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Effect of cryogenic treatment on tensile behavior of case carburized steel-815M17
Materials Characterization 58 (2007) 485 – 491
Effect of cryogenic treatment on tensile behavior of case carburized steel-815M17 A. Bensely a,⁎, D. Senthilkumar a , D. Mohan Lal a , G. Nagarajan a , A. Rajadurai b a
b
Department of Mechanical Engineering, Anna University, Sardar Patel Road, Chennai-600 025, India Department of Production Engineering, Madras Institute of Technology, Chrompet, Anna University, Chennai-600 044, India Received 25 April 2006; accepted 20 June 2006
Abstract The crown wheel and pinion represent the most highly stressed parts of a heavy vehicle; these are typically made of 815M17 steel. The reasons for the frequent failure of these components are due to tooth bending impact, wear and fatigue. The modern processes employed to produce these as high, durable components include cryogenic treatment as well as conventional heat treatment. It helps to convert retained austenite into martensite as well as promote carbide precipitation. This paper deals with the influence of cryogenic treatment on the tensile behavior of case carburized steel 815M17. The impetus for studying the tensile properties of gear steels is to ensure that steels used in gears have sufficient tensile strength to prevent failure when gears are subjected to tensile or fatigue loads, and to provide basic design information on the strength of 815M17 steel. A comparative study on the effects of deep cryogenic treatment (DCT), shallow cryogenic treatment (SCT) and conventional heat treatment (CHT) was made by means of tension testing. This test was conducted as per ASTM standard designation E 8M. The present results confirm that the tensile behavior is marginally reduced after cryogenic treatment (i.e. both shallow and deep cryogenic treatment) for 815M17 when compared with conventional heat treatment. Scanning electron microscopic (SEM) analysis of the fracture surface indicates the presence of dimples and flat fracture regions are more common in SCT specimens than for CHT and DCT-processed material. © 2006 Elsevier Inc. All rights reserved. Keywords: Deep cryogenic treatment; Subzero treatment; Case carburized steel; Gear failure; Tensile strength
1. Introduction Owing to globalization, industry must avoid compromising quality in order to compete in the market. Presently, the growth of the automobile industry is increasing tremendously. Research efforts are widespread with a view to improving the life and performance of components in automotive, aircraft, racing engine, firearms, etc. applications by various treatments. Over the past few ⁎ Corresponding author. Tel.: +91 44 2220 3262; fax: +91 44 2220 3255. E-mail address: [email protected] (A. Bensely). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.06.019
decades, research interest has been shown in the effect of cryogenic treatment on the performance of steels. Supplementing cryogenic treatment to conventional heat treatment processes may help the manufacturer to achieve high durable component. Cryogenic treatment is an inexpensive one-time treatment that influences the core properties of the component, unlike purely surface treatments. In the present research work, improving the mechanical properties of crown wheel and pinion components by cryogenic treatment is considered. Both the crown wheel and the pinion are made from high chromium nickel steel. They are highly stressed,
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A. Bensely et al. / Materials Characterization 58 (2007) 485–491
expensive components of an automobile. If one fails both have to be replaced. These components undergo frequent fatigue failures and wear failures due to overloading, poor heat treatment, improper material characteristics, etc. [1]. Cryogenic treatment is generally classified as either shallow cryogenic treatment (SCT), sometimes referred as subzero treatment (193 K), or deep cryogenic treatment (DCT)(77 K) based on the treatment temperature. Normally, SCT or subzero treatment are widely used for high precision parts in order to have high dimensional stability. However, DCT may also be employed due to the increased benefits reported in terms of wear resistance and compressive residual stress compared with SCT [2,3]. 2. Background and supplemental information In the present study, the material studied is case carburized steel (815M17). Typical applications of this material are heavy-duty gears, shafts, pinions, rocker arms, camshafts, gudgeon pins, push rod leads, levers, bushes, and other small arm components. The conventional heat treatment of 815M17 includes carburizing, hardening, quenching and tempering. Due to carburization, the carbon content in the case of the component will increase to approximately 1% from the base composition 0.17%. This increased carbon content leads to retention of austenite after hardening, which is not beneficial for the component under service. Hence, this retained austenite has to be converted to martensite. To eliminate the retained austenite, cryogenic treatment (SCT or DCT) is employed. The conversion of retained austenite to martensite increases the toughness and dimensional stability of the component [4]. The formation of a higher carbide concentration by the cryogenic treatment will result in increased wear resistance, reduced friction, and improved dimensional stability [5]. In order to determine the potential benefits of cryogenic treatments on the performance of tensile strength and plastic elongation, tension tests were carried out for CHT, SCT and DCT samples. 3. Literature survey The results of a literature survey on the effects of cryogenic treatment of materials is discussed below. Barron [6] conducted preliminary tests to determine the effect of cryogenic treatment on lathe tools, end mills, and zone punches and concluded that an increase in tool life from 50% to more than 200% was observed for the tools which had been soaked in liquid nitrogen for 12 h. Meng et al. [7] studied the effect of cryogenic treatment on the wear behavior of Fe–12Cr–Mo–V–1.4 C tool steel. The results
