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Hardness and toughness investigations of deep cryogenic treated cold work die steel
Cryogenics 50 (2010) 89–92
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Hardness and toughness investigations of deep cryogenic treated cold work die steel Shaohong Li *, Yinzi Xie, Xiaochun Wu School of Materials Science and Engineering, Shanghai Univ., Shanghai 200072, PR China
a r t i c l e
i n f o
Article history: Received 19 July 2009 Received in revised form 5 December 2009 Accepted 5 December 2009
Keywords: Metals (A) Mechanical properties (C) Phase transitions (C) Nitrogen (B)
a b s t r a c t In consideration of good results about the application of deep cryogenic treatment (DCT) on materials, the effect on the microstructure and properties (hardness, toughness and the content of retained austenite) of a new developed cold work die steel (Cr8Mo2SiV) was examined. The execution of the deep cryogenic treatment in different processes showed a varying effect on materials. It was shown that the hardness of the DCT specimens was higher (+0.5HRC to +2HRC) whereas the toughness was lower when compared with the conventionally treated specimens (quenching and tempering). Following the DCT process retained austenite transformed into martensite, however, not completely. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In cold work die steels, the primary objective of the heat treatment is to obtain high wear resistance. 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 [1]. The retained austenite as a soft phase in steels could reduce the product life and, in working conditions, it can be transformed into martensite [2]. This transformation results to a size changing during use. In order to resolve the problems mentioned above, the deep cryogenic treatment is used to transform the retained austenite into martensite. As a result, the retained austenite is reduced and higher wear resistance is obtained [3]. The execution of the deep cryogenic treatment on quenched and tempered high speed steel tools increases hardness, reduces tool consumption and down time for the equipment set up, thus leading to cost reductions of about 50% [4]. In order to improve the service lifetime of tools, the DCT has become more and more interesting in recent years and allied industries, its effects on the static mechanical properties of die steels were investigated in particular hardness, toughness and microstructure. However, more research about cryogenic treatment on cold work die steel focus on Cr12MoV. This steel is well known with its high wear resistance but low toughness. The tool tends to tipping or brittle fracture in use. Because of this, some new high hardness & high toughness cold work die steels were developed as DAIDO DC53, ASSAB 88, and Hitachi SLD Magic in recent years. And
* Corresponding author. Tel.: +86 021 5633 1153. E-mail address: [email protected] (S. Li). 0011-2275/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2009.12.005
this type of cold work die steel is being the main trend but no information can be available about DCT properties in literature. Due to the lack of research carried out in the field of the DCT properties on this type die steel, this paper focuses on the DCT properties of this type cold work die steel. The greatest improvement in properties is obtained by carrying out the deep cryogenic treatment between quenching and tempering [5]. However some researchers pointed out that there is a good effect when the samples repeated several times [4]. In order to consider complete with all of DCT cycles that is desired, the samples with repeating several times were carried out and the mechanical properties were also compared with the traditional die steel Cr12MoV. 2. Materials and test methods Material provided in this study is designed at the School of Materials Science and Engineering in Shanghai University and produced in Baosteel Co., Ltd. of China. The material is a new developed cold work die steel (Cr8Mo2SiV) in the type of DAIDO DC53. It is fabricated by vacuum induction melting and electroslag remelting prior to forging to a rod with a diameter of 180 mm. The chemical composition is listed in Table 1. Samples of the investigated steel with the sizes of 10 10 55 mm were austenitized at 1040 °C, hardened in oil and then DCT treated in different cycles at 196 °C in liquid nitrogen. In order to evaluate the effects of DCT properties, conventional hardening was used as a reference. In this regard, a group of specimens were subjected to conventional hardening including, austenitizing at 1040 °C for 30 min in a vacuum furnace under flowing argon atmosphere, followed by oil quenching. And then tempering was carried out at 210 °C, using the same protective argon atmosphere
90
S. Li et al. / Cryogenics 50 (2010) 89–92
Table 1 Chemical composition of the used steel (in wt.%).
70
C
Si
Mn
Cr
Mo
V
S
P
Fe
%
0.98
0.98
0.30
8.60
2.0
0.50
0.0008
0.01
Balance
50
Toughness,J
Elements
30 20
Details of treatment
Nomenclature
No. No. No. No. No.
