Thermodynamic Prediction and Experimental Verification of Multiphase Composition of Scale Formed on Reheated Alloy Steels

A variety of industrially produced steels were used in this study (Table I). The steels studied included carbon steels (Class 1) with varying alloying elements (Si, Mn, Cr, and Ni), advanced high strength steels (Class 2) with high alloying elements (Si and Mn), ferritic stainless steels (Class 3) with varying Cr, Al and Ti contents, and high Ni and Cr austenitic stainless steel (Class 4). These cast steels were supplied by industrial sponsors. Samples used for the oxidation test were machined to 50×20×3.5 mm and finished using wet machine grinding with 60 grit silicon carbide, the surface quality of the ground sample was measured to have Ra of 0.271 µm using 3D optical profiler (Nanovea, Model PS50 Micro Photonic Inc.)

Oxidation testing was carried out using a laboratory thermo-gravimetric setup in a vertical MoSi₂ resistance-heating tube furnace (Model D900438 ATS Inc.). The oxidation tests were conducted in a combustion gas atmosphere by mixing gases in proportions that reproduced the reacted natural gas + air combustion atmosphere used in an industrial furnace (Table II). Thermal conditions (temperature and time) and combustion atmosphere were designed to mimic a typical industrial reheating used these grades before hot rolling.

Samples after oxidation were mounted in epoxy to protect scale layer, carefully cross-sectioned using wet cutting, and mechanically polished using diamond paste. The morphology, thickness, microstructure, and chemistry of the oxide layers were analyzed using a scanning electron microscopy (SEM) with an ASPEX-PIXA 1020 system equipped energy-dispersive spectroscopy (EDS). EDS analysis was performed using several points in each scale layer. HORIBA Jobin-Yvon Raman spectroscopy (LabRam ARAMIS) was also used to identify the oxide phases that were present and compared to reported phases in RRUFF database.[39]

FactSage 7.2 thermodynamic software[38] was used to simulate steel oxidation under the local equilibrium conditions. The databases used included FSsteel, FToxid, and FasctPS. The local equilibrium condition was examined by making stepwise oxygen additions into the simulated streams from previous step assuming irreversible reactions. The predicted stable oxide phases were identified at each calculation step. This method allows one to predict different phases that form at low (inner layer) and high (outer layer) added oxygen levels. The thermodynamic simulations were performed assuming that the local equilibrium conditions were reached between gas component, the formed oxide phases, and the steel. Preliminary simulations showed that for the studied thermodynamic conditions, oxygen was the predominant oxidant in the combustion gas. Using different oxygen/steel mass ratio, different oxide phases which form under the local equilibrium condition during oxidation were traced from low to high added oxygen levels and compared to the oxide phases reported from the experiments. The equilibrium oxygen partial pressure after formation of the various oxides phases was also reported for each oxygen/steel ratio. The temperatures used in the thermodynamic simulations were similar to those used in laboratory experimental oxidation.

Figure 1 shows the cross-section microstructure of the scale that formed after 60 minutes at 1230 °C in a laboratory-simulated combustion gas atmosphere (Table II) for two carbon steels having different alloying elements (Table I). Average scale thickness on these steels was between 500 µm and 700 µm for steel 1a and 1b, respectively. The scale structure was stratified into 4 different layers (1—outer, 2—intermediate, 3—inner, and 4—subsurface). Each layer was completely (Figure 1(a)) or partially (Figure 1(b)) separated from the other. Both the outer and intermediate layers had dense structures with mainly iron oxide as the predominant oxide phase. The inner scale layer had voids and porosity and also included entrapped droplets of unoxidized metal. This layer was formed next to scale/metal boundary and had isolated islands composed of different oxides of Mn and Si (Table III). In steel 1b, which had higher Si and Mn when compared to low alloyed steel 1a, Si and Mn oxide, droplets of entrapped metal (Fe-Ni-Mn) and porosity in the scale structure increased toward the subsurface layer which had an uneven boundary at the scale/metal interface. A randomly scattered dotted oxide structure was formed in subsurface area below scale/metal interface in the steel 1b. In this case, the subsurface oxidized layer in the metal substrate had a depth ranging between 15 and 50 µm.

