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Investigating the effect of the quench environment on the final microstructure and wear behavior of 1.2080 tool steel after deep cryogenic heat treatment
Materials and Design 45 (2013) 316–322
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Investigating the effect of the quench environment on the final microstructure and wear behavior of 1.2080 tool steel after deep cryogenic heat treatment Kamran Amini a, Amin Akhbarizadeh b,⇑, Sirus Javadpour b a b
Department of Materials Science and Engineering, Majlesi Branch, Islamic Azad University, Isfahan, Iran Department of Materials Science and Engineering, School of Engineering, Shiraz University, Zand Ave., Shiraz, Fars, Iran
a r t i c l e
i n f o
Article history: Received 2 May 2012 Accepted 2 August 2012 Available online 16 August 2012 Keywords: Quench severity Wear behavior Microstructure Nano-sized carbide
a b s t r a c t Deep cryogenic heat treatment is an add-on heat treatment which has been added to the conventional heat treatment to improve the wear behavior of cold worked tool steels in recent years. In this study, the effect of the different quench environments with different quench severities, including water, oil, air, 30 °C ethanol and 195 °C liquid nitrogen upon the final microstructure and wear behavior of the 1.2080 tool steel was investigated. Results showed that increasing the quench severity decreases the retained austenite before the deep cryogenic heat treatment, and the final microstructure shows a more homogenous carbide distribution with higher carbide percentages. Despite the low quench severity of liquid nitrogen, the samples quenched in this environment show the highest wear resistance and hardness after the ethanol-quenched samples. This behavior is a function of a very low quenching temperature and a long incubation time for the nucleation of other phases except the martensite. The wear rate and hardness of the ethanol-quenched samples shows the highest values due to the low temperature, higher thermal conductivity (as compared with the liquid nitrogen) and a less stable martensite structure. The formation of nano-sized carbide also shows an important role in the improving mechanical properties. The predominant wear mechanism is adhesive wear. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Deep cryogenic heat treatment is a one-time and bulk hardening supplementary heat treatment that is regularly performed between the quench and temper during the conventional heat treatment [1]. For this purpose, the samples are cooled down to liquid nitrogen temperature, held in that cool environment for some hours and then gradually warmed up to room temperature to prohibit severe thermal shocks [1,2]. The most important materials subjected to this add-on process (added to the conventional hardening process) are tool steels, including M2, 1.2080, T1 and HSS tools, as well as tungsten carbide components [3–9]. Austenite commonly remains in high alloy steels after the conventional heat treatment as a result of low martensite finish temperature (Mf) which is lower than room temperature. This austenite which is called retained austenite might be transformed to martensite due to the fluctuating loads applied to a component during its working life (martensite mechanical transformation) [10,11]. This virgin martensite is not acceptable owing to its high brittleness, as well as the dimensional stability of the components which is annihilated on account of the 4% vol expansion during the ⇑ Corresponding author. Tel.: +98 9131651659; fax: +98 3112351525. E-mail addresses: [email protected] (K. Amini), [email protected] (A. Akhbarizadeh). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.08.006
austenite- martensite transformation [12]. All of the mentioned phenomena reduce the component life and subsequently increase the production cost [10–12]. One of the common methods to eliminate retained austenite is the deep cryogenic heat treatment. Besides the elimination or reduction of retained austenite at low temperatures (<125 °C), the carbide percentage increases and shows a more homogenous distribution, as compared with the conventionally heat treated samples [11–13]. The wear resistance and hardness of the samples are improved significantly after the deep cryogenic heat treatment as a consequence of these microstructural changes [11,14,15]. At low temperatures, the martensite unit cells endure a high degree of contraction which forces the carbon atoms to jump to the nearby defects to decrease the structure internal stresses. These carbon atoms act as preferential sites for carbide nucleation during tempering. This phenomenon leads to an increase in the carbide percentage and improves its distribution [6,12,16,17]. A great number of researches were carried out into the effect of the deep cryogenic heat treatment on the wear behavior and microstructural changes of tool steels. These studies investigated the effect of cryogenic temperature, austenizing and tempering temperature, and holding durations [12,18]. Despite this, the effect of the quenching environments on the final microstructure, hardness and wear behavior of 1.2080 tool steel was not specifically investigated. For this purpose, different quenching environments
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K. Amini et al. / Materials and Design 45 (2013) 316–322 Table 1 Retained austenite percentage in different quenching environments. Quench environment
Environment temperature (°C)
Thermal conductivity (W/ m K)
Retained austenite percentage before the deep cryogenic heat treatment (%)
Retained austenite percentage after the deep cryogenic heat treatment (%)
Air Oil Water Ethanol Liquid nitrogen
25 25 25 30 195
0.0257 [23] 0.145 [23] 0.612 [23] 0.182 [23] 3.9 104 [24]
17 12 8 Below 1 Below 1
Below Below Below Below Below
