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Influence of different cryotreatments on tribological behavior of 80CrMo12 5 cold work tool steel
Materials and Design 31 (2010) 4666–4675
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Influence of different cryotreatments on tribological behavior of 80CrMo12 5 cold work tool steel K. Amini a,*, S. Nategh a, A. Shafyei b a b
Department of Materials Engineering, Science and Research Branch, Islamic Azad University (IAU), Tehran, Iran Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran
a r t i c l e
i n f o
Article history: Received 1 March 2010 Accepted 13 May 2010 Available online 20 May 2010 Keywords: Ferrous metals and alloys (A) Heat treatments (C) Wear (E)
a b s t r a c t This experimental study investigated the effect of cryogenic treatments on the wear behavior of 80CrMo12 5 tool steel. For this purpose, two different cryogenic temperatures were used: 80 °C as the shallow cryogenic temperature and 196 °C as the deep cryogenic temperature. The results showed that the cryogenic treatments decrease retained austenite, which is more effective in the case of the deep cryogenic treatment (DCT). As a result, a remarkable improvement in the wear resistance of the cryogenically treated specimens was observed. In addition, DCT increases the percentage of carbides and their homogeneity in distribution. An optimum holding time was found in the deep cryogenic temperature, in which the hardness and wear resistance show maximum values. Moreover, the wear debris and worn surfaces showed that the dominant mechanism in the wear test is adhesive. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Cryogenic treatment is a supplementary heat treatment that is performed on some tool steels before tempering as an effective method for decreasing residual stress, retained austenite and increasing wear life. Two types of cryogenic treatments are generally applied as follows: (1) the shallow cryogenic treatment which is performed between 60 °C and 90 °C; (2) the deep cryogenic treatment that is conducted at temperatures below 125 °C [1]. The cryogenic treatment consists of controlled cooling of conventionally hardened specimens to a selected temperature, holding for a certain period, followed by controlled heating back to the ambient temperature for subsequent tempering. The cryogenic treatment enhances the transformation of retained austenite (as a soft and unstable phase) to martensite (as a promising phase) and subsequently increases the hardness and wear resistance [2]. In the deep cryogenic temperatures, as well as retained austenite elimination, fine dispersed eta (g) carbides are precipitated. This higher proportion and more homogenized distribution of carbides are due to the crystal lattice contraction. In the deep cryogenic temperatures, the lattice contraction forces carbon atoms to diffuse out to neighbor dislocations and defects [3]. Moreover, some new dislocations are created in the deep cryogenic treatment as a result of a difference in the thermal expansion of austenite and martensite. This new dislocations * Corresponding author. Tel.: +98 9131651659; fax: +98 3112351525. E-mail address: [email protected] (K. Amini). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.05.028
provide suitable places for the segregation of carbon atoms and subsequently carbide nucleation in tempering. Thus, during tempering, these carbon atoms would produce new carbides, thereby leading to more homogenized carbide distribution [4–6]. Several investigators studied the cryogenic process and compared the effect of the cryogenic treatment on the wear behavior, hardness, tensile and other mechanical properties of different materials. The cryogenic treatment is conducted on different materials including tools steels [7–10], carburized steels [11,12], maraging steel [4,13], cast iron [14,15], tungsten carbide [16] and polymers [17]. Recent studies showed that the cryogenic treatment generally improves the wear resistance and hardness of tool steels. The wear resistance improvement varies from a few to a few hundred percentages in different kinds of steel [2–10]. In contrast, some researchers have been skeptical about the process and claimed that there is no noticeable difference in steels after and before the cryogenic treatment [18,19]. The present work compared the wear resistance of 80CrMo12 5 tool steel samples processed by conventional heat treatment (CHT), shallow cryogenic treatment (SCT) and deep cryogenic treatment (DCT) using a pin-on-disk wear tester. Moreover, these experiments tried to unfold the influence of the duration of the deep cryogenic treatment on the structure and wear behavior of the 80CrMo12 5 tool steel. To reveal the effect of cryogenic holding time and temperature on the wear behavior of the 80CrMo12 5 tool steel, the microstructure, hardness, morphology of wear surface and wear derbies were evaluated.
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Table 1 Chemical composition of the 80CrMo12 5 tool steel (wt.%). Elements
Wt.%
%C %Si %Mn %Mo %Cr %S %P %Fe
0.8 0.85 0.25 0.5 3.06 <0.005 <0.009 Remaining
Table 2 Heat treatment condition of the 80CrMo12 5 tool steel. Sample no.
