THE EFFECT OF HEAT TREATMENT ON THE STRUCTURE OF SCALE LAYER AND ITS REMOVING BY PICKLING IN REDUCING MELT

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1 THE EFFECT OF HEAT TREATMENT ON THE STRUCTURE OF SCALE LAYER AND ITS REMOVING BY PICKLING IN REDUCING MELT Petra VÁŇOVÁ 1, Roman PĚNČÍK 2, David ČEMPEL 1, Kateřina KONEČNÁ 1 1 VŠB Technical University of Ostrava, Faculty of Metallurgy and Materials Engineering, 17. listopadu 15, CZ Ostrava, Czech Republic, 2 Bochemie a.s., Lidická 326, Bohumín, petra.vanova@vsb.cz Abstract This paper deals with the structure and composition of scale layer of stainless steel AISI 316 and AISI 430 after heat treatment at temperatures of 800, 1000 and 1200 C with holding time at temperature for 1 or 2 hours and after air and water cooling. Subsequently, pickling in reducing melt was carried out. The structure of scale layers after heat treatment and after the following pickling in reducing melt, and its chemical composition using X-ray microanalysis is described, the thickness of scale layer is measured and the weight decrease after pickling in reducing melt is documented. After heat treatment, thicker and more compact oxide layers were formed on the surface of the wire made of AISI 430 stainless steel than on the surface of that made of AISI 316 steel. Oxide layers showed greater thickness after cooling in air than after cooling in water. In the case of stainless steel AISI 316, the oxide layers contained relatively high amount of nickel, which is rather difficult to eliminate by the action of reducing melt. As for AISI 430 steel, the layers are formed on the iron oxide and chromium oxide basis. These layers are more easily distort when exposed to reducing melt and they tend to crack and spall. Keywords scale layer, oxide layer, heat treatment, steel pickling, reducing melt, X-ray microanalysis 1. INTRODUCTION Scale layer is a thin oxide layer on the material surface, formed mainly during higher temperatures of heat treatment. Formation of scales on the stainless steel surface depends on its composition (i.e. alloying elements content) and also on the surface condition and the type of atmosphere, in which the steel is heattreated. The scales are formed by chemical composition of iron or other elements contained in steel with oxygen from air or from the atmosphere of annealing oven. The estimation is that during steel annealing one to five percent of material is changed into scale. The scale layer formed during steel heating at the temperatures needed for its heat treatment is never chemically homogenous. It is made by two or three stages with different composition and quality. The stages are separated from each other by quite acute boundary. They are arranged as follows: the closer to the metal, the poorer in oxygen and the richer in iron. This composition corresponds to the diffusion of iron and oxygen in the scale layer [1-3]. Iron and oxygen form together three stable oxides: iron (II) oxide FeO, magnetite Fe 3 O 4 and iron (III) oxide Fe 2 O 3. Iron (II) oxide FeO is not invariable under temperatures lower than 575 C, because it slowly changes into iron and magnetite Fe 3 O 4. Therefore, the oxide layers formed under lower temperatures than mentioned limit of 575 C do not contain FeO at all. It is important for pickling that iron (II) oxide FeO is easier to dissolve in acids than other iron oxides. Thus the scale layers containing a great amount of iron (II) oxide are in general easier to remove, while layers without this component are more difficult to remove, although they are often much thinner. Magnetite Fe 3 O 4 is the most invariable of the iron oxides and it does not change even in a large range of temperatures. It is significantly much more difficult to dissolve in acids than iron (II) oxide. Iron (III) oxide Fe 2 O 3 is the iron oxide the richest in oxygen. It loses part of oxygen and turns into magnetite during heating over 1200 C. Therefore, the scale layers formed under the temperatures higher than 1200 C do not contain iron (III) oxide. Among the three oxides, this one is the most difficult to dissolve in acids [1-3].

