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Microstructure and mechanical properties of spheroidized D6AC steel
Materials Science & Engineering A 585 (2013) 94–99
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure and mechanical properties of spheroidized D6AC steel Young-Won Lee a, Young-Il Son a, Seok-Jae Lee b,n a b
Advanced Propulsion Technology Center, Agency for Defense Development, Yuseong, PO Box 35-16, Daejeon 305-600, Republic of Korea Division of Advanced Materials Engineering, Chonbuk National University, Deokjin-gu, Jeonju 561-756, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 7 May 2013 Received in revised form 18 July 2013 Accepted 22 July 2013 Available online 30 July 2013
The influence of spheroidization on the microstructure and mechanical properties of D6AC steel was investigated by means of dilatometry, micrograph analysis, and tensile testing. The continuous cooling transformation diagram was presented, and the critical cooling rate to obtain a fully martensitic microstructure was determined. Undissolved Mo carbide particles in the martensite matrix were observed after the solid solution treatment at 860 1C. The kinetics of the spheroidized cementite particles accelerated due to the carbide particles that were undissolved during the solid solution treatment. As the holding time for the spheroidization was increased, the total elongation increased, while the tensile strength and hardness were reduced according to a proportional relationship. & 2013 Elsevier B.V. All rights reserved.
Keywords: Spheroidization Microstructure Mechanical properties Continuous-cooling-transformation (CCT) diagram D6AC steel
1. Introduction Spheroidization treatment in steels is carried out to obtain spheroidized carbide particles that are uniformly spread in a ferrite matrix [1,2]. A long isothermal holding below the Ae1 temperature is a typical spheroidizing process, which could be influenced by initial microstructure of steel. It is known that the microstructure with spheroidized carbide particles in the ferrite matrix shows a high ductility among various microstructures of steels. The high ductility of the spheroidized microstructure is correlated with a ductile ferrite matrix. The spheroidized microstructure has a lower hardness and higher ductility than the pearlite microstructure, which consists of a plate-type lamellar cementite and ferrite matrix. The superior ductility and low hardness of the spheroidized microstructure play an important role in the cold rolling and cold forming of low and medium carbon steels and in the machinability of high carbon steels before the final hardening processes. The relationship between the spheroidizing treatment and cold formability is briefly summarized as follows: First, spheroidized carbide particles are formed, and coarse particles are observed after a long holding time, resulting in a yield strength (YS) and ultimate tensile strength (UTS) reduction, and elongation increases as spheroidizing continues. Finally, the cold formability is improved.
n
Corresponding author. Tel.: +82 63 270 2298; fax: +82 63 270 2305. E-mail address: [email protected] (S.-J. Lee).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.07.040
The spheroidized microstructure is the most stable due to the stress relief in the ferrite matrix and the minimized interfacial surface area per unit volume between the cementite particle and the ferrite matrix. The high ratio of the interfacial surface area to the unit volume for lamellar cementite in pearlite results in high surface energy. The tendency to reduce the surface energy is the driving force for dividing cementite lamellae into small spherical particles. The small cementite particles grow or merge each other to minimize the total interfacial surface area of a system as the equilibrium volume fraction of cementite is preserved at a given temperature, resulting in coarse particles. This growing process is referred to as coarsening or Ostwald ripening [3]. The rate of spheroidization is directly influenced by the carbon diffusion in ferrite and is gradually decreased as the average size of the spheroidized particles is increased. The addition of an alloying element, especially a strong carbide forming element such as Ti, V, Nb, etc. which could be as a solute or precipitate, decreases the diffusivity of carbon in ferrite, so the spheroidization reaction becomes slow. The effect of carbon diffusivity on the coarsening rate of spheroidized cementite particle is described in the following equation [4]: r 3 r 30 ¼
8γDC CV m t 9RT
ð1Þ
where r is the particle radius, r0 is the initial particle radius, γ is the interfacial energy between ferrite and cementite, DC is the diffusion coefficient of carbon, C is the equilibrium carbon concentration in ferrite, Vm is the molar volume of particle, R is the gas constant, T is temperature, and t is time. In fact, the coarsening of
Y.-W. Lee et al. / Materials Science & Engineering A 585 (2013) 94–99
particle means the cementite/ferrite boundary movement. The addition of alloying element influences the cementite/ferrite boundary movement not only by varying the carbon diffusivity but also by changing the thermodynamic conditions [5]. For instance, Si can dissolve in cementite and increase the solubility of carbon in ferrite, resulting in the retardance of cementite formation and growth in para-equilibrium condition. The spheroidized microstructure is formed at a proper temperature with a sufficiently long holding time for a carbon diffusion reaction. As a result, various spheroidizing treatment methods have been proposed to obtain an optimal spheroidized microstructure for steels. Joung and Chung investigated several heat treatment process conditions for spheroidized AISI 9260 (Fe– 0.62%C–0.93%Mn–1.87%Si) steel [6]. They found that martensite as the initial microstructure and a repeated cycling heat treatment around the Ae1 temperature could maximize the rate of spheroidized carbide particle formation. Hundreds of hours may be Table 1 Chemical composition of D6AC steel used in the present study (in wt%). C
Mn
Si
P
S
Cr
Mo
V
Fe
0.46
0.75
0.25
0.01
0.003
1.1
1.0
0.1
Bal.