showed a dramatic increase in wear resistance, especially at high sliding speeds. Microstructural analysis of the sample after cryogenic treatment showed the presence of fine carbide precipitates of size in the range of 10 nm; these were characterized as η-carbide. The formation of ηcarbide helps to improve the wear resistance. Molinari and Pellizzari [8] mentioned that cryogenic treatment improves the mechanical properties of material by allowing the molecules of the material to compress and expand in a uniform, homogeneous manner and then to be realigned in a more coherent fashion, thus reducing internal stress and thereby increasing the life of components. Mohanlal et al. [9] studied the effect of cryogenic treatment on T1 type-high speed steel and found that the conversion of retained austenite to martensite is an isothermal process. The maximum hardness was attained by soaking at 203 K, which led to a hardness of 67 HRc. Microstructural analysis showed that large alloy carbides were broken into finely dispersed carbides when treated for more than 2 h, irrespective of soaking temperature. Wilson [10] concluded that cryogenically treating slitter knives in paper mills increases the lifetime by more than 500%. The improvement in wear life is due to complete transformation of the retained austenite to martensite at cryogenic temperatures; the amount of retained austenite in typical steel is reduced by a factor of three by the cryogenic treatment. This leads to a small increase in the size of the component, and enhanced stability of the component. Alexandru Ailincai and Baciu [11] mentioned that the structure of cryogenically cooled metallic materials has a more uniform and dense microstructure than non-cryogenically treated samples. In addition, cryogenic cooling induced the occurrence of very fine carbides with dimensions less than 1 μm, which occupy microvoids and contribute to an increase of the density. Bensely et al. [12] studied the wear resistance of case carburized EN 353 steel after shallow and deep cryogenic treatment and concluded that the improvement over conventional heat treatment was 85% and 372% for SCT and DCT, respectively. Based on the above literature survey, it is concluded that cryogenic treatment has the potential to improve the mechanical properties of steel. In order to study the effect of cryogenic treatment on tensile behavior, the present research work has been carried out. 4. Experimental procedure 4.1. Specimen preparation In the present study, 15 tensile specimens were machined from raw material (815M17). The chemical
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487
Table 1 Chemical composition of 815M17 (wt.%) Carbon
Silicon
Manganese
Phosphorus
Sulphur
Chromium
Nickel
Molybdenum
0.17
0.19
0.92
0.018
0.031
1.09
1.05
0.11
composition of this steel is tabulated in Table 1 and the tensile test specimen is shown in Fig. 1. The 15 machined samples were divided into 4 groups, namely Group A, Group B, Group C and Group D. Group A consists of 3 samples whereas all other groups consist of 4 samples each. The Group A samples were kept as such to determine the raw material (815M17) behavior and the remaining Group B, Group C and Group D samples were subjected to CHT, SCT and DCT, respectively. After various thermal treatments it was found that one Group D sample had a machining error. Hence, it was rejected and only 3 samples were included in Group D. The tensile tests were conducted as per ASTM standard “Standard test methods for tension testing of metallic materialsdesignation: E 8 M” [13]. The tensile testing machine used for the test was an Instron 5500 series which has a test frame with a 100 kN load cell, calibrated to ASTM E 4 requirements. The extensometer, calibrated to ASTM E 83 requirements, was a 20-mm clip-on device (not dual averaging due to the brittle nature of the material failing in the elastic region), Class B-1. The specimens were placed in the grips of the tensile testing machine at a specified grip separation and pulled until failure. A specimen in the grips of the machine with extensometer attached is shown in Fig. 2. The test was conducted by the strain control method up to the yield point with a speed of 0.005 min− 1. As the extensometer is an expensive, highly sensitive, accurate measuring device, it was removed prior to fracture to avoid
damage. The test setup was not affected by the removal of extensometer. Fig. 3 shows the specimen being measured for the change in gauge length and reduction in area after fracture. 4.2. Heat treatment of 815M17 Numerous industrial applications such as gears, crown wheel and pinion required a hard wear-resistant surface and a soft tough core. Carburization is one process to achieve these properties. 4.2.1. Conventional heat treatment (CHT) The conventional heat treatment cycle consists of carburizing, air-cooling, quench-hardening in oil, followed by tempering. The machined tensile specimens (Group B, Group C and Group D) were placed in a bath of molten cyanide. Carbon will diffuse from the molten cyanide bath into the specimen. The carburizing temperature was 1203 K (930 °C) and the cycle time was 4 h. The material after carburizing was air-cooled. This was followed by quench-hardening process. In the quenchhardening process, the specimens were heated at (1105 K) 832 °C and soaked for 1 h and rapidly quenched in oil at (303 K) 30 °C. After quench-hardening, the 3 groups were segregated. Group B samples were immediately subjected to tempering. This process consists of reheating the hardened steel to 150 °C for 1.5 h and cooling it in air in order to impart toughness. It is carried out to reduce
Fig. 1. Tensile test specimen.
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A. Bensely et al. / Materials Characterization 58 (2007) 485–491 Table 2 Tensile test results Conditions
Specimen Tensile Average % Average identification strength (MPa) Elongation % (MPa)
Raw material (Group A) CHT (Group B)
A B C
755 785 765
D E F G H I J K L M N
1420 1300 1230 1230 1300 1300 1243 1260 1110 1070 1342
SCT (Group C)
Fig. 2. Tensile testing machine.
brittleness, even though sacrificing some hardness and tensile strength, to relieve internal stresses, and to increase toughness and ductility. The segregated Group C and Group D samples were immediately subjected to the SCT and DCT processes, respectively. 4.2.2. Cryogenic treatment The retained austenite present after the conventional heat treatment process after hardening can be alleviated by means of cryogenic treatment. Because in most of the steel, the martensite finish temperature doesn't lie above room temperature, the steel has to be cooled still further from room temperature to achieve 100% martensite. Cryogenic treatment is an extension of conventional heat treatment by which it is possible to achieve 100% martensite. This treatment alters the material microstructure and enhances the strength and wear properties. Presently, two types of practice are available — SCT or DCT. For maximum
Fig. 3. Experimental setup for gauge length measurement.
DCT (Group D)
22 18 20
20
0.4 0.2 0.1 0.1 1275.75 – – 0.1 0.1 1174 – – –
0.2
768.3
1295
0.1
–
benefit, the cryogenic treatment should be introduced between the hardening and tempering process. 4.2.2.1. Shallow cryogenic treatment (SCT). Group C samples, after quench-hardening in the conventional process, were directly placed in a mechanical freezer, which was at 193 K, and soaked for 5 h. Then it was removed from the freezer and allowed to reach ambient temperature. The samples were then tempered at 423 K (150 °C) for 1.5 h. The tempering process is used to ensure that there is no brittle, untempered martensite when the component is put into service. 4.2.2.2. Deep cryogenic treatment (DCT). Group D samples, after quench-hardening in the conventional process, were directly subjected to deep cryogenic
Fig. 4. SEM fractograph of a Group A sample.
A. Bensely et al. / Materials Characterization 58 (2007) 485–491
489
Fig. 5. SEM fractograph of the case region of a Group B CHT sample.
Fig. 7. SEM fractograph of the case region of a Group D DCT sample.
treatment. The samples were slowly cooled from room temperature to 77 K at 1.24 K/min, soaked at 77 K for 24 h, and finally heated back to room temperature at 0.64 K/min. These very low temperatures were achieved using computer controls in a well-insulated treatment chamber with liquid nitrogen as the working fluid. This treatment enhances the desired metallurgical and structural properties by completing the transformation of austenite to martensite. The samples were then tempered at 423 K (150 °C) for 1.5 h.
The tensile strength is the value most frequently recorded and cited from the results of a tension test. However, it is a value of little fundamental significance with
regard to the strength of a metal. For ductile metals, the tensile strength should be regarded as a measure of the maximum load that a metal can withstand under the very restrictive conditions of uniaxial loading. This value bears little relation to the useful strength of structural members. However, because of the long practice of using the tensile strength to describe the strength of materials, it has become a familiar property, and as such, it is a useful identification of a material in the same sense that the chemical composition serves to identify a metal or alloy. Furthermore, because the tensile strength is easy to determine and is a reproducible property, it is useful for the purposes of specification, to design components to withstand application forces and for quality control of a product. For brittle materials, the tensile strength is a valid design criterion. The results of the tensile test for the material 815M17 are given in Table 2 for the raw material, as well as the CHT, SCT and DCT samples.
Fig. 6. SEM fractograph of the case region of a Group C SCT sample.
Fig. 8. SEM fractograph of the core region of a Group B CHT sample.
5. Results and discussion 5.1. Tensile strength
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5.2. Percent elongation Percent elongation is an indication to the designer, in a general way, of the ability of the metal to flow plastically before fracture. A high ductility indicates to the designer that the material is “forgiving” and likely to deform locally without fracture. Ductility measurements may be specified to assess material “Quality” even though no direct relationship exists between the ductility measurement and performance in service. The percent elongation of all the CHT and SCT specimens is quite low and approximately the same. For the DCT specimens, the percent elongation was sufficiently negligible that it was not detected. 5.3. Fractography Fracture surfaces of all treated specimens were observed in detail by scanning electron microscopy (SEM). The SEM pictures were taken in the central region of the fracture as well as at the case of the fractured CHT, SCT and DCT specimens. Fig. 4 shows the presence of large dimples and microvoids on the fracture surface of a Group A, non-heat treated sample. The widely distributed dimples and voids attest to the extensive plastic deformation prior to fracture. Figs. 5–7 are micrographs of the nearsurface case regions of fractures of CHT, SCT and DCT samples, respectively. Figs. 5 and 6 clearly indicate the existence of both transgranular and intergranular facets on the CHT and SCT samples. Fig. 7 shows primarily intergranular brittle fracture near the surface of a DCT sample. These features are reflected in the slightly higher tensile strength for the CHT and SCT samples compared to the DCT samples. Figs. 8–10 show scanning micrographs of the core fracture regions of CHT, SCT and CDT samples, respectively. Fig. 8, for a CHT sample, shows the presence
Fig. 10. SEM fractograph of the core region of a Group D DCT sample.
of mixed mode, ductile brittle fracture, with about 50% ductile dimpled appearance, and 50% flat facets indicating the occurrence of brittle fracture. Fig. 9, a SCT sample, shows more ductility than the DCT in Fig. 10. The SCT sample shows the presence of about 75% dimpled fracture and 25% flat facets. Fig. 10, the DCT sample, shows 50% dimpled appearance and 50% flat facets, indicating limited ductility. 6. Conclusions The study indicates a reduction in tensile strength for SCT and DCT samples over CHT by a factor of 1.5% and 9.34%, respectively. However, considering the improvement achieved in earlier studies for wear resistance, where improvements of 85% for SCT and 372% for DCT were reported, the marginal reduction in tensile strength suggests the reduced tensile strength is an acceptable tradeoff in the optimization efforts. The SEM analyses of the fracture surfaces indicate the occurrence of more extensive microvoids and microcracks formation for the SCT samples compared to those prepared by the cryogenic SCT and DCT heat treatments. Acknowledgements
Fig. 9. SEM fractograph of the core region of a Group C SCT sample.
The Westmoreland Mechanical and Testing and Research Limited, Westmoreland Building, Beaumont Road, Industrial Estate, Banbury, Oxon, UK has supported this research work by conducting the test as per ASTM standard. The authors would like to thank Chris Wiseman, Technical and Development Manager and Maggie Beal, Quality Manager and the company for the help rendered towards the successful completion of the work without any commercial aspects.
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References [1] ASM. Mechanical testing and evaluation. ASM handbook, vol. 8; 2000. p. 861–72. [2] Prabhakaran A, Bensely A, Nagarajan G, Mohanlal D. Effect of cryogenic treatment on impact strength of case carburized steelEn353. Proceedings of IMEC2004 international mechanical engineering conference; 2004. p. 1–5. [3] Johan Singh P, Guha B. Fatigue life improvement of AISI 304L cruciform welded joints by cryogenic treatment. Eng Fail Anal 2003;10:1–12. [4] Mohan Lal D, Renganarayanan S, Kalanidhi A. Cryogenic treatment to augment wear resistance of tool and die steels. Cryogenics 2001;41:149–55. [5] Huag JY, Zhu YT. Microstructure of cryogenic treated M2 tool steel. Mater Sci Eng, A Struct Mater: Prop Microstruct Process 2003;339:241–4. [6] Barron RF. Effect of cryogenic treatment on lathe tool wear. Prog Refrig Sci Technol 1973;1:529–33. [7] Meng Fanju, Tagashira Kohsuke, Azuma Ryo, Kishoma Hidea. Role of eta-carbide precipitations in the wear resistance improve-
[8]
[9]
[10] [11]
[12]
[13]
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ments of Fe–12Cr–Mo–V–1.4C tool steel by the cryogenic treatment. ISIJ Int 1994;34:205–10. Molinari M, Pellizzari. Effect of deep cryogenic treatment on the mechanical properties of tool steels. Mater Process Technol 2001;118:350–5. Mohanlal D, Renganarayanan S, Kalanidhi. Effect of cryotreatment on T1 type high speed steel tool material. Indian J Cryog 1996;21(2):41–4. Wilson V. Ultra-cold treatment up heavy duty tool wear. Iron Age 1971;207(6):58. Alexandru, Ailincai G, Baciu C. Influence of cryogenic treatments on life of alloyed high speed steels. Mem Etud Sci Rev Metall 1990;4:203–6. Bensely A, Prabhakaran A, Mohan Lal D, Nagarajan G. Enhancing the wear resistance of case carburized steel (En 353) by cryogenic treatment. Cryogenics 2005;45:747–54. Standard test methods for tension testing of metallic materialsdesignation: E 8, 2004, Annual Book of ASTM Standard, Volume 03.01.
Effect of cryogenic treatment on tensile behavior of case carburized steel-815M17 A. Bensely a,⁎, D. Senthilkumar a , D. Mohan Lal a , G. Nagarajan a , A. Rajadurai b a
b
Department of Mechanical Engineering, Anna University, Sardar Patel Road, Chennai-600 025, India Department of Production Engineering, Madras Institute of Technology, Chrompet, Anna University, Chennai-600 044, India Received 25 April 2006; accepted 20 June 2006
Abstract The crown wheel and pinion represent the most highly stressed parts of a heavy vehicle; these are typically made of 815M17 steel. The reasons for the frequent failure of these components are due to tooth bending impact, wear and fatigue. The modern processes employed to produce these as high, durable components include cryogenic treatment as well as conventional heat treatment. It helps to convert retained austenite into martensite as well as promote carbide precipitation. This paper deals with the influence of cryogenic treatment on the tensile behavior of case carburized steel 815M17. The impetus for studying the tensile properties of gear steels is to ensure that steels used in gears have sufficient tensile strength to prevent failure when gears are subjected to tensile or fatigue loads, and to provide basic design information on the strength of 815M17 steel. A comparative study on the effects of deep cryogenic treatment (DCT), shallow cryogenic treatment (SCT) and conventional heat treatment (CHT) was made by means of tension testing. This test was conducted as per ASTM standard designation E 8M. The present results confirm that the tensile behavior is marginally reduced after cryogenic treatment (i.e. both shallow and deep cryogenic treatment) for 815M17 when compared with conventional heat treatment. Scanning electron microscopic (SEM) analysis of the fracture surface indicates the presence of dimples and flat fracture regions are more common in SCT specimens than for CHT and DCT-processed material. © 2006 Elsevier Inc. All rights reserved. Keywords: Deep cryogenic treatment; Subzero treatment; Case carburized steel; Gear failure; Tensile strength
1. Introduction Owing to globalization, industry must avoid compromising quality in order to compete in the market. Presently, the growth of the automobile industry is increasing tremendously. Research efforts are widespread with a view to improving the life and performance of components in automotive, aircraft, racing engine, firearms, etc. applications by various treatments. Over the past few ⁎ Corresponding author. Tel.: +91 44 2220 3262; fax: +91 44 2220 3255. E-mail address: [email protected] (A. Bensely). 1044-5803/$ - see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.matchar.2006.06.019
decades, research interest has been shown in the effect of cryogenic treatment on the performance of steels. Supplementing cryogenic treatment to conventional heat treatment processes may help the manufacturer to achieve high durable component. Cryogenic treatment is an inexpensive one-time treatment that influences the core properties of the component, unlike purely surface treatments. In the present research work, improving the mechanical properties of crown wheel and pinion components by cryogenic treatment is considered. Both the crown wheel and the pinion are made from high chromium nickel steel. They are highly stressed,