2 Other elements, contained in steel, mainly the so called alloying elements do take part in scales forming processes. The easiest proportion we find in elements with similar chemical properties as iron. Manganese is the typical example. The rapidity of its infiltration into scale layer is approximately the same as in the case of iron. The rapidity of its oxidation is also the same as in the case of iron and the manganese oxides are spread out in scale layer uniformly and in the amount corresponding to the percentage of manganese in relevant material. As for other alloying elements, the diffusion is slower than in the case of iron, which leads to settling of these elements in the lowest layer of the coating. If these metals are nobler (nickel, copper), iron takes away the oxygen from its oxides and oxide layer therefore contains these elements in the form of pure metals. Other elements (chromium, silicon, manganese) are in the oxide layer in the form of oxides. If the steel is highly alloyed with e.g. chromium, the lowest layer of its oxide layer is so enriched with this metal, so that the chromium oxide forms a significant component on the steel-scale boundary. Another iron atoms are not able to penetrate through this boundary, which gives reasons for heat resistance of highly alloyed steel [1-5]. Steel pickling is a part of final production process of some steel products. During this the scales, formed during metal heat treatment under high temperatures, are removed from the surface of steel belts, wires and other forms of steel by dissolving in acids or melts. Chemical removing of scales is based mainly on their dissolving in appropriate solution, sometimes with the use of electric current. In the case of carbon steel the hydrochloric acid (HCl) or the sulfuric acid (H 2 SO 4 ) is commonly used, while the combination of hydrofluoric acid and nitric acid (HF+HNO 3 ) is often used for stainless steel. Acids dissolve scales chemically and at the same time the iron oxides turn into corresponding iron salts. For pickling in the reducing melt the melt of sodium hydroxide (NaOH), containing sodium hydride (NaH) as reducing agent, is used. Sodium hydride is a highly reducing agent, changing iron oxides into lower oxides and iron. A great advantage of pickling in sodium hydride is that it does not damage the basic metal. Oxidation melts are efficient in action of oxidation agent from dissolved sodium hydroxide (NaOH) on the oxide layer. Above-valent oxides with larger specific capacity are formed during oxidation, which provokes changes of structure of scales on the surface of the material, tension and following cracking. Only a small part of scales is separated or dissolved in the melt, where it settles like sediment. Greater part of scales is removed only during following operation cooling and rinsing in water while the scales are partially torn away. The rest of scales is removed by final pickling in acids [6-8]. 2. EXPERIMENT DESCRIPTION Two types of steel with different chemical composition and structures, i.e. AISI 316 and AISI 430, hot drawn wires, have been used for the experimental finding of the effect of heat treatment on the formation of scale layer on stainless steels. AISI 316 is austenitic chromium-nickel-molybdenum steel, AISI 430 is ferritic stainless steel with 17% of chromium content. Chemical composition of both steels is given in Table 1. Individual samples in form of 100 mm long and 7 mm diameter wires (sample surface was 0,0022 m 2 ) were pickled in reducing melt and afterwards in mixed acid in order to make their surface clean and ready for following laboratory heat treatment. Tab. 1 Chemical composition of AISI 316 and AISI 430 steels in weight % AISI C Si Mn P S Cr Mo Ni Samples warmed to 800 C and 1000 C temperatures were left in the oven during two hours, samples warmed to 1200 C temperature during one hour. The holding time was followed by air or water cooling.