Table III reports the composition of the oxide scale layers formed in the two studied carbon steels. In both steels, precipitates of alloying element oxides were observed to be more frequent in the inner layer and subsurface layer compared to the outer and intermediate layers. The compositions of the oxides were obtained using EDX point analysis to report chemistry differences in the various scale layers, and the oxide phases were identified by Raman spectroscopy analysis. Fayalite (Fe₂SiO_4-) and tephroite (Mn₂SiO₄) were the predominant phases in the inner and subsurface scale layers. The alloying elements in the steels affected the oxide phases formed in subsurface layers, with the higher alloy steel (1b) exhibiting an increased presence of Si-containing fayalite and Mn and Si-containing tephroite phases.

Thermodynamic simulations of the local equilibrium at different positions through the multiphase oxide layers were performed by varying the amount of oxygen available to react using different oxygen/steel mass ratios, as shown in (Figure 2). Approximately, four different phases were predicted at low oxygen/steel ratios that mimicked the local equilibrium conditions for oxide phase formation in the subsurface and inner scale layer. The oxide phases predicted at this low ratio showed strong presence of oxides of alloying elements (Mn and Si) as well as impurities (Al). Complexity of the oxide phases decreased toward the external (intermediate and outer) scale layers having higher oxygen/steel ratio with the predicted oxide phase being predominantly hematite (Fe₂O₃) with some traces of tephroite (Mn₂SiO₄). Thermodynamics predicted the formation of different oxides in the inner scale layers (subsurface and inner) but similar oxides in the external layers (intermediate and outer) for the two alloy steels. In the steel 1b with an elevated concentration of alloying elements, simulations predicted formation of complex Mn and Si oxides in the form of MnSiO₃ and Mn₃Al₂Si₃O₁₂ in the inner scale layer, while mono-oxides MnO and Cr₂O₃ and also spinel-MnAl₂O₄ were predicted in the lower alloy steel 1a.

In the case of AHSS, the cross-section of the scale layer in Figure 3 revealed a dense continuous scale structure with partial detachment of outer and intermediate layers from inner and sublayer in steel 2a and complete detachment in higher alloyed by Si and Mn steel 2b. The average scale thickness on the AHSS after 60 minutes at 1230 °C in the combustion atmosphere noted in Table II was between 870 and 950 µm with a thicker scale layer observed on high alloy steel 2b. The internal porosity observed in the scale layers formed on the carbon steels was not observed in the scale structure formed in AHSS and penetrating “root-like” dark oxide phases originating from the intermediate and outer layers into the inner and subsurface layers of the sample were observed. The round shape of subsurface oxides indicated possible liquefaction. The results from composition analysis and Raman spectroscopy on the various phases in scale layers are reported in Table II. The dark continuous oxide phases present in the inner and subsurface layers were identified as fayalite (Fe₂SiO₄) and silica (SiO₂). The inner and subsurface layers contained thick continuous dark Fe-Si oxide phases of about 100 µm with embedded metal (Fe-Ni-Mn) entrapment and scatted precipitated gray Fe-Mn oxide phases were observed by adjusting contrast in SEM/BSE analysis. The subsurface structure contained a superimposed oxide scale in the metal substrate with strong adhesion to inner layer and minimal porosity, cracks, and voids. The oxide phases identified by Raman spectroscopy were similar in the two steels (2a and 2b) with the major difference being the presence of SiO₂ oxide in the inner layer of the high Si and Mn steel 2b, which was not observed in the medium Si and Mn steel 2a (Table IV). Both steels contained phases of MnO and Fe₂SiO₄.

Figure 4 reports the thermodynamically predicted solid oxides and liquid oxide solutions (slags) at varying oxygen/steel mass ratio for the AHSS samples under the local equilibrium conditions at reheating temperature. The outer oxide phase in both AHSS grades was predicted to be pure iron oxide (about 95 pct) with traces of solid Mn-Si oxide-based phases. The phases predicted in the subsurface and the inner layer contained a mixture of solid oxides of alloying elements (Si and Mn) and liquid oxide phases. A greater amount of silicon-based solid oxide phases and liquid oxide solutions in the subsurface through to the intermediate layer were predicted in steel 2b which had higher Si and Mn concentrations in the steel. These liquid solutions had average compositions: Slag 1 (51 wt pct SiO₂, 34 wt pct MnO, 15 wt pct Al₂O₃), Slag 2 (69 wt pct FeO, 15 wt pct SiO₂, 11 wt pct MnO, 5 wt pct Fe₂O₃), and Slag 3 (33 wt pct Fe₂O₃, 32 wt pct SiO₂, 18 wt pct MnO, 13 wt pct Mn₂O₃, 4 wt pct FeO).