486
A. Bensely et al. / Materials Characterization 58 (2007) 485–491
expensive components of an automobile. If one fails both have to be replaced. These components undergo frequent fatigue failures and wear failures due to overloading, poor heat treatment, improper material characteristics, etc. [1]. Cryogenic treatment is generally classified as either shallow cryogenic treatment (SCT), sometimes referred as subzero treatment (193 K), or deep cryogenic treatment (DCT)(77 K) based on the treatment temperature. Normally, SCT or subzero treatment are widely used for high precision parts in order to have high dimensional stability. However, DCT may also be employed due to the increased benefits reported in terms of wear resistance and compressive residual stress compared with SCT [2,3]. 2. Background and supplemental information In the present study, the material studied is case carburized steel (815M17). Typical applications of this material are heavy-duty gears, shafts, pinions, rocker arms, camshafts, gudgeon pins, push rod leads, levers, bushes, and other small arm components. The conventional heat treatment of 815M17 includes carburizing, hardening, quenching and tempering. Due to carburization, the carbon content in the case of the component will increase to approximately 1% from the base composition 0.17%. This increased carbon content leads to retention of austenite after hardening, which is not beneficial for the component under service. Hence, this retained austenite has to be converted to martensite. To eliminate the retained austenite, cryogenic treatment (SCT or DCT) is employed. The conversion of retained austenite to martensite increases the toughness and dimensional stability of the component [4]. The formation of a higher carbide concentration by the cryogenic treatment will result in increased wear resistance, reduced friction, and improved dimensional stability [5]. In order to determine the potential benefits of cryogenic treatments on the performance of tensile strength and plastic elongation, tension tests were carried out for CHT, SCT and DCT samples. 3. Literature survey The results of a literature survey on the effects of cryogenic treatment of materials is discussed below. Barron [6] conducted preliminary tests to determine the effect of cryogenic treatment on lathe tools, end mills, and zone punches and concluded that an increase in tool life from 50% to more than 200% was observed for the tools which had been soaked in liquid nitrogen for 12 h. Meng et al. [7] studied the effect of cryogenic treatment on the wear behavior of Fe–12Cr–Mo–V–1.4 C tool steel. The results
showed a dramatic increase in wear resistance, especially at high sliding speeds. Microstructural analysis of the sample after cryogenic treatment showed the presence of fine carbide precipitates of size in the range of 10 nm; these were characterized as η-carbide. The formation of ηcarbide helps to improve the wear resistance. Molinari and Pellizzari [8] mentioned that cryogenic treatment improves the mechanical properties of material by allowing the molecules of the material to compress and expand in a uniform, homogeneous manner and then to be realigned in a more coherent fashion, thus reducing internal stress and thereby increasing the life of components. Mohanlal et al. [9] studied the effect of cryogenic treatment on T1 type-high speed steel and found that the conversion of retained austenite to martensite is an isothermal process. The maximum hardness was attained by soaking at 203 K, which led to a hardness of 67 HRc. Microstructural analysis showed that large alloy carbides were broken into finely dispersed carbides when treated for more than 2 h, irrespective of soaking temperature. Wilson [10] concluded that cryogenically treating slitter knives in paper mills increases the lifetime by more than 500%. The improvement in wear life is due to complete transformation of the retained austenite to martensite at cryogenic temperatures; the amount of retained austenite in typical steel is reduced by a factor of three by the cryogenic treatment. This leads to a small increase in the size of the component, and enhanced stability of the component. Alexandru Ailincai and Baciu [11] mentioned that the structure of cryogenically cooled metallic materials has a more uniform and dense microstructure than non-cryogenically treated samples. In addition, cryogenic cooling induced the occurrence of very fine carbides with dimensions less than 1 μm, which occupy microvoids and contribute to an increase of the density. Bensely et al. [12] studied the wear resistance of case carburized EN 353 steel after shallow and deep cryogenic treatment and concluded that the improvement over conventional heat treatment was 85% and 372% for SCT and DCT, respectively. Based on the above literature survey, it is concluded that cryogenic treatment has the potential to improve the mechanical properties of steel. In order to study the effect of cryogenic treatment on tensile behavior, the present research work has been carried out. 4. Experimental procedure 4.1. Specimen preparation In the present study, 15 tensile specimens were machined from raw material (815M17). The chemical
A. Bensely et al. / Materials Characterization 58 (2007) 485–491
487
Table 1 Chemical composition of 815M17 (wt.%) Carbon
Silicon