Heat treatment
Nomenclature
1 2 3
Conventional heat treatment Shallow cryogenically treated at 80 °C for 24 h Deep cryogenically treated at 196 °C for 0 h (warm up the sample after reaching at 196 °C) Deep cryogenically treated at 196 °C for 6 h Deep cryogenically treated at 196 °C for 24 h Deep cryogenically treated at 196 °C for 48 h Deep cryogenically treated at 196 °C for 72 h Deep cryogenically treated at 196 °C for 168 h
CHT SCT24 Instant DCT
4 5 6 7 8
DCT6 DCT24 DCT48 DCT72 DCT168
2. Experiments The experimental testing was conducted on the 80CrMo12 5 commercial tool steel with a nominal composition reported in Table 1. The wear test samples were cut into disk shapes (dia 5 cm 0.4 cm) using wire electron-discharge machining. To reach a uniform and smooth wear surface, the samples were machined and then ground up to 600 mesh papers, reaching a 0.4 lm surface roughness. For conventional heat treatment (CHT), the samples were preheated at 620 °C for 20 min and then austenitized at 920 °C for 20 min. Then, the samples were quenched in oil to room temperature and tempered at 150 °C for 3 h. To compare the effect of the cryogenic treatment on the mechanical properties of the 80CrMo12 5 tool steel, the samples were cryogenically treated in two different temperatures. For the shallow cryogenic treatment, after quenching, the samples were
Fig. 2. Optical microscope images of the carbide particles: (a) CHT and (b) DCT24 samples.
cooled to 80 °C with the cooling rate of 1 °C/min. The samples were held at the same temperature for 24 h and then heated up to room temperature. Tempering was conducted at 150 °C for 3 h (SCT24). For the deep cryogenic treatment, the samples were uniformly cooled to 196 °C with the cooling rate of 1 °C/min, held for 24 h and heated up to room temperature with the heating rate of 1 °C/min. Afterward, the samples were tempered at 150 °C for 3 h (DCT24). The details of DCT24 are illustrated in Fig. 1.
Fig. 1. Schematic presentation of deep cryogenic processing cycle for DCT24 sample.
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Fig. 4. FSEM micrographs of carbides: (a) CHT and (b) DCT24 samples.
Fig. 3. SEM micrographs of carbides: (a) CHT, (b) DCT24 and (C) SCT24 samples.
To compare the effect of holding time in the deep cryogenic temperature, six different holding times (0, 6, 24, 48, 72 and 168 h) were employed. The nomenclature of the heat treatment samples, which is referred henceforth, is summarized in Table 2. The surfaces were observed by an optical microscope (Olympus PGM3) after etching in a 100 ml H2O, 10 g K3Fe(CN)6 and 10 g NaOH. This etchant is used to darken carbides. The carbides percentage have been estimated using an image analyzing software. For more studies, the microstructure was analyzed using field emission scanning electron microscopy (FSEM JEOL2010). For better contrast, the samples were etched in 100 ml ethanol, 100 ml HCl and 5 g CuCl2, for 20 s before bringing into the FSEM chamber.
The phase analyses were conducted by an X-ray diffractometer (Philips PW3710 diffractometer) with Cu Ka radiation. The volume fraction of retained austenite was estimated in according with ASTM standard E975-00 [20]. The bulk hardness was measured using Rockwell and Vickers hardness testers. At least 12 readings were considered in each sample for estimating the average value of the macro-hardness. The wear behavior was studied to determine: (i) wear rate, (ii) morphology of wear derbies and (iii) the morphology of the wornout surface of the specimens. The wear tests were carried out using a pin-on-disk wear tester. In this type of apparatus, the disk rotates and the pin is fixed. The wear tests were carried out using two normal loads (FN) of 120 and 160 N at three different sliding velocities (Vs) of 0.1, 0.15 and 0.2 m/s at 25 ± 4 °C up to 1000 m. The wear rate was calculated as a non-dimensional parameter incorporating the volume loss, distance and load by the following equation:
W r ¼ Dm=ðqLF N Þ 103
ð1Þ
where Wr is the wear rate in mm3/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. After the wear experiments, the disks worn-out surfaces were observed by scanning electron microscopy (SEM Seron AIS-2100) to determine the extent of the surface damage, change in the surface morphology and the dominant wear mechanism. The wear derbies also examined by SEM to investigate the morphological characteristics.
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Fig. 5. XRD pattern of the 80CrMo12 5 tool steel: (a) after quenching and (b) after conventional and cryogenic heat treatment.
3. Results and discussion 3.1. Microstructure Fig. 2 shows the optical micrographs of the 80CrMo12 5 tool steel. After etching in 100 ml H2O, 10 g K3Fe(CN)6 and 10 g NaOH,
darkened carbides can be distinct from other phases. This micrograph shows that the carbides percentage of the DCT24 and CHT samples is different. In the CHT sample (Fig. 2a), the carbides percentage was 3.8–4%, but in the DCT24 sample (Fig. 2b) this percentage increased up to about 6%. As a result of austenite to martensite transformation at the deep cryogenic temperature
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Table 3 The retained austenite percentage of the 80CrMo12 5 tool steel after heat treatment. Sample
Retained austenite percentage (%)
CHT SCT24 DCT24
12 6 Less than the diffractometer detection limit (<1%)
Table 4 The hardness (HRC and HV) of the heat treated 80CrMo12 5 tool steel. Sample
Mean hardness (HV)
Mean hardness (HRC)
CHT SCT24 Instant DCT DCT6 DCT24 DCT48 DCT72 DCT168
785 ± 15 805 ± 7 828 ± 9 876 ± 12 895 ± 6 912 ± 8 905 ± 6 886 ± 7
63.5 ± 0.1 64.2 ± 0.3 65.1 ± 0.2 65.8 ± 0.4 66.2 ± 0.4 66.6 ± 0.3 66.3 ± 0.1 66.1 ± 0.2
accompanied by high lattice contraction in the long holding times, new twins and dislocations are produced [7]. This is also due to micro-internal stresses and the different expansion coefficient of martensite and neighbor austenite [7]. The lattice distortion forces carbon atoms to diffuse or segregate at the nearby crystal defects. These segregated regions are reputed as new sites for carbides nucleation. This new carbides increase the percentage and homogeneity of the carbides in DCTs [4,5,10]. In the shallow cryogenic temperature (80 °C), this contraction is not enough to affect the carbide distribution and percentage (Fig. 3). The better carbide distribution in the deep cryogenic treatment temperature is revealed in the FSEM micrographs of the DCT24 and CHT samples (Fig. 4). These pictures clearly show that new carbides appear in the deep cryogenically treated samples. The X-ray diffraction (XRD) analysis of the CHT sample (Fig. 5) shows that the microstructure of the 80CrMo12 5 tool steel consists of austenite, martensite, Cr3C and Si5C3. The 100 ml H2O, 10 g K3Fe(CN)6 and 10 g NaOH etchant
Fig. 7. FSEM micrographs of carbides: (a) DCT48 and (b) DCT168.