3 Afterwards transverse metallographic cuts were prepared in order to find the character and structure of scale layer after different heat treatments. As for the second set of samples, the scales were removed by pickling in hydride reducing melt, to which the reducing component (NaH) was dosed using a special product Feropur. Samples with scale layers after the heat treatment were put into reducing melt for 15 minutes and afterwards cooled by a thermal shock due to water immersion. When removed from pickling bath, the samples were rid of alkalinity by washing in hot water and drying. The samples were weighed before and after pickling and their weight loss was documented. The samples without scale layers were also metallographically prepared in transverse cuts and they were observed using scanning electron microscope. The scale layers were observed using JEOL JSM-6490LV scanning electron microscope in backscatter electrons (BE). Semi quantitative x-ray microanalysis was carried out as surface analysis, using Inca X-act detector. Evaluation of scale layer in transverse cut using scanning electron microscopy (SEM) included documentation of the scale layer in different parts of the wire and semi quantitative x-ray microanalysis. As for the carrying out of microanalysis, different stages of oxide layer and basic material to the depth of 20 to 40 m under the surface were evaluated. Carbon was not included in the semi quantitative x-ray microanalysis. 3. RESULTS 3.1 Evaluation of wires surface after the heat treatment During heat treatment at 800 C temperature with holding time for 2 hours a layer of iron oxide (57 % Fe, 1 % Cr, 1-3 % Ni, 1-2 % Mn, 38 % O) was formed on the AISI 316 steel wire surface. After air cooling the layer was cracked (Fig. 1), while after water cooling it was more compact (Fig. 2). Chromium, iron (nickel, molybdenum, silicon) oxides (27-35 % Cr, % Fe, 5-7 % Ni, % Mo, % O) were separated under the layer. The material right under the oxide layer was not chromium depleted. A thin layer of chromium and iron oxides (44 % Cr, 17 % Fe, 7 % Mn, 32 % O) appeared on the surface of AISI 430 steel wires after air cooling (Fig. 3), while after water cooling (Fig. 4) it was a thin layer of iron and chromium oxides (52 % Fe, 20 % Cr, 7 % Mn, 20 % O). Right under the surface the matrix was chromium depleted (14.5 % Cr). Fig. 1 Oxide layer AISI 316 steel Heat treatment: 800 C/2 hours/air Fig. 2 Oxide layer AISI 316 steel Heat treatment: 800 C/2 hours/water

4 Fig. 3 Oxide layer AISI 430 steel Heat treatment: 800 C/2 hours/air Fig. 4 Oxide layer AISI 430 steel Heat treatment: 800 C/2 hours/water During heat treatment at 1000 C temperature with holding time for 2 hours and air cooling a broader layer was formed on the AISI 316 steel wire surface. On the very surface it was formed by rough crystals of iron oxide (55 % Fe, 1.5 % Cr, 0,5 % Ni, 43 % O). Under this layer more porous sublayer of iron and chromium oxides (30% Cr, 28 % Fe, 36 % O, 4 % Ni, Mn, Si) was found. This layer did not contain molybdenum. Right under the layer the chromium depletion (13 % Cr) has taken place, but on the other hand the nickel and molybdenum content (17 % Ni, 5 % Mo) increased. In the 20 m depth under the layer the matrix was still chromium depleted (15.8 % Cr). After water cooling the layer seemed to be more compact. On the surface there was a thin sublayer of tiny iron oxides crystals (52 % Fe, 4 % Mn, 5 % Ni, 1 % Cr). Rough crystals in the layer were formed by iron oxides (63 % Fe, 1 % Cr, 36 %). The composition of the porous layer corresponded to chromium and iron oxides (34 % Cr, 20 % Fe, 5.5 % Ni, 1,4 % Mn, 39 % O, Si). Molybdenum was not separated in the layer. On the scale layer basic material boundary chromium oxides without molybdenum presence were found (52 % Cr, 37 % Fe, 4 % Mn, 51 % Ni, 4 % O). The chromium depletion of basic material under the layer was minimal. In the case of AISI 430 steel wire the oxide layer after air cooling was formed by a consistent iron and chromium oxides layer (57 % Fe, 4 % Cr, 39 % O). After water cooling the oxide layer was more disrupted and the closer to the matrix, the higher the chromium content in the scale layer. Right under the layer the matrix was chromium depleted (9-11 % Cr). During heat treatment at 1200 C temperature with holding time for 1 hour and air cooling was the layer on the AISI 316 steel wire surface horizontally cracked (Fig. 5). Chromium, iron and molybdenum oxides (35 % Cr, 23 % Fe, 3 % Mo 1 % Ni, 1 % Mn, 38 % O) formed the scale layer. These oxides were interspersed with discontinuous structures on the pure nickel and iron base (60 % Ni, 35 % Fe, 3 % Cr, 2 %). Under the consistent layer tiny oxides interfering into the matrix were found. The matrix was chromium depleted (11,5 % Cr). After the water cooling the scale layer was more consistent and less porous (Fig. 6). On the surface it consisted of iron and nickel oxides (55 % Fe, 7 % Ni, 1.5 % Cr, 3 % Mn, 35 % O). There were more compact chromium, iron and nickel oxides structures (25 % Cr, 25 % Fe, 7 % Ni, 35 % O, without Mo) in the layer content. Chromium, iron and nickel oxides with lower oxygen content (27 % Cr, 28 % Fe, 17 % Ni, 28 % O) formed the porous parts of the layer. White