Hardening & tempering (2 times) Hardening & DCT (2 h) & tempering (2 times) Hardening & DCT (24 h) & tempering (2 times) Hardening & tempering (2 times) & DCT (24 h) Hardening & DCT (3 times, 1 h) & tempering (2 times) Hardening & tempering (1 time) & DCT (24 h) & tempering (1 time)
HT HCT2 HCT24 HTC HC3T HTCT
Hardness,HRC
0
Groups
No. 6
40
10
Table 2 Details of treatment for groups.
1 2 3 4 5
Cr8Mo2SiV Cr12MoV
60
67 66 65 64 63 62 61 60 59 58 57 56 55
Cr8Mo2SiV Cr12MoV
HT for 2 h. The details of each step being illustrated in Table 2. The cryogenic processing was done by uniform cooling of the samples to 196 °C in liquid nitrogen directly and holding the samples in for different time durations or in different cycles, following by uniform heating to room temperature unaffectedly. A typical deep cryogenic processing cycle is shown in Fig. 1. After quenching and tempering, the hardness and toughness were measured and by means of X-ray quantitative phase analysis, the volume fraction of retained austenite was evaluated on the earlier polished surfaces. The contents of retained austenite and martensite were measured with the X-ray diffraction instrument (D/ Max-2200, Rigaku, Japan) that uses a Cu Ka (k = 1.5418 1010 m) X-ray source. The microstructure of the material was analyzed by scanning electron microscopy (S-570, Hitachi, Japan). 3. Results and discussion 3.1. Mechanical properties Mechanical properties were measured on the different specimens, obtaining the results reported as in Fig. 2. For all hardness measurement, Rockwell hardness tester was used. The major load of 150 kg was applied for HRC scale for duration of 60 s and the depth of resistance to indentation was automatically recorded on the dial gauge. The HRC test results point out that the hardness has been increased by the DCT treating. The higher hardness of the cryogenic treated samples was due to the decrease of the retained austenite. In addition, the higher chromium carbide percentage increases the hardness, too [5]. Fig. 2 shows the
HCT2
HCT24
HTC
HC3T
HTCT
Fig. 2. The toughness and hardness at different treatment groups.
relationship between hardness and toughness, that is, higher hardness but lower toughness after deep cryogenic treatment. The hardness increases +0.5HRC when the samples carried out after tempering (HTC), because of carbides precipitation and the carbon content in the retained austenite are depleted after tempering. When the samples carried out by the DCT treating, the retained austenite transform into martensite which has lower hardness because of carbon depletion after tempering. The carbon content of retained austenite is higher in quenching samples than the tempering because of the carbides precipitation during tempering. While the DCT groups of samples which carried out before tempering (HCT2 and HCT24), the hardness of HCT2 (+2.5HRC) and HCT24 (+3.5HRC) is higher than the samples which carried out after tempering. And the toughness has no significant difference when prolonger the soaking time. The greatest improvement in hardness is obtained by carrying out the deep cryogenic treatment between quenching and tempering. When carried out the deep cryogenic treatment after tempering, the hardness increased not as high as before tempering. When the samples repeated several times in the liquid nitrogen, the hardness increased as high as carrying out the deep cryogenic treatment between quenching and tempering for 2 h, and the lower toughness is obtained. The reason is probably the internal stress increased and micro cracks produced when the samples repeated several times in the liquid nitrogen. It is indicated that the hardness increased obviously when carried out the deep cryogenic treatment between quenching and tempering. To compare with conventional Cr12MoV steel, the new developed die steel exerts high toughness after DCT treating even though Cr12MoV shows higher hardness at all cycles. The low toughness of Cr12MoV tends to tipping or brittle fracture in use. While the new developed die steel has a longer service lifetime on the same conditions after DCT treating. 3.2. Retained austenite
Fig. 1. Schematic representation of the heat treatment history consisting of hardening (H), deep cryogenic treatment (DCT) and tempering (T) cycles.
