Effect of Subzero and Tempering Treatments on Corrosion Resistance of Medium Carbon Steel in Acidic Medium

Effect of Subzero and Tempering Treatments on Corrosion Resistance of Medium Carbon Steel in Acidic Medium

Isadare. D. A., Ajao. A. A, Oyesakin. T. H

Department of Materials Science and Engineering, Faculty of Technology, Obafemi Awolowo University, Ile-Ife, Nigeria

Abstract – This study investigated the effect of subzero and tempering treatments on the corrosion resistance of hardened medium carbon steel with a view of improving its performance in service condition. Standard corrosion specimens of medium carbon steel were austenitized at 8500C, soaked for 2 hours and quenched in water to room temperature. The quenched samples were immediately subjected to cryogenic treatment in dry ice (solid CO2) at -780C for 24 hours, followed by tempering at 5000C for 1-5 hours and then air cooled to ambient temperature. The heat-treated and as-received samples were characterized with the Autolab PG STAT 204 Potentiostat & FRA32 EIS Module, X-ray diffractometer (XRD) and Smart EVO 10 Scanning Electron microscope/Energy dispersive spectroscope (SEM/EDS). The results show that tempering of hardened and subzero treated medium carbon steel generally improves its corrosion resistance in acidic medium, transformed the retained austenite to martensite, and modifies the morphology of

the transformed martensite to tempered martensite. The results also depict that the retained austenite stability is different from one crystallographic plane to another and the driving force required for transformation is different from plane to plane. It can be concluded that tempering treatment can be used to improve the corrosion resistance of hardened and subzero treated medium steel in acidic environment.

Keywords: cryogenic; austenite; martensite; corrosion; tempering; subzero

1. INTRODUCTION

   Medium carbon steels are steels with carbon content varying between 0.30 and 0.60 wt. %. These steels have good responsive to heat treatment unlike low carbon steels. They are used in shaft, gear, crankshafts, simple tools, railway axles and railway wheels. For use in these applications, medium carbon steel has to be quench hardened in order to meet the required service performance [1]. Upon full hardening, the microstructure of a steel should contain martensite. However, in practice, it is usually impossible to obtain a complete martensitic structure upon full hardening, there are still traces of retained austenite in the hardened steel. The presence of retained austenite greatly reduces mechanical properties.

   The presence of retained austenite in hardened steel is deleterious to its mechanical properties, and it cause dimensional instability in components made from them. Researchers have beamed their searchlight on the use of subzero treatment to mitigate this problem. However, there seems to be little or no information on the effect of this treatment on the corrosion resistivity of hardened medium carbon steel. Subzero treatment also known as cryogenic treatment or cold treatment can be used to eliminate this problem of retained austenite. Subzero treatment is the process of treating workpiece at temperatures well below room temperature in order to transform retained austenite to martensite. However, residual stresses are developed during subzero treatment due to sudden temperature difference that could aggravate corrosion failure of steel in service environment.

   Handoko et al. [2] worked on the effect of retained austenite stability in the corrosion mechanism of dual phase high carbon steel. From their experiment, preferential attack to the retained austenite phase was observed during corrosion. It was concluded that increase in the stability of retained austenite in the dual phase steels led to decreasing weight loss rate.

   Dubovan et al. [3] researched on the effect of subzero treatment applied to sintered steels. The researchers carried out subzero treatment at -760C using dry ice to four sample groups sintered; sintered and oil quenched; sintered, oil quenched and cooled in dry ice for 60 min, sintered, oil quenched and cooled in dry ice for 120 minutes. Sintering was done at 1150°C for an hour in an argon atmosphere. The best improvement was obtained in sintered + oil quenched + cooled in ice for 2 hours, in which all of the retained austenite were converted to martensite.

   Karthikeyan et al. [4] conducted a study on the effect of subzero treatment on the microstructure and material properties of EN24 steel. Shallow cryogenic treatment was caried out on annealed EN24 medium carbon steels (C-0.35%, Mn-0.45%, Si-0.11%, Cr- 0.91%, Ni-1.31%, Mo-0.21%). Two sets of specimens with four samples each were used for both the conventional hardening treatment and cryogenic treatment. In the first group, the samples were austenitised at 850°C followed by quenching in a carburizing atmosphere. The samples were either double or triple tempered at 450°C and 650°C for 90 minutes before finally air-cooled to room temperature. The other group were conventional heat treated followed by shallow cryogenic treatment at -78°C for 24 hours in an insulated container with solid CO2, followed by double or triple tempering at 450°C and 650°C before finally air-cooled to room temperature. The results of their experiment revealed substantial improvement in hardness, toughness, tensile strength and yield strength. Fractography tests reveal the mode of failure of the cryogened samples as being ductile.