structures represented reduced metal Ni, Fe, Mo (53 % Ni, 17 % Fe. 15 % Mo the O and Cr content was influenced by environs). Right under the layer the basic material was chromium depleted (9.5 % Cr). In the case of AISI 430 steel wire after air cooling a consistent thick scale layer was formed (Fig. 7). At the very surface it consisted of iron and chromium oxides (35 % Fe, 27 % Cr, 1 % Mn, 36 % O). The deeper, the more the Cr content in the layer increased, the more the Fe content decreased and the more the Mo content increased. The oxygen content decreased at 27 % and molybdenum (1.2 % Mo) was analyzed in the interlayer. Right under the layer the matrix was Cr depleted (11 % Cr). In a more detailed scale Cr and Fe oxides (33 % Cr, 23 % Fe, 1 % Mo, 40 % O) and white

5 structures of almost pure iron (94 % Fe, 3 % Cr, 1% Mo, 1% Ni) formed the interlayer. Air cooling led to spalling of the scale layer. The rests of the spalled layer were analyzed separately. In the scales the layer was bright, crystalline based on iron (66 % Fe, 33 % O), smooth formed of Fe oxides (63 % Fe, 36 % O), which on the reverse contained except Fe oxides also Cr (61 % Fe, 6.5 % Cr, 1 % Mn, 30 %). After water cooling the layer of Cr, Fe and Mo was evident on the surface (Fig. 8). Interlayer composition and basic material under the layer depletion was similar as in the case of air cooling. Fig. 5 Oxide layer AISI 316 steel Heat treatment: 1200 C/1 hour/air Fig. 6 Oxide layer AISI 316 steel Heat treatment: 1200 C/1 hour/water Fig. 7 Oxide layer AISI 430 steel Heat treatment: 1200 C/1 hour/air Fig. 8 Oxide layer AISI 430 steel Heat treatment: 1200 C/1 hour/water 3.2 Evaluation of wires surfaces after pickling in reducing melt Evaluation of scale layer after pickling in reducing melt was carried out by the same method as after heat treatment, i.e. by using scanning electron microscopy (SEM) on the transverse cuts. Only the air cooled samples were evaluated. The oxides were present in surface defects in the case of AISI 316 steel samples, heat treated in 800 C/2 hours/air mode and subsequently pickled in reducing melt (Fig. 9). Calcium and sodium as remains of the pickling process and molybdenum were analyzed in the oxides. The surface without oxide layer was not Cr depleted. A thin layer of Cr and Fe oxides (27-36 % Cr, % Fe, 2-3 % Ni, 35 % O, without Mo) appeared in the case of samples heat treated in 1000 C/2 hours/air mode and subsequently pickled in reducing melt. Right under the layer the Cr depletion (13.5 % Cr) was still apparent. In the case of samples

6 heat treated in 1200 C/1 hour/air mode and subsequently pickled in reducing melt (Fig. 10), the wire surface showed more significant oxide remains, somewhere based on Fe and Ni (52 % Fe, 11 % Ni, 4 % Cr, 23 % O), somewhere based on Fe and Cr (42 % Fe, 20 % Cr, 38 % O), or the original interlayer with presence of pure nickel and iron. The iron, chromium and manganese oxides (29 % Fe, 25 % Cr, 18 % Mn, 26 % O, without Mo) were present in surface inequalities in the case of AISI 430 steel samples, heat treated in 800 C/2 hours/air mode and subsequently pickled in reducing melt (Fig. 11). Calcium and sodium as remains of the pickling process were analyzed in the oxides. The surface without oxide layer was slightly Cr depleted (14 % Cr). A thin layer of Cr and Fe oxides (46 % Cr, 19 % Fe, 30 % O) appeared in the case of samples heat treated in 1000 C/2 hours/air mode and subsequently pickled in reducing melt. In some parts of the surface thin layers of pure metal (66 % Fe, 18 % Cr, 11 % Ni) were apparent. Right under the layer the Cr depletion (13.8 % Cr) was still apparent. In the case of samples heat treated in 1200 C/1 hour/air mode and subsequently pickled in reducing melt (Fig. 12), the wire surface showed a broader layer which morphology corresponded to the interlayer of annealed samples. There were Cr and Fe oxides (37 % Cr, 26 % Fe, 38 % O) on the surface. Fig. 9 AISI 316 steel sample after pickling Original heat treatment: 800 C/2 hours/air Fig. 10 AISI 430 steel sample after pickling Original heat treatment: 800 C/2 hours/air Fig. 11 AISI 316 steel sample after pickling Original heat treatment: 1200 C/1 hour/air Fig. 12 AISI 430 steel sample after pickling Original heat treatment: 1200 C/1 hour/air

7 3.3 Scale layer thickness and weight loss As apparent from given figures (Fig. 1 Fig. 8), the scale layer was thicker with higher temperatures of the heat treatment. After samples air cooling the oxide layer grew thicker because of additional oxidation of the surface, which often led to spalling of scales. From Fig. 13 it is evident that scale layer thickness on AISI 430 steel is bigger than in the case of AISI 316 steel despite the loss of a part of layer during air cooling. The weight loss was determined by samples weighing before and after the pickling. The higher the temperature of heat treatment was, the bigger was the weight loss of given material after the pickling in reducing melt. After previous heat treatment at the same temperature the weight loss was lower in the case of air cooling than in the case of water cooling. There were certain divergences in the case of samples treated at the 1200 C temperature when during air cooling from the 1200 C temperature massive cracking and spalling of scales from the material surface appeared, mainly in the case of AISI 430 steel. That led to bigger weight loss. It is evident from Tables (Fig. 14) that there was bigger weight loss in the case of AISI 430 steel. Fig. 13 Scale layer thickness Fig. 14 Weight loss after pickling 4. CONCLUSION During the heat treatment of given stainless steel wires a oxide layer was formed on their surface. The higher temperature was used, the thicker the scale layer was. The very surface of the scale layer contained iron oxides, but approaching the matrix the chromium (Cr) fixed in oxides content was increasing. In high annealing temperatures (1200 C) the molybdenum (Mo) was fixed in oxides too. In this temperature an interlayer with pure metal particles based on nickel (Ni) and iron (Fe) in the case of AISI 316 steel or pure iron only in the case of AISI 430 steel was formed between the matrix and the oxide layer. The layer seemed to be more cracked after air cooling, it often spalled and was therefore thinner in the final state. On the contrary, after water cooling the oxide layers stayed thicker and more compact. Chromium (Cr) depletion right under the scale layer was more significant by AISI 430 steel. The oxide layer decreased after pickling in reducing melt depending on its thickness in the after heat treatment state. Thin layers formed under low temperature annealing were removed by pickling and therefore the depletion on the material surface was the least possible. The remains of oxides stuck only in deeper surface defects. After the pickling of layers formed under higher temperature annealing, in some parts of the surface areas of reduced oxides or small layers of pure metals were observed. In the interlayer area a higher amount of pure metal particles was evident. In the case of AISI 316 steel, the oxide layers contained relatively high amount of nickel (Ni), which is quite difficult to eliminate by the sodium hydride action. As for the AISI 430 steel, the layer is more complex, but formed mainly by iron and chromium oxides. Such a layer is more easily distort by hydride action and it cracks and spalls. The rest of scales is removed during final pickling in acids.

8 ACKNOWLEDGEMENT The authors are grateful to the Ministry of Education of the Czech Republic for the financial support of the project No. CZ.1.05/2.1.00/ Regional Materials Science and Technology Centre research activity New sources of strength and toughness of materials for high technological applications within the frame of the operation program Research and Development for Innovations financed by the Structural Funds and from the state budget of the Czech Republic. LITERATURE [1] RONEŠ, J.; JAROŠ, M.. Moření oceli a litiny. 1. vyd. Praha: SNTL- Nakladatelství technické literatury, s. [2] ŠTURC, J. a kol. Moření ocelí. 1. vyd. Praha: SNTL Nakladatelství technické literatury, s. [3] RITUPER, R. Beizen von Metallen, 1. vyd. Weingarten; Eugen G. Leuze Verlag, s. ISBN-10: [4] YANG, C. W.; KIM, J. H.; TRIAMBULO, R., E.; et al.. The mechanical property of the oxide scale on Fe-Cr alloy steels. Journal of alloys and compounds, 2013, vol. 549, p ISSN: [5] Matsuhashi, T; Okada, H; Kiya, S: Effects of Si, Mn contents on the descalability behavior of the scale of annealed austenitic stainless steels. Tetsu to hagane-journal of the iron and steel institute of japan, 2004, vol. 90, Is. 7, p ISSN: [6] ULLRYCH, J.; TRŽIL, J.. Mechanismus odokujování ocelí v taveninové redukční lázni. In Moření ocelí v redukčních taveninách Frýdek-Místek: Ekomor; Bochemie; ZP HS Válcoven plechu, s. [7] LINDELL, D.; PETTERSSON, R.. Pickling of Process-Oxidised Austenitic Stainless Steels in HNO 3-HF Mixed Acid. Steel research international, 2010, vol.: 81, Is. 7. p ISSN: [8] LI, L. F.; CELIS, J. P.. Pickling of austenitic stainless steels (a review). Canadian metallurgical quarterly, 2003, vol. 42, Is. 3, p ISSN: