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Characterization of cryogenic heat treated Vanadis 4 PM cold work tool steel
Vacuum 86 (2011) 370e373
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Characterization of cryogenic heat treated Vanadis 4 PM cold work tool steel F.K. Arslan a, I. Altinsoy b, A. Hatman a, M. Ipek b, S. Zeytin b, C. Bindal b, * a b
Assab Steel and Heat Treatment Company (Turkish Branch), Dudullu-Istanbul, Turkey Sakarya University, Engineering Faculty, Department of Metallurgical and Materials Engineering, Esentepe Campus, 54187 Esentepe-Sakarya, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2011 Received in revised form 22 July 2011 Accepted 23 July 2011
The amount of retained austenite in the quenched cold work tool steel sample is 17.7%, in the condition of sub-zero heat treated and double tempered samples following by quenching is 1.9% determined by XRD analysis. The types of carbides (MC, M7C3, M23C6) present in the structure was determined by XRD and SEM-EDS analysis. The hardness of test samples were 865 HV(0.1) for quenched sample and 785 HV(0.1) forth sample subjected to sub-zero treatment and double tempered after the quenching. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Retained austenite Hardness Sub-zero treatment Cold work tool steel
1. Introduction Tool steels produced by powder metallurgy has extremely refined, homogenous microstructure such as homogenous distribution of primer alloying carbides came from production resulting as alloying process compared with their conventionally produced cast and wrought counterparts. Therefore, this tool steels perform more unique properties under the usage conditions than conventional ones. Cold work tool steels are highly alloyed type of steels in comparison with conventional standard steels, having properties such as high hardness, toughness and wear resistance and used in cutting blades, rolling planes, forming, deep drawing and spinning applications, die materials. Cold work tool steels find a very wide range of applications. In order to meet large demand varying compositions are essential, they are alloyed with chromium, vanadium, chrome-tungsten, chrome-vanadium, etc. In higher carbon content alloyed steels, the martensite finish temperature (Mf) is below 0 C, which means that at the end of the heat treatment, a low percentage of austenite is retained at room temperature. The retained austenite as a soft phase in steels could reduce the product life and, in working conditions, it can be transformed into martensite. This transformation results to a volume changing during use. In order to resolve the problems mentioned above, the deep cryogenic treatment is used to transform the retained
* Corresponding author. Tel.: þ90 264 295 57 59; fax: þ90 264 295 56 01. E-mail address: [email protected] (C. Bindal). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.07.066
austenite into martensite. As a result, the retained austenite is reduced and higher wear resistance is obtained. Cryogenic treatment, as an effective way to improve the properties of tool steels, is widely used for high precision parts and components since it enhances the transformation of austenite to martensite. Several papers have been published that cryogenic treatment can improve wear resistance of tool steels. The common practice identifiese60 Ce80 C as treatment temperature called shallow cryogenic treatment (SCT). Deep cryogenic treatment (DCT) ranging from 125 C to 196 C improves certain properties beyond the improvement obtained by normal cryogenic treatment. The greatest improvement in properties is obtained by carrying out the deep cryogenic treatment between quenching and tempering due to completion of transformation from austenite into martensite in addition to the formation of very fine carbides dispersed in the tempered martensite structure. Compared to alternative methods to extend tool life, DCT is an inexpensive one-time process. In contrast to coatings, it affects the whole volume of the treated materials. In recent years, a new kind of Vanadis 4 PM cold work tool steel manufactured by powder metallurgy has been widely used due to its high strength and high-toughness. The steel have secondary hardening effect after two times tempering at 525 C. However, the effect of cryogenic treatment on this type steel has not been studied extensively. In this work, DCT were applied to the patented Vanadis 4 PM cold work tool steel for investigating their effect on the properties of this steel which used in applications such as powder pressing, knives, substrate steel for surface coating and cold extrusion tooling [1e9].
F.K. Arslan et al. / Vacuum 86 (2011) 370e373 Table 1 The chemical composition of steel used (wt.%). Steel
C
Mn
Cr
Mo
V
Si
Vanadis 4 PM
1.40
0.40
4.70
3.50
3.70
0.40
2. Experimental details In this study, deep cryogenic treatment was applied to a commercial powder metallurgically produced Vanadis 4 PM cold work tool steel. The powder metallurgical (PM) route was chosen because segregation-free and more homogeneous microstructures with a higher cleanness can be achieved compared to a conventional metallurgical route. Furthermore, tool steels produced by conventional metallurgical route usually exhibit a banded carbide microstructure, which leads to differences in the mechanical properties and has to be considered when sampling. Accordingly, different mechanical properties can be achieved depending on whether samples were taken parallel or perpendicular to the direction of hot deformation. In addition, PM tool steels provide uniform spacing between single carbides in all directions. Thus, the scattering of test results due to anisotropic effects and an inhomogeneous distribution and size of carbides in small specimens is minimized [4].