95
required to complete the spheroidization from pearlite, and separate small carbide particles in bainite or martensite can reduce the holding time. In the case of the initial pearlite microstructure, a smaller interlamellar spacing between the cementite and pearlitic ferrite results in a faster spheroidizing rate due to the diffusion distance of carbon [7]. The spheroidization of martensite is occasionally carried out in high alloy tool steels, which easily form martensite during slow cooling such as natural air cooling. Undissolved carbide particles accelerate the formation of the spherodized microstructure, so it is important to restrict the solid solution temperature in order to maintain some undissolved carbide particles. When the size of a spheroidized cementite particle is greater than 4 μm, the particle acts as a nucleation site for microcracks [8]. D6AC steel, a medium carbon steel with low alloy content and high strength, has been widely used in aerospace and defense components due to the high yield to tensile strength ratio and the superior ductility [9]. A typical heat treatment process of D6AC steel is direct quenching from a high austenitic temperature to room temperature, followed by conventional tempering. The chemical composition of D6AC steel is similar to AISI 4340 and AISI 4140, but D6AC steel contains additional V and Cu, which are a strong carbide forming element and a solid solution strengthening element, respectively. Several studies on the fracture of D6AC steel have been reported [10–12]. Recently, Lian investigated the effects of tempering condition on microstructure and mechanical properties of D6AC steel [13]. Even though a typical microstructure of spheroidized cementite with a ferrite matrix improves the cold formability, few studies about the spheroidization of D6AC steel related to the microstructure and mechanical properties have been reported. Therefore, this paper addresses the influence of spheroidization on the microstructure and mechanical properties of D6AC steel to improve the cold formability during cold working.
2. Experimental procedure
Fig. 1. CCT diagram of D6AC steel austenitized at 860 1C for 10 min.
The chemical composition of the D6AC steel used in this study is listed in Table 1. The material is a medium carbon low alloy steel containing carbide-forming alloying elements such as Cr, Mo, and V. The solid solution treatment was carried out in a tube furnace filled with Ar gas to prevent oxidation. Two different solid solution temperatures were adopted: 860 1C and 1000 1C. The holding time for the solid solution treatment was 10 min. The sample was directly quenched in room temperature oil to achieve a full martensite microstructure. The spheroidizing temperature was 700 1C, which is below the Ae1 temperature, and the
Fig. 2. (a) Optical micrograph and (b) SEM micrographs of the as-received D6AC sample.
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maximum holding time was 10 h, which was followed by air cooling. The transformation kinetics of the austenite decomposition in D6AC steel was investigated with various cooling rates using a dilatometer. The sample was heated to 860 1C with a heating rate of 3 1C/s, and was then held for 10 min. The dilatometer sample dimensions were 3 1 10 mm3. The microstructure of the heattreated sample was observed by means of an optical microscope (OM) and a scanning electron microscope (SEM). The mechanical properties were evaluated using a room temperature tensile test and a microhardness measurement. An ASTM E8 tensile sample was used, and the tensile rate was 1 10 3 s 1.
