Investigation of Surface Oxidation of Cast Austenitic 304 Stainless Steel at High Temperatures

Academic Editor: Koh-ichi Sugimoto Received: 23 May 2025 Revised: 27 June 2025 Accepted: 29 June 2025 Published: 2 July 2025 Citation: Ivanova, T.; Koˇrenek, M.; Mashlan, M.; Fryšák, M. Investigation of Surface Oxidation of Cast Austenitic 304 Stainless Steel at High Temperatures. Metals 2025,15, 748. https://doi.org/10.3390/ met15070748 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article Investigation of Surface Oxidation of Cast Austenitic 304 Stainless Steel at High Temperatures Tatiana Ivanova 1,* , Michal Koˇrenek 1, Miroslav Mashlan 1,* and Martin Fryšák 2,3 1Department of Experimental Physics, Faculty of Science, PalackýUniversity, 17. listopadu 12, 77900 Olomouc, Czech Republic; [email protected] 2SIGMA GROUP a.s., Jana Sigmunda 313, 78349 Lutín, Czech Republic; [email protected] 3Department of Mechanical Technology, Faculty of Mechanical Engineering, Technical University of Ostrava, 17. listopadu 15, 70800 Ostrava, Czech Republic *Correspondence: [email protected] (T.I.); miroslav[email protected] (M.M.) Abstract The microstructure and surface behavior of iron-based 304 stainless steel after temperature exposure was studied by Mössbauer spectroscopy, powder X-ray diffraction, scanning electron microscopy, energy dispersive analysis and positron annihilation. The tested specimens were in the form of cylinders produced by the casting process. The samples were annealed in air in the 600–1000 ◦ C temperature range for 36 h. Under the influence of temperature, cast 304 stainless steel underwent austenitic–ferritic transformation and tended to form an oxide layer on the surface. The oxides were mainly found in the thin surface layer (0.3 µ m) and consisted of Fe oxides and oxides of alloying elements (Cr and Mn) in the form of corundum, while, in the bulk region (10 µ m), the phase transformation of austenite to ferrite occurred. Surface phase inhomogeneity was studied by Mössbauer spectroscopy. The method of positron annihilation was used to study defects and the effect of annealing on the formation and removal of a defect structure. Keywords: cast304 stainlesssteel; annealing; Mössbauerspectroscopy; positron annihilation ; powder X-ray diffraction; scanning electron microscopy; energy-dispersive analysis 1. Introduction Stainless steel is widely used in industry due to its exceptional properties [ 1 , 2 ]. Of all the different types of stainless steel, 304 stainless steel is the most widely used and well known for its sufficient resistance to corrosion, which is achieved by the formation of a protective layer of chromium oxide under the influence of oxygen. The corrosion resistance of 304 stainless steel at different temperatures, atmospheres and solutions can be improved by adding elements (boron, TiO2) to the 304 stainless steel [3–5]. Temperature annealing, as part of post-processing treatment, is necessary for 304 austenitic stainless steel to relieve residual stresses and improve mechanical and microstructural properties [ 6 , 7 ]. A number of papers have been devoted to the study of the effect of temperature on the corrosion properties of 304 stainless steel using Mössbauer spectroscopy [ 8 – 14 ]. Mössbauer spectroscopy (MS) has been used to identify phase changes during the heat treatment of 304 steel [ 15 – 17 ]. Positron electron annihilation spectroscopy (PAS) was also used to study corrosion processes and phase transformations in 304 steel. Additionally, Doppler broadening spectroscopy (DBS) [ 18 – 25 ] and positron annihilation lifetime spectroscopy (PALS) [ 25 , 26 ] can be applied. Nonetheless, the combination of Metals 2025,15, 748 https://doi.org/10.3390/met15070748 Metals 2025,15, 748 2 of 15 Mössbauer spectroscopy and positron annihilation has not yet been utilized to study the corrosion and phase changes of 304 steel. MS and PAS can be excellent tools for studying the microstructural properties of metallic materials. MS provides qualitative and quantitative phase analysis and leads to information about the structure, phases and magnetic properties of the studied material. MS [ 27 ] is a non-destructive research method that can analyze surface and bulk by recording the free path of conversion electrons and the penetration depth of X-rays. This method is well suited to iron alloys due to the measurable effect of the resonant absorption of a gamma-ray isotope 57 Fe. Moreover, a high resolution of MS can provide information about local features of the atomic environment of an alloy or compound by recording changes in the energy of nuclear transitions near the resonant nucleus. PAS is also a nondestructive testing method that allows for the analysis of a defective structure and the