reveals the carbides and does not affect the other phases. The volume fraction of the carbides was determined by image analyses on
Fig. 6. Variation of HRC versus holding time for DCT samples.
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Fig. 8. Variation of wear rate of the heat treated 80CrMo12 5 tool steel versus sliding distance under 120 N load at V = 0.1 m/s.
Fig. 9. Variation of wear rate of the heat treated 80CrMo12 5 tool steel versus sliding velocity under 120 N load.
the optical microscope micrographs (Fig. 2). The austenite and martensite volume fraction was determined using direct method according to ASTM standard E975-00 standard [20]. The retained austenite percentages are listed in Table 3. It can be seen that the retained austenite percentage decreases due to the cryogenic treatments. In the DCT samples, the retained austenite percentage is lower than the diffractometer detection limit.
3.2. Hardness The bulk hardness variation (HRC and HV) of the samples is summarized in Table 4. These results showed that in the cryogenic treatments, the hardness increases. This increase in the SCT samples is slight owing to the retained austenite reduction. In the DCT samples, the increase in hardness continues, attributed to
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Fig. 10. Variation of wear rate of the heat treated 80CrMo12 5 tool steel versus sliding distance under 160 N load at V = 0.1 m/s.
the retained austenite elimination, more homogenized carbide distribution and higher carbide percentage. The hardness variation versus the holding time at 196 °C is demonstrated in Fig. 6. This figure depicts that the hardness of the DCT samples initially increases with increasing the holding time, reaches a maximum at 48 h and then decreases with further holding. This hardness reduction is a result of the diffusion of the atoms that involved in the precipitation process. It means that holding time equal to 48 h is the saturation limit of carbide dispersion. In the longer holding times, the progress of the carbon segregation increases carbide
precipitates sizes, thereby decreasing the number of the carbides and their homogenized distribution (Fig. 7). This weaker carbides distribution decreases the hardness of the samples that are kept for the longer periods in the deep cryogenic temperatures. These results showed that in spite of general though that more holding times make more homogenized carbide distribution, there is an optimum holding time that beyond that the hardness and mechanical properties decrease. In this tool steel, this optimum time is 48 h. Most of researches, that concluded that more holding times are better, studied only DCT for two or three holding times and
Fig. 11. Variation of wear rate of the heat treated 80CrMo12 5 tool steel versus sliding velocity under 160 N load.
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Fig. 12. Wear rate improvement percentage of the heat treated samples under 120 N load at V = 0.1 m/s.
did not consider long ranges of times. However, Das et al. [21] showed that the optimum time for the best mechanical properties of deep cryogenically treated D2 tool steel is 36 h.
