Effect of Cryogenic Treatment on Corrosion Resistance and Thermal Expansion of Valve Steels

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1 305 Effect of Cryogenic Treatment on Corrosion Resistance and Thermal Expansion of Valve Steels M. Arockia Jaswin Department of Mechanical Engineering, Aksheyaa College of Engineering, Chennai , India. D. Mohan Lal Department of Mechanical Engineering, College of Engineering Guindy, Anna University Chennai, Chennai , India. ABSTRACT This experimental study investigates the effects of cryogenic treatment on the corrosion resistance and thermal expansion of En 52 and 21-4N valve steels. The materials are subjected to deep cryogenic treatment and the corrosion behaviour and thermal expansion are compared with that of the conventional heat treatment. The valve steel samples are slowly cooled from the room temperature to 85 K at 1 K/min, soaked at 85 K for 24 hours, and heated back to room temperature at 0.6 K/min. The corrosion resistance of the En 52 and 21-4N valve steels is measured using the ASTM G3-89 standard 1. The dimensional stability of the En 52 and 21-4N valve steels after the and is studied using the Mettler thermo-mechanical analyzer as per the ASTM E831 standard 2. The potential for the En 52 specimen has increased and is due to the repassivation effect; this may be due to the presence of more chromium carbides in the martensitic structure. The corrosion rate of En 52 sample has an improvement of 8.44 % than that of the sample. The 21-4N sample has a marginal reduction in the corrosion rate than that of the samples. The En 52 and 21-4N valve steels has a lower coefficient of the thermal expansion for the specimens, which indicates that the dimensional stability is better for the samples when compared to that of the sample. Key words: Cryogenic treatment; corrosion resistance; thermal expansion; valve steel 1. INTRODUCTION Continuous research works are directed to improve the service life of the engineering components by varying the processing routes. Recently, interest in the cryogenic treatments of ferritic and non-ferritic alloys has increased due to the large potential/promise of enhanced performance. Potential property enhancements include increased hardness, fracture toughness, wear resistance, dimensional stability and intergranular corrosion resistance 3, 4,5. If some or all of these property enhancements can be achieved, it should be possible to utilize a more energy efficient manufacturing process to produce higher quality and lower cost components. Cryogenic treatment, unlike coatings, is an inexpensive one-time permanent treatment affecting the entire section of the component. The treatment is an add-on process over the conventional heat treatment in which the samples are cooled down to the prescribed cryogenic temperature level at a slow rate, maintained at this temperature for a long time and then heated back to room temperature, and tempered to decrease the brittleness of the martensite 3,7. The notable effects of the cryogenic treatment include changes in the mechanical properties and the microstructure of materials. Intake and exhaust valves are

2 precision engine components used to block gas flow ports and to control the exchange of gases in internal combustion engines. They have to operate satisfactorily over long periods of time at elevated temperatures, under high stress, and in corrosive environments. While the engine is in operation, the valve and the seat insert comes into direct contact at the seating face. The contact between the two seating faces causes wear and the wear of both faces reduces the sealing ability of the valve and the seat insert. Valve wear has been a problem to engine designers and manufacturers for many years. The implication of valve seat wear is that when it exceeds the preset lash, the valve will be held open and will not be able to seal the combustion chamber. The engine can lose its combustion pressure and thus its power. Furthermore, gas leakage could lead to valve burning or guttering. Typical wear modes at valve seats include adhesion, abrasion, deformation, corrosion, and contact fatigue 6. The most effective process for cryoprocessing steel is still under great debate. The basic undisputed process has been to austenitize the steel, quench it to room temperature, cool to around -185 C, dwell for some period of time, warm to room temperature, and temper it to decrease the brittleness of the martensite 7, 8, 9. The focal point of this experimental used for intake valves and exhaust valve stems in internal combustion engines. The study is to investigate the influence of cryogenic treatment on the corrosion resistance and thermal expansion of En 52 and 21-4N valve steels. The En 52 valve steel has been widely 21-4N valve steel is heat-resistant steel, used as the plate material of diesel engine exhaust valves in the automobile industry. 2. EXPERIMENTAL PROCEDURE For conducting the experimental study the chemical compositions of the selected valve steel materials are confirmed using the optical emission spectroscope (OES). The result of the chemical analysis confirms the chemical compositions of the En 52 and 21-4N valve steels. The specimens required for the corrosion test and thermo-mechanical analysis are machined as per the ASTM standard. The machined specimens are subjected to conventional heat treatment [] and deep cryogenic treatment [] processes. 2.1 Conventional Heat Treatment The conventional heat treatment for the En 52 and 21-4N valve steels is conducted based on the valve alloy heat treatment procedures discussed by Yushu Wang 6 and Alok Nayar 10. The following heat treatment is given to one group of the En 52 specimens. Hardening (austenitizing) at 1248 K (975 ºC) for one hour, is followed by oil quench and tempering at 923 K (650 C) temperature for one hour. One group of the 21-4N specimens is solution-heated for 40 min at 1413 K (1140 ºC) and then water-quenched; the subsequent ageing is carried out at 1018 K (745 C) for 5 hours. 2.2 Deep Cryogenic Treatment In the present investigation the En 52 valve steel material which has undergone the conventional hardening and the solutionized 21-4N valve steel are slowly cooled from room temperature to 85 K at 1 K/min, soaked at 85 K for 24 hours, and finally heated back to room temperature at 0.6 K/min. This controlled process is achieved using the computerized control A.C.I. CP-200vi (Massachusetts, U.S.A) cryogenic treatment processor. The schematic diagram of the cryogenic processor is shown in Figure