3. Results and discussion 3.1. Dilatometry analysis Fig. 1 shows the continuous cooling transformation (CCT) diagram of D6AC steel cooled from 860 1C with different cooling rates ranging from 0.1 to 100 1C/s. The martensite transformation without any other diffusional transformations only occurs when the cooling rate is greater than 1 1C/s because there is enough hardenability due to the addition of the alloying elements to retard the diffusional ferrite and bainite transformations. The observed martensite start (Ms) temperature was about 320 1C regardless of the cooling rate, since the martensite transformation is an athermal transformation that only relies on the transformation temperature. The transformation strain observed around 508 1C at the cooling rate of 0.1 1C/s represents the bainite start (Bs) temperature. According to the empirical equation for the Bs temperature prediction proposed by Kunitake and Okada [14], the predicted Bs temperature for the chemical composition in Table 1, except for the amount of V, is 539 1C. This calculation value indicates the highest temperature at which the upper bainite transformation occurs, and the actual Bs temperature is decreased as the cooling rate increases since the bainite transformation is diffusion controlled. As the cooling rate increases to 0.5 1C/s, the observed Bs temperature is about 407 1C, while the Ms temperature is also around 320 1C. The austenite formation kinetics of D6AC steel was investigated using heating dilation curves. The start temperature of the austenite formation (Ac1) was 745 1C, and the finish temperature of the austenite formation (Ac3) was 813 1C with a heating rate of 0.3 1C/s. However, as the heating rate increased to 30 1C/s, the Ac1 temperature was 768 1C and the Ac3 temperature was 834 1C. The increases in the transformation temperatures reveal that the austenite formation kinetics involves a diffusion-controlled transformation.
3.2. Microstructure Fig. 2 shows the optical and SEM micrographs of the as-received D6AC sample before the solid solution treatment. The microstructure observed consisted of mainly ferrite and pearlite, a small amount of bainite, and some small particles like carbides. This sample was heated to the different solution temperatures (860 1C and 1000 1C), held for 10 min, and was then immediately quenched to room temperature at a cooling rate of 100 1C/s. Fig. 3 compares the microstructural changes affected by the solid solution temperature. The samples show similar martensitic microstructures, but some areas of spherical particles exist are observed in the sample annealed at 860 1C, as indicated in Fig. 3a. It is thought that the particles are Mo- and V-carbides which are not fully dissolved during the solid solution treatment at the relatively low temperature. On the other hand, no spherical carbide particles are observed in the sample annealed at 1000 1C, as seen in Fig. 3b. This indicates that the carbide particles that existed in the initial condition have
Fig. 4. SEM-EDS analysis of carbide particles in the as-quenched D6AC sample after austenitizing at 860 1C for 10 min. The red line (bottom) indicates the counts for V, the green line (middle) indicates the counts for Cr, and the blue line (top) indicates the counts for Mo. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. SEM micrographs of the as-quenched D6AC samples austenitized at different temperatures: (a) 860 1C and (b) 1000 1C. The holding time is 10 min.
Y.-W. Lee et al. / Materials Science & Engineering A 585 (2013) 94–99
been fully dissolved during the solid solution treatment at the relatively high temperature. The spherical particle that is expected as a carbide particle is line scanned using SEM-EDS (Energy Dispersive Spectrometry), and the result is shown in Fig. 4. The spherical particle is an undissolved Mo carbide, because the concentration of Mo in the particle is two times higher than that in a matrix. It is reported that the dissolution temperature of Mo carbide in austenite is between 900 and 1000 1C [15]. The dissolution temperature of VC in austenite can be predicted by using the solute product equation
97
proposed by Narita [16]. The solubility product equation for VC is given by log10 ½V½C ¼ 9500=T þ 6:72
ð2Þ
where V and C are the alloy contents in wt%, and T is the dissolution temperature in Kelvin. Considering the chemical composition of D6AC steel in Table 1, the dissolution temperature of VC that is calculated from Eq. (1) is 999 1C. Therefore, most of the VC particles are dissolved during the solid solution treatment at 1000 1C, and a small amount of very fine VC particles remains.
Fig. 5. SEM micrographs of the spheroidized D6AC samples with different heat treatment conditions: (a) austenitized at 860 1C and spheroidized for 1 h, (b) austenitized at 860 1C and spheroidized for 5 h, (c) austenitized at 1000 1C and spheroidized for 1 h, and (d) austenitized at 1000 1C and spheroidized for 5 h. The austenitizing time is 10 min, and the spheroidizing temperature is 700 1C.
Fig. 6. Engineering stress–strain curves of D6AC steel with various heat treatment conditions. The time indicates the spheroidizing time at 700 1C. The austenitizing temperatures are (a) 860 1C and (b) 1000 1C.
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Fig. 5 shows the SEM micrographs of the spheroidized microstructures in the samples that were annealed at different solid solution temperatures. The average size and volume fraction of the carbide particles increase as the holding time increases to 5 h. The observed average particle size in the sample annealed at 860 1C seems relatively larger, which could be due to undissolved Mo carbides. Ata and Meisam have reported that undissolved carbide particles accelerate spheroidization [7]. Thus, the undissolved particles due to the low solid solution temperature brought about the faster kinetics of spheroidization in D6AC steel.