Three stainless steels with low Al (steel 3a), high Al (steel 3b), and Ti-bearing (steel 3b) were studied. All these steels were highly alloyed with Cr in addition to Mn. Cross-section analysis of the oxidized steels revealed a complex scale structure that could best be classified into four layers:1—outer, 2—intermediate, 3—inner, and 4—subsurface (Figure 5). The average total scale thickness for these 3 stainless steels oxidized under the conditions noted in Table II was measured to be 1000, 900, and 600 µm for the steels 3a, 3b, and 3c, respectively. The wide variation in scale thickness was due to the different steel chemistry and the different applied industrial oxidation conditions (Tables I and II). In all cases, the outer, intermediate, and inner layers represented nearly 95 to 96 pct of the total scale thickness and the subsurface layer made up of the remaining 4 to 5 pct. The typical structure of the outer scale layer on the ferritic stainless steels consisted of dense continuous layers containing iron oxide without any traces of alloying elements. However, the intermediate layer structure had porosity and voids coupled with microscopic and macroscopic transverse and lateral cracks. The chemistry and oxide phases in the intermediate layer were predominantly hematite (Fe₂O₃) and chromia (Cr₂O₃), while the inner layer contained a mixture of chromite (FeCr₂O₄), hematite (Fe₂O₃), chromia (Cr₂O₃), and fayalite (Fe₂SiO₄) phases with impurities of Ni and Cu (Table V). The subsurface scale layer was strongly attached to the metal substrate with heavily superimposed oxides in the metal matrix. In general, two Cr-based phases were observed in the subsurface region, high Cr with traces of Si, and low Cr with high Si content (Table V). The stainless steel with Al (steel 3b) developed a subsurface scale layer with a significant presence of Al oxides. Titanium oxide was not detected in analysis of the scale chemistry in the Ti-bearing steel 3c.

Thermodynamic simulations (Figure 6) of the oxidation sequence in these steels predicted the formation of specific oxides at the scale/metal boundary at low oxygen/metal ratios. In the case of the low Al steel 3a, the predicted subsurface oxides phases where predominantly Cr and Si-based oxides with trace of Al-Mn-Si oxide phase. Aluminum oxide (Al₂O₃) and other Al-based complex Al-Mn-Si oxide were formed in the subsurface layer of the high Al steel 3b. In the case of steel grade 3c having traces of Ti, thermodynamics predicted a complex multicomponent subsurface scale formation, having evidence of Ti-based oxides mixed with Al and Si-based oxide phases which extended to the inner scale layer. A mixture of Cr- and Fe-based oxide phases identified in the experimental investigation was predicted by thermodynamic for these steels in the intermediate to outer scale layer with traces of a Si-based phase mixture as described in Table V.

Scale structures formed on the studied austenitic stainless steel that was highly alloyed with Cr, Ni, and Mn are shown in Figure 7. This scale at the outer and intermediate layers was completely separated from the inner and subsurface layers and high levels of porosity were observed in the intermediate scale layer. The inner-subsurface layer had a thickness of about 270 µm with a dense oxide structure superimposed in the metal matrix. The total scale thickness in this steel averaged about 900 µm. The composition and phases measured in these two layers consisted mainly of Fe- and Mn-based oxides (Table VI). The inner and subsurface layers where fused together and strongly attached to the base metal substrate, having a complex oxide chemistry comprising of different ratios of Fe, Mn, Si, and Cr (Table VI).

Thermodynamic simulation of the oxidation of this stainless steel predicted the formation of several layers with different oxide phases (Figure 8). A monoxide phase (Mn2SiO₄) was predicted in the subsurface and through to the inner oxide layer. Multiphases having Cr, Mn, Fe, and Ni-based oxides were predicted in the intermediate and outer scale layer.