3 Pressure Gauge LN 2 line Circulating Fan Temperature Indicator Computer LN 2 Tank CRYOGENIC CHAMBER Controller DATA ACQUISITIO Figure 1 Schematic diagram of cryogenic processor The processor is a well-insulated chamber with liquid nitrogen as the working fluid. The cryogenic processor consists of a treatment chamber, which is connected to a liquid nitrogen tank (MVE DURA-CYL 160MP) through a vacuum insulated hose. The temperature sensors inside the chamber sense the temperature and accordingly the PID temperature controller operates the solenoid valve to regulate the liquid nitrogen flow. The liquid nitrogen passes through the spiral heat exchanger and enters the duct leading to the bottom of the chamber as nitrogen gas. The blower at the top of the chamber sucks the gas coming out at the bottom and makes it circulate effectively inside the chamber and reduces the chamber temperature. The programmable temperature controller (Partlow 1462) of the cryogenic processor is used to set the deep cryogenic treatment parameters, which in turn, control the process parameters like soaking time, temperature and cooling rate. Through the data acquisition system, the deep cryogenic treatment processes are recorded and stored. The En 52 samples taken out from the processor are tempered at 923 K (650 o C) for an hour and the 21-4N are aged at 1018 K (745 o C) for 5 hours. 2.3 Electrochemical Corrosion Test Electrochemical methods are useful tools for studying the corrosion behaviour of the materials, since most corrosion processes are electrochemical in nature. Electrochemical techniques can be used to measure the kinetics of electrochemical processes (e.g., corrosion rates). An electrochemical corrosion study is carried out using a three electrode cell assembly ( Potentiostat Galvanostat 12 ). The corrosion resistance of the En 52 and 21-4N valve steels is measured using the ASTM G3-89 standard 1. The En 52 and 21-4N valve steel materials are machined to 10 mm diameter and 10 mm height and subjected to conventional and deep cryogenic treatments. The electrochemical corrosion test setup is shown in Figure 2. Figure 2 Electrochemical corrosion test setup 307

4 The cryo-treated and samples are polished to 1200 grit finish, cleaned in acetone, methanol, and deionized water. The specimen is mounted onto the Teflon specimen holder to expose 1.00 cm 2 area of polished surface to the test solution (electrolyte). In the three electrode electrochemical flat cell the En 52 and 21-4N specimens are taken as the working electrode, the saturated calomel electrode as a reference electrode and platinum as the counter electrode. The reference electrode is connected to the salt bridge which is connected to the Luggin probe. The electrolyte selected for the study is 3.5% NaCl solution. The corrosion behaviour of the valve steels is studied with the open circuit potential (OCP) curve. The OCP measurement is carried out by exposing the working electrode (specimen) to the 3.5% NaCl medium for half an hour. The potential is measured as a function of time with respect to saturated calomel electrode (SCE), until the potential of the specimen reached a stable value. The cathodic and anodic polarization of specimens is carried out at a sweep rate of 1mV/sec. An electrochemical impedance spectra study is carried out at Ecorr with the electrochemical system frequency response analyzer. Potentiometeric polarization and tafel plots are used to determine the corrosion rate of the valve steels. This technique allows the determination of the corrosion potential Ecorr and corrosion current Icorr. Corrosion current Icorr is determined by the Tafel extrapolation method, by carrying out scans in both the positive and negative directions with respect to the corrosion potential of 250 mv each. Based on the Icorr values, the corrosion rate of the valve steels is calculated. 2.3 Thermo-mechanical Analysis The aim of the thermo-mechanical analysis is to measure the variations in the dimensions of a sample, while it is heated at a constant heating rate and subjected to a non-oscillatory loading. The dimensional stability of the En 52 and 21-4N valve steels after the and is studied using the Mettler thermo-mechanical analyzer as per the ASTM E831 standard 2. Figure 3 shows the photographic image of the thermo mechanical analyzer. The specimen to be tested is machined to 5 mm length and 5 mm diameter and subjected to and. The treated specimen is held in an enclosure and is contacted by a probe leading to a displacement sensor, with the temperature sensor in contact with the specimen. An appropriate load force of 20 mn is applied to the sensing probe to ensure that it is in contact with the specimen. The specimen is heated at a constant heating rate of 20 K/min from room temperature to 1083 K (810 o C). The expansion of the specimen is measured using the linear variable differential transformer over the temperature range and the data obtained is used for comparing and describing the dimensional stability of En 52 and 21-4N valve steels during service. Figure 3 Thermo-mechanical Analyzer 308

5 Potential / V (SCE) Potential / V (SCE) International Journal of Engineering Technology, Management and Applied Sciences The coefficient of linear thermal expansion (α) is calculated using the formula: where ΔL α= L o ΔT ΔL is the change in the length of the specimen, L 0 is the original length of the specimen, ΔT is the temperature change during the test. (1) 3. RESULT AND DISCUSSION The results of the electro chemical corrosion test and thermo-mechanical analysis are presented in this section. The Open Current Potential (OCP) curves for the En 52 and 21-4N valve steels at the and optimized conditions are presented in Figures 4 and Time (Sec) Figure 4 Open Current Potential curves for the En 52 valve steel Time (Sec) Figure 5 Open Current Potential curves for the 21-4N valve steel For the En 52 valve steel material, the potential of the specimen shows mv and the specimen shows mv at the initial condition. The potential of the specimen decreases continuously and reaches a stable value, and for the specimen the potential decreases 309

6 Potential V / (SCE) Potential V / (SCE) International Journal of Engineering Technology, Management and Applied Sciences upto around 1300 sec and increases to some extent and gets stable at the end. The increase in the potential for the specimen is due to the repassivation effect; this may be due to the presence of more chromium carbides in the martensitic structure. From figure 5 the initial potential for the 21-4N valve steel at the condition is mv and at the it shows a value of mv. The potential value for both the specimen decreases continuously with time and the specimen reaches a stable value at the end; the potential of the specimen increases after around 1500 sec and reaches a stable condition. The higher potential value in the OCP curve indicates the higher corrosion resistance of the En 52 and 21-4N specimen Current density (A / cm -2 ) Figure 6 Polarization curve for the En 52 valve steel at and condition Current density (A / cm -2 ) Figure 7 Polarization curve for the En 52 valve steel at and condition

7 The polarization curves for the and optimized specimens of both the materials are shown in Figures 6 and 7. The corrosion potential E corr and corrosion current I corr are determined by the Tafel extrapolation method, by carrying out scans in both the positive and negative directions. Based on the I corr values, the corrosion rate of the alloy in ionic liquids is calculated and presented in Table 1. Table 1 Specimen Electrochemical corrosion test results for the En 52 and 21-4N valve steels at and conditions 21-4N En 52 I corr E corr Corrosion rate I corr (µa) (mv) (mmpy) (µa) E corr (mv) Corrosion (mmpy) e e e e-3 From this table, the corrosion rate for the En 52 sample shows a lower value of x 10-3 mm/year when compared to that of the. The corrosion rate of the 21-4N sample shows a slightly higher corrosion rate of e-3 mm/year when compared to that of the samples. The precipitation of more M 23 C 6 carbides in the grain boundaries of the austenitic valve steel through cryogenic treatment induces intergranular corrosion and reduces the corrosion resistance. From the polarization curve, the shift in the E corr value of the En 52 specimen indicates a marginal improvement in the corrosion resistance; this may be due to the precipitation of more fine carbides in the martensitic matrix. rate Conversion of retained austenite Precipitation of transition carbides Precipitation of secondary carbides Figure 8 Linear expansion coefficient of the En 52 valve steel for and specimens 311

8 Precipitation of transition carbides Precipitation of secondary carbides Figure 9 Linear expansion coefficient of the 21-4N valve steel for and specimens The coefficient of thermal expansion (CTE) estimated for the En 52 and 21-4N valve steels at the and optimized conditions are shown in Figures 8 and 9. The coefficient of the thermal expansion of the En 52 and 21-4N valve steel specimens is low for the specimens, which indicate that the dimensional stability is high. The CTE increases in the and specimens till around 450 K for both the materials, and remains the same which is due to a pure thermal effect. For both the materials around 450 K to 560 K, a decrease in CTE is observed in the specimens; this is due to the precipitation of the transition carbides. Again, a reduction in the CTE is noted around 600 K to 650 K in both the materials. This is due to the effect of the conversion of the retained austenite. Due to the precipitation of fine secondary carbides in the specimens the CTE is less from 750 K to 1000 K on both the materials. At around 900 K the En 52 material shows a sudden fall in the CTE; this can be attributed to the precipitation of alloy carbides and grain coarsening. 4 CONCLUSION The electro-chemical corrosion test concludes that the corrosion rate of En 52 sample has an improvement of 8.44 % than that of the sample. The 21-4N sample has a marginal reduction in the corrosion rate than that of the samples. From the study it was observed that the increase in the potential for the En 52 specimen is due to the repassivation effect; this may be due to the presence of more chromium carbides in the martensitic structure. It is infer that the precipitation of more M 23 C 6 carbides in the grain boundaries of the austenitic valve steel (21-4N) through cryogenic treatment induces intergranular corrosion and reduces the corrosion resistance. The En 52 and 21-4N valve steels has an average of 15.78% and 16.36% lower coefficient of the thermal expansion for the specimens, which indicates that the dimensional stability is better for the samples when compared to that of the sample. The study also concludes that the increase in the dimensional stability can be attributed to the precipitation of alloy carbides and grain coarsening. 312

9 ACKNOWLEDGEMENT The authors gratefully acknowledge their indebtedness to the DST-FIST, Government of India, for providing funds for procuring the cryogenic treatment facility, Kalyani Carpenter Special Steels Ltd., Pune, India for providing the valve steel materials and Rane Engine Valves Ltd., Chennai, India, for extending their material-testing facilities required for the study. REFERENCES 1. ASTM International (2004), Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing, G3-89, pp ASTM International (2000), Standard test method for linear thermal expansion of solid materials by thermo-mechanical analysis, E381, pp Mohan Lal D, Renganarayanan S and Kalanidhi A. Cryogenic treatment to augment wear resistance of tool and die steels. Cryogenics 2001; 41: Paolo Baldissera and Cristiana Delprete (2009) Effects of deep cryogenic treatment on static mechanical properties of 18NiCrMo5 carburized steel, Materials and Design, Vol.30, No.5, pp Wayne Reitz and John Pendray (2001) Cryo-processing of materials: A review of current status, Materials and manufacturing processes, Vol. 16(6), pp Yushu Wang (2007) Introduction to engine valvetrains, SAE International, United States of America. 7. Das D., Dutta A.K., Toppo V. and Ray K.K. (2007), Effect of Deep Cryogenic Treatment on the Carbide Precipitation and Tribological Behavior of D2 Steel, Materials and Manufacturing Processes, Vol.22, pp Preciado M., Bravo P.M. and Alegre J.M. (2006), Effect of low temperature tempering prior cryogenic treatment on carburized steels, Journal of Materials Processing Technology, Vol.176, pp Haohuai Liu, Jun Wang, Hongshan Yang, and Baoluo Shen. (2008), Effects of cryogenic treatment on microstructure and abrasion resistance of CrMnB high-chromium cast iron subjected to sub-critical treatment, Materials Science and Engineering A 478, pp Alok Nayar (2007) The steel handbook, Tata McGraw-Hill, New Delhi, India. 313