Retained austenite was measured by X-ray diffraction (XRD) measurement which is based on the change in the interplanar crystal spacing (strain) by virtue of load or temperature or both. Diffraction effects are produced when a beam of X-rays of specific wavelength passes through the three dimensional array of atoms, which constitutes the crystal [6]. Retained austenite content was determined by XRD analysis at room temperature using Cu Ka (k = 1.5418 1010 m) radiation with vanadium filters. The volume fraction of retained austenite was estimated in accordance
S. Li et al. / Cryogenics 50 (2010) 89–92
improves the wear resistance and hardness. Due to the homogenized carbide distribution as well as the elimination of the retained austenite, the deep cryogenic treatment demonstrated more improvement in wear resistance and hardness [5]. No evidence shows more retained austenite was transformed when the samples were treated in DCT for a longer time (i.e. HCT2 which cryotreated 2 h while HCT24 cryotreated 24 h); it is indicated that keeping time at lower temperature is as not important parameter as the different technical cycles. In order to get rid of the effects of carbides on the volume fraction of retained austenite, the crystal plane (3 1 1) of retained austenite and (2 1 1) of martensite were used to calculate the volume fraction of retained austenite as following. Some investigators reported in literature have pointed out that the retained austenite will transform into martensite completely [8,9]. But Fig. 4 shows retained austenite transformation is not complete after DCT, even though keeping longer time in liquid nitrogen (the arrows are the peaks of retained austenite). Table 3 shows the volume fraction of retained austenite which treated by DCT is related to the sequence between cryogenic treating and tempering. The deep cryogenic treatment when executed after tempering (i.e. HTC and HTCT), the volume fraction retained austenite is higher than the groups of sample which carried out prior to tempering (i.e. HCT2).The possible reason is the retained austenite stabilized after tempering and the stabilized retained austenite is beneficial to toughness.
1600 1400
(110)M
(111)A
HT
1200
I,cps
1000 800 600 Cr7C3 400
(211)M
(200)A
200
(200)M
(311)A (220)M
(220)A
0 40
50
60
70 2 degrees
80
90
100
I,cps
Fig. 3. The XRD diffraction for tempered sample.
3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0
91
HT HCT2 HCT24 HTC HC3T HTCT
3.3. Microstructure analysis
40
50
60
70
80
90
100
110
2 degrees Fig. 4. The XRD diffraction for DCT groups.
with ASTM standard E975-00, [7] considering the diffraction crystal plane (1 1 1), (2 0 0), (2 2 0), (3 1 1) of retained austenite and (1 1 0), (2 0 0), (2 1 1), (2 2 0) of martensite. The peaks of retained austenite and martensite were measured using XRD with a Cu Ka (k = 1.5418 1010 m) X-ray source as shown in Figs. 3 and 4. The volume fraction of retained austenite was calculated in Table 3. The formula of retained austenite calculated as follow:
fr ¼ ð1 V c Þ=ð1 þ Rr =Ra Ia =Ir Þ
ð1Þ
The mechanism behind the claimed improvements in ferrous alloys has not been totally clarified, different hypotheses with the microstructure observations have been reported in the literature. Some researchers have pointed out that fine dispersed carbides precipitated during the deep cryogenic treatment [10,11]. And also some researchers have reported that carbide percentage was increased to about 2% after DCT [5]. The reason is martensite and austenite lattice contraction, the chromium carbide distribution was more homogenized after the deep cryogenic treatment. Because of this shrinkage, carbon atoms are forced to diffuse and make a new carbide nucleus [5]. This new carbide nucleus increases the carbide percentage and the homogenous distribution [12,13]. Sample blanks (10 10 10 mm) for metallographic examinations were cut using wire electro discharge machining. Digital micrographs were recorded using scanning electron microscope (S-570, Hitachi, Japan). Figs. 5 and 6 depict the graphs of tempered
where fr is the volume fraction of retained austenite, Vc is the volume fraction of carbide, Ra & Rr is the structure factor of austenite and martensite respectively, Ir & Ir is accumulative intensity of austenite and martensite respectively. As shown in Fig. 4, the diffraction maximums of austenite are decreased extremely after DCT treating, which indicated that the retained austenite transformed into martensite in quality at lower temperature. The retained austenite decreases in DCT and hence
Table 3 The volume fraction of retained austenite with different groups. Nomenclature
Retain austenite percent (%)
HT HCT2 HCT24 HTC HC3T HTCT
23.25 2.87 2.79 4.65 3.05 5.72
Fig. 5. The SEM graph of the high-alloyed steel tempered at 210 °C.