concentration of vacancies. This technique can detect vacancy-type defects in a crystal structure that are beyond the resolving power of a scanning electron microscope [ 28 ]. In 304 stainless steel, PAS was also used to study fatigue [ 28 , 29 ], irradiation effects [ 30 ], hydrogen-induced defects [ 31 ] and surface damage after sandblasting [ 32 ]. The principle of positron annihilation is the capture of a positron implanted into a sample by structural defects. There is an increase in the lifetime of the positron and a narrowing of the pulse distribution of the positron ray (Doppler broadening). This study is devoted to the investigation of the surface behavior of cast austenitic 304 stainless steel after exposure to high temperatures. The temperature effect consisted of annealing in air at a temperature of 600–1000 ◦ C for 36 h. It is well known that austenitic stainless steels are annealed in various atmospheres and temperatures to relieve internal stresses. After exposure to a temperature in air, both oxidation and the ‘austenite ferrite’ phase transformations occur on the surface of cast austenitic 304 stainless steel. These transformations proceed in accordance with the Fe-Cr-Ni binary phase diagram for 304 austenitic stainless steel [ 33 ] (Figure 4). The effect of heat treatment on the microstructure and mechanical properties of 304 stainless steel was studied in various publications [ 33 – 36 ], in which the authors identified ‘austenite–ferrite’ phase transformations and chromium carbide formation. The oxidation products of the stainless steel surface after exposure to temperature were also studied in these publications [ 37 – 39 ]. The morphology of the oxide layer and the distribution of alloying elements after temperature exposure were studied using scanning electron microscopy (SEM) and energy-dispersive spectrometry (EDS), respectively. Conversion electron Mössbauer spectroscopy (CEMS) was used to study phase transformations in the thin surface layer (0.3 µ m) under the influence of temperature. Conversion X-ray Mössbauer spectroscopy (CXMS) was used to study the phase transformations that occurred in the bulk (10 µ m) of the material. In addition, X-ray diffraction (XRD) was used for phase composition as well as for the purpose of comparison with Mössbauer data. The formation of a defect structure after exposure to temperature in cast austenitic 304 stainless steel was investigated by positron annihilation spectroscopy. 2. Materials and Methods The test samples were made of cast 304 stainless steel, the chemical composition of which is given in Table 1. The samples studied were cut from a cast 304 austenitic stainless steel cylinder. The samples were annealed for 36 h in air at a temperature range of 600–1000 ◦C. Temperature annealing was carried out in an LE05/11 laboratory furnace (LAC, Židlochovice, Czech Republic). Thermal treatment consisted of 1 h ramp-up to the required temperature, dwell time of 36 h and spontaneous cooling to RT. The sample was then left in the furnace until it had completely cooled. The samples showed visible changes in color depending on the annealing temperature (Figure 1). Metals 2025,15, 748 3 of 15 Table 1. Chemical composition of 304 cast stainless steel. Steel Element Concentration, wt % Fe C Si Mn P S Cr Ni N 304 Bal. 0.07 1.0 2.0 0.045 ≤0.01 17.50–19.50 8.00–10.50 0.10 Figure 1. Annealed samples of cast 304 austenitic steel (sample diameter 25 mm, height 5 mm). The backscattering 57 Fe Mössbauer spectra were measured by a backscattering Mössbauer spectrometer at room temperature. The Mössbauer spectrometer operated in constant acceleration mode and was equipped with a 57 Co(Rh) source. The spectra were processed and accumulated using MS96 Mössbauer spectrometer software (V.3.6.3, Department of Experimental Physics, PalackýUniversity in Olomouc, Olomouc, Czech Republic) [ 40 ]. The spectra were recorded on 512 channels. Two types of detectors were used to record the CXMS spectra and CEMS spectra. An air scintillation detector was used to register CEMS spectra, and for registration of the CXMS spectra, a proportional gas detector registering 6.4 keV X-rays was used [41]. MossWinn 4.0 software was used to analyze the Mössbauer spectra with least-squares fitting of the lines [ 42 , 43 ]. The centroid of the α -Fe foil (thickness 30 µ m) spectrum was used as the standard for determining the value of the isomer shift values at room temperature. The samples were studied by means of positron annihilation spectroscopy, specifically by Doppler broadening of the annihilation spectral line (DBS) measurements. The values of the Doppler broadening ∆ E were units of keV, so a spectrometer with an energy resolution of 1–2 keV was needed to measure them. An HPGe detector (BE2825, Canberra, Atlanta, GA, USA) with a resolution (FWHM) of 1.9 keV for 1332.5 keV was used for DBS measurements. 22 Na with an activity of about 1 MBq was used as a positron source. The broadening of the annihilation spectral line was characterized by means of the standard parameters S and W, which were defined according to Figure 2. The S parameter was the integral of the central part of the annihilation line (511 keV) normalized to the total area of the line. The S parameter contained information on the size of defects and their concentration [ 44 ]. The W parameter was an integral of the wing-shaped part of the annihilation line at 511 keV and provided information about the chemical environment of the vacancy defects [ 45 ]. The evolution of the S and W positron annihilation parameters could best be monitored by changes in the so-called S-W correlation plots. In these plots, different slopes in the data sets correlated with the vacancy defects at different locations [46]. The phase composition of cast 304 stainless steel after temperature exposure was studied by the X-ray diffraction technique. All measurements were performed on a Bruker Advance D8 X-ray diffractometer (Bruker, Billerica, MA, USA) with a Co K α X-ray source (wavelength 1.79026 Å, voltage 35 kV, current 40 mA). The diffractometer operated with Bregg–Brentano parafocusing geometry and was equipped with a LYNXEYE positionsensitive detector. On the primary beam path, the instrument was equipped with a 0.6 mm divergence slit and 2.5 ◦ axial Soller slits. A 20 µ m Fe K β filter and 2.5 ◦ axial Soller slits were used on the secondary beam path. Metals 2025,15, 748 4 of 15 Figure 2. Definition of parameters S and W extracted from the 511 keV annihilation peak. Values for the windows were from 510.2 to 511.8 keV for region A. For the wings C, they were from 507.7 to 508.9 keV and 512.6 to 514.0 keV. To obtain information about the surface morphology and elemental composition, a scanning electron microscope, VEGA3 LMU (TESCAN, Brno, Czech Republic), was used. The SEM system was equipped with an Everhart–Thornley secondary electron detector (TESCAN, Brno, Czech Republic) and an XFlash 410-M silicon drift detector (Bruker Nano GmbH, Berlin, Germany). A detailed description of the methods used in this research can be found in the Supplementary Materials. 3. Results and Discussion 3.1. Oxide Morphology and Element Distribution in the Surface Layer The surface morphology and oxide scales formed on the cast 304 stainless steel surface under high temperatures are shown in Figures 3and 4. SEM images were obtained at different magnifications. As can be seen in both figures, under the influence of temperature and atmosphere, the surface was covered with a broad oxide layer. Globular oxide particles started to grow at 600 ◦ C; the same growth of globular particles was observed in [ 47 ]. The tendency to increase the oxide layer also increased with higher annealing temperatures. At higher magnification (Figure 4), sheet-shaped crystals were observed at an annealing temperature of 800 ◦ C. Additionally, it can be seen that the surface of the sample at temperatures of 900 ◦ C and 1000 ◦ C was fully covered with globular and octahedral particles. In [ 48 ], the oxidation behavior of additive-made 316L stainless steel after annealing in an air atmosphere was investigated. The formation of octahedral particles was observed under the temperature impact of 900 ◦ C and 1000 ◦ C for both 304 [ 47 ] and 316L [ 48 ] stainless steel grades. In the case of 316L stainless steel, this surface diversity was explained by the formation of oxides containing chromium and manganese. The octahedral particles at the top of the surface were identified as manganese-containing oxides on additive-manufactured 316L stainless steel. In [ 49 ], the authors studied the behavior of 304 austenitic stainless steel under the influence of temperature and atmosphere. The 304 stainless steel was annealed in an air/CH 4 atmosphere at a temperature of 1373 K for 20 min. After annealing and atmospheric impact, as shown in [ 49 ], spherical crystals were formed on the surface of the material. The surface oxide layer was shown to be a layer of iron oxide and a thin layer of chromium oxide. Thus, spherical-shaped crystals belonged to the Fe-Cr mixed oxide. In [ 36 , 49 ], it was mentioned that chromium oxide can evaporate under the influence of temperature (annealing). As a result, the protective layer of chromium oxide is destroyed and the surface of 304 stainless steel is covered with a porous layer of iron oxide. Thus, at an annealing temperature of 600 ◦ C in air, intensive oxidation of the surface began, and at an annealing temperature of Metals 2025,15, 748 5 of 15 600 ◦ C–800 ◦ C, the surface