371
The chemical composition of test materials is given in Table 1. Test materials used with nominal sizes of 10 10 10 mm3 were preheated at 650 C for 10 min and austenitized at 1040 C for 30 min, then hold at 170 C for period of 15 min and exposed to deep cryogenic treatment at 196 C in liquid nitrogen for 1 h and then double tempered for 30 min and 60 min, respectively. Fig. 1aeb show heat treatment cycles for quenched and quenched, deep cryogenic heat treated and double tempered test materials. Samples are coded as V1 for just quenched one and V2 for quenched, deep heat treated and double tempered one. The presence of carbide types (MC, M7C3, M23C6) were determined by SEMEDS analysis. SEM-EDS results were confirmed with XRD analysis. Alloy carbides (MC, M7C3, M23C6) in V2 samples were determined by SEM-EDS analysis. The amount of retained austenite of test materials determined by XRD and microhardness of them measured by microhardness technique.
3. Results and discussion It was observed that there are about.17.7 v/o retained austenite in quenched sample, and 1.9 v/o retained austenite in quenched, sub-zero heat treated in liquid nitrogen for 60 min and double
Fig. 1. Heat treatment cycles for test materials (a)V1; Only quenched (b) V2; quenched / sub-zero treatment / double tempered.
Fig. 2. XRD diffraction patterns of quenched and quenched, sub-zero heat treated in liquid nitrogen for 60 min and double tempered test materials.
372
F.K. Arslan et al. / Vacuum 86 (2011) 370e373
tempered test material which confirmed by XRD analysis technique (Fig. 2). That is, cryogenic treatment decreases the amount of retained austenite. Vanadis 4 cold work tool steels contain high amount of alloying elements (Cr, V, Mo, etc.) which remarkably reduce Mf temperature of the steel. Therefore, high amount of retained austenite remains in the body of the steel after quenching. In our study, the amount of retained austenite were calculated as 17.7% by volume percentage after quenching and this is high amount for his steel because of high proportion retained austenite decreases the hardness and wear resistance of this steel and it must be eliminated by the help of method such as deep cryogenic treatment. In the present study, the amount of retained austenite was reduced to a value of 1.9 v/o. that is acceptable level, by applying deep cryogenic treatment after quenching and also double tempering at 525 C contributed to reducing the amount of this austenite, too. SEM studies on quenched sample indicated that appeared massive regions around carbides are probably austenite (Fig. 3). It is possible to claim that during austenitizing period (phase-stage), some of primary (alloy) carbides dissolve in austenite which has a greater solubility and carbon and alloying elements diffuse to austenite close to carbides. So that closer regions to carbides have more C and carbide forming alloying elements. Therefore, these austenite regions have lower Ms temperature. It was found that there are spherical shaped alloying carbides (MC, M7C3, and M23C6) in both microstructures of test materials. While, in quenched, subzero heat treated and double tempered sample the amount of carbides increases and secondary carbides having submicron sizes take place during tempering. SEM-EDS results revealed that in quenched sample sharp cornered plates reflect martensite, spherical shaped regions belong to chromium carbide (M7C3eM23C6), Fig. 4. SEM-EDS analysis of quenched, sub-zero heat treated in liquid nitrogen for 60 min and double tempered sample.
and spherical shaped and small sized dark regions reflect the vanadium carbide (MC) (Fig. 4). It was found that in quenched, sub-zero heat treated and double tempered sample, gray and white regions which surrounding network-shaped carbides are tempered martensite. SEM-EDS studies showed that similar carbides are present in the microstructure as the same quenched sample but the amount of hard VC increases owing to secondary hardening during tempering process. Also, XRD results indicated that the amounts of retained austenite decreases as well as the amount of MC type carbides increase in quenched, sub-zero heat treated and double tempered test material. As it is known well cryogenic treatment increases hardness by decreasing retained austenite. In present study, it was found that the hardness of only quenched sample is about 865 HV whereas the hardness of quenched, sub-zero heat treated and double tempered sample is about 785 HV (Table 2). It is possible to say that the high hardness of only quenched sample is due to martensite morphology and dissolved alloying elements in martensite and austenite even if it has higher amount of retained austenite. The increase in martensite volume fraction is most likely responsible however cryogenic treatment could have encouraged carbide formation during tempering, leading to Table 2 The microhardness and the amount of retained austenite of test materials as a function of heat treatment.