3.3. Wear behavior The wear characteristics of the specimens were assessed by estimating the wear rate with respect to sliding distance, and observing the worn surface and wear derbies. The results obtained from the wear tests at FN = 120 N are presented in Figs. 8 and 9. It is observed in Fig. 8 that the cryogenic treatments decrease the wear rate. In the SCT samples, the wear resistance is improved as a result of the decrease of retained austenite. The wear resistance improvement of the DCT samples is higher than that of the SCT samples. The localized diffusion process generated by microscopic internal stresses during cooling causes to the segregation of carbon and alloying elements to defects [5,7]. Hence, more improvement in the DCT samples is a result of the retained austenite elimination, higher carbide percentage and more homogenized carbide distribution. The wear resistance of the DCT samples increases with increasing holding time in the deep cryogenically treated temperature (196 °C) to 48 h. In this interval (0–48 h), the longer holding times force more atoms to diffuse out to neighbor defects, resulting in the creation of more carbide nucleation sites [5]. In the holding times longer than 48 h, this segregation gave rise to the extreme growth of the precipitated carbides, thereby weakening the carbides distribution. The weaker carbide distribution affects the wear resistance of the specimens and increases the wear rate. Fig. 9 suggests that the wear rate of all the specimens increases with increasing the sliding velocity at a constant FN. Because of more interface contact between the wear samples and pin, the wear rate increases with the load. The variation in the wear rate for the identical sliding distance and velocities for FN = 160 N is illustrated in Figs. 10 and 11. These diagrams show that the wear resistance of the SCT sample is better than that of CHT samples. Additionally, in the deep cryogenic treatments, the DCT48 sample exhibits the highest wear resistance and hardness amongst all the specimens considered in this study. Figs. 10 and 11 also display that in the deep cryogenic temperature, the
wear rate initially decreases up to 48 h and then increases with increasing the holding time; i.e. the DCT48 sample exhibits the minimum wear rate for all the velocities and loads. In the holding times longer than 48 h, the wear resistance decreases due to the weaker carbide distribution. The increase of FN from 120 to 160 N enhances the wear rate by 1.2–1.35 times. The friction coefficient of all the samples was 0.6 and did not change during the wear tests. The wear rates improvement percentage of heat treated samples are represented in Fig. 12 for FN = 120 N and V = 0.1 m/s. This figure clearly shows that the cryogenic treatments improve the wear resistance of the samples compared with the CHT. Comparing the DCT and SCT implies that the DCT increases the wear resistance drastically. It is observed that the wear resistance is improved by 5–12% due to the shallow cryogenic treatment (SCT24) and 37–52% due to the deep cryogenic treatment (DCT 24) with respect to the CHT. The SEM micrograph of the worn-out surfaces of the specimens (Fig. 13) shows that the predominant wear mechanism is adhesive. In the CHT samples (Fig. 13a) many projections are obviously observed. The DCT samples exhibit fewer projections; typically, the worn surface roughness of the DCT 48 sample is lowest among that of all the samples. The morphology of the collected wear derbies generated during the wear tests (Fig. 14) shows that in the cryogenically treated samples, the wear derbies are smaller and do not show any plastic deformation, in contrast to the CHT sample derbies. In the CHT sample, the collected derbies are big metallic plates with plastic deformation signs and dimples (Fig. 14a). As a result of the more brittle structure of the DCT samples, the collected wear derbies are smaller and show fewer plastic deformation edges compared with the CHT sample (Fig. 14b).
4. Conclusion The effect of cryogenic treatments on the wear behavior of 80CrMo12 5 tool steel was studied using two different temperatures: 80 °C as the shallow cryogenic temperature and 196 °C as the deep cryogenic temperature. The cryogenic treatments increase the wear resistance and harnesses. Shallow cryogenic treatment (SCT) decreased the retained austenite by 6% for the SCT24 sample. In deep cryogenic treatment (DCT), as well as the retained
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Fig. 13. SEM image of the worn-out surface of 80CrMo12 5 tool steel sample under 120 N load and 0.1 m/s sliding velocity after 1000 m sliding: (a) CHT, (b) SCT24, (c) instant DCT, (d) DCT6, (e) DCT24, (f) DCT48, (g) DCT72 and (h) DCT168.
austenite elimination, the carbides percentage increased. This increase was about 2% and the carbides distribution became more homogenized. This led to an increase in the hardness and the wear resistance of the samples. The hardness and wear resistance of the DCT samples depend on the holding time, showing a maximum at
48 h holding time. The decrease beyond 48 h was attributed to the decrease in the carbides homogeneous distribution. The predominant wear mechanism was adhesive; and the collected wear derbies of the cryogenically treated samples were more brittle and smaller.
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Fig. 14. SEM micrograph of wear derbies of the heat treated samples under 120 N load and sliding velocity of 0.1 m/s: (a) CHT and (b) DCT24.
References [1] Molinari A, Pellizzari M, Gialanella S, Straffelini G, Stiasny KH. Effect of deep cryogenic treatment on the mechanical properties of tool steels. J Mater Process Technol 2001;118:350–5.
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[2] Leskovsek V, Kalin M, Vizˇintin J. Influence of deep-cryogenic treatment on wear resistance of vacuum heat-treated HSS. Vacuum 2006;80:507–18. [3] Meng F, Tagashira K, Azuma R, Sohma H. Role of eta-carbide precipitation in the wear resistance improvements of Fe-12-Cr-Mo-V-1.4C tool steel by cryogenic treatment. ISIJ Int 1994;34:205–11. [4] He Y, Yang K, Qu W, Kong F, Su G. Strengthening and toughening of a 2800MPa grade maraging steel. J Mater Lett 2002;56:763–9. [5] Stratton PF. Optimising nano-carbide precipitation in tool steels. Mater Sci Eng A 2007;809:449–51. [6] Das D, Ray KK, Dutta AK. Influence of temperature of sub-zero treatments on the wear behaviour of die steel. Wear 2009;267:1361–70. [7] Huang JF, Zhu YT, Liao XZ, Beyerlein IJ, Bourje MA, Mitchell TE. Microstructure of cryogenic treated M2 tool steel. Mater Sci Eng A 2003;339:241–4. [8] Firouzdor V, Nejati E, Khomamizadeh F. Effect of deep cryogenic treatment on wear resistance and tool life of M2 HSS