92
S. Li et al. / Cryogenics 50 (2010) 89–92
4. Conclusions The effects of DCT treated on the static mechanical properties and microstructure of a new developed cold work die steel are investigated. The results of investigation are given as following:
C,%
Fig. 6. The SEM graph of DCT treated samples following destabilization treatment in liquid nitrogen.
0.28 0.26 0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
C% in martensite
The effects of DCT on static mechanical properties show an increase of hardness and decrease of toughness. The hardness increases during the samples carrying out DCT when prolonger time in liquid nitrogen. The greatest improvement in hardness is obtained by carrying out the deep cryogenic treatment between quenching and tempering with a longer soaking time. Following the DCT process retained austenite transformed to martensite, however, not completely. The carbon content decreased with the soaking time prolonging when carried out the deep cryogenic treatment between quenching and tempering. The greatest decreasement in carbon content is obtained by carrying out the deep cryogenic treatment with repeating several times. To compare with Cr12MoV the new developed die steel has a high toughness after deep cryogenic treatment while the hardness is well-matched each other. The low toughness of Cr12MoV steel tends to tipping or brittle fracture in use. Because of the high toughness and wear resistance, the new developed die steel leads to a longer service lifetime on the same conditions after DCT treating. It indicated that the new developed die steel has good results about the application of deep cryogenic treatment.
Acknowledgements
1
2
3
4 Groups
5
6
7
Fig. 7. the carbon content in martensite for all heat treatment cycles.
The authors would like furthermore acknowledge the Chinese ‘‘National Science and Technology Plan” for financial support (Project 2007BAE51B04) and the company Baosteel Co., Ltd. Special steel subsidiary company (China) for support and samples supply (Project: high-performance die steel products R&D, 2007.92010.12). References
sample (HT) and deep cryogenic treated respectively under scanning electron microscope. From the graphs of SEM as following, there are more white regions appear on the matrix after DCT treating. The white regions are most probably the carbides which precipitated form matrix after DCT treating. Fig. 7 shows the XRD profiles associated with the (1 1 0) martensite reflections and the carbon content of martensite for all DCT treating cycles. The results show the carbon content in the martensite decreased when prolonger the soaking time in the liquid nitrogen for the quenched samples. The deep cryogenic treatment when carried out after tempering, increases toughness and does not influence hardness because of the carbides precipitation during tempering. In other words, the high toughness is attained without reducing hardness. A toughness increase can be obtained, even if lower, by carrying out after tempering. It indicated that the content of precipitation carbides is dependent on the soaking time. It is clearly seen that there is a definite reduction in carbon content when carried out repeating several times in DCT treating (HCT3 in Fig. 7). The martensite and austenite lattice contraction during DCT cycles and the carbide distribution was more homogenized after the deep cryogenic treatment. Because of this change, carbon atoms are forced to diffuse and make a new carbide nucleus during DCT treating. It is indicated that the content of carbides precipitation is also dependent on the repeating times. Therefore, the content of carbides precipitation is related to the soaking time and the repeating times during the deep cryogenic treatment.
[1] Mayr P. Residual stresses in science and technology, vol. 1. GarmischPartenkirschen, FRG; 1986. [2] Mohan Lal D, Renganarayanan S, Kalanidhi A. Cryogenic treatment to augment wear resistance of tool and die steels. Cryogenics 2001;41:149. [3] 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. [4] Xiaoping L, Yun Dong. Study on cryogenic treatment technology and mechanism of high speed steels. Trans Metal Heat Treat 1998;19:21. [5] Akhbarizadeh A, Shafyei A, Golozar MA. Effects of cryogenic treatment on wear behavior of D6 tool steel. Mater Des 2009;30:3259. [6] Clarke AJ, Speer JG, Miller MK, Hackenberg RE, Edmonds DV, Matlock DK, et al. Carbon partitioning to austenite from martensite or bainite during the quench and partition (Q&P) process: a critical assessment. Acta Mater 2008;56:16. [7] Chowdhury ASMF, Mari D, Schaller R. Analysis of transient mechanical loss due to thermal stresses in metal matrix composites. Mater Sci Eng: A 2009;521– 522:310. [8] Bensely A, Shyamala L, Harish S, Mohan Lal D, Nagarajan G, Junik K, et al. Fatigue behaviour and fracture mechanism of cryogenically treated En 353 steel. Mater Des 2009;30:2955. [9] Darwin JD, Mohan Lal D, Nagarajan G. Optimization of cryogenic treatment to maximize the wear resistance of 18% Cr martensitic stainless steel by Taguchi method. J Mater Process Technol 2008;195:241. [10] Preciado M, Bravo PM, Alegre JM. Effect of low temperature tempering prior cryogenic treatment on carburized steels. J Mater Process Technol 2006;176:41. [11] Da Silva FJ, Franco SD, Machado ÁR, Ezugwu EO, Souza JAM. Performance of cryogenically treated HSS tools. Wear 2006;261:674. [12] Buss K, Mari D. High temperature deformation mechanisms in cemented carbides and cermets studied by mechanical spectroscopy. Mater Sci Eng: A 2004;370:163. [13] Zhirafar S, Rezaeian A, Pugh M. Effect of cryogenic treatment on the mechanical properties of 4340 steel. J Mater Process Technol 2007;186:298.