such as water, oil, air, ethanol 30 °C and liquid nitrogen (195 °C) were investigated. After quenching, the deep cryogenic heat treatment was performed on the samples for 36 h followed by tempering to investigate the effect of the quenching environment on the final microstructure, hardness and wear behavior of 1.2080 tool steel. 2. Experimental procedure A 20 mm bar of 1.2080 tool steel with the nominal composition of (%wt): 2.2% C, 0.6% Si, 0.6% Mn, 12% Cr, 0.5 V, 0.4% W and 83.7% Fe was cut into the disks of 5 mm in height. The samples were austenized at 950 °C for 15 min and then quenched in different environments, including air, water and oil at room temperature, 30 °C ethanol and 195 °C liquid nitrogen. The samples were then deep cryogenically treated at liquid nitrogen temperature for 36 h and then tempered at 180 °C for 3 h. The samples quenched in air, water, oil, ethanol and liquid nitrogen before the deep cryogenic heat treatment were named ADCT, WDCT, ODCT, EDCT and NDCT, respectively. The samples surface was then polished up to 1000 emery paper to reach a smooth and uniform surface. To study the microstructural changes of the samples, the samples surface was etched in (a) 100 ml H2O, 10 g K3Fe(CN)6 and10 g NaOH, (b) Mixed acid (3 ml HF, 53 ml H2NO3, 2 ml CH3COOH and 42 ml H20) and (c) Nital 4% (96 ml ethanol and 4 ml HNO3). The samples were etched in each of these etchants for 2 s and for 3–4 times. This combination only clarifies the carbides and does not affect other phases. The samples surface were then analyzed via the scanning electron microscope (SEM Ser on AIS-2100). The SEM micrographs were then analyzed via an image analyzing software of Clemex Vision (version 3.5.025) to calculate the carbide percentage. X-ray diffraction (XRD) with Cu Ka radiation was used to clarify the phases and the retained austenite percentage. The XRD analysis of the samples shows that in addition to the austenite and martensite, chromium carbide exists in the structure. The retained austenite percentage was calculated with respect to ASTM E975-00 standard [19]. The combination of the phase’s percentage should be equal to 100%. The carbide percentage was evaluated via the SEM micrographs and the austenite percentage was calculated with the following equation:
Vc ¼
ð1 V c ÞðIc =Rc Þ þ ðIc =Rc Þ ðIa =Ra Þ
ð1Þ
where Vc and Vc are the retained austenite and carbide percentage respectively, Ic and Ia are the integrated intensity per angular diffraction peak (hkl) in the austenite and martensite phases respectively and Rc and Ra are the austenite and martensite constants respectively. The value of R, can be calculated with respect to the (hkl) plane in combination with the polarization, multiplicity and structure factor of the phases according to the ASTM E975-00 standard [19].
1 1 1 1 1
The microstructural changes were also examined via scanning electron microscope (SEM Ser on AIS-2100). For more studies the carbides type were also examined via energy dispersive spectroscopy (EDX, Oxford Instrument Stereoscan 120). To analyze the nano-sized carbides, the samples surface was examined via the transition electron microscope (TEM, Philips CM200). To do this the samples were mechanically polished to 1 lm height and then further thinning was continued by jet polishing and ion beam thinning. At last the samples were punched to disks of 3 mm in diameter. The hardness of the samples was evaluated via a KOOPA UV1 hardness tester as the simulated Vickers method. In the simulated Vickers method the load of 30 kg was applied for 15 s and the hardness was evaluated with regards to the depth of indentation. The wear tests were carried out with a pin-on-disk wear tester with steel pins with the hardness of 68 HRC. The wear tests were carried out at 25 ± 5 °C with 30 ± 10% humidity and the applied load of 160 N in three sliding velocities of 0.05, 0.1 and 0.15 m/s. The sliding distance was 1000 m and the samples weight loss was calculated via an electronic balance with an accuracy of 0.0001 gr. The wear rate was calculated by the following equation:
Wr ¼ Dm=ðqLF N Þ 103
ð2Þ 3
where Wr is the wear rate in mm /Nm; Dm, the weight loss in gr, q, the steel density in gr/cm3; L, the wear distance in meter and FN, the load in Newton. The worn-out surface of the samples was analyzed via SEM to study the extent of wear damages and topography of the surface as well as to clarify the predominant wear mechanism. 3. Results and discussion 3.1. Retained austenite The XRD analysis of the samples shows that retained austenite percentage varies in different quenching environments as a function of the environment severity of quench (Table 1). The severity of quench is a numeral quantity that expresses the ability of an environment to cool down the sample during quench via the heat transfer ability of the environment. This function is related to the thermal conductivity coefficient and subsequently can be classified as water, ethanol, oil, air and nitrogen from higher to lower values (Table 1 and Fig. 1) [20–25]. Another factor to influence the quench severity of an environment is its temperature that increases by decreasing the temperature. In this study the ethanol was cooled down to 30 °C via a refrigerator, the liquid nitrogen temperature was naturally at 195 °C and the other environments were kept at room temperature (Table 1). The highest value of retained austenite belongs to the air cooled samples with 17%. The oil and waterquenched samples show lower values of retained austenite with 12% and 8%, respectively and finally in the ethanol and liquid nitrogen quenched samples, retained austenite is lower than the detection limit of the XRD technique (<1%) (Table 1 and Fig. 1). The
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Fig. 1. Variation of retained austenite after etching and thermal conductivity of different quenching environments.
retained austenite percentage in the air, oil and water decreases due to a decrease in the thermal conductivity of the quench environment. Retained austenite was eliminated in the ethanol due to its high conductivity and lower temperature simultaneously. Despite the fact that the ethanol conductivity is lower than that of the water, the lower temperature of ethanol facilitates the transformation and reduces the retained austenite percentage. In spite of a high difference between the liquid nitrogen thermal conductivity coefficient and the other environments (at a ratio of less than 1/1000) [22–24], retained austenite vanishes after quenching in liquid nitrogen. This unexpected behavior can be explained through the Time – Temperature – Transformation (TTT) diagram of the 1.2080 tool steel (Fig. 2). Due to the high percentage
of alloying elements, the TTT diagram was shifted to the right side and to very long incubations times for pearlite or bainite nucleation (more than 200 s) [25]. This means that at lower cooling rates, a fully martensitic structure is still accessible in 1.2080 tool steel. Although the liquid nitrogen thermal conductivity is too low [25], the cooling rate in this environment is sufficient to prohibit the nucleation of the other phases. The low temperature of the environment (195 °C) eliminated the retained austenite due to the low temperatures (below the Mf) and subsequently the microstructure consisted of a fully martensitic structure with chromium carbide. The long incubation time of diffusional transformation is the main factor in complete austenite transformation in the EDCT and NDCT samples (Table 1).
Fig. 2. Time – Temperature – Transformation (TTT) diagram of 1.2080 tool steel (with permission from metal Ravne).
K. Amini et al. / Materials and Design 45 (2013) 316–322
After the deep cryogenic heat treatment, retained austenite was completely transformed to martensite. In other words, the retained austenite percentage decreases to the percentage lower than the detection limit of the XRD technique in all of the ADCT, ODCT, WDCT, EDCT and NDCT samples (Fig. 3a). The surface of the samples were also examined via the EDX to clarify the existence of other phases with lower percentages. Fig. 3b shows that except martensite as the main phase of the samples, the carbide is the
Fig. 3. XRD pattern of the ODCT sample (a), and EDX analysis of the carbides (b).
319
only phase existed in the structure after the deep cryogenic heat treatment. For more assurance, the EDX analysis was performed on different places of the samples for 10 times. 3.2. Hardness and microstructural changes The hardness of the samples was evaluated via the simulated Vickers hardness tester (Fig. 4). The hardness of the EDCT sample shows the highest value, as compared with the other samples. The hardness decreases in the NDCT, WDCT, ODCT and ADCT samples, respectively (Fig. 4). The retained austenite is lower than the detection limit of the XRD technique in the whole of the samples and consequently the carbides distribution and percentage are the main effective factors in the hardness variation. The SEM micrographs of the samples demonstrate that the carbide percentage increases, and the carbide distribution improves due to an increase in the quench severity factor of the environments. Fig. 5 reveals that the carbide percentage increases from 14% in the ADCT sample to 18%, 20%, 21% and 22.5% in the ODCT, WDCT, NDCT and EDCT samples respectively. At low temperatures, martensite structure endures a high degree of contraction which forces the carbon atoms in the saturated structure to jump to the nearby defects including dislocations and as-quenched vacancies [18,26]. In other words; the martensite structure decomposes at low temperatures due to the structure instability. These wandering carbon atoms would act as preferential sites for carbide formation during tempering. The virgin martensite (the newly formed martensite which is formed when the samples are cooled down during the deep cryogenic heat treatment) shows a less saturated structure due to the formation of martensite at lower temperatures. At low temperatures, the probability of filling the octahedral sites (the places which carbon atoms occupy during martensite transformation) decreases due to the structure contraction. Alternatively, the newly formed martensite shows a less distorted structure due to less carbon content in each unit cell, or the structure of virgin martensite is more stable, as compared with the early formed martensite. The contraction in the martensite structure is the most crucial factor to produce the instability in the structure. The carbon atoms in this semi-stable martensite (virgin martensite) show a less tendency toward jumping due to a less saturated structure and subsequently the carbides population decreases in the adjacent defects in virgin martensite. It
Fig. 4. Schematic representation of Vickers hardness of the samples.
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Fig. 5. SEM micrograph of (a) ADCT, (b) ODCT, (c) WDCT, (d) EDCT and (e) NDCT samples at 750 after etching in 100 ml H2O, 10 g K3Fe(CN)6 and 10 g NaOH, mixed acid and Nital 4%.
0.018 ODCT
ADCT
NDCT
EDCT
Wear rate ×1000 (mm/Nm3)
WDCT
0.012
0.006
0 0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
0.15
0.16
Sliding velocity (m/s) Fig. 7. Wear rate of the samples versus sliding speed after 1000 m sliding distance under 160 N normal load.
Fig. 6. TEM micrograph of the DCT48 sample at 200000.
means that in the samples with fewer percentage of retained austenite after quenching, the carbide percentage improves and a more homogenous distribution is accessible after the deep cryogenic heat treatment. In comparison between the EDCT and NDCT
samples, the later shows a weaker distribution, even though the retained austenite was eliminated completely. During the quenching of the 1.2080 tool steel in the liquid nitrogen, martensite transformation takes place in longer periods due to the lower conduction coefficient of liquid nitrogen. In other words, the transformed martensite in the liquid nitrogen environment has enough time to be transformed in a less saturated-distorted structure. The martensite
K. Amini et al. / Materials and Design 45 (2013) 316–322
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Fig. 8. SEM micrograph of the worn out surface of (a) ADCT, (b) ODCT, (c) WDCT, (d) EDCT and (e) NDCT samples at 400 after 1000 m sliding distance under 160 N applied load.