Fig. 3. SEM micrograph and concerning EDS analysis of only quenched sample.
Heat Treatment
Retained Austenite (vol.%)
Microhardness (HV)
Q QþDCTþþDT
17.7 1.9
865 785
Q: Quenching, DCT: Deep Cryogenic treatment, DT: Double tempering.
F.K. Arslan et al. / Vacuum 86 (2011) 370e373
increased hardness. As a matter of fact that, previous studies done by Chi et al. [2] revealed that the degree of decreasing retained austenite by deep cryogenic treatment is lower than that by two times tempering. So, the mainly responsible for peak hardness during tempering at 520 C is the secondary precipitates of MC and/ or M6 C-carbide. Acknowledgements The authors thank to ASSAB Steel and Heat Treatment Company (Turkish Branch), ITU, GYTE and Sakarya University, Engineering Faculty, Department of Metallurgy and Materials Engineering for their valuable supports.
373
References [1] Prabhudev KH. Handbook of Heat Treatment of Steels. New Delhi: Tata McGraw-Hill Publishing Company Limited; 1992. [2] Chi H-X, Ma D-S, Yong Q-L, Wu L-Z, Zhang Z-P, Wang Y-W. Journal of Iron and Steel Research Internatıonal 2010;17(6):43e6. 59. [3] Molinari A, Pellizzari M, Gialanella S, Straffelini G, Stiasny KH. Journal of Materials Processing Technology 2001;118:350e5. [4] Oppenkowski A, Weber S, Theisen W. Journal of Materials Processing Technology 2010;210:1949e55. [5] Shaohong L, Yinzi X, Xiaochun W. Cryogenics 2010;50:89e92. [6] http://www.bohler-uddeholm.co.za/english/files/vanadis_4E-english_040403. pdf- July 2011. [7] http://www.ipmd.net/shop/PM_high_alloy_steels_and_PM_superalloys- July 2011. [8] Koneshlou M, Asl KM, Khomamizadeh F. Cryogenics 2011;51:55e61. [9] Zhirafar S, Rezaeian A, Pugh M. Journal of Materials Processing Technology 2007;186:298e303.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Characterization of cryogenic heat treated Vanadis 4 PM cold work tool steel F.K. Arslan a, I. Altinsoy b, A. Hatman a, M. Ipek b, S. Zeytin b, C. Bindal b, * a b
Assab Steel and Heat Treatment Company (Turkish Branch), Dudullu-Istanbul, Turkey Sakarya University, Engineering Faculty, Department of Metallurgical and Materials Engineering, Esentepe Campus, 54187 Esentepe-Sakarya, Turkey
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2011 Received in revised form 22 July 2011 Accepted 23 July 2011
The amount of retained austenite in the quenched cold work tool steel sample is 17.7%, in the condition of sub-zero heat treated and double tempered samples following by quenching is 1.9% determined by XRD analysis. The types of carbides (MC, M7C3, M23C6) present in the structure was determined by XRD and SEM-EDS analysis. The hardness of test samples were 865 HV(0.1) for quenched sample and 785 HV(0.1) forth sample subjected to sub-zero treatment and double tempered after the quenching. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Retained austenite Hardness Sub-zero treatment Cold work tool steel
1. Introduction Tool steels produced by powder metallurgy has extremely refined, homogenous microstructure such as homogenous distribution of primer alloying carbides came from production resulting as alloying process compared with their conventionally produced cast and wrought counterparts. Therefore, this tool steels perform more unique properties under the usage conditions than conventional ones. Cold work tool steels are highly alloyed type of steels in comparison with conventional standard steels, having properties such as high hardness, toughness and wear resistance and used in cutting blades, rolling planes, forming, deep drawing and spinning applications, die materials. Cold work tool steels find a very wide range of applications. In order to meet large demand varying compositions are essential, they are alloyed with chromium, vanadium, chrome-tungsten, chrome-vanadium, etc. In higher carbon content alloyed steels, the martensite finish temperature (Mf) is below 0 C, which means that at the end of the heat treatment, a low percentage of austenite is retained at room temperature. The retained austenite as a soft phase in steels could reduce the product life and, in working conditions, it can be transformed into martensite. This transformation results to a volume changing during use. In order to resolve the problems mentioned above, the deep cryogenic treatment is used to transform the retained
* Corresponding author. Tel.: þ90 264 295 57 59; fax: þ90 264 295 56 01. E-mail address: [email protected] (C. Bindal). 0042-207X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2011.07.066