Contents lists available at ScienceDirect
Cryogenics journal homepage: www.elsevier.com/locate/cryogenics
Hardness and toughness investigations of deep cryogenic treated cold work die steel Shaohong Li *, Yinzi Xie, Xiaochun Wu School of Materials Science and Engineering, Shanghai Univ., Shanghai 200072, PR China
a r t i c l e
i n f o
Article history: Received 19 July 2009 Received in revised form 5 December 2009 Accepted 5 December 2009
Keywords: Metals (A) Mechanical properties (C) Phase transitions (C) Nitrogen (B)
a b s t r a c t In consideration of good results about the application of deep cryogenic treatment (DCT) on materials, the effect on the microstructure and properties (hardness, toughness and the content of retained austenite) of a new developed cold work die steel (Cr8Mo2SiV) was examined. The execution of the deep cryogenic treatment in different processes showed a varying effect on materials. It was shown that the hardness of the DCT specimens was higher (+0.5HRC to +2HRC) whereas the toughness was lower when compared with the conventionally treated specimens (quenching and tempering). Following the DCT process retained austenite transformed into martensite, however, not completely. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction In cold work die steels, the primary objective of the heat treatment is to obtain high wear resistance. 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 [1]. The retained austenite as a soft phase in steels could reduce the product life and, in working conditions, it can be transformed into martensite [2]. This transformation results to a size changing during use. In order to resolve the problems mentioned above, the deep cryogenic treatment is used to transform the retained austenite into martensite. As a result, the retained austenite is reduced and higher wear resistance is obtained [3]. The execution of the deep cryogenic treatment on quenched and tempered high speed steel tools increases hardness, reduces tool consumption and down time for the equipment set up, thus leading to cost reductions of about 50% [4]. In order to improve the service lifetime of tools, the DCT has become more and more interesting in recent years and allied industries, its effects on the static mechanical properties of die steels were investigated in particular hardness, toughness and microstructure. However, more research about cryogenic treatment on cold work die steel focus on Cr12MoV. This steel is well known with its high wear resistance but low toughness. The tool tends to tipping or brittle fracture in use. Because of this, some new high hardness & high toughness cold work die steels were developed as DAIDO DC53, ASSAB 88, and Hitachi SLD Magic in recent years. And
* Corresponding author. Tel.: +86 021 5633 1153. E-mail address: [email protected] (S. Li). 0011-2275/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.cryogenics.2009.12.005
this type of cold work die steel is being the main trend but no information can be available about DCT properties in literature. Due to the lack of research carried out in the field of the DCT properties on this type die steel, this paper focuses on the DCT properties of this type cold work die steel. The greatest improvement in properties is obtained by carrying out the deep cryogenic treatment between quenching and tempering [5]. However some researchers pointed out that there is a good effect when the samples repeated several times [4]. In order to consider complete with all of DCT cycles that is desired, the samples with repeating several times were carried out and the mechanical properties were also compared with the traditional die steel Cr12MoV. 2. Materials and test methods Material provided in this study is designed at the School of Materials Science and Engineering in Shanghai University and produced in Baosteel Co., Ltd. of China. The material is a new developed cold work die steel (Cr8Mo2SiV) in the type of DAIDO DC53. It is fabricated by vacuum induction melting and electroslag remelting prior to forging to a rod with a diameter of 180 mm. The chemical composition is listed in Table 1. Samples of the investigated steel with the sizes of 10 10 55 mm were austenitized at 1040 °C, hardened in oil and then DCT treated in different cycles at 196 °C in liquid nitrogen. In order to evaluate the effects of DCT properties, conventional hardening was used as a reference. In this regard, a group of specimens were subjected to conventional hardening including, austenitizing at 1040 °C for 30 min in a vacuum furnace under flowing argon atmosphere, followed by oil quenching. And then tempering was carried out at 210 °C, using the same protective argon atmosphere
90
S. Li et al. / Cryogenics 50 (2010) 89–92
Table 1 Chemical composition of the used steel (in wt.%).