produced in the ethanol environment shows a higher saturated and distorted structure and subsequently more carbon atoms can jump to the nearby defects during the deep cryogenic treatment. This leads to a higher hardness, more carbide percentage and a more homogenous carbide distribution in the EDCT sample in comparison with the NDCT sample. The TEM micrograph indicates that some newly formed nanosized carbides were also produced during the deep cryogenic heat treatment of the samples which are an important factor in hardness improvement (Fig. 6). These nano-sized carbides were not observed in the conventionally heat treated samples and their population increased in the same manner as the micron sized carbides due to the fact that the formation of both of them followed the same rules. 3.3. Wear behavior The wear rate was evaluated via a pin- on- disk wear testing machine under a 160 N applied load and at three different velocities of 0.05, 0.1 and 0.15 m/s after 1000 m sliding distance (Fig. 7). Fig. 7 shows that the wear rate decreases in the samples with higher harnesses. The lowest wear rate was concerned with the ADCT sample, and the ODCT, WDCT, NDCT and EDCT samples filled the next places in the wear rate-sliding velocity diagram, respectively (Fig. 7). The EDCT sample showed the best wear
behavior due to its highest carbide percentage and the most uniform carbide distribution in comparison with the other samples. Due to the elimination of retained austenite, carbide is the only factor that affects the wear behavior; in other words, the wear rate is a direct function of the carbide percentage and its distribution in the deep cryogenically treated samples (Fig. 7). Its worth to mention that the micro-crack and thermal distortion are negligible in these small samples due to their size, but it should be considered in larger tools. These factors are the restrictive factors in quench environment selection. The formation of nano- sized carbides is another function to be taken into account as a crucial factor in the wear resistance of the samples. These nano-sized carbides improve the wear resistance of the material (Fig. 6). Fig. 8 shows the SEM micrograph of the worn- out surface of the samples after the wear test under the 160 N applied load, at 0.1 m/ s sliding velocity and after 1000 m sliding. The micrographs show that the predominant wear mechanism is adhesive wear and that the surface damages are fewer in the samples with lower wear rates (NDCT and EDCT samples). 4. Conclusion The effect of the quench media before the deep cryogenic heat treatment on the microstructural changes, hardness and wear
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behavior of the 1.2080 tool steel was investigated. These studies have pointed out the following results: 1. Increasing the quench severity of an environment decreased the retained austenite. This reduction decreased the virgin martensite percentage transformed from retained austenite to martensite during the deep cryogenic heat treatment. 2. Decreasing the virgin martensite percentage increased the carbide percentage and made a more uniform carbide distribution. This behavior is a result of the ability of the early martensite in producing the carbides (the ability which has been lost in virgin martensite). 3. The wear resistance and hardness were increased by increasing the quench severity. Liquid nitrogen conductivity is too low, but due to the long incubation time in the TTT diagram and small size of samples, austenite was completely transformed into martensite. Despite this through transformation, which was similar with the ethanol quenched samples, the wear resistance and hardness of the ethanol-quenched samples were higher due to a more stable martensite structure in the liquid nitrogenquenched samples. 4. The predominant wear mechanism in these samples was adhesive wear.
Acknowledgment The authors are thankful to the Majlesi Branch, Islamic Azad University for the support of this work. References [1] Popandopulo N, Zhukova LT. Transformation in high speed steels during cold treatment. Met Sci Heat Treat 1980;22(10):708–10. [2] Carlson EA. Cold treating and cryogenic treatment of steel in ASM handbook. In: Heat treating, vol. 4. 10th ed. Metals Park, OH: ASM International; 1990. [3] Leskovsek V, Kalin M, Vizintin J. Influence of deep cryogenic treatment on wear resistance of vacuum heat-treated HSS. Vacuum 2006;80:507–18. [4] MohanLal D, Renganarayanan S, Kalanidhi A. Cryogenic treatment to augment wear resistance of tool and die steels. Cryogenics 2001;41:149–55. [5] Silva DFJ, Franco SD, Machado AR, Ezugwu EO, Souza AM. Performance of cryogenically treated HSS tools. Wear 2006;261:674–85.