   Singh et al. [5] worked on improvement in the corrosion rate and mechanical properties of low carbon steel through deep cryogenic treatment. The material used were all heat treated in a rotary furnace at a temperature of 950°C for about 1 hour followed by water quenching. The specimen was grouped into three groups: Group A is regarded as untreated; Group B were further tempered at 150°C for 1hour while Group C were cryogenically treated at a temperature of -193°C and was soaked for 36 hours and later tempered at a temperature of 150°C for 1 hour. All samples were subjected to tensile test, hardness test, corrosion and the microstructure was examined. Samples were subjected to a salt solution for seven days however weight loss was obtained daily. Their results showed that the corrosion rate of the material is governed by a number of factors such as environmental conditions, hardness of the material, chemical composition and the type of treatment process.; also, mechanical properties of all samples were improved. Microstructural examinations revealed the microstructure consisting of bainite, martensite and retained austenite as a result of deep cryogenic treatment.

   Baldiserra and Delprete [6] in their work titled deep cryogenic treatment of AISI 302 stainless steel, fatigue and corrosion studied the effect of deep cryogenic treatment on fatigue and corrosion resistance of AISI 302 stainless steel. Their experiment was carried out on both solubilized and hardened steel. The results of their experiment showed an increase in the fatigue limit of the solubilized stainless steel however no improvement was seen on the hardened steel. No significant result was seen in the corrosion resistance of either of the two steel sample groups.

   Zhirafar et al. [7] worked on effect of cryogenic treatment on the mechanical properties of 4340 steel. After hardening at 845°C for 15 minutes, followed by oil quenching. cryogenic treatment was performed at a rate of 1.8°C/min up to -196°C and was held at that temperature for 2 hours, then tempered at temperatures 200°C, 300°C and 495°C for 2 hours. Slight reduction in retained austenite was observed through neutron diffraction which in turn led to an ncrease in hardness, fatigue limit and decrease in impact energy/toughness.

   Collins and Domer [8] worked on deep cryogenic treatment of a cold-worked tool steel. All test specimen were unnotched Charpy samples, of dimensions 7 x 10 x 55 mm. All samples were quenched in water followed by cooling to -196°C, then warmed up to ambient temperature in air. All samples were subjected to Charpy and Rockwell tests to measure the toughness and hardness respectively. The result obtained from their experiment showed an improvement in both toughness and wear resistance.

   The application of cryogenic treatment to transform retained austenite to martensite and improving the mechanical properties of steels such as stainless steels, high carbon steels, low carbon steels have been well studied and reported. However, there have been only very few studies on the effect of cryogenic and tempering treatments on the corrosion resistance of medium carbon steels. There have also been inconsistencies in the selection of heat treatment variables to be implemented and as such there is no consensus on this.

2. MATERIALS AND METHOD

   1. Sample preparation

      The materials to be used for this research work are medium carbon steel, dry ice, HCl, abrasive papers,

      Nital etchant and diamond pastes. Spectrometric analysis of the as received medium carbon steel was carried out to determine the elemental composition of the steel. The as received medium carbon steel rods were machined and cut into standard specimens for XRD, SEM and corrosion rate analysis. The prepared samples were then classified into 7 categories based on the various heat treatments they will undergo. These categories are as shown in Table 1.

      Table 3.1: Table describing the various categories of samples

      Acronym Description

      As received As received from the supplier HC Hardened and cryogened

      HCT1 Hardened, cryogened and tempered for 1 hour HCT2 Hardened, cryogened and tempered for 2 hours HCT3 Hardened, cryogened and tempered for 3 hours HCT4 Hardened, cryogened and tempered for 4 hours HCT5 Hardened, cryogened and tempered for 5 hours

   2. Heat Treatments

      Hardening: Some of the machined samples were heated in the muffle furnace to a temperature of 850°C at heating rate of 9.740C/mins, held for 60 minutes and quenched rapidly to room temperature in water.

      Subzero treatment: The quenched samples for subzero treatment (HC, HCT1, HCT2, HCT3, HCT4, and HCT5) were immediately immersed into a cryogenic cooler containing dry ice at -78°C and held at this temperature for 24 hours.

      Tempering: Some of the cryogened samples were reheated to about 500°C inside a muffle furnace and held at this temperature for 1 – 5 hours.

   3. Corrosion Tests

      Corrosion rate determination: Representative specimens from all samples above were subjected to a corrosive environment (1M HCl) and were examined via the Autolab PGSTAT204 potentiostat &FRA32 EIS Module to determine the rate and extent of corrosion on them.