70
C
Si
Mn
Cr
Mo
V
S
P
Fe
%
0.98
0.98
0.30
8.60
2.0
0.50
0.0008
0.01
Balance
50
Toughness,J
Elements
30 20
Details of treatment
Nomenclature
No. No. No. No. No.
Hardening & tempering (2 times) Hardening & DCT (2 h) & tempering (2 times) Hardening & DCT (24 h) & tempering (2 times) Hardening & tempering (2 times) & DCT (24 h) Hardening & DCT (3 times, 1 h) & tempering (2 times) Hardening & tempering (1 time) & DCT (24 h) & tempering (1 time)
HT HCT2 HCT24 HTC HC3T HTCT
Hardness,HRC
0
Groups
No. 6
40
10
Table 2 Details of treatment for groups.
1 2 3 4 5
Cr8Mo2SiV Cr12MoV
60
67 66 65 64 63 62 61 60 59 58 57 56 55
Cr8Mo2SiV Cr12MoV
HT for 2 h. The details of each step being illustrated in Table 2. The cryogenic processing was done by uniform cooling of the samples to 196 °C in liquid nitrogen directly and holding the samples in for different time durations or in different cycles, following by uniform heating to room temperature unaffectedly. A typical deep cryogenic processing cycle is shown in Fig. 1. After quenching and tempering, the hardness and toughness were measured and by means of X-ray quantitative phase analysis, the volume fraction of retained austenite was evaluated on the earlier polished surfaces. The contents of retained austenite and martensite were measured with the X-ray diffraction instrument (D/ Max-2200, Rigaku, Japan) that uses a Cu Ka (k = 1.5418 1010 m) X-ray source. The microstructure of the material was analyzed by scanning electron microscopy (S-570, Hitachi, Japan). 3. Results and discussion 3.1. Mechanical properties Mechanical properties were measured on the different specimens, obtaining the results reported as in Fig. 2. For all hardness measurement, Rockwell hardness tester was used. The major load of 150 kg was applied for HRC scale for duration of 60 s and the depth of resistance to indentation was automatically recorded on the dial gauge. The HRC test results point out that the hardness has been increased by the DCT treating. The higher hardness of the cryogenic treated samples was due to the decrease of the retained austenite. In addition, the higher chromium carbide percentage increases the hardness, too [5]. Fig. 2 shows the
HCT2
HCT24
HTC
HC3T
HTCT
Fig. 2. The toughness and hardness at different treatment groups.
relationship between hardness and toughness, that is, higher hardness but lower toughness after deep cryogenic treatment. The hardness increases +0.5HRC when the samples carried out after tempering (HTC), because of carbides precipitation and the carbon content in the retained austenite are depleted after tempering. When the samples carried out by the DCT treating, the retained austenite transform into martensite which has lower hardness because of carbon depletion after tempering. The carbon content of retained austenite is higher in quenching samples than the tempering because of the carbides precipitation during tempering. While the DCT groups of samples which carried out before tempering (HCT2 and HCT24), the hardness of HCT2 (+2.5HRC) and HCT24 (+3.5HRC) is higher than the samples which carried out after tempering. And the toughness has no significant difference when prolonger the soaking time. The greatest improvement in hardness is obtained by carrying out the deep cryogenic treatment between quenching and tempering. When carried out the deep cryogenic treatment after tempering, the hardness increased not as high as before tempering. When the samples repeated several times in the liquid nitrogen, the hardness increased as high as carrying out the deep cryogenic treatment between quenching and tempering for 2 h, and the lower toughness is obtained. The reason is probably the internal stress increased and micro cracks produced when the samples repeated several times in the liquid nitrogen. It is indicated that the hardness increased obviously when carried out the deep cryogenic treatment between quenching and tempering. To compare with conventional Cr12MoV steel, the new developed die steel exerts high toughness after DCT treating even though Cr12MoV shows higher hardness at all cycles. The low toughness of Cr12MoV tends to tipping or brittle fracture in use. While the new developed die steel has a longer service lifetime on the same conditions after DCT treating. 3.2. Retained austenite
Fig. 1. Schematic representation of the heat treatment history consisting of hardening (H), deep cryogenic treatment (DCT) and tempering (T) cycles.