[6] Das D, Dutta AK, Ray KK. Influence of varied cryotreatment on the wear behavior of AISI 1.2080 steel. Wear 2009;266:297–309. [7] AYLYong, Seah KHW, Rahman M. Performance evaluation of cryogenically treated tungsten carbide tools in turning. Int J Mach Tools Manuf 2006;46:2051–6. [8] Gisipa J, Gazob R, Stewart HA. Effects of cryogenic treatment and refrigerated air on tool wear when machining medium density fiberboard. J Mater Process Technol 2009;209:5117–22. [9] Thakur D, Ramamoorthy B, Vijayaraghavan L. Influence of different post treatments on tungsten carbide-cobalt inserts. Mater Lett 2008;62:4403–6. [10] Bensely A, Venkatesh S, MohanLal D, Nagarajan G, Rajadurai A, Junik K. Effect of cryogenic treatment on distribution of residual stress in case carburized En 353 steel. Mater Sci Eng A 2008;479:229–35. [11] Akhbarizadeh A, Amini K, Javadpour S. Effect of simultaneous magnetic field and deep cryogenic heat treatment on the microstructure of 1.2080 tool steel. Mater Des 2012;35:484–90. [12] Reitz W, Pendray J. Cryoprocessing of materials: a review of current status. Mater Manuf Proc 2001;16:829–40. [13] Bensely A, Prabhakaran A, Lal D Mohan, Nagarajan G. Enhancing the wear resistance of case carburized steel (En 353) by cryogenic treatment. Cryogenics 2005;45(12):747–54. [14] Das D, Dutta AK, Ray KK. On the enhancement of wear resistance of tool steels by cryogenic treatment. Philos Mag Lett 2008;88:801–11. [15] Kalin M, Leskovsek V, Vizintin J. Wear behavior of deep cryogenic treated high speed steel at different loads. Mater Manuf Proc 2006;21:741–6. [16] Das D, Dutta AK, Ray KK. On the refinement of carbide precipitates by cryotreatment in AISI 1.2080 steel. Philos Mag 2009;89:55–76. [17] Amini K, Akhbarizadeh A, Javadpour S. Investigating the effect of holding duration on the microstructure of 1.2080 tool steel during the deep cryogenic heat treatment. Vacuum 2012;86:1534–40. [18] Gill SS, Singh J, Singh R, Singh H. Metallurgical principles of cryogenically treated tool steels—a review on the current state of science. Int J Adv Manuf Technol 2011;54:59–82. [19] ASTM standard E 975-00. Standard practice for X-Ray determination of retained austenite in steel with near random crystallographic orientation. In: ASTM Int, West Conshohocken, PA, United States; 2000. [20] Totten GE, Bates CE, Clinton NA. Handbook of quenchants and quenching. Berlin: Springer; 1993. [21] Rajan TV, Sharma CP, Sharma A. Heat treatment: principles and techniques. 2nd ed. Berlin: Springer; 2004. [22] Young HD, Freedman RA. University physics. 7th ed. Berlin: Springer; 1992. [23] http://www.engineeringtoolbox.com/thermal-conductivity-liquidsd_1260.html. [24] Powers RW, Mattox RW, Johnston HL. Thermal conductivity of liquid nitrogen. J Am Chem Soc 1954;76:5968–73. [25] chandler H. Heat theater’s guide practices and procedures for irons and steels. In: ASM int ASM international. Berlin: Springer; 1995. [26] Akhbarizadeh A, Amini K, Javadpour S. Effects of applying an external magnetic field during the deep cryogenic heat treatment on the corrosion resistance and wear behavior of 1.2080 tool steel. Mater Des 2012;41:114–23.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Investigating the effect of the quench environment on the final microstructure and wear behavior of 1.2080 tool steel after deep cryogenic heat treatment Kamran Amini a, Amin Akhbarizadeh b,⇑, Sirus Javadpour b a b
Department of Materials Science and Engineering, Majlesi Branch, Islamic Azad University, Isfahan, Iran Department of Materials Science and Engineering, School of Engineering, Shiraz University, Zand Ave., Shiraz, Fars, Iran
a r t i c l e
i n f o
Article history: Received 2 May 2012 Accepted 2 August 2012 Available online 16 August 2012 Keywords: Quench severity Wear behavior Microstructure Nano-sized carbide
a b s t r a c t Deep cryogenic heat treatment is an add-on heat treatment which has been added to the conventional heat treatment to improve the wear behavior of cold worked tool steels in recent years. In this study, the effect of the different quench environments with different quench severities, including water, oil, air, 30 °C ethanol and 195 °C liquid nitrogen upon the final microstructure and wear behavior of the 1.2080 tool steel was investigated. Results showed that increasing the quench severity decreases the retained austenite before the deep cryogenic heat treatment, and the final microstructure shows a more homogenous carbide distribution with higher carbide percentages. Despite the low quench severity of liquid nitrogen, the samples quenched in this environment show the highest wear resistance and hardness after the ethanol-quenched samples. This behavior is a function of a very low quenching temperature and a long incubation time for the nucleation of other phases except the martensite. The wear rate and hardness of the ethanol-quenched samples shows the highest values due to the low temperature, higher thermal conductivity (as compared with the liquid nitrogen) and a less stable martensite structure. The formation of nano-sized carbide also shows an important role in the improving mechanical properties. The predominant wear mechanism is adhesive wear. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Deep cryogenic heat treatment is a one-time and bulk hardening supplementary heat treatment that is regularly performed between the quench and temper during the conventional heat treatment [1]. For this purpose, the samples are cooled down to liquid nitrogen temperature, held in that cool environment for some hours and then gradually warmed up to room temperature to prohibit severe thermal shocks [1,2]. The most important materials subjected to this add-on process (added to the conventional hardening process) are tool steels, including M2, 1.2080, T1 and HSS tools, as well as tungsten carbide components [3–9]. Austenite commonly remains in high alloy steels after the conventional heat treatment as a result of low martensite finish temperature (Mf) which is lower than room temperature. This austenite which is called retained austenite might be transformed to martensite due to the fluctuating loads applied to a component during its working life (martensite mechanical transformation) [10,11]. This virgin martensite is not acceptable owing to its high brittleness, as well as the dimensional stability of the components which is annihilated on account of the 4% vol expansion during the ⇑ Corresponding author. Tel.: +98 9131651659; fax: +98 3112351525. E-mail addresses: [email protected] (K. Amini), [email protected] (A. Akhbarizadeh). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.08.006
austenite- martensite transformation [12]. All of the mentioned phenomena reduce the component life and subsequently increase the production cost [10–12]. One of the common methods to eliminate retained austenite is the deep cryogenic heat treatment. Besides the elimination or reduction of retained austenite at low temperatures (<125 °C), the carbide percentage increases and shows a more homogenous distribution, as compared with the conventionally heat treated samples [11–13]. The wear resistance and hardness of the samples are improved significantly after the deep cryogenic heat treatment as a consequence of these microstructural changes [11,14,15]. At low temperatures, the martensite unit cells endure a high degree of contraction which forces the carbon atoms to jump to the nearby defects to decrease the structure internal stresses. These carbon atoms act as preferential sites for carbide nucleation during tempering. This phenomenon leads to an increase in the carbide percentage and improves its distribution [6,12,16,17]. A great number of researches were carried out into the effect of the deep cryogenic heat treatment on the wear behavior and microstructural changes of tool steels. These studies investigated the effect of cryogenic temperature, austenizing and tempering temperature, and holding durations [12,18]. Despite this, the effect of the quenching environments on the final microstructure, hardness and wear behavior of 1.2080 tool steel was not specifically investigated. For this purpose, different quenching environments
317
K. Amini et al. / Materials and Design 45 (2013) 316–322 Table 1 Retained austenite percentage in different quenching environments. Quench environment
Environment temperature (°C)
Thermal conductivity (W/ m K)
Retained austenite percentage before the deep cryogenic heat treatment (%)
Retained austenite percentage after the deep cryogenic heat treatment (%)
Air Oil Water Ethanol Liquid nitrogen
25 25 25 30 195
0.0257 [23] 0.145 [23] 0.612 [23] 0.182 [23] 3.9 104 [24]
17 12 8 Below 1 Below 1
Below Below Below Below Below
such as water, oil, air, ethanol 30 °C and liquid nitrogen (195 °C) were investigated. After quenching, the deep cryogenic heat treatment was performed on the samples for 36 h followed by tempering to investigate the effect of the quenching environment on the final microstructure, hardness and wear behavior of 1.2080 tool steel. 2. Experimental procedure A 20 mm bar of 1.2080 tool steel with the nominal composition of (%wt): 2.2% C, 0.6% Si, 0.6% Mn, 12% Cr, 0.5 V, 0.4% W and 83.7% Fe was cut into the disks of 5 mm in height. The samples were austenized at 950 °C for 15 min and then quenched in different environments, including air, water and oil at room temperature, 30 °C ethanol and 195 °C liquid nitrogen. The samples were then deep cryogenically treated at liquid nitrogen temperature for 36 h and then tempered at 180 °C for 3 h. The samples quenched in air, water, oil, ethanol and liquid nitrogen before the deep cryogenic heat treatment were named ADCT, WDCT, ODCT, EDCT and NDCT, respectively. The samples surface was then polished up to 1000 emery paper to reach a smooth and uniform surface. To study the microstructural changes of the samples, the samples surface was etched in (a) 100 ml H2O, 10 g K3Fe(CN)6 and10 g NaOH, (b) Mixed acid (3 ml HF, 53 ml H2NO3, 2 ml CH3COOH and 42 ml H20) and (c) Nital 4% (96 ml ethanol and 4 ml HNO3). The samples were etched in each of these etchants for 2 s and for 3–4 times. This combination only clarifies the carbides and does not affect other phases. The samples surface were then analyzed via the scanning electron microscope (SEM Ser on AIS-2100). The SEM micrographs were then analyzed via an image analyzing software of Clemex Vision (version 3.5.025) to calculate the carbide percentage. X-ray diffraction (XRD) with Cu Ka radiation was used to clarify the phases and the retained austenite percentage. The XRD analysis of the samples shows that in addition to the austenite and martensite, chromium carbide exists in the structure. The retained austenite percentage was calculated with respect to ASTM E975-00 standard [19]. The combination of the phase’s percentage should be equal to 100%. The carbide percentage was evaluated via the SEM micrographs and the austenite percentage was calculated with the following equation:
Vc ¼
ð1 V c ÞðIc =Rc Þ þ ðIc =Rc Þ ðIa =Ra Þ
ð1Þ
where Vc and Vc are the retained austenite and carbide percentage respectively, Ic and Ia are the integrated intensity per angular diffraction peak (hkl) in the austenite and martensite phases respectively and Rc and Ra are the austenite and martensite constants respectively. The value of R, can be calculated with respect to the (hkl) plane in combination with the polarization, multiplicity and structure factor of the phases according to the ASTM E975-00 standard [19].
1 1 1 1 1
The microstructural changes were also examined via scanning electron microscope (SEM Ser on AIS-2100). For more studies the carbides type were also examined via energy dispersive spectroscopy (EDX, Oxford Instrument Stereoscan 120). To analyze the nano-sized carbides, the samples surface was examined via the transition electron microscope (TEM, Philips CM200). To do this the samples were mechanically polished to 1 lm height and then further thinning was continued by jet polishing and ion beam thinning. At last the samples were punched to disks of 3 mm in diameter. The hardness of the samples was evaluated via a KOOPA UV1 hardness tester as the simulated Vickers method. In the simulated Vickers method the load of 30 kg was applied for 15 s and the hardness was evaluated with regards to the depth of indentation. The wear tests were carried out with a pin-on-disk wear tester with steel pins with the hardness of 68 HRC. The wear tests were carried out at 25 ± 5 °C with 30 ± 10% humidity and the applied load of 160 N in three sliding velocities of 0.05, 0.1 and 0.15 m/s. The sliding distance was 1000 m and the samples weight loss was calculated via an electronic balance with an accuracy of 0.0001 gr. The wear rate was calculated by the following equation:
Wr ¼ Dm=ðqLF N Þ 103
ð2Þ 3
where Wr is the wear rate in mm /Nm; Dm, the weight loss in gr, q, the steel density in gr/cm3; L, the wear distance in meter and FN, the load in Newton. The worn-out surface of the samples was analyzed via SEM to study the extent of wear damages and topography of the surface as well as to clarify the predominant wear mechanism. 3. Results and discussion 3.1. Retained austenite The XRD analysis of the samples shows that retained austenite percentage varies in different quenching environments as a function of the environment severity of quench (Table 1). The severity of quench is a numeral quantity that expresses the ability of an environment to cool down the sample during quench via the heat transfer ability of the environment. This function is related to the thermal conductivity coefficient and subsequently can be classified as water, ethanol, oil, air and nitrogen from higher to lower values (Table 1 and Fig. 1) [20–25]. Another factor to influence the quench severity of an environment is its temperature that increases by decreasing the temperature. In this study the ethanol was cooled down to 30 °C via a refrigerator, the liquid nitrogen temperature was naturally at 195 °C and the other environments were kept at room temperature (Table 1). The highest value of retained austenite belongs to the air cooled samples with 17%. The oil and waterquenched samples show lower values of retained austenite with 12% and 8%, respectively and finally in the ethanol and liquid nitrogen quenched samples, retained austenite is lower than the detection limit of the XRD technique (<1%) (Table 1 and Fig. 1). The
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Fig. 1. Variation of retained austenite after etching and thermal conductivity of different quenching environments.
retained austenite percentage in the air, oil and water decreases due to a decrease in the thermal conductivity of the quench environment. Retained austenite was eliminated in the ethanol due to its high conductivity and lower temperature simultaneously. Despite the fact that the ethanol conductivity is lower than that of the water, the lower temperature of ethanol facilitates the transformation and reduces the retained austenite percentage. In spite of a high difference between the liquid nitrogen thermal conductivity coefficient and the other environments (at a ratio of less than 1/1000) [22–24], retained austenite vanishes after quenching in liquid nitrogen. This unexpected behavior can be explained through the Time – Temperature – Transformation (TTT) diagram of the 1.2080 tool steel (Fig. 2). Due to the high percentage
of alloying elements, the TTT diagram was shifted to the right side and to very long incubations times for pearlite or bainite nucleation (more than 200 s) [25]. This means that at lower cooling rates, a fully martensitic structure is still accessible in 1.2080 tool steel. Although the liquid nitrogen thermal conductivity is too low [25], the cooling rate in this environment is sufficient to prohibit the nucleation of the other phases. The low temperature of the environment (195 °C) eliminated the retained austenite due to the low temperatures (below the Mf) and subsequently the microstructure consisted of a fully martensitic structure with chromium carbide. The long incubation time of diffusional transformation is the main factor in complete austenite transformation in the EDCT and NDCT samples (Table 1).
Fig. 2. Time – Temperature – Transformation (TTT) diagram of 1.2080 tool steel (with permission from metal Ravne).
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After the deep cryogenic heat treatment, retained austenite was completely transformed to martensite. In other words, the retained austenite percentage decreases to the percentage lower than the detection limit of the XRD technique in all of the ADCT, ODCT, WDCT, EDCT and NDCT samples (Fig. 3a). The surface of the samples were also examined via the EDX to clarify the existence of other phases with lower percentages. Fig. 3b shows that except martensite as the main phase of the samples, the carbide is the
Fig. 3. XRD pattern of the ODCT sample (a), and EDX analysis of the carbides (b).
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only phase existed in the structure after the deep cryogenic heat treatment. For more assurance, the EDX analysis was performed on different places of the samples for 10 times. 3.2. Hardness and microstructural changes The hardness of the samples was evaluated via the simulated Vickers hardness tester (Fig. 4). The hardness of the EDCT sample shows the highest value, as compared with the other samples. The hardness decreases in the NDCT, WDCT, ODCT and ADCT samples, respectively (Fig. 4). The retained austenite is lower than the detection limit of the XRD technique in the whole of the samples and consequently the carbides distribution and percentage are the main effective factors in the hardness variation. The SEM micrographs of the samples demonstrate that the carbide percentage increases, and the carbide distribution improves due to an increase in the quench severity factor of the environments. Fig. 5 reveals that the carbide percentage increases from 14% in the ADCT sample to 18%, 20%, 21% and 22.5% in the ODCT, WDCT, NDCT and EDCT samples respectively. At low temperatures, martensite structure endures a high degree of contraction which forces the carbon atoms in the saturated structure to jump to the nearby defects including dislocations and as-quenched vacancies [18,26]. In other words; the martensite structure decomposes at low temperatures due to the structure instability. These wandering carbon atoms would act as preferential sites for carbide formation during tempering. The virgin martensite (the newly formed martensite which is formed when the samples are cooled down during the deep cryogenic heat treatment) shows a less saturated structure due to the formation of martensite at lower temperatures. At low temperatures, the probability of filling the octahedral sites (the places which carbon atoms occupy during martensite transformation) decreases due to the structure contraction. Alternatively, the newly formed martensite shows a less distorted structure due to less carbon content in each unit cell, or the structure of virgin martensite is more stable, as compared with the early formed martensite. The contraction in the martensite structure is the most crucial factor to produce the instability in the structure. The carbon atoms in this semi-stable martensite (virgin martensite) show a less tendency toward jumping due to a less saturated structure and subsequently the carbides population decreases in the adjacent defects in virgin martensite. It
Fig. 4. Schematic representation of Vickers hardness of the samples.