drill. J Mater Process Technol 2008;206:467–72. [9] Mohan Lal D, Renganarayanan S, Kalanidhi A. Cryogenic treatment to augment wear resistance of tool and die steel. Cryogenics 2001;41:149–55. [10] Akhbarizadeh A, Shafyei A, Golozar MA. Effects of cryogenic treatment on wear behavior of D6 tool steel. Mater Design 2009;30:3259–64. [11] 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. [12] Preciado M, Bravo PM, Alegre JM. Effect of low temperature tempering prior cryogenic treatment on carburized steels. J Mater Process Technol 2006;176:41–4. [13] Wagner JA. Correlation of mechanical properties with metallurgical structure for 18Ni 200 grade maraging steel at room and cryogenic temperatures. Cryogenics 1991;31:780–5. [14] Liu HH, Wang J, Yang HS, Shen BL, Gao SJ, Huang SJ. Effects of deep cryogenic treatment on property of 3Cr13Mo1V1.5 high chromium cast iron. Mater Design 2007;28:1059–64. [15] Liu HH, Wang J, Yang HS, Shen BL, Gao SJ, Huang SJ. Effect of cryogenic treatment on property of 14Cr2Mn2V high chromium cast iron subjected to subcritical treatment. J Iron Steel Res Int 2006;13:43–8. [16] Yong AYL, Seah KHW, Rahman M. Performance evaluation of cryogenically treated tungsten carbide tools in turning. Int J Mach Tool Manu 2006;46:2051–6. [17] Indumathi J, Bijwe J, Ghosh AK, Fahim M, Krishnaraj N. Wear of cryo-treated engineering polymers and composites. Wear 1999;Part1:343:225–9. [18] Speich GR. Ferrous martensitic structures. ASM handbook, vol. 9; 1985. [19] Smoloikov EA, Tkachenko VI. How cold treatment in liquid nitrogen affects cutting tool life. Mach Tooling 1980;51:22–4. [20] ASTM E975-00: standard practice for X-ray determination of retained austenite in steel with near random crystallographic orientation, ASTM book of standards, V 03.01, West Conshohocken, PA, United States; 2004. [21] Das D, Dutta AK, Ray KK. Optimization of the duration of cryogenic processing to maximize wear resistance of AISI D2 steel. Cryogenics 2009;49:184.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Influence of different cryotreatments on tribological behavior of 80CrMo12 5 cold work tool steel K. Amini a,*, S. Nategh a, A. Shafyei b a b
Department of Materials Engineering, Science and Research Branch, Islamic Azad University (IAU), Tehran, Iran Department of Materials Engineering, Isfahan University of Technology, Isfahan, Iran
a r t i c l e
i n f o
Article history: Received 1 March 2010 Accepted 13 May 2010 Available online 20 May 2010 Keywords: Ferrous metals and alloys (A) Heat treatments (C) Wear (E)
a b s t r a c t This experimental study investigated the effect of cryogenic treatments on the wear behavior of 80CrMo12 5 tool steel. For this purpose, two different cryogenic temperatures were used: 80 °C as the shallow cryogenic temperature and 196 °C as the deep cryogenic temperature. The results showed that the cryogenic treatments decrease retained austenite, which is more effective in the case of the deep cryogenic treatment (DCT). As a result, a remarkable improvement in the wear resistance of the cryogenically treated specimens was observed. In addition, DCT increases the percentage of carbides and their homogeneity in distribution. An optimum holding time was found in the deep cryogenic temperature, in which the hardness and wear resistance show maximum values. Moreover, the wear debris and worn surfaces showed that the dominant mechanism in the wear test is adhesive. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Cryogenic treatment is a supplementary heat treatment that is performed on some tool steels before tempering as an effective method for decreasing residual stress, retained austenite and increasing wear life. Two types of cryogenic treatments are generally applied as follows: (1) the shallow cryogenic treatment which is performed between 60 °C and 90 °C; (2) the deep cryogenic treatment that is conducted at temperatures below 125 °C [1]. The cryogenic treatment consists of controlled cooling of conventionally hardened specimens to a selected temperature, holding for a certain period, followed by controlled heating back to the ambient temperature for subsequent tempering. The cryogenic treatment enhances the transformation of retained austenite (as a soft and unstable phase) to martensite (as a promising phase) and subsequently increases the hardness and wear resistance [2]. In the deep cryogenic temperatures, as well as retained austenite elimination, fine dispersed eta (g) carbides are precipitated. This higher proportion and more homogenized distribution of carbides are due to the crystal lattice contraction. In the deep cryogenic temperatures, the lattice contraction forces carbon atoms to diffuse out to neighbor dislocations and defects [3]. Moreover, some new dislocations are created in the deep cryogenic treatment as a result of a difference in the thermal expansion of austenite and martensite. This new dislocations * Corresponding author. Tel.: +98 9131651659; fax: +98 3112351525. E-mail address: [email protected] (K. Amini). 0261-3069/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2010.05.028
provide suitable places for the segregation of carbon atoms and subsequently carbide nucleation in tempering. Thus, during tempering, these carbon atoms would produce new carbides, thereby leading to more homogenized carbide distribution [4–6]. Several investigators studied the cryogenic process and compared the effect of the cryogenic treatment on the wear behavior, hardness, tensile and other mechanical properties of different materials. The cryogenic treatment is conducted on different materials including tools steels [7–10], carburized steels [11,12], maraging steel [4,13], cast iron [14,15], tungsten carbide [16] and polymers [17]. Recent studies showed that the cryogenic treatment generally improves the wear resistance and hardness of tool steels. The wear resistance improvement varies from a few to a few hundred percentages in different kinds of steel [2–10]. In contrast, some researchers have been skeptical about the process and claimed that there is no noticeable difference in steels after and before the cryogenic treatment [18,19]. The present work compared the wear resistance of 80CrMo12 5 tool steel samples processed by conventional heat treatment (CHT), shallow cryogenic treatment (SCT) and deep cryogenic treatment (DCT) using a pin-on-disk wear tester. Moreover, these experiments tried to unfold the influence of the duration of the deep cryogenic treatment on the structure and wear behavior of the 80CrMo12 5 tool steel. To reveal the effect of cryogenic holding time and temperature on the wear behavior of the 80CrMo12 5 tool steel, the microstructure, hardness, morphology of wear surface and wear derbies were evaluated.