Retained austenite was measured by X-ray diffraction (XRD) measurement which is based on the change in the interplanar crystal spacing (strain) by virtue of load or temperature or both. Diffraction effects are produced when a beam of X-rays of specific wavelength passes through the three dimensional array of atoms, which constitutes the crystal [6]. Retained austenite content was determined by XRD analysis at room temperature using Cu Ka (k = 1.5418 1010 m) radiation with vanadium filters. The volume fraction of retained austenite was estimated in accordance
S. Li et al. / Cryogenics 50 (2010) 89–92
improves the wear resistance and hardness. Due to the homogenized carbide distribution as well as the elimination of the retained austenite, the deep cryogenic treatment demonstrated more improvement in wear resistance and hardness [5]. No evidence shows more retained austenite was transformed when the samples were treated in DCT for a longer time (i.e. HCT2 which cryotreated 2 h while HCT24 cryotreated 24 h); it is indicated that keeping time at lower temperature is as not important parameter as the different technical cycles. In order to get rid of the effects of carbides on the volume fraction of retained austenite, the crystal plane (3 1 1) of retained austenite and (2 1 1) of martensite were used to calculate the volume fraction of retained austenite as following. Some investigators reported in literature have pointed out that the retained austenite will transform into martensite completely [8,9]. But Fig. 4 shows retained austenite transformation is not complete after DCT, even though keeping longer time in liquid nitrogen (the arrows are the peaks of retained austenite). Table 3 shows the volume fraction of retained austenite which treated by DCT is related to the sequence between cryogenic treating and tempering. The deep cryogenic treatment when executed after tempering (i.e. HTC and HTCT), the volume fraction retained austenite is higher than the groups of sample which carried out prior to tempering (i.e. HCT2).The possible reason is the retained austenite stabilized after tempering and the stabilized retained austenite is beneficial to toughness.
1600 1400
(110)M
(111)A
HT
1200
I,cps
1000 800 600 Cr7C3 400
(211)M
(200)A
200
(200)M
(311)A (220)M
(220)A
0 40
50
60
70 2 degrees
80
90
100
I,cps
Fig. 3. The XRD diffraction for tempered sample.
3000 2800 2600 2400 2200 2000 1800 1600 1400 1200 1000 800 600 400 200 0
91
HT HCT2 HCT24 HTC HC3T HTCT
3.3. Microstructure analysis
40
50
60
70
80
90
100
110
2 degrees Fig. 4. The XRD diffraction for DCT groups.