austenite into martensite. As a result, the retained austenite is reduced and higher wear resistance is obtained. Cryogenic treatment, as an effective way to improve the properties of tool steels, is widely used for high precision parts and components since it enhances the transformation of austenite to martensite. Several papers have been published that cryogenic treatment can improve wear resistance of tool steels. The common practice identifiese60 Ce80 C as treatment temperature called shallow cryogenic treatment (SCT). Deep cryogenic treatment (DCT) ranging from 125 C to 196 C improves certain properties beyond the improvement obtained by normal cryogenic treatment. The greatest improvement in properties is obtained by carrying out the deep cryogenic treatment between quenching and tempering due to completion of transformation from austenite into martensite in addition to the formation of very fine carbides dispersed in the tempered martensite structure. Compared to alternative methods to extend tool life, DCT is an inexpensive one-time process. In contrast to coatings, it affects the whole volume of the treated materials. In recent years, a new kind of Vanadis 4 PM cold work tool steel manufactured by powder metallurgy has been widely used due to its high strength and high-toughness. The steel have secondary hardening effect after two times tempering at 525 C. However, the effect of cryogenic treatment on this type steel has not been studied extensively. In this work, DCT were applied to the patented Vanadis 4 PM cold work tool steel for investigating their effect on the properties of this steel which used in applications such as powder pressing, knives, substrate steel for surface coating and cold extrusion tooling [1e9].
F.K. Arslan et al. / Vacuum 86 (2011) 370e373 Table 1 The chemical composition of steel used (wt.%). Steel
C
Mn
Cr
Mo
V
Si
Vanadis 4 PM
1.40
0.40
4.70
3.50
3.70
0.40
2. Experimental details In this study, deep cryogenic treatment was applied to a commercial powder metallurgically produced Vanadis 4 PM cold work tool steel. The powder metallurgical (PM) route was chosen because segregation-free and more homogeneous microstructures with a higher cleanness can be achieved compared to a conventional metallurgical route. Furthermore, tool steels produced by conventional metallurgical route usually exhibit a banded carbide microstructure, which leads to differences in the mechanical properties and has to be considered when sampling. Accordingly, different mechanical properties can be achieved depending on whether samples were taken parallel or perpendicular to the direction of hot deformation. In addition, PM tool steels provide uniform spacing between single carbides in all directions. Thus, the scattering of test results due to anisotropic effects and an inhomogeneous distribution and size of carbides in small specimens is minimized [4].
371
The chemical composition of test materials is given in Table 1. Test materials used with nominal sizes of 10 10 10 mm3 were preheated at 650 C for 10 min and austenitized at 1040 C for 30 min, then hold at 170 C for period of 15 min and exposed to deep cryogenic treatment at 196 C in liquid nitrogen for 1 h and then double tempered for 30 min and 60 min, respectively. Fig. 1aeb show heat treatment cycles for quenched and quenched, deep cryogenic heat treated and double tempered test materials. Samples are coded as V1 for just quenched one and V2 for quenched, deep heat treated and double tempered one. The presence of carbide types (MC, M7C3, M23C6) were determined by SEMEDS analysis. SEM-EDS results were confirmed with XRD analysis. Alloy carbides (MC, M7C3, M23C6) in V2 samples were determined by SEM-EDS analysis. The amount of retained austenite of test materials determined by XRD and microhardness of them measured by microhardness technique.
3. Results and discussion It was observed that there are about.17.7 v/o retained austenite in quenched sample, and 1.9 v/o retained austenite in quenched, sub-zero heat treated in liquid nitrogen for 60 min and double
Fig. 1. Heat treatment cycles for test materials (a)V1; Only quenched (b) V2; quenched / sub-zero treatment / double tempered.
Fig. 2. XRD diffraction patterns of quenched and quenched, sub-zero heat treated in liquid nitrogen for 60 min and double tempered test materials.