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Table 1 Chemical composition of the 80CrMo12 5 tool steel (wt.%). Elements
Wt.%
%C %Si %Mn %Mo %Cr %S %P %Fe
0.8 0.85 0.25 0.5 3.06 <0.005 <0.009 Remaining
Table 2 Heat treatment condition of the 80CrMo12 5 tool steel. Sample no.
Heat treatment
Nomenclature
1 2 3
Conventional heat treatment Shallow cryogenically treated at 80 °C for 24 h Deep cryogenically treated at 196 °C for 0 h (warm up the sample after reaching at 196 °C) Deep cryogenically treated at 196 °C for 6 h Deep cryogenically treated at 196 °C for 24 h Deep cryogenically treated at 196 °C for 48 h Deep cryogenically treated at 196 °C for 72 h Deep cryogenically treated at 196 °C for 168 h
CHT SCT24 Instant DCT
4 5 6 7 8
DCT6 DCT24 DCT48 DCT72 DCT168
2. Experiments The experimental testing was conducted on the 80CrMo12 5 commercial tool steel with a nominal composition reported in Table 1. The wear test samples were cut into disk shapes (dia 5 cm 0.4 cm) using wire electron-discharge machining. To reach a uniform and smooth wear surface, the samples were machined and then ground up to 600 mesh papers, reaching a 0.4 lm surface roughness. For conventional heat treatment (CHT), the samples were preheated at 620 °C for 20 min and then austenitized at 920 °C for 20 min. Then, the samples were quenched in oil to room temperature and tempered at 150 °C for 3 h. To compare the effect of the cryogenic treatment on the mechanical properties of the 80CrMo12 5 tool steel, the samples were cryogenically treated in two different temperatures. For the shallow cryogenic treatment, after quenching, the samples were
Fig. 2. Optical microscope images of the carbide particles: (a) CHT and (b) DCT24 samples.
cooled to 80 °C with the cooling rate of 1 °C/min. The samples were held at the same temperature for 24 h and then heated up to room temperature. Tempering was conducted at 150 °C for 3 h (SCT24). For the deep cryogenic treatment, the samples were uniformly cooled to 196 °C with the cooling rate of 1 °C/min, held for 24 h and heated up to room temperature with the heating rate of 1 °C/min. Afterward, the samples were tempered at 150 °C for 3 h (DCT24). The details of DCT24 are illustrated in Fig. 1.
Fig. 1. Schematic presentation of deep cryogenic processing cycle for DCT24 sample.
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Fig. 4. FSEM micrographs of carbides: (a) CHT and (b) DCT24 samples.
Fig. 3. SEM micrographs of carbides: (a) CHT, (b) DCT24 and (C) SCT24 samples.
To compare the effect of holding time in the deep cryogenic temperature, six different holding times (0, 6, 24, 48, 72 and 168 h) were employed. The nomenclature of the heat treatment samples, which is referred henceforth, is summarized in Table 2. The surfaces were observed by an optical microscope (Olympus PGM3) after etching in a 100 ml H2O, 10 g K3Fe(CN)6 and 10 g NaOH. This etchant is used to darken carbides. The carbides percentage have been estimated using an image analyzing software. For more studies, the microstructure was analyzed using field emission scanning electron microscopy (FSEM JEOL2010). For better contrast, the samples were etched in 100 ml ethanol, 100 ml HCl and 5 g CuCl2, for 20 s before bringing into the FSEM chamber.
The phase analyses were conducted by an X-ray diffractometer (Philips PW3710 diffractometer) with Cu Ka radiation. The volume fraction of retained austenite was estimated in according with ASTM standard E975-00 [20]. The bulk hardness was measured using Rockwell and Vickers hardness testers. At least 12 readings were considered in each sample for estimating the average value of the macro-hardness. The wear behavior was studied to determine: (i) wear rate, (ii) morphology of wear derbies and (iii) the morphology of the wornout surface of the specimens. The wear tests were carried out using a pin-on-disk wear tester. In this type of apparatus, the disk rotates and the pin is fixed. The wear tests were carried out using two normal loads (FN) of 120 and 160 N at three different sliding velocities (Vs) of 0.1, 0.15 and 0.2 m/s at 25 ± 4 °C up to 1000 m. The wear rate was calculated as a non-dimensional parameter incorporating the volume loss, distance and load by the following equation:
W r ¼ Dm=ðqLF N Þ 103
ð1Þ
where Wr is the wear rate in mm3/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. After the wear experiments, the disks worn-out surfaces were observed by scanning electron microscopy (SEM Seron AIS-2100) to determine the extent of the surface damage, change in the surface morphology and the dominant wear mechanism. The wear derbies also examined by SEM to investigate the morphological characteristics.