with ASTM standard E975-00, [7] considering the diffraction crystal plane (1 1 1), (2 0 0), (2 2 0), (3 1 1) of retained austenite and (1 1 0), (2 0 0), (2 1 1), (2 2 0) of martensite. The peaks of retained austenite and martensite were measured using XRD with a Cu Ka (k = 1.5418 1010 m) X-ray source as shown in Figs. 3 and 4. The volume fraction of retained austenite was calculated in Table 3. The formula of retained austenite calculated as follow:
fr ¼ ð1 V c Þ=ð1 þ Rr =Ra Ia =Ir Þ
ð1Þ
The mechanism behind the claimed improvements in ferrous alloys has not been totally clarified, different hypotheses with the microstructure observations have been reported in the literature. Some researchers have pointed out that fine dispersed carbides precipitated during the deep cryogenic treatment [10,11]. And also some researchers have reported that carbide percentage was increased to about 2% after DCT [5]. The reason is martensite and austenite lattice contraction, the chromium carbide distribution was more homogenized after the deep cryogenic treatment. Because of this shrinkage, carbon atoms are forced to diffuse and make a new carbide nucleus [5]. This new carbide nucleus increases the carbide percentage and the homogenous distribution [12,13]. Sample blanks (10 10 10 mm) for metallographic examinations were cut using wire electro discharge machining. Digital micrographs were recorded using scanning electron microscope (S-570, Hitachi, Japan). Figs. 5 and 6 depict the graphs of tempered
where fr is the volume fraction of retained austenite, Vc is the volume fraction of carbide, Ra & Rr is the structure factor of austenite and martensite respectively, Ir & Ir is accumulative intensity of austenite and martensite respectively. As shown in Fig. 4, the diffraction maximums of austenite are decreased extremely after DCT treating, which indicated that the retained austenite transformed into martensite in quality at lower temperature. The retained austenite decreases in DCT and hence
Table 3 The volume fraction of retained austenite with different groups. Nomenclature
Retain austenite percent (%)
HT HCT2 HCT24 HTC HC3T HTCT
23.25 2.87 2.79 4.65 3.05 5.72
Fig. 5. The SEM graph of the high-alloyed steel tempered at 210 °C.
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4. Conclusions The effects of DCT treated on the static mechanical properties and microstructure of a new developed cold work die steel are investigated. The results of investigation are given as following:
C,%
Fig. 6. The SEM graph of DCT treated samples following destabilization treatment in liquid nitrogen.
0.28 0.26 0.24 0.22 0.20 0.18 0.16 0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00
C% in martensite
The effects of DCT on static mechanical properties show an increase of hardness and decrease of toughness. The hardness increases during the samples carrying out DCT when prolonger time in liquid nitrogen. The greatest improvement in hardness is obtained by carrying out the deep cryogenic treatment between quenching and tempering with a longer soaking time. Following the DCT process retained austenite transformed to martensite, however, not completely. The carbon content decreased with the soaking time prolonging when carried out the deep cryogenic treatment between quenching and tempering. The greatest decreasement in carbon content is obtained by carrying out the deep cryogenic treatment with repeating several times. To compare with Cr12MoV the new developed die steel has a high toughness after deep cryogenic treatment while the hardness is well-matched each other. The low toughness of Cr12MoV steel tends to tipping or brittle fracture in use. Because of the high toughness and wear resistance, the new developed die steel leads to a longer service lifetime on the same conditions after DCT treating. It indicated that the new developed die steel has good results about the application of deep cryogenic treatment.
Acknowledgements
1
2
3
4 Groups
5
6
7
Fig. 7. the carbon content in martensite for all heat treatment cycles.
The authors would like furthermore acknowledge the Chinese ‘‘National Science and Technology Plan” for financial support (Project 2007BAE51B04) and the company Baosteel Co., Ltd. Special steel subsidiary company (China) for support and samples supply (Project: high-performance die steel products R&D, 2007.92010.12). References
sample (HT) and deep cryogenic treated respectively under scanning electron microscope. From the graphs of SEM as following, there are more white regions appear on the matrix after DCT treating. The white regions are most probably the carbides which precipitated form matrix after DCT treating. Fig. 7 shows the XRD profiles associated with the (1 1 0) martensite reflections and the carbon content of martensite for all DCT treating cycles. The results show the carbon content in the martensite decreased when prolonger the soaking time in the liquid nitrogen for the quenched samples. The deep cryogenic treatment when carried out after tempering, increases toughness and does not influence hardness because of the carbides precipitation during tempering. In other words, the high toughness is attained without reducing hardness. A toughness increase can be obtained, even if lower, by carrying out after tempering. It indicated that the content of precipitation carbides is dependent on the soaking time. It is clearly seen that there is a definite reduction in carbon content when carried out repeating several times in DCT treating (HCT3 in Fig. 7). The martensite and austenite lattice contraction during DCT cycles and the carbide distribution was more homogenized after the deep cryogenic treatment. Because of this change, carbon atoms are forced to diffuse and make a new carbide nucleus during DCT treating. It is indicated that the content of carbides precipitation is also dependent on the repeating times. Therefore, the content of carbides precipitation is related to the soaking time and the repeating times during the deep cryogenic treatment.
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