372
F.K. Arslan et al. / Vacuum 86 (2011) 370e373
tempered test material which confirmed by XRD analysis technique (Fig. 2). That is, cryogenic treatment decreases the amount of retained austenite. Vanadis 4 cold work tool steels contain high amount of alloying elements (Cr, V, Mo, etc.) which remarkably reduce Mf temperature of the steel. Therefore, high amount of retained austenite remains in the body of the steel after quenching. In our study, the amount of retained austenite were calculated as 17.7% by volume percentage after quenching and this is high amount for his steel because of high proportion retained austenite decreases the hardness and wear resistance of this steel and it must be eliminated by the help of method such as deep cryogenic treatment. In the present study, the amount of retained austenite was reduced to a value of 1.9 v/o. that is acceptable level, by applying deep cryogenic treatment after quenching and also double tempering at 525 C contributed to reducing the amount of this austenite, too. SEM studies on quenched sample indicated that appeared massive regions around carbides are probably austenite (Fig. 3). It is possible to claim that during austenitizing period (phase-stage), some of primary (alloy) carbides dissolve in austenite which has a greater solubility and carbon and alloying elements diffuse to austenite close to carbides. So that closer regions to carbides have more C and carbide forming alloying elements. Therefore, these austenite regions have lower Ms temperature. It was found that there are spherical shaped alloying carbides (MC, M7C3, and M23C6) in both microstructures of test materials. While, in quenched, subzero heat treated and double tempered sample the amount of carbides increases and secondary carbides having submicron sizes take place during tempering. SEM-EDS results revealed that in quenched sample sharp cornered plates reflect martensite, spherical shaped regions belong to chromium carbide (M7C3eM23C6), Fig. 4. SEM-EDS analysis of quenched, sub-zero heat treated in liquid nitrogen for 60 min and double tempered sample.
and spherical shaped and small sized dark regions reflect the vanadium carbide (MC) (Fig. 4). It was found that in quenched, sub-zero heat treated and double tempered sample, gray and white regions which surrounding network-shaped carbides are tempered martensite. SEM-EDS studies showed that similar carbides are present in the microstructure as the same quenched sample but the amount of hard VC increases owing to secondary hardening during tempering process. Also, XRD results indicated that the amounts of retained austenite decreases as well as the amount of MC type carbides increase in quenched, sub-zero heat treated and double tempered test material. As it is known well cryogenic treatment increases hardness by decreasing retained austenite. In present study, it was found that the hardness of only quenched sample is about 865 HV whereas the hardness of quenched, sub-zero heat treated and double tempered sample is about 785 HV (Table 2). It is possible to say that the high hardness of only quenched sample is due to martensite morphology and dissolved alloying elements in martensite and austenite even if it has higher amount of retained austenite. The increase in martensite volume fraction is most likely responsible however cryogenic treatment could have encouraged carbide formation during tempering, leading to Table 2 The microhardness and the amount of retained austenite of test materials as a function of heat treatment.
Fig. 3. SEM micrograph and concerning EDS analysis of only quenched sample.
Heat Treatment
Retained Austenite (vol.%)
Microhardness (HV)
Q QþDCTþþDT
17.7 1.9
865 785
Q: Quenching, DCT: Deep Cryogenic treatment, DT: Double tempering.
F.K. Arslan et al. / Vacuum 86 (2011) 370e373
increased hardness. As a matter of fact that, previous studies done by Chi et al. [2] revealed that the degree of decreasing retained austenite by deep cryogenic treatment is lower than that by two times tempering. So, the mainly responsible for peak hardness during tempering at 520 C is the secondary precipitates of MC and/ or M6 C-carbide. Acknowledgements The authors thank to ASSAB Steel and Heat Treatment Company (Turkish Branch), ITU, GYTE and Sakarya University, Engineering Faculty, Department of Metallurgy and Materials Engineering for their valuable supports.
373
References [1] Prabhudev KH. Handbook of Heat Treatment of Steels. New Delhi: Tata McGraw-Hill Publishing Company Limited; 1992. [2] Chi H-X, Ma D-S, Yong Q-L, Wu L-Z, Zhang Z-P, Wang Y-W. Journal of Iron and Steel Research Internatıonal 2010;17(6):43e6. 59. [3] Molinari A, Pellizzari M, Gialanella S, Straffelini G, Stiasny KH. Journal of Materials Processing Technology 2001;118:350e5. [4] Oppenkowski A, Weber S, Theisen W. Journal of Materials Processing Technology 2010;210:1949e55. [5] Shaohong L, Yinzi X, Xiaochun W. Cryogenics 2010;50:89e92. [6] http://www.bohler-uddeholm.co.za/english/files/vanadis_4E-english_040403. pdf- July 2011. [7] http://www.ipmd.net/shop/PM_high_alloy_steels_and_PM_superalloys- July 2011. [8] Koneshlou M, Asl KM, Khomamizadeh F. Cryogenics 2011;51:55e61. [9] Zhirafar S, Rezaeian A, Pugh M. Journal of Materials Processing Technology 2007;186:298e303.