by the character and composition of the oxides and their spatial locations within a particle [10]. Additionally, successful sintering of CrMn steel requires consideration of the high vapor pressure of manganese. Manganese sublimation can lead to elemental loss [14], [21]. As shown by means of thermodynamic calculations performed by Hryha et al. [14], manganese sublimation already starts at around 700 °C when its partial pressure is 10− 3 Pa. The equilibrium partial pressure is significantly higher when the temperature exceeds 900 °C. Additionally, one has to consider that the nitrogen solubility of the steel decreases with increasing temperature. Owing to the fact that the nitrogen content in the steel is dependent on the nitrogen partial pressure of the atmosphere, degassing of nitrogen can occur, especially if a vacuum is applied. Degassing of nitrogen decreases the strength and may destabilize the austenitic phase and thus change the material's properties.
This work deals with the high-strength CrMn austenitic steel Fe–19Mn–18Cr–C–N developed at the Chair of Materials Technology of the Ruhr-Universität Bochum, Germany. Former studies based on XPS and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM + EDX) were performed at Chalmers University. They reveal that the surface of the investigated steel is covered by a homogeneous iron oxide layer with a thickness of ~ 4 nm. This layer is discontinued by relatively large islands of Mn-rich oxide. Additionally, chromium was found to be enriched in the first 7 nm of the steel, although surface enrichment of chromium is less pronounced than enrichment of manganese [9]. The present work focused on the surface characteristics of the austenitic steel powder Fe–19Mn–18Cr–C–N and its evolution during vacuum sintering. The behavior of Mn and Mn-rich oxides is of special interest. The results will lead to a more profound understanding of the reduction behavior. Furthermore, the feasibility of vacuum sintering for producing Mn-rich austenitic PM parts will be assessed.
The following questions are to be answered:
- •
Does vacuum processing enable reduction of manganese oxides at the surface zone without a detrimental decrease in the manganese content of the bulk?
- •
Does reduction of less stable oxides take place before Mn-rich and/or mixed oxides become thermodynamically stable, thus avoiding a shift of oxidation?
- •
What is the morphology of the remaining oxides (if any)?
Section snippets
Experimental
The austenitic steel was gas-atomized in a vacuum atomization unit under nitrogen gas and then sieved to a maximum grain diameter of 200 μm. The chemical composition was measured by means of optical emission spectroscopy (OES) of a HIPed specimen and is listed in Table 1. Combustion analysis of the powder material gave an oxygen content of 360 ppm.
The chemical analysis of the powder surface was performed at Chalmers University of Technology with the help of X-ray photoelectron spectroscopy (XPS)
XPS
The survey scan as well as the concentration profiles of the powder surface composition before heat treatment has been presented in [9]. An overview of the survey scans of the powder surface before and after heat treatment at the applied temperatures is shown in Fig. 1. Due to the influence of contamination layer at room temperature, the survey scan of the unannealed sample which was etched at 1 nm is shown. Changes in the relative peak height indicate a change in chemical composition of the
Discussion
The results allow postulation of the following statements, which will be discussed below:
- 1.
The iron oxide layer was reduced at temperatures ≤ 500 °C.
- 2.
A shift of oxidation took place between 500 °C and 600 °C.
- 3.
Progressive reduction took place with increasing temperature above 700 °C.
- 4.
This reduction process was accompanied by strong evaporation of Mn, which started at 700 °C.
- 5.
Mn-rich oxides transformed during heat treatment in a high vacuum. This was accompanied by a change of oxide morphology and
Summary and Conclusion
This study deals with characterization of the surface of the austenitic steel powder Fe–19Mn–18Cr–C–N after heat treatment at annealing temperatures between 500 and 1000 °C under a high vacuum by means of XPS, SEM, and EDX analyses.
Previous studies indicated that the powder is initially covered by relatively large islands of Mn-rich oxide surrounded by a homogeneous iron oxide layer. Enrichment of chromium oxides in the first 7 nm was also detected. This study revealed the reduction of iron
Acknowledgment
Mr. Urban Jelvestam is greatly acknowledged for the assistance/performance of most of the XPS analyses. The work at the Chair of Material Technology (LWT) of the Ruhr-Universität Bochum, Germany was supported by the Deutsche Forschungsgemeinschaft within the project “Untersuchung des Flüssigphasensinterns vorlegierter Metallpulver auf Eisenbasis und dessen Beeinflussung durch Gas-Festkörperreaktionen” (TH 531/8-1 and WE 4436/3-1).
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