3.3. Mechanical properties
Fig. 7 shows the relationship between the ultimate tensile strength (UTS) and the total elongation depending on the heat treatment conditions. The mechanical properties of D6AC steel which is quenched and then tempered at various temperatures [13] are compared. The spheroidized microstructure shows better ductility than the tempered martensite microstructure in D6AC steel. This means that the spheroidized D6AC steel can have better cold formability than the D6AC steel after a conventional quenching and tempering treatment. Fig. 8 compares the microhardness values of the samples with different heat treatment conditions. The hardness of the as-received sample before the heat treatment was 447 HV, while that of the as-quenched sample was greater than 900 HV due to the martensite microstructure. A higher hardness value of the asquenched sample austenitized at 860 1C is observed, which is similar to the tendency of the strength change, as shown in Fig. 6.
Fig. 6 compares the engineering stress–strain curves obtained using different heat treatment conditions. The maximum tensile strength and total elongation of the as-received sample are 1133 MPa and 7.9%, respectively. The sample was fractured without plastic deformation in the as-quenched sample. The increase in strength was about 100 MPa in the sample annealed at 860 1C, which is due to the carbide precipitation strengthening effect. After the samples are spheroidized for 1 h, the sample annealed at 1000 1C shows relatively higher strength and lower ductility. The effect of the solid solution temperature on the strength and ductility disappear as the samples are held for 5 h. When the samples are spheroidized for 10 h, the sample annealed at 1000 1C shows good elongation greater than 25%, whereas the ductility of the sample annealed at 860 1C decreases slightly. Therefore, the ductility of the spheroidized D6AC steel can be improved by increasing the holding time after the carbide particles are fully dissolved in the high austenite temperature region. Also, the yield point elongation was observed in the sample with a long holding time after annealing at 860 1C. Xiong et al. observe the occurrence of the yield point elongation as the spheroidization of cementite [17]. Yield point elongation is also observed when the spheroidization of cementite is completed in hypereutectoid steel [18]. Thus, it could be supposed that the observation of yield point elongation in spheroidized D6AC steel results from the coarsening of spheroidized cementite particles. A more detailed analysis is required for the correlation between yield point elongation and particle coarsening.
Fig. 8. Variations of hardness for D6AC steel with different heat treatment conditions: (a) as-quenched, (b) spheroidized for 1 h, (c) spheroidized for 5 h, and (d) spheroidized for 10 h. The spheroidizing temperature is 700 1C.
Fig. 7. Combinations of ultimate tensile strength and total elongation for D6AC steel with various heat treatment conditions. The tempering temperature was varied from 250 1C to 650 1C [13].
Fig. 9. Linear relationship between ultimate tensile strength and hardness for D6AC steel regardless of heat treatment condition.
Y.-W. Lee et al. / Materials Science & Engineering A 585 (2013) 94–99
These results according to the solid solution condition are related to undissolved carbide particles at the lower solid solution temperature. The microhardness values are significantly decreased by spheroidization, but the variation of hardness is insensitive to the solid solution temperature and holding time for spheroidization. In general, strength and hardness have a proportional relationship. The linear relationship between the UTS and hardness of D6AC steel is observed, as shown in Fig. 9. These mechanical properties are mainly influenced by the microstructure controlled by the heat treatment conditions. 4. Conclusions The effects of the spheroidizing treatment on the microstructure and mechanical properties in D6AC steel were investigated. The transformation temperatures and the phase transformations during cooling were investigated by means of dilatometer testing, and the CCT diagram for D6AC steel was presented. The critical cooling rate to obtain a fully martensitic microstructure was 1 1C/s, and the bainite transformation occurred with a cooling rate that was slower than the critical cooling rate. Undissolved carbide particles in the martensite matrix were observed in the sample quenched from 860 1C. These undissolved carbide particles accelerated the kinetics of the spheroidized cementite particles. The ductility of the spheroidized D6AC steel was improved, and the strength and hardness decreased together as the holding time
99
increased for the spheroidization at 700 1C. The spheroidizing treatment in D6AC steel increased the elongation compared with the quenching and tempering treatment, so it is expected that the spheroidization of D6AC steel can improve the cold formability for cold working, such as flow forming.
References [1] G. Krauss, Steels-Processing, Structure, and Performance, ASM International, