The results of the influence of the cooling rate of the casting made of the acid-resistant ferritic - austenitic cast steel on the microstructure and corrosion resistance are presented in the paper. Samples cut out from the walls of the casting being cooled at the cooling rate of 3,2 - 0,5 ºC/s were used in the study. Different cooling rates create favorable conditions for the segregation processes lowering properties of castings. It was found, that differences in the polarization curves occur only in the more aggressive corrosive environment. The reason of such behaviour of cast steel is the segregation of elements dissolved in austenite and the difference in the volume fraction of ferrite and austenite in the walls of the different thickness

B. Kalandyk

M. Kawalec

M. Starowicz

R. Zapała

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75METALURGIJA 52 (2013) 1, 75-78B. KALANDYK, M. STAROWICZ, M. KAWALEC, R. ZAPAŁAINFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELReceived – Prispjelo: 2012-03-15Accepted – Prihvaćeno: 2012-08-10Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 52(1) 75-78 (2013)UDC – UDK 620.193:621.747:620.18=111B. Kalandyk, M. Starowicz, M. Kawalec, R. Zapała AGH University of Science and Technology, Krakow, PolandThe results of the infl uence of the cooling rate of the casting made of the acid-resistant ferritic - austenitic cast steel on the microstructure and corrosion resistance are presented in the paper. Samples cut out from the walls of the casting being cooled at the cooling rate of 3,2 - 0,5 ºC/s were used in the study. Diff erent cooling rates create favo-rable conditions for the segregation processes lowering properties of castings. It was found, that diff erences in the polarization curves occur only in the more aggressive corrosive environment. The reason of such behaviour of cast steel is the segregation of elements dissolved in austenite and the diff erence in the volume fraction of ferrite and austenite in the walls of the diff erent thickness.Key words: electrochemical corrosion, casting, stainless steel, microstructuresINTRODUCTIONThe characteristic feature of castings is their diversi-ﬁ ed thickness and complexity of shapes. In the case of castings made of acid resistant Cr-Ni cast steel, various cooling rates lead to the segregation of alloy components and differences in the precipitation processes rates within the temperature range: 300 - 1 000 ºC [1,2]. As the result of these processes phases of different structure and chem-ical composition (e.g. carbides, nitrides, σ, α’) can nucle-ate in the casting walls [3-5]. Their occurrence in the mi-crostructure worsens the corrosive resistance and passi-vation ability of castings made of Cr-Ni cast steel, espe-cially in the environment containing Cl¯ ions [6]. Per-forming the proper heat treatment of castings eliminates, in the majority of cases, a precipitation of those phases, however in the case of cast steel containing ferrite and austenite in its structure the volume fraction of those phases can vary in dependence of the hyper quenching temperature and casting wall thickness [7-9]. Operating conditions of castings require that the basic properties be retained in all thickness of its walls. This means that in thin-wall castings (e.g. rotors, steering gears), as well as in castings of a wall thickness of 50 - 70 mm (valve footings and housings, large industrial pumps), material structure should not reveal essential differences. Otherwise castings will not satisfy the requirements both at the surface and inside the casting wall.Therefore an attempt to present the inﬂ uence of cooling rates of the F-A cast steel castings on their cor-rosion resistance – in two different corrosive environ-ments was undertaken in this paper.MATERIALS AND METHODSThe inﬂ uence of the cooling rate of the F-A cast steel casting containing 0,06 % C, 24,0 % Cr, 5,2 % Ni, 2,5 % Mo, 2,7 % Cu and 0,2 % Nb on its microstructure and corrosive resistance was investigated. The casting of different wall thickness: 12, 24 and 45 mm was made under laboratory conditions. After pouring the mould with liquid metal the temperature change was recorded by means of thermoelements Pt-PtRh10, and on this basis the cooling rate of each wall was determined. Samples for metallographic and cor-rosive tests were cut off from the walls of a thickness 12 and 45 mm, cooling with rates of 3,2 - 3,4 ºC/s and 0,4-0,5 ºC /s – respectively. Electrochemical measurements were performed for samples after the heat treatment (solution annealing) at 1 060 °C for 1 hour for each 25 mm. Samples polariza-tion was performed with the application of the cyclic vol-tammetry in 3 % NaCl solution and 0,1M H2SO4. In both cases solutions were deaerated with argon. The polarisa-tion curves were performed in a potential range from -1,5 to 1,0 V at the polarisation rate: 1 V/min. The chronopo-tentiostatic curve was performed for a potential of 0,5 V in 3 % NaCl solution. The reference electrode in 3 % NaCl solution was the calomel electrode, while in 0,1M H2SO4 the silver-silver chloride electrode. The observations of the cast steel surface after the corrosion tests were carried out by the optical micro-scope Neophot 32 and the scanning electron micro-scope. The chemical composition analysis of ferrite, austenite and corrosion products was performed by means of the scanning microscope equipped with the EDX system of the IXRF Company for the X-ray micro-analysis.76  METALURGIJA 52 (2013) 1, 75-78B. KALANDYK et al.: INFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELDISCUSSION AND RESULTSThe microstructure of the investigated F-A cast steel consists of the ferritic matrix and austenite ‘islands’ (Figures 1 and 2) [8,10]. On the grounds of the investi-gations of the relative volume fraction of austenite in F-A cast steel cooling with a rate of 3,2 - 3,4 ºC/s and of  0,4 - 0,5 ºC/s, the difference of 1,6 % in the volume fraction was found. A lower cooling rate results in in-creasing the austenite volume fraction at the expense of ferrite. On the bases of the X-ray EDX microanalysis the differences in the chemical composition of the fer-ritic and austenitic phase, being the result of the alloy-ing elements segregation, were found. The average con-tent of the main elements not exceeding 1 %, excluding Si and Mn, are shown in Table 1. On the bases of the obtained results of the ferrite chemical composition the difference in the Ni content in walls of a thickness of 12 and 45 mm was found. In the case of austenite occurring in the wall of a thickness of 45 mm, an increased Cr and Mo content and a decreased Ni content was noticed as compared to austenite occurring in the wall of a thick-ness of 12 mm – Table 1.Table 1  Chemical compositions of the ferrite and austenite in the investigated cast steel / mas.%Phase Wall thickness / mmCr Mo Ni Cu Fe/ mas. %Ferrite 12 24,9 5,1 3,6 2,8 61,245 25,1 4,7 4,3 2,9 61,0Austenite 12 20,1 1,4 7,4 3,2 66,645 21,7 2,1 6,5 2,7 66,9Corrosion investigations The performed corrosion investigations indicated very good corrosion resistance of the investigated cast steel in 0,1M H2SO4 solution as compared with 3 % NaCl solution (Figure 3a). The breakdown potential in 0,1M H2SO4 for the tested cooling rates was 1 V. The passive zone range was from –0,5 to 1,0 V (Figure 3b).On the other hand, the difference in breakdown po-tentials for the tested cooling rates of castings in 3 % NaCl solution was noticed. The breakdown potential value for the casting cooling faster (wall: 12 mm) was app. –0,8 V, while for the casting cooling slower (wall: 45 mm) this potential shifted into positive values. Ad-ditional corrosion tests were performed at a constant abFigure 1  Microstructure of the investigated cast steel in the wall of a thickness: a) 12 mm, b) 45 mm; optical microscope, etchant - Beraha IIabFigure 2  Microstructure of the investigated cast steel in the wall of a thickness: a) 12 mm, b) 45 mm; SEM77METALURGIJA 52 (2013) 1, 75-78B. KALANDYK et al.: INFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELpotential value, being 0,5 V, during 120 minutes (Figure 4). On the basis of the chronopotentiostatic curve, ob-tained for the sample cut off from the 12 mm thick wall, it was found that the corrosion process is the multistage one with a high velocity at the process beginning (up to ∼5 minutes) and slowing down further on. Violent cur-rent oscillations recorded at the beginning of the curve, indicate an intensive sample etching accompanied by an intensive emissivity of hydrogen (the most probably) on the sample surface [11]. Further mild pathway of the chronopotentiostatic curve, at a constant potential of 0,5 V indicates pitting formation on the surface of tested materials. In turn, for the sample cut off from the 45 mm thick wall none increasing of the anodic reaction was found in the very ﬁ rst minutes of the measurement. At the ﬁ nal stage of the measurement the current density in both walls (12 and 45 mm thick) was similar. Microstructural investigations performed after cor-rosion tests in 3 % NaCl solution indicated an intensive etching of austenite on the sample cut off from the 12 mm wall (Figure 5). The reason of such behaviour was the most probably a decreased Cr, Ni and Mo content in austenite (Table 1). Layers of corrosion products formed on the cast steel surface are heterogeneous and display corrosion damages in forms of clusters, dark spots, Figure 3  Infl uence of the casting cooling rate on the polarisation curves of F-A cast steel in 3 % NaCl solution -a) and 0,1M H2SO4 -b)abFigure 4  Chronopotentiostatic curves for the tested F-A cast steel at a potential of 0,5 V in 3 % NaCl solutionpores and pitting. The selective system of corrosive zones corresponds to the austenite arrangement. The SEM-EDX analysis of these zones indicated that they are strongly oxidised (20 - 34 % of oxygen) and en-riched with Cr (17 - 19 %) and Fe (40 - 50 %). The wall of the casting, which cooled more slowly (thickness of 45 mm), had only deep, ﬁ ne spherical pit-ting without an intensive austenite etching as it was in the case of sample cut off from the 12 mm wall.  CONCLUSIONSNone essential inﬂ uence of the cooling rate of the investigated F-A cast steel on the polarization curves pathway in 0,1M H2SO4 solution was seen. The break-down potential in both cases was 1 V.The investigated cast steel cooled with the cooling rate of 3,2 - 3,4 ºC/s (12 mm wall) retains its corrosion resistance - in 3 % NaCl solution - up to a value of –0,8 V, while the one cooled with the rate of 0,4 - 0,5 ºC/s (45 mm wall) up to a value of 0,01 V. The chronopotentio-static curves recorded at a potential of 0,5 V during 120 minutes indicate the inﬂ uence of the cooling rate in the initial period of curve pathways. For the cooling rate of 3,2 - 4,3 ºC/s the intensive corrosion processes were ob-Figure 5  Surface of the investigated cast steel after corrosion tests in 3 % NaCl solution at a potential of 0,5 V78  METALURGIJA 52 (2013) 1, 75-78B. KALANDYK et al.: INFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELserved in the ﬁ rst minutes of investigations. The curves obtained at the ﬁ nal stage of investigations were similar for both samples. The research part of the study has been partially ex-ecuted under a Statutory Work no 11.11.170.318 Task no.5 (2011).REFERENCES[1] M. Pohl, O. Storz, T. Glogowski, Materials Characteriza-tion vol.58 (2007) 65-71.[2] J.A. Jimenez, M. Carsi, A. Ruano, F. Penabla, Journal of Materials Science vol. 35 (2000) 907-915.[3] A.J. Sedriks, Corrosion of Stainless Steel. Edited by A Wiley-Interscience Publ., (1996), 437 p. [4] Stainless steel castings. ASTM STP 756, (1982), 444 p. [5] R.N. Gunn, Duplex stainless steels. Edited by Abington Publ., (1999), 204 p. [6] E. Otero, A. Pardo, M. Utrilla, E. Saenez, F. Perez, Mate-rials Characterization vol.35 (1995) 145-153.[7] J. Głownia, D. Szczypkowski, B. Kalandyk, J. Górniok, S. Sobula, Foundry Journal of the Polish Foundrymen’s As-sociation, 12 (2008), 616-621.[8] J. Głownia, B. Kalandyk, K. Hübner, Materials Characteri-zation, 47 (2001), 149-155.[9] M.T. Leger, Stainless Steel Castings, American Society for Testing and Materials, (1982), 105-125.[10] P. Malatyńska, J. Głownia, The inﬂ uence of carbon content on change peritectic transformation temperature in chro-mium-nickel stainless steel, Foundry of Archives Enginee-ring, SFEROID Scientiﬁ c Student, 11, Especial Volume, (2011), 121-126.[11] D. Landolt, Corrosion and surface chemistry of metals. Lausanne: EPFL Press, (2007).Note:  The responsible translator for English language: “ANGOS” Translation Ofﬁ ce, Krakow, Poland

Influence of the cooling rate on the corrosion resistance of duplex cast steel

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75METALURGIJA 52 (2013) 1, 75-78B. KALANDYK, M. STAROWICZ, M. KAWALEC, R. ZAPAŁAINFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELReceived – Prispjelo: 2012-03-15Accepted – Prihvaćeno: 2012-08-10Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 52(1) 75-78 (2013)UDC – UDK 620.193:621.747:620.18=111B. Kalandyk, M. Starowicz, M. Kawalec, R. Zapała AGH University of Science and Technology, Krakow, PolandThe results of the infl uence of the cooling rate of the casting made of the acid-resistant ferritic - austenitic cast steel on the microstructure and corrosion resistance are presented in the paper. Samples cut out from the walls of the casting being cooled at the cooling rate of 3,2 - 0,5 ºC/s were used in the study. Diff erent cooling rates create favo-rable conditions for the segregation processes lowering properties of castings. It was found, that diff erences in the polarization curves occur only in the more aggressive corrosive environment. The reason of such behaviour of cast steel is the segregation of elements dissolved in austenite and the diff erence in the volume fraction of ferrite and austenite in the walls of the diff erent thickness.Key words: electrochemical corrosion, casting, stainless steel, microstructuresINTRODUCTIONThe characteristic feature of castings is their diversi-fi ed thickness and complexity of shapes. In the case of castings made of acid resistant Cr-Ni cast steel, various cooling rates lead to the segregation of alloy components and differences in the precipitation processes rates within the temperature range: 300 - 1 000 ºC [1,2]. As the result of these processes phases of different structure and chem-ical composition (e.g. carbides, nitrides, σ, α’) can nucle-ate in the casting walls [3-5]. Their occurrence in the mi-crostructure worsens the corrosive resistance and passi-vation ability of castings made of Cr-Ni cast steel, espe-cially in the environment containing Cl¯ ions [6]. Per-forming the proper heat treatment of castings eliminates, in the majority of cases, a precipitation of those phases, however in the case of cast steel containing ferrite and austenite in its structure the volume fraction of those phases can vary in dependence of the hyper quenching temperature and casting wall thickness [7-9]. Operating conditions of castings require that the basic properties be retained in all thickness of its walls. This means that in thin-wall castings (e.g. rotors, steering gears), as well as in castings of a wall thickness of 50 - 70 mm (valve footings and housings, large industrial pumps), material structure should not reveal essential differences. Otherwise castings will not satisfy the requirements both at the surface and inside the casting wall.Therefore an attempt to present the infl uence of cooling rates of the F-A cast steel castings on their cor-rosion resistance – in two different corrosive environ-ments was undertaken in this paper.MATERIALS AND METHODSThe infl uence of the cooling rate of the F-A cast steel casting containing 0,06 % C, 24,0 % Cr, 5,2 % Ni, 2,5 % Mo, 2,7 % Cu and 0,2 % Nb on its microstructure and corrosive resistance was investigated. The casting of different wall thickness: 12, 24 and 45 mm was made under laboratory conditions. After pouring the mould with liquid metal the temperature change was recorded by means of thermoelements Pt-PtRh10, and on this basis the cooling rate of each wall was determined. Samples for metallographic and cor-rosive tests were cut off from the walls of a thickness 12 and 45 mm, cooling with rates of 3,2 - 3,4 ºC/s and 0,4-0,5 ºC /s – respectively. Electrochemical measurements were performed for samples after the heat treatment (solution annealing) at 1 060 °C for 1 hour for each 25 mm. Samples polariza-tion was performed with the application of the cyclic vol-tammetry in 3 % NaCl solution and 0,1M H2SO4. In both cases solutions were deaerated with argon. The polarisa-tion curves were performed in a potential range from -1,5 to 1,0 V at the polarisation rate: 1 V/min. The chronopo-tentiostatic curve was performed for a potential of 0,5 V in 3 % NaCl solution. The reference electrode in 3 % NaCl solution was the calomel electrode, while in 0,1M H2SO4 the silver-silver chloride electrode. The observations of the cast steel surface after the corrosion tests were carried out by the optical micro-scope Neophot 32 and the scanning electron micro-scope. The chemical composition analysis of ferrite, austenite and corrosion products was performed by means of the scanning microscope equipped with the EDX system of the IXRF Company for the X-ray micro-analysis.76  METALURGIJA 52 (2013) 1, 75-78B. KALANDYK et al.: INFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELDISCUSSION AND RESULTSThe microstructure of the investigated F-A cast steel consists of the ferritic matrix and austenite ‘islands’ (Figures 1 and 2) [8,10]. On the grounds of the investi-gations of the relative volume fraction of austenite in F-A cast steel cooling with a rate of 3,2 - 3,4 ºC/s and of  0,4 - 0,5 ºC/s, the difference of 1,6 % in the volume fraction was found. A lower cooling rate results in in-creasing the austenite volume fraction at the expense of ferrite. On the bases of the X-ray EDX microanalysis the differences in the chemical composition of the fer-ritic and austenitic phase, being the result of the alloy-ing elements segregation, were found. The average con-tent of the main elements not exceeding 1 %, excluding Si and Mn, are shown in Table 1. On the bases of the obtained results of the ferrite chemical composition the difference in the Ni content in walls of a thickness of 12 and 45 mm was found. In the case of austenite occurring in the wall of a thickness of 45 mm, an increased Cr and Mo content and a decreased Ni content was noticed as compared to austenite occurring in the wall of a thick-ness of 12 mm – Table 1.Table 1  Chemical compositions of the ferrite and austenite in the investigated cast steel / mas.%Phase Wall thickness / mmCr Mo Ni Cu Fe/ mas. %Ferrite 12 24,9 5,1 3,6 2,8 61,245 25,1 4,7 4,3 2,9 61,0Austenite 12 20,1 1,4 7,4 3,2 66,645 21,7 2,1 6,5 2,7 66,9Corrosion investigations The performed corrosion investigations indicated very good corrosion resistance of the investigated cast steel in 0,1M H2SO4 solution as compared with 3 % NaCl solution (Figure 3a). The breakdown potential in 0,1M H2SO4 for the tested cooling rates was 1 V. The passive zone range was from –0,5 to 1,0 V (Figure 3b).On the other hand, the difference in breakdown po-tentials for the tested cooling rates of castings in 3 % NaCl solution was noticed. The breakdown potential value for the casting cooling faster (wall: 12 mm) was app. –0,8 V, while for the casting cooling slower (wall: 45 mm) this potential shifted into positive values. Ad-ditional corrosion tests were performed at a constant abFigure 1  Microstructure of the investigated cast steel in the wall of a thickness: a) 12 mm, b) 45 mm; optical microscope, etchant - Beraha IIabFigure 2  Microstructure of the investigated cast steel in the wall of a thickness: a) 12 mm, b) 45 mm; SEM77METALURGIJA 52 (2013) 1, 75-78B. KALANDYK et al.: INFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELpotential value, being 0,5 V, during 120 minutes (Figure 4). On the basis of the chronopotentiostatic curve, ob-tained for the sample cut off from the 12 mm thick wall, it was found that the corrosion process is the multistage one with a high velocity at the process beginning (up to ∼5 minutes) and slowing down further on. Violent cur-rent oscillations recorded at the beginning of the curve, indicate an intensive sample etching accompanied by an intensive emissivity of hydrogen (the most probably) on the sample surface [11]. Further mild pathway of the chronopotentiostatic curve, at a constant potential of 0,5 V indicates pitting formation on the surface of tested materials. In turn, for the sample cut off from the 45 mm thick wall none increasing of the anodic reaction was found in the very fi rst minutes of the measurement. At the fi nal stage of the measurement the current density in both walls (12 and 45 mm thick) was similar. Microstructural investigations performed after cor-rosion tests in 3 % NaCl solution indicated an intensive etching of austenite on the sample cut off from the 12 mm wall (Figure 5). The reason of such behaviour was the most probably a decreased Cr, Ni and Mo content in austenite (Table 1). Layers of corrosion products formed on the cast steel surface are heterogeneous and display corrosion damages in forms of clusters, dark spots, Figure 3  Infl uence of the casting cooling rate on the polarisation curves of F-A cast steel in 3 % NaCl solution -a) and 0,1M H2SO4 -b)abFigure 4  Chronopotentiostatic curves for the tested F-A cast steel at a potential of 0,5 V in 3 % NaCl solutionpores and pitting. The selective system of corrosive zones corresponds to the austenite arrangement. The SEM-EDX analysis of these zones indicated that they are strongly oxidised (20 - 34 % of oxygen) and en-riched with Cr (17 - 19 %) and Fe (40 - 50 %). The wall of the casting, which cooled more slowly (thickness of 45 mm), had only deep, fi ne spherical pit-ting without an intensive austenite etching as it was in the case of sample cut off from the 12 mm wall.  CONCLUSIONSNone essential infl uence of the cooling rate of the investigated F-A cast steel on the polarization curves pathway in 0,1M H2SO4 solution was seen. The break-down potential in both cases was 1 V.The investigated cast steel cooled with the cooling rate of 3,2 - 3,4 ºC/s (12 mm wall) retains its corrosion resistance - in 3 % NaCl solution - up to a value of –0,8 V, while the one cooled with the rate of 0,4 - 0,5 ºC/s (45 mm wall) up to a value of 0,01 V. The chronopotentio-static curves recorded at a potential of 0,5 V during 120 minutes indicate the infl uence of the cooling rate in the initial period of curve pathways. For the cooling rate of 3,2 - 4,3 ºC/s the intensive corrosion processes were ob-Figure 5  Surface of the investigated cast steel after corrosion tests in 3 % NaCl solution at a potential of 0,5 V78  METALURGIJA 52 (2013) 1, 75-78B. KALANDYK et al.: INFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELserved in the fi rst minutes of investigations. The curves obtained at the fi nal stage of investigations were similar for both samples. The research part of the study has been partially ex-ecuted under a Statutory Work no 11.11.170.318 Task no.5 (2011).REFERENCES[1] M. Pohl, O. Storz, T. Glogowski, Materials Characteriza-tion vol.58 (2007) 65-71.[2] J.A. Jimenez, M. Carsi, A. Ruano, F. Penabla, Journal of Materials Science vol. 35 (2000) 907-915.[3] A.J. Sedriks, Corrosion of Stainless Steel. Edited by A Wiley-Interscience Publ., (1996), 437 p. [4] Stainless steel castings. ASTM STP 756, (1982), 444 p. [5] R.N. Gunn, Duplex stainless steels. Edited by Abington Publ., (1999), 204 p. [6] E. Otero, A. Pardo, M. Utrilla, E. Saenez, F. Perez, Mate-rials Characterization vol.35 (1995) 145-153.[7] J. Głownia, D. Szczypkowski, B. Kalandyk, J. Górniok, S. Sobula, Foundry Journal of the Polish Foundrymen’s As-sociation, 12 (2008), 616-621.[8] J. Głownia, B. Kalandyk, K. Hübner, Materials Characteri-zation, 47 (2001), 149-155.[9] M.T. Leger, Stainless Steel Castings, American Society for Testing and Materials, (1982), 105-125.[10] P. Malatyńska, J. Głownia, The infl uence of carbon content on change peritectic transformation temperature in chro-mium-nickel stainless steel, Foundry of Archives Enginee-ring, SFEROID Scientifi c Student, 11, Especial Volume, (2011), 121-126.[11] D. Landolt, Corrosion and surface chemistry of metals. Lausanne: EPFL Press, (2007).Note:  The responsible translator for English language: “ANGOS” Translation Offi ce, Krakow, Poland

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75 METALURGIJA 52 (2013) 1, 75-78B. KALANDYK, M. STAROWICZ, M. KAWALEC, R. ZAPAŁAINFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELReceived – Prispjelo: 2012-03-15Accepted – Prihvaćeno: 2012-08-10Preliminary Note – Prethodno priopćenjeISSN 0543-5846METABK 52(1) 75-78 (2013)UDC – UDK 620.193:621.747:620.18=111B. Kalandyk, M. Starowicz, M. Kawalec, R. Zapała AGH University of Science and Technology, Krakow, PolandThe results of the infl  uence of the cooling rate of the casting made of the acid-resistant ferritic - austenitic cast steel on the microstructure and corrosion resistance are presented in the paper. Samples cut out from the walls of the casting being cooled at the cooling rate of 3,2 - 0,5 ºC/s were used in the study. Diff  erent cooling rates create favo-rable conditions for the segregation processes lowering properties of castings. It was found, that diff  erences in the polarization curves occur only in the more aggressive corrosive environment. The reason of such behaviour of cast steel is the segregation of elements dissolved in austenite and the diff  erence in the volume fraction of ferrite and austenite in the walls of the diff  erent thickness.Key words: electrochemical corrosion, casting, stainless steel, microstructuresINTRODUCTIONThe characteristic feature of castings is their diversi-ﬁ  ed thickness and complexity of shapes. In the case of castings made of acid resistant Cr-Ni cast steel, various cooling rates lead to the segregation of alloy components and differences in the precipitation processes rates within the temperature range: 300 - 1 000 ºC [1,2]. As the result of these processes phases of different structure and chem-ical composition (e.g. carbides, nitrides, σ, α’) can nucle-ate in the casting walls [3-5]. Their occurrence in the mi-crostructure worsens the corrosive resistance and passi-vation ability of castings made of Cr-Ni cast steel, espe-cially in the environment containing Cl¯ ions [6]. Per-forming the proper heat treatment of castings eliminates, in the majority of cases, a precipitation of those phases, however in the case of cast steel containing ferrite and austenite in its structure the volume fraction of those phases can vary in dependence of the hyper quenching temperature and casting wall thickness [7-9]. Operating conditions of castings require that the basic properties be retained in all thickness of its walls. This means that in thin-wall castings (e.g. rotors, steering gears), as well as in castings of a wall thickness of 50 - 70 mm (valve footings and housings, large industrial pumps), material structure should not reveal essential differences. Otherwise castings will not satisfy the requirements both at the surface and inside the casting wall.Therefore an attempt to present the inﬂ  uence  of cooling rates of the F-A cast steel castings on their cor-rosion resistance – in two different corrosive environ-ments was undertaken in this paper.MATERIALS AND METHODSThe inﬂ  uence of the cooling rate of the F-A cast steel casting containing 0,06 % C, 24,0 % Cr, 5,2 % Ni, 2,5 % Mo, 2,7 % Cu and 0,2 % Nb on its microstructure and corrosive resistance was investigated. The casting of different wall thickness: 12, 24 and 45 mm was made under laboratory conditions. After pouring the mould with liquid metal the temperature change was recorded by means of thermoelements Pt-PtRh10, and on this basis the cooling rate of each wall was determined. Samples for metallographic and cor-rosive tests were cut off from the walls of a thickness 12 and 45 mm, cooling with rates of 3,2 - 3,4 ºC/s and 0,4-0,5 ºC /s – respectively. Electrochemical measurements were performed for samples after the heat treatment (solution annealing) at 1 060 °C for 1 hour for each 25 mm. Samples polariza-tion was performed with the application of the cyclic vol-tammetry in 3 % NaCl solution and 0,1M H2SO4. In both cases solutions were deaerated with argon. The polarisa-tion curves were performed in a potential range from -1,5 to 1,0 V at the polarisation rate: 1 V/min. The chronopo-tentiostatic curve was performed for a potential of 0,5 V in 3 % NaCl solution. The reference electrode in 3 % NaCl solution was the calomel electrode, while in 0,1M H2SO4 the silver-silver chloride electrode. The observations of the cast steel surface after the corrosion tests were carried out by the optical micro-scope Neophot 32 and the scanning electron micro-scope. The chemical composition analysis of ferrite, austenite and corrosion products was performed by means of the scanning microscope equipped with the EDX system of the IXRF Company for the X-ray micro-analysis.76   METALURGIJA 52 (2013) 1, 75-78B. KALANDYK et al.: INFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELDISCUSSION AND RESULTSThe microstructure of the investigated F-A cast steel consists of the ferritic matrix and austenite ‘islands’ (Figures 1 and 2) [8,10]. On the grounds of the investi-gations of the relative volume fraction of austenite in F-A cast steel cooling with a rate of 3,2 - 3,4 ºC/s and of  0,4 - 0,5 ºC/s, the difference of 1,6 % in the volume fraction was found. A lower cooling rate results in in-creasing the austenite volume fraction at the expense of ferrite. On the bases of the X-ray EDX microanalysis the differences in the chemical composition of the fer-ritic and austenitic phase, being the result of the alloy-ing elements segregation, were found. The average con-tent of the main elements not exceeding 1 %, excluding Si and Mn, are shown in Table 1. On the bases of the obtained results of the ferrite chemical composition the difference in the Ni content in walls of a thickness of 12 and 45 mm was found. In the case of austenite occurring in the wall of a thickness of 45 mm, an increased Cr and Mo content and a decreased Ni content was noticed as compared to austenite occurring in the wall of a thick-ness of 12 mm – Table 1.Table 1   Chemical compositions of the ferrite and austenite in the investigated cast steel / mas.%Phase Wall thickness / mmCr Mo Ni Cu Fe/ mas. %Ferrite 12 24,9 5,1 3,6 2,8 61,245 25,1 4,7 4,3 2,9 61,0Austenite 12 20,1 1,4 7,4 3,2 66,645 21,7 2,1 6,5 2,7 66,9Corrosion investigations The performed corrosion investigations indicated very good corrosion resistance of the investigated cast steel in 0,1M H2SO4 solution as compared with 3 % NaCl solution (Figure 3a). The breakdown potential in 0,1M H2SO4 for the tested cooling rates was 1 V. The passive zone range was from –0,5 to 1,0 V (Figure 3b).On the other hand, the difference in breakdown po-tentials for the tested cooling rates of castings in 3 % NaCl solution was noticed. The breakdown potential value for the casting cooling faster (wall: 12 mm) was app. –0,8 V, while for the casting cooling slower (wall: 45 mm) this potential shifted into positive values. Ad-ditional corrosion tests were performed at a constant abFigure 1   Microstructure of the investigated cast steel in the wall of a thickness: a) 12 mm, b) 45 mm; optical microscope, etchant - Beraha IIabFigure 2   Microstructure of the investigated cast steel in the wall of a thickness: a) 12 mm, b) 45 mm; SEM77 METALURGIJA 52 (2013) 1, 75-78B. KALANDYK et al.: INFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELpotential value, being 0,5 V, during 120 minutes (Figure 4). On the basis of the chronopotentiostatic curve, ob-tained for the sample cut off from the 12 mm thick wall, it was found that the corrosion process is the multistage one with a high velocity at the process beginning (up to ∼5 minutes) and slowing down further on. Violent cur-rent oscillations recorded at the beginning of the curve, indicate an intensive sample etching accompanied by an intensive emissivity of hydrogen (the most probably) on the sample surface [11]. Further mild pathway of the chronopotentiostatic curve, at a constant potential of 0,5 V indicates pitting formation on the surface of tested materials. In turn, for the sample cut off from the 45 mm thick wall none increasing of the anodic reaction was found in the very ﬁ  rst minutes of the measurement. At the ﬁ  nal stage of the measurement the current density in both walls (12 and 45 mm thick) was similar. Microstructural investigations performed after cor-rosion tests in 3 % NaCl solution indicated an intensive etching of austenite on the sample cut off from the 12 mm wall (Figure 5). The reason of such behaviour was the most probably a decreased Cr, Ni and Mo content in austenite (Table 1). Layers of corrosion products formed on the cast steel surface are heterogeneous and display corrosion damages in forms of clusters, dark spots, Figure 3   Infl  uence of the casting cooling rate on the polarisation curves of F-A cast steel in 3 % NaCl solution -a) and 0,1M H2SO4 -b)abFigure 4   Chronopotentiostatic curves for the tested F-A cast steel at a potential of 0,5 V in 3 % NaCl solutionpores and pitting. The selective system of corrosive zones corresponds to the austenite arrangement. The SEM-EDX analysis of these zones indicated that they are strongly oxidised (20 - 34 % of oxygen) and en-riched with Cr (17 - 19 %) and Fe (40 - 50 %). The wall of the casting, which cooled more slowly (thickness of 45 mm), had only deep, ﬁ  ne spherical pit-ting without an intensive austenite etching as it was in the case of sample cut off from the 12 mm wall.  CONCLUSIONSNone essential inﬂ  uence of the cooling rate of the investigated F-A cast steel on the polarization curves pathway in 0,1M H2SO4 solution was seen. The break-down potential in both cases was 1 V.The investigated cast steel cooled with the cooling rate of 3,2 - 3,4 ºC/s (12 mm wall) retains its corrosion resistance - in 3 % NaCl solution - up to a value of –0,8 V, while the one cooled with the rate of 0,4 - 0,5 ºC/s (45 mm wall) up to a value of 0,01 V. The chronopotentio-static curves recorded at a potential of 0,5 V during 120 minutes indicate the inﬂ  uence of the cooling rate in the initial period of curve pathways. For the cooling rate of 3,2 - 4,3 ºC/s the intensive corrosion processes were ob-Figure 5   Surface of the investigated cast steel after corrosion tests in 3 % NaCl solution at a potential of 0,5 V78   METALURGIJA 52 (2013) 1, 75-78B. KALANDYK et al.: INFLUENCE OF THE COOLING RATE ON THE CORROSION RESISTANCE OF DUPLEX CAST STEELserved in the ﬁ  rst minutes of investigations. The curves obtained at the ﬁ  nal stage of investigations were similar for both samples. The research part of the study has been partially ex-ecuted under a Statutory Work no 11.11.170.318 Task no.5 (2011).REFERENCES[1]  M. Pohl, O. Storz, T. Glogowski, Materials Characteriza-tion vol.58 (2007) 65-71.[2]  J.A. Jimenez, M. Carsi, A. Ruano, F. Penabla, Journal of Materials Science vol. 35 (2000) 907-915.[3]  A.J. Sedriks, Corrosion of Stainless Steel. Edited by A Wiley-Interscience Publ., (1996), 437 p. [4]  Stainless steel castings. ASTM STP 756, (1982), 444 p. [5]  R.N. Gunn, Duplex stainless steels. Edited by Abington Publ., (1999), 204 p. [6]  E. Otero, A. Pardo, M. Utrilla, E. Saenez, F. Perez, Mate-rials Characterization vol.35 (1995) 145-153.[7]  J. Głownia, D. Szczypkowski, B. Kalandyk, J. Górniok, S. Sobula, Foundry Journal of the Polish Foundrymen’s As-sociation, 12 (2008), 616-621.[8]  J. Głownia, B. Kalandyk, K. Hübner, Materials Characteri-zation, 47 (2001), 149-155.[9]  M.T. Leger, Stainless Steel Castings, American Society for Testing and Materials, (1982), 105-125.[10]  P. Malatyńska, J. Głownia, The inﬂ  uence of carbon content on change peritectic transformation temperature in chro-mium-nickel stainless steel, Foundry of Archives Enginee-ring, SFEROID Scientiﬁ  c Student, 11, Especial Volume, (2011), 121-126.[11]  D. Landolt, Corrosion and surface chemistry of metals. Lausanne: EPFL Press, (2007).Note:   The responsible translator for English language: “ANGOS” Translation Ofﬁ  ce, Krakow, Poland

Influence of the cooling rate on the corrosion resistance of duplex cast steel

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