   4. Microstructural Characterization

      Phase determination (X-Ray Diffraction): The type and percent composition of various phases present in the quenched, cryogened and tempered samples were examined and determined using an x-ray diffractometer. This was performed using Shimadzu XDS 2400H diffractometer with Cu anode control, 40 KV, 30 M.A, optics: Automatic divergence slit) with Cu K radiation =1.5418°A over a wide range of Bragg angles (30°280°). The XRD patterns of all specimens were recorded in the 0°- 70° 2 range with a step size of 0.017° and a counting time of 14 s per step.

      Microscopy: Representative samples of HC, HCT1, HCT2, HCT3, HCT4, and HCT5 were ground, polished and etched with Nital etchant (98% Ethanol and 2% nitric acid) for 1 minute and air-dried for microscopy examination. SEM micrographs of the treated samples were obtained from the ZEISS SMART EVO 10 Scanning Electron Microscope to study the morphology of the phases present.

3. Results and Discussion

Tables 2 shows the elemental composition of the medium carbon steel used for this research. Table 3 shows the corrosion test results, while Figure 1 shows the variation of corrosion rate with tempering time. Figures 2-8 show the results of SEM imaging, XRD pattern and Table for each of the sample categories. Figure 9 present typical Tafel Plot and result for the corrosion experiment. This serves as a representative of the results obtained in this experiment.

Table 2: Elemental Composition of the As-received Medium Carbon Steel Used.

Please note that the corrosion rate for the as-received sample is 47.013 mm/yr.

Corrosion Resistance versus Tempering Time

Figure 1: Variation of Corrosion Rate with Tempering Time of Hardened and Crogened Medium Carbon Steel

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Figure 2: a) SEM micrograph, b) XRD Pattern and c) XRD Result Table for the As-received Medium Carbon Steel

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Figure 3: a) SEM micrograph, b) XRD Pattern and c) XRD Result Table for Hardened and Cryogened Medium Carbon Steel (- Martensite = 85.67%, Austenite = 14.33%).

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Figure 4: a) SEM Micrograph, b) XRD Pattern and c) XRD Result Table for Medium Carbon Steel Hardened, Cryogened and Tempered for 1 Hour (-Martensite = 85.38%, Austenite = 14.62%).

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Figure 5: a) SEM Micrograph, b) XRD Pattern and c) XRD Result Table for Medium Carbon Steel Hardened, Cryogened and Tempered for 2 Hours (-Martensite = 80.18%, Austenite = 19.82%).

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Figure 6: a) SEM Micrograph, b) XRD Pattern and c) XRD Result Table for Medium Carbon Steel Hardened, Cryogened and Tempered for 3 Hours (-Martensite = 96.55%, Austenite = 3.36%).

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Figure 7: a) SEM Micrograph, b) XRD Pattern and c) XRD Result Table for Medium Carbon Steel Hardened, Cryogened and Tempered for 4 Hours (-Martensite = 95.99%, Austenite = 4.01%).

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Figure 8: a) SEM Micrograph, b) XRD Pattern and c) XRD Result Table for Medium Carbon Steel Hardened, Cryogened and Tempered for 5 Hours (-Martensite = 92.40%, Austenite = 7.60%)

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Figure 9: a) Tafel Plot, and b) Corrosion Test Results of the Medium Carbon Steel Hardened, Cryogened, Tempered for 3 Hours

1. DISCUSSION OF RESULTS

   The microstructure and XRD results of the as-received sample as shown in Fig. 2 reveals the presence of austenite, transition carbide (Fe2.4C), and Ferrite. The onlyhard phase here is the transition carbide, which is just 8.85%. The bulk phases austenite and ferrite are soft in nature and will be susceptible to corrosion failure. This definitely resulted in the very high corrosion rate of the as- received sample (47.013mm/yr).

   1. Variation of Corrosion Rate with Tempering Time

      Figure 1 shows the effect of tempering time on the corrosion rate of the medium carbon steel. From this figure, it can be seen that the as hardened and cryogened steel had the highest corrosion rate based on the fact that it has not been subjected to tempering. The microstructural examination of the sample revealed the presence of plate martensite as shown in Fig. 3a, and the XRD result revealed that the sample has a martensite composition of 85.67% as seen in Fig. 3c. By virtue of this low martensite level and their plate-like shape, the corrosion rate of the material was higher. It is seen in Fig. 1 that corrosion rate begins to reduce with increase in tempering time until it gets to tempering time of 3 hours where we have the least corrosion rate (0.00026028 mm/yr). As shown in Fig. 6b, it is seen that the presence of martensite is highest (96.6%) when tempering is carried out for 3 hours. This is the reason for the very low corrosion rate of this specimen. This high composition of martensite increases the hardness of the sample, thereby improves it corrosion resistance. It can also be seen in the micrograph (Fig. 6a), an increased in number of tempered martensite which aligns with the XRD results. This high corrosion resistance could also be attributed to stress relieving effect of tempering treatment. It could be that complete stress relieving was achieved at 3 hours soaking time at this temperature (5000C). Furthermore, tempered martensite consists of ferrite and fine, uniformly dispersed cementite (Fe3C) particles. Cementite is a very strong phase in steel that combined good strength/hardness with toughness, which improves the corrosion resistance of the sample. Upon tempering for longer

      hours of 4 and 5, it was observed that the corrosion rate started increases again as seen in Table 2. Figures 7a and 8a show the morphologies of the tempered martensite for samples tempered at 4 and 5 hours, respectively, while Figures 7c and 8c indicates a reduction in the percentage composition of martensite in these samples, and thus the reason for the increased corrosion rate. The reduction in the volume of martensite shows that the optimum tempering time for this material at 5000C for corrosion resistance application in acidic medium is 3 hours. The increase in corrosion resistance of as a result of cryogenic treatment was in accordance with the literature [9, 10]. However, some researchers reported a decrease in corrosion resistance of steel due to tempering [11-13].

   2. Effect of Tempering on Microstructure of Treated Samples

      Figures 2a to 8a show the micrographs obtained from the scanning electron microscope. The presence of needle-like or plate or biconvex structures denote plate martensite [1]. During tempering operation, a modified form of martensite is formed which is called tempered martensite. The formation of tempered martensite take place according to the following reaction:

      (, ) ( + ).

      The formation of tempered martensite resulted in the modification of martensite shape and improvement in the corrosion resistance. This can be seen in Fig. 1.

      However, based on the tempering temperature (5000C) used for this treatment, which implies medium temperature tempering. It is expected that the retained austenite should transforms to bainite. According to Rajan et al. [1], this bainite is different from the conventional bainite, because it consists of ferrite and epsilon carbide (Fe2.4C).

   3. Effect of Tempering on the Phase Evolution for the Treated Samples

      As shown in Figure 2, the as-received sample was seen to contain austenite on planes (110) & (220), alpha ferrite on planes (200) & (310) and Transition carbides (Fe2.4C) on plane (211). Percentage composition of martensite fluctuates as tempering time increases across board. However, there was general increase in the martensite composition. Although, all the retained austenite was supposed to be transformed during cryogenic treatment but some retained austenite was not, this maybe as a result of insufficient soaking time and the presence of austenite stabilizers alloying elements such as nickel, manganese.

      During tempering, martensite composition keeps increasing, this maybe as a result of tempering been a mode of converting retained austenite to a modified martensite called tempered martensite, and also due to the fact that an increase in temperature will aids diffusion of atoms, unlike extremely cold condition prevailing during cryogenic treatment.

      Transformation of retained austenite to martensite takes place from plane to plane, as shown in Figs. 3 to 8. Planes (110), (200) &

      (310) totally transform to martensite while plane (211) start transforming at tempering time of 3 hours and totally at 5 hours. It was observed that the retained austenite on plane (220) does not transform completely to martensite at all the soaking times adopted for tempering treatment. This could be attributed to chemical stabilization as a result of high concentration of alloying elements such as manganese, nickel and chromium, which are all present in the steel used for this research. This alloying element except aluminium and cobalt lower the martensite start and martensite finish temperatures, thereby retarding retained austenite transformation to martensite [1, 5]. However, the most probable reason in this case could be that the stability of retained austenite is highly dependent on plane (220) crystallographic orientation relative to the surrounding matrix and the stress applied. This (220) crystallographic orientation might have a higher critical driving force required for transformation of retained austenite to martensite compared to other planes as a result of favourable atomic packing factor or alignment with residual stresses [14-16]. Literature also has it that austenite transformation to martensite never goes to completion [1].

      CONCLUSION

      The following conclusion can be drawn from the results of this research:

      1. Subzero and tempering treatments has a complimenting effect on the corrosion resistance of hardened medium carbon steel. When tempered, the corrosion resistance is improved. However, there is a peak tempering time when highest corrosion resistance is observed;
      2. The results revealed that the optimum tempering time for the best corrosion resistance when medium carbon steel is hardened at 850oC and cryogened at – 78oC for 24 hours, and tempered at 500oC is 3 hours;
      3. The results further revealed that the stability of retained austenite is highly dependent on crystallographic orientation of planes, and that the drive force required for transformation of retained austenite to martensite or bainite differs from one plane to another.

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