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Fig. 5. SEM micrograph of (a) ADCT, (b) ODCT, (c) WDCT, (d) EDCT and (e) NDCT samples at 750 after etching in 100 ml H2O, 10 g K3Fe(CN)6 and 10 g NaOH, mixed acid and Nital 4%.
0.018 ODCT
ADCT
NDCT
EDCT
Wear rate ×1000 (mm/Nm3)
WDCT
0.012
0.006
0 0.04
0.05
0.06
0.07
0.08
0.09
0.1
0.11
0.12
0.13
0.14
0.15
0.16
Sliding velocity (m/s) Fig. 7. Wear rate of the samples versus sliding speed after 1000 m sliding distance under 160 N normal load.
Fig. 6. TEM micrograph of the DCT48 sample at 200000.
means that in the samples with fewer percentage of retained austenite after quenching, the carbide percentage improves and a more homogenous distribution is accessible after the deep cryogenic heat treatment. In comparison between the EDCT and NDCT
samples, the later shows a weaker distribution, even though the retained austenite was eliminated completely. During the quenching of the 1.2080 tool steel in the liquid nitrogen, martensite transformation takes place in longer periods due to the lower conduction coefficient of liquid nitrogen. In other words, the transformed martensite in the liquid nitrogen environment has enough time to be transformed in a less saturated-distorted structure. The martensite
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Fig. 8. SEM micrograph of the worn out surface of (a) ADCT, (b) ODCT, (c) WDCT, (d) EDCT and (e) NDCT samples at 400 after 1000 m sliding distance under 160 N applied load.
produced in the ethanol environment shows a higher saturated and distorted structure and subsequently more carbon atoms can jump to the nearby defects during the deep cryogenic treatment. This leads to a higher hardness, more carbide percentage and a more homogenous carbide distribution in the EDCT sample in comparison with the NDCT sample. The TEM micrograph indicates that some newly formed nanosized carbides were also produced during the deep cryogenic heat treatment of the samples which are an important factor in hardness improvement (Fig. 6). These nano-sized carbides were not observed in the conventionally heat treated samples and their population increased in the same manner as the micron sized carbides due to the fact that the formation of both of them followed the same rules. 3.3. Wear behavior The wear rate was evaluated via a pin- on- disk wear testing machine under a 160 N applied load and at three different velocities of 0.05, 0.1 and 0.15 m/s after 1000 m sliding distance (Fig. 7). Fig. 7 shows that the wear rate decreases in the samples with higher harnesses. The lowest wear rate was concerned with the ADCT sample, and the ODCT, WDCT, NDCT and EDCT samples filled the next places in the wear rate-sliding velocity diagram, respectively (Fig. 7). The EDCT sample showed the best wear
behavior due to its highest carbide percentage and the most uniform carbide distribution in comparison with the other samples. Due to the elimination of retained austenite, carbide is the only factor that affects the wear behavior; in other words, the wear rate is a direct function of the carbide percentage and its distribution in the deep cryogenically treated samples (Fig. 7). Its worth to mention that the micro-crack and thermal distortion are negligible in these small samples due to their size, but it should be considered in larger tools. These factors are the restrictive factors in quench environment selection. The formation of nano- sized carbides is another function to be taken into account as a crucial factor in the wear resistance of the samples. These nano-sized carbides improve the wear resistance of the material (Fig. 6). Fig. 8 shows the SEM micrograph of the worn- out surface of the samples after the wear test under the 160 N applied load, at 0.1 m/ s sliding velocity and after 1000 m sliding. The micrographs show that the predominant wear mechanism is adhesive wear and that the surface damages are fewer in the samples with lower wear rates (NDCT and EDCT samples). 4. Conclusion The effect of the quench media before the deep cryogenic heat treatment on the microstructural changes, hardness and wear
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behavior of the 1.2080 tool steel was investigated. These studies have pointed out the following results: 1. Increasing the quench severity of an environment decreased the retained austenite. This reduction decreased the virgin martensite percentage transformed from retained austenite to martensite during the deep cryogenic heat treatment. 2. Decreasing the virgin martensite percentage increased the carbide percentage and made a more uniform carbide distribution. This behavior is a result of the ability of the early martensite in producing the carbides (the ability which has been lost in virgin martensite). 3. The wear resistance and hardness were increased by increasing the quench severity. Liquid nitrogen conductivity is too low, but due to the long incubation time in the TTT diagram and small size of samples, austenite was completely transformed into martensite. Despite this through transformation, which was similar with the ethanol quenched samples, the wear resistance and hardness of the ethanol-quenched samples were higher due to a more stable martensite structure in the liquid nitrogenquenched samples. 4. The predominant wear mechanism in these samples was adhesive wear.
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