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Fig. 5. XRD pattern of the 80CrMo12 5 tool steel: (a) after quenching and (b) after conventional and cryogenic heat treatment.
3. Results and discussion 3.1. Microstructure Fig. 2 shows the optical micrographs of the 80CrMo12 5 tool steel. After etching in 100 ml H2O, 10 g K3Fe(CN)6 and 10 g NaOH,
darkened carbides can be distinct from other phases. This micrograph shows that the carbides percentage of the DCT24 and CHT samples is different. In the CHT sample (Fig. 2a), the carbides percentage was 3.8–4%, but in the DCT24 sample (Fig. 2b) this percentage increased up to about 6%. As a result of austenite to martensite transformation at the deep cryogenic temperature
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Table 3 The retained austenite percentage of the 80CrMo12 5 tool steel after heat treatment. Sample
Retained austenite percentage (%)
CHT SCT24 DCT24
12 6 Less than the diffractometer detection limit (<1%)
Table 4 The hardness (HRC and HV) of the heat treated 80CrMo12 5 tool steel. Sample
Mean hardness (HV)
Mean hardness (HRC)
CHT SCT24 Instant DCT DCT6 DCT24 DCT48 DCT72 DCT168
785 ± 15 805 ± 7 828 ± 9 876 ± 12 895 ± 6 912 ± 8 905 ± 6 886 ± 7
63.5 ± 0.1 64.2 ± 0.3 65.1 ± 0.2 65.8 ± 0.4 66.2 ± 0.4 66.6 ± 0.3 66.3 ± 0.1 66.1 ± 0.2
accompanied by high lattice contraction in the long holding times, new twins and dislocations are produced [7]. This is also due to micro-internal stresses and the different expansion coefficient of martensite and neighbor austenite [7]. The lattice distortion forces carbon atoms to diffuse or segregate at the nearby crystal defects. These segregated regions are reputed as new sites for carbides nucleation. This new carbides increase the percentage and homogeneity of the carbides in DCTs [4,5,10]. In the shallow cryogenic temperature (80 °C), this contraction is not enough to affect the carbide distribution and percentage (Fig. 3). The better carbide distribution in the deep cryogenic treatment temperature is revealed in the FSEM micrographs of the DCT24 and CHT samples (Fig. 4). These pictures clearly show that new carbides appear in the deep cryogenically treated samples. The X-ray diffraction (XRD) analysis of the CHT sample (Fig. 5) shows that the microstructure of the 80CrMo12 5 tool steel consists of austenite, martensite, Cr3C and Si5C3. The 100 ml H2O, 10 g K3Fe(CN)6 and 10 g NaOH etchant
Fig. 7. FSEM micrographs of carbides: (a) DCT48 and (b) DCT168.
reveals the carbides and does not affect the other phases. The volume fraction of the carbides was determined by image analyses on
Fig. 6. Variation of HRC versus holding time for DCT samples.
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Fig. 8. Variation of wear rate of the heat treated 80CrMo12 5 tool steel versus sliding distance under 120 N load at V = 0.1 m/s.
Fig. 9. Variation of wear rate of the heat treated 80CrMo12 5 tool steel versus sliding velocity under 120 N load.
the optical microscope micrographs (Fig. 2). The austenite and martensite volume fraction was determined using direct method according to ASTM standard E975-00 standard [20]. The retained austenite percentages are listed in Table 3. It can be seen that the retained austenite percentage decreases due to the cryogenic treatments. In the DCT samples, the retained austenite percentage is lower than the diffractometer detection limit.
3.2. Hardness The bulk hardness variation (HRC and HV) of the samples is summarized in Table 4. These results showed that in the cryogenic treatments, the hardness increases. This increase in the SCT samples is slight owing to the retained austenite reduction. In the DCT samples, the increase in hardness continues, attributed to
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Fig. 10. Variation of wear rate of the heat treated 80CrMo12 5 tool steel versus sliding distance under 160 N load at V = 0.1 m/s.
the retained austenite elimination, more homogenized carbide distribution and higher carbide percentage. The hardness variation versus the holding time at 196 °C is demonstrated in Fig. 6. This figure depicts that the hardness of the DCT samples initially increases with increasing the holding time, reaches a maximum at 48 h and then decreases with further holding. This hardness reduction is a result of the diffusion of the atoms that involved in the precipitation process. It means that holding time equal to 48 h is the saturation limit of carbide dispersion. In the longer holding times, the progress of the carbon segregation increases carbide
precipitates sizes, thereby decreasing the number of the carbides and their homogenized distribution (Fig. 7). This weaker carbides distribution decreases the hardness of the samples that are kept for the longer periods in the deep cryogenic temperatures. These results showed that in spite of general though that more holding times make more homogenized carbide distribution, there is an optimum holding time that beyond that the hardness and mechanical properties decrease. In this tool steel, this optimum time is 48 h. Most of researches, that concluded that more holding times are better, studied only DCT for two or three holding times and
Fig. 11. Variation of wear rate of the heat treated 80CrMo12 5 tool steel versus sliding velocity under 160 N load.
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Fig. 12. Wear rate improvement percentage of the heat treated samples under 120 N load at V = 0.1 m/s.
did not consider long ranges of times. However, Das et al. [21] showed that the optimum time for the best mechanical properties of deep cryogenically treated D2 tool steel is 36 h.
3.3. Wear behavior The wear characteristics of the specimens were assessed by estimating the wear rate with respect to sliding distance, and observing the worn surface and wear derbies. The results obtained from the wear tests at FN = 120 N are presented in Figs. 8 and 9. It is observed in Fig. 8 that the cryogenic treatments decrease the wear rate. In the SCT samples, the wear resistance is improved as a result of the decrease of retained austenite. The wear resistance improvement of the DCT samples is higher than that of the SCT samples. The localized diffusion process generated by microscopic internal stresses during cooling causes to the segregation of carbon and alloying elements to defects [5,7]. Hence, more improvement in the DCT samples is a result of the retained austenite elimination, higher carbide percentage and more homogenized carbide distribution. The wear resistance of the DCT samples increases with increasing holding time in the deep cryogenically treated temperature (196 °C) to 48 h. In this interval (0–48 h), the longer holding times force more atoms to diffuse out to neighbor defects, resulting in the creation of more carbide nucleation sites [5]. In the holding times longer than 48 h, this segregation gave rise to the extreme growth of the precipitated carbides, thereby weakening the carbides distribution. The weaker carbide distribution affects the wear resistance of the specimens and increases the wear rate. Fig. 9 suggests that the wear rate of all the specimens increases with increasing the sliding velocity at a constant FN. Because of more interface contact between the wear samples and pin, the wear rate increases with the load. The variation in the wear rate for the identical sliding distance and velocities for FN = 160 N is illustrated in Figs. 10 and 11. These diagrams show that the wear resistance of the SCT sample is better than that of CHT samples. Additionally, in the deep cryogenic treatments, the DCT48 sample exhibits the highest wear resistance and hardness amongst all the specimens considered in this study. Figs. 10 and 11 also display that in the deep cryogenic temperature, the
wear rate initially decreases up to 48 h and then increases with increasing the holding time; i.e. the DCT48 sample exhibits the minimum wear rate for all the velocities and loads. In the holding times longer than 48 h, the wear resistance decreases due to the weaker carbide distribution. The increase of FN from 120 to 160 N enhances the wear rate by 1.2–1.35 times. The friction coefficient of all the samples was 0.6 and did not change during the wear tests. The wear rates improvement percentage of heat treated samples are represented in Fig. 12 for FN = 120 N and V = 0.1 m/s. This figure clearly shows that the cryogenic treatments improve the wear resistance of the samples compared with the CHT. Comparing the DCT and SCT implies that the DCT increases the wear resistance drastically. It is observed that the wear resistance is improved by 5–12% due to the shallow cryogenic treatment (SCT24) and 37–52% due to the deep cryogenic treatment (DCT 24) with respect to the CHT. The SEM micrograph of the worn-out surfaces of the specimens (Fig. 13) shows that the predominant wear mechanism is adhesive. In the CHT samples (Fig. 13a) many projections are obviously observed. The DCT samples exhibit fewer projections; typically, the worn surface roughness of the DCT 48 sample is lowest among that of all the samples. The morphology of the collected wear derbies generated during the wear tests (Fig. 14) shows that in the cryogenically treated samples, the wear derbies are smaller and do not show any plastic deformation, in contrast to the CHT sample derbies. In the CHT sample, the collected derbies are big metallic plates with plastic deformation signs and dimples (Fig. 14a). As a result of the more brittle structure of the DCT samples, the collected wear derbies are smaller and show fewer plastic deformation edges compared with the CHT sample (Fig. 14b).
4. Conclusion The effect of cryogenic treatments on the wear behavior of 80CrMo12 5 tool steel was studied using two different temperatures: 80 °C as the shallow cryogenic temperature and 196 °C as the deep cryogenic temperature. The cryogenic treatments increase the wear resistance and harnesses. Shallow cryogenic treatment (SCT) decreased the retained austenite by 6% for the SCT24 sample. In deep cryogenic treatment (DCT), as well as the retained
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Fig. 13. SEM image of the worn-out surface of 80CrMo12 5 tool steel sample under 120 N load and 0.1 m/s sliding velocity after 1000 m sliding: (a) CHT, (b) SCT24, (c) instant DCT, (d) DCT6, (e) DCT24, (f) DCT48, (g) DCT72 and (h) DCT168.
austenite elimination, the carbides percentage increased. This increase was about 2% and the carbides distribution became more homogenized. This led to an increase in the hardness and the wear resistance of the samples. The hardness and wear resistance of the DCT samples depend on the holding time, showing a maximum at
48 h holding time. The decrease beyond 48 h was attributed to the decrease in the carbides homogeneous distribution. The predominant wear mechanism was adhesive; and the collected wear derbies of the cryogenically treated samples were more brittle and smaller.
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Fig. 14. SEM micrograph of wear derbies of the heat treated samples under 120 N load and sliding velocity of 0.1 m/s: (a) CHT and (b) DCT24.
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