of 304 stainless steel was covered with Fe-Cr oxide particles. As the exposure temperature increased (900–1000 ◦ C), particles of Mn-containing oxide also appeared on the surface. Figure 3. Surface morphology of cast 304 stainless steel annealed at different temperatures in air for 36 h (SEM images at 1000×magnification). Figure 4. Surface morphology of cast 304 stainless steel annealed at different temperatures in air for 36 h (SEM images at 10,000×magnification). Metals 2025,15, 748 6 of 15 According to Table 1, the main alloying elements in the chemical composition of 304 stainless steel were Cr, Ni and Mn. These elements were selected for elemental analysis using the energy-dispersive spectroscopy (EDS) method. Additionally, these alloying elements were present in all stainless steel grades and were also investigated in [ 48 ] additive-manufactured 316L stainless steel. In 304 stainless steel, the effect of temperature caused a redistribution of the main alloying elements in the surface layer [ 50 ]. The EDS technique is a tool for the determination of the concentration of elements at different depths and their redistribution under the influence of temperature and atmosphere. Cast 304 stainless steel specimens were examined at accelerating voltages of 11 to 30 keV, which corresponded to penetration depths of 0.3 to 2.6 µ m according to Castaing’s formula [ 5 ]. The elemental distribution in the surface layer of cast 304 stainless steel after annealing for 36 h in an air atmosphere is shown in Figure 5. As can be seen, at the initial exposure temperature of 600 ◦ C, only at a surface depth of 0.3 µ m was there a slight increase in chromium concentration with a decrease in iron concentration. This phenomenon can be related to the affinity of chromium atoms for oxygen and the ability of stainless steel to form a corrosion-protective chromium oxide layer. At the temperature impact of 700 ◦C and 800 ◦ C in the bulk region (2.6 µ m), iron was still the predominant element in the chemical composition. At a temperature of 800 ◦ C, a significant increase in the content of chromium and manganese with a proportional decrease in iron could also be observed in the thin surface layer. With annealing at 900 ◦ C and 1000 ◦ C, there were noticeable changes in the distribution of the main alloying elements. At this point, cast 304 stainless steel behaved differently than additive-manufactured 316L stainless steel in [ 48 ]. In Figure 5, it can be seen that the concentration of iron content decreased and stabilized at annealing at 900 ◦C and 1000 ◦ C. In addition, the chromium content increased threefold throughout the depths examined in this study compared to the original composition. There was also a stabilization of the concentration at depth and an almost tenfold increase in the concentration of manganese. The surface manganese content was different for cast 304 stainles s steel and additively manufactured 316L stainless steel [ 48 ], where only a twofold increase in manganese concentration was observed after annealing. In addition, nickel was not susceptible to concentration fluctuations under the influence of temperature. The results obtained by the elemental analysis correlated with the results of the SEM. Based on this, it can be assumed that the surface of the cast 304 stainless steel was coated with chromium and manganese oxides under the influence of temperature. 3.2. Surface Phase Transformation Analysis One of the methods used to study the composition of the oxide phases was XRD at room temperature. Furthermore, the application of Mössbauer spectroscopy to the analysis of phases with iron content is shown in the Section 3.3. The XRD patterns of the cast 304 stainless steel samples annealed in air for 36 h in the temperature range of 600–1000 ◦ C are shown in Figure 6. The austenitic stainless steel family has the predominant phase as a face-centered cubic (FCC) structure at room temperature. In addition, Ni is known to be an FCC stabilizing element. Regarding the chemical composition of 304 stainless steel, Ni accounts for 8–10 wt % , which is sufficient to stabilize the FCC phase. Meanwhile, Cr (17.5–19.5 wt %) is known as a body-centered cubic (BCC) stabilizer [ 52 , 53 ]. Austenitic stainless steels undergo phase transformations as a result of exposure to temperature and plastic deformation. In the case of plastic deformation, an austenite–martensitic transformation occurs (γ→α′) [54,55]. Metals 2025,15, 748 7 of 15 Figure 5. Depth dependency of elemental concentration of 304 stainless steel after 36 h of annealing in air. The electron penetration depth was related to the accelerating voltage, given by Castaing’s formula, reprinted from Ref. [51], and ranged from 0.3 µm for HV 11 kV to 2.6 µm for HV 30 kV. As can be seen, the original sample annealed at a temperature of 600 ◦ C contained a dominant austenitic phase ( γ ) and a ferritic phase ( α ). Surface oxidation started at a temperature of 700 ◦ C. According to the XRD data, the austenitic–ferrite transformation (γ→α) took place at a temperature of 800–1000 ◦ C. Austenite–ferritic transformations were observed for all annealing temperatures except 700 ◦ C, where the cast 304 stainless steel retained only the austenitic phase. As the exposure temperature increased, the surface was actively oxidized, and, as can be seen with annealing at 1000 ◦ C, the sample surface consisted mainly of an oxide phase. The oxide phases were identified as (Fe-Cr) 2 O 3 and (Fe-Cr)2MnO4. The formation of chromium oxide on the surface can occur due to the high diffusion of chromium under the influence of temperature [ 56 ]. Cr 2 O 3 and Fe 2 O 3 oxides can form on the surface of stainless steel. On the XRD patterns, these oxides overlap and it is difficult to distinguish them due to similarity in crystallographic parameters and crystal lattice type (trigonal). In addition, a Mn-containing oxide ((Fe-Cr) 2 MnO 4 ) was found in the XRD pattern. The formation of this oxide was correlated with the high diffusion rate of Mn Metals 2025,15, 748 8 of 15 under the temperature influence [ 57 ] and the octahedral particles observed on the surface by the SEM method. Figure 6. The XRD patterns of cast 304 stainless steel annealed in air for 36 h at temperatures of 600–1000 ◦C. 3.3. Application of Mössbauer Spectroscopy for Phase Transformation Analysis 3.3.1. Conversion X-Ray Mössbauer Spectroscopy Conversion X-ray Mössbauer spectroscopy (CXMS) is applicable to the investigation of bulk parts of a sample due to the detection of X-ray escape depth. By the CXMS method, the surface layer of the original and annealed cast 304 stainless steel samples was examined at a depth of 10 µ m. Figure 7shows all the CXMS spectra of the cast 304 stainless steel annealed at temperatures of 600–1000 ◦C for 36 h. The results obtained by fitting the Mössbauer spectra using MOSSWIN 4.0 are shown in Table 2. The model with small quadrupole splitting due to the presence of alloying elements [ 58 ] was used for the fitting process of Fe atoms arranged in a face-centered (FCC) lattice (austenitic phase). As can be seen in Figure 7, the CXMS spectrum of the unannealed sample was the same as for the samples after annealing at 600 ◦ C and 700 ◦ C, and only the austenitic phase ( γ ) was present in the bulk region. The austenitic phase in the Mössbauer spectrum corresponded to a doublet with small quadrupole splitting, and its hyperfine parameters are shown in Table 2. With increasing exposure temperature (800–1000 ◦ C), austenitic–ferritic transformations ( γ→α ) began in the bulk region. As can be seen from the spectra obtained, the resulting singlet spectral line amplitude decreased with increasing annealing temperature and increasing austenitic–ferritic transformation. The ferritic phase ( α ) in Figure 7is shown as a sextet (blue spectrum) and corresponds to the iron atoms arranged in a body-centered (BCC) crystal lattice. The hyperfine parameters of the ferritic phase are also shown in Table 2. According to the quantitative analysis represented by Metals 2025,15, 748 9 of 15 coefficient A in Table 2, during annealing at 900 ◦ C and 1000 ◦ C, a large proportion of the bulk region belonged to the ferritic phase. Figure 7. CXMS spectra of cast 304 stainless steel annealed in air for 36 h. Table 2. Hyperfine parameters for CXMS spectra of cast 304 stainless steel annealed in air for 36 h (IS—isomer shift; QS—quadrupole splitting; W—full width at half maximum; A—spectrum area). Annealing Temperature Phase δ[mm/s] ∆[mm/s] B [T] W [mm/s] A [%] Unannealed austenite −0.08 ±0.01 0.15 ±0.01 - 0.28 ±0.01 100 600 ◦C austenite −0.08 ±0.01 0.14 ±0.01 - 0.27 ±0.01 100 700 ◦C austenite −0.08 ±0.01 0.15 ±0.01 - 0.27 ±0.01 100 800 ◦Caustenite −0.08 ±0.01 0.14 ±0.01 - 0.28 ±0.01 70 ±2 ferrite 0.09 ±0.05 - 27.4 ±0.5 0.91 ±0.14 30 ±2 900 ◦Caustenite −0.09 ±0.01 0.16 ±0.01 - 0.22 ±0.03 9 ±2 ferrite 0.01 ±0.01 - 27.7 * 0.25 ±0.02 91 ±2 1000 ◦Caustenite −0.05 ±0.03 0.17 ±0.01 - 0.19 ±0.03 5 ±2 ferrite -0.01 ±0.01 - 26.8 * 0.22 ±0.02 95 ±2 * Fixed parameter. 3.3.2. Conversion Electron Mössbauer Spectroscopy In comparison to CXMS, conversion electron Mössbauer spectroscopy (CEMS) was based on the diffusion depth in the material of conversion electrons emitted from resonance atoms. The penetration depth was approximately 0.3 µ m and could be calculated using an equation [59]. The CEMS spectra for all annealed 304 stainless steel samples and the fitting results from the MOSSWINN 4.0 software are shown in Figure 8and Table 3, respectively. As