Manganese
Phosphorus
Sulphur
Chromium
Nickel
Molybdenum
0.17
0.19
0.92
0.018
0.031
1.09
1.05
0.11
composition of this steel is tabulated in Table 1 and the tensile test specimen is shown in Fig. 1. The 15 machined samples were divided into 4 groups, namely Group A, Group B, Group C and Group D. Group A consists of 3 samples whereas all other groups consist of 4 samples each. The Group A samples were kept as such to determine the raw material (815M17) behavior and the remaining Group B, Group C and Group D samples were subjected to CHT, SCT and DCT, respectively. After various thermal treatments it was found that one Group D sample had a machining error. Hence, it was rejected and only 3 samples were included in Group D. The tensile tests were conducted as per ASTM standard “Standard test methods for tension testing of metallic materialsdesignation: E 8 M” [13]. The tensile testing machine used for the test was an Instron 5500 series which has a test frame with a 100 kN load cell, calibrated to ASTM E 4 requirements. The extensometer, calibrated to ASTM E 83 requirements, was a 20-mm clip-on device (not dual averaging due to the brittle nature of the material failing in the elastic region), Class B-1. The specimens were placed in the grips of the tensile testing machine at a specified grip separation and pulled until failure. A specimen in the grips of the machine with extensometer attached is shown in Fig. 2. The test was conducted by the strain control method up to the yield point with a speed of 0.005 min− 1. As the extensometer is an expensive, highly sensitive, accurate measuring device, it was removed prior to fracture to avoid
damage. The test setup was not affected by the removal of extensometer. Fig. 3 shows the specimen being measured for the change in gauge length and reduction in area after fracture. 4.2. Heat treatment of 815M17 Numerous industrial applications such as gears, crown wheel and pinion required a hard wear-resistant surface and a soft tough core. Carburization is one process to achieve these properties. 4.2.1. Conventional heat treatment (CHT) The conventional heat treatment cycle consists of carburizing, air-cooling, quench-hardening in oil, followed by tempering. The machined tensile specimens (Group B, Group C and Group D) were placed in a bath of molten cyanide. Carbon will diffuse from the molten cyanide bath into the specimen. The carburizing temperature was 1203 K (930 °C) and the cycle time was 4 h. The material after carburizing was air-cooled. This was followed by quench-hardening process. In the quenchhardening process, the specimens were heated at (1105 K) 832 °C and soaked for 1 h and rapidly quenched in oil at (303 K) 30 °C. After quench-hardening, the 3 groups were segregated. Group B samples were immediately subjected to tempering. This process consists of reheating the hardened steel to 150 °C for 1.5 h and cooling it in air in order to impart toughness. It is carried out to reduce
Fig. 1. Tensile test specimen.
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A. Bensely et al. / Materials Characterization 58 (2007) 485–491 Table 2 Tensile test results Conditions
Specimen Tensile Average % Average identification strength (MPa) Elongation % (MPa)
Raw material (Group A) CHT (Group B)
A B C
755 785 765
D E F G H I J K L M N
1420 1300 1230 1230 1300 1300 1243 1260 1110 1070 1342
SCT (Group C)
Fig. 2. Tensile testing machine.
brittleness, even though sacrificing some hardness and tensile strength, to relieve internal stresses, and to increase toughness and ductility. The segregated Group C and Group D samples were immediately subjected to the SCT and DCT processes, respectively. 4.2.2. Cryogenic treatment The retained austenite present after the conventional heat treatment process after hardening can be alleviated by means of cryogenic treatment. Because in most of the steel, the martensite finish temperature doesn't lie above room temperature, the steel has to be cooled still further from room temperature to achieve 100% martensite. Cryogenic treatment is an extension of conventional heat treatment by which it is possible to achieve 100% martensite. This treatment alters the material microstructure and enhances the strength and wear properties. Presently, two types of practice are available — SCT or DCT. For maximum
Fig. 3. Experimental setup for gauge length measurement.
DCT (Group D)
22 18 20
20
0.4 0.2 0.1 0.1 1275.75 – – 0.1 0.1 1174 – – –
0.2
768.3
1295
0.1
–
benefit, the cryogenic treatment should be introduced between the hardening and tempering process. 4.2.2.1. Shallow cryogenic treatment (SCT). Group C samples, after quench-hardening in the conventional process, were directly placed in a mechanical freezer, which was at 193 K, and soaked for 5 h. Then it was removed from the freezer and allowed to reach ambient temperature. The samples were then tempered at 423 K (150 °C) for 1.5 h. The tempering process is used to ensure that there is no brittle, untempered martensite when the component is put into service. 4.2.2.2. Deep cryogenic treatment (DCT). Group D samples, after quench-hardening in the conventional process, were directly subjected to deep cryogenic
Fig. 4. SEM fractograph of a Group A sample.
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Fig. 5. SEM fractograph of the case region of a Group B CHT sample.
Fig. 7. SEM fractograph of the case region of a Group D DCT sample.
treatment. The samples were slowly cooled from room temperature to 77 K at 1.24 K/min, soaked at 77 K for 24 h, and finally heated back to room temperature at 0.64 K/min. These very low temperatures were achieved using computer controls in a well-insulated treatment chamber with liquid nitrogen as the working fluid. This treatment enhances the desired metallurgical and structural properties by completing the transformation of austenite to martensite. The samples were then tempered at 423 K (150 °C) for 1.5 h.
The tensile strength is the value most frequently recorded and cited from the results of a tension test. However, it is a value of little fundamental significance with
regard to the strength of a metal. For ductile metals, the tensile strength should be regarded as a measure of the maximum load that a metal can withstand under the very restrictive conditions of uniaxial loading. This value bears little relation to the useful strength of structural members. However, because of the long practice of using the tensile strength to describe the strength of materials, it has become a familiar property, and as such, it is a useful identification of a material in the same sense that the chemical composition serves to identify a metal or alloy. Furthermore, because the tensile strength is easy to determine and is a reproducible property, it is useful for the purposes of specification, to design components to withstand application forces and for quality control of a product. For brittle materials, the tensile strength is a valid design criterion. The results of the tensile test for the material 815M17 are given in Table 2 for the raw material, as well as the CHT, SCT and DCT samples.
Fig. 6. SEM fractograph of the case region of a Group C SCT sample.
Fig. 8. SEM fractograph of the core region of a Group B CHT sample.
5. Results and discussion 5.1. Tensile strength
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5.2. Percent elongation Percent elongation is an indication to the designer, in a general way, of the ability of the metal to flow plastically before fracture. A high ductility indicates to the designer that the material is “forgiving” and likely to deform locally without fracture. Ductility measurements may be specified to assess material “Quality” even though no direct relationship exists between the ductility measurement and performance in service. The percent elongation of all the CHT and SCT specimens is quite low and approximately the same. For the DCT specimens, the percent elongation was sufficiently negligible that it was not detected. 5.3. Fractography Fracture surfaces of all treated specimens were observed in detail by scanning electron microscopy (SEM). The SEM pictures were taken in the central region of the fracture as well as at the case of the fractured CHT, SCT and DCT specimens. Fig. 4 shows the presence of large dimples and microvoids on the fracture surface of a Group A, non-heat treated sample. The widely distributed dimples and voids attest to the extensive plastic deformation prior to fracture. Figs. 5–7 are micrographs of the nearsurface case regions of fractures of CHT, SCT and DCT samples, respectively. Figs. 5 and 6 clearly indicate the existence of both transgranular and intergranular facets on the CHT and SCT samples. Fig. 7 shows primarily intergranular brittle fracture near the surface of a DCT sample. These features are reflected in the slightly higher tensile strength for the CHT and SCT samples compared to the DCT samples. Figs. 8–10 show scanning micrographs of the core fracture regions of CHT, SCT and CDT samples, respectively. Fig. 8, for a CHT sample, shows the presence
Fig. 10. SEM fractograph of the core region of a Group D DCT sample.
of mixed mode, ductile brittle fracture, with about 50% ductile dimpled appearance, and 50% flat facets indicating the occurrence of brittle fracture. Fig. 9, a SCT sample, shows more ductility than the DCT in Fig. 10. The SCT sample shows the presence of about 75% dimpled fracture and 25% flat facets. Fig. 10, the DCT sample, shows 50% dimpled appearance and 50% flat facets, indicating limited ductility. 6. Conclusions The study indicates a reduction in tensile strength for SCT and DCT samples over CHT by a factor of 1.5% and 9.34%, respectively. However, considering the improvement achieved in earlier studies for wear resistance, where improvements of 85% for SCT and 372% for DCT were reported, the marginal reduction in tensile strength suggests the reduced tensile strength is an acceptable tradeoff in the optimization efforts. The SEM analyses of the fracture surfaces indicate the occurrence of more extensive microvoids and microcracks formation for the SCT samples compared to those prepared by the cryogenic SCT and DCT heat treatments. Acknowledgements
Fig. 9. SEM fractograph of the core region of a Group C SCT sample.
The Westmoreland Mechanical and Testing and Research Limited, Westmoreland Building, Beaumont Road, Industrial Estate, Banbury, Oxon, UK has supported this research work by conducting the test as per ASTM standard. The authors would like to thank Chris Wiseman, Technical and Development Manager and Maggie Beal, Quality Manager and the company for the help rendered towards the successful completion of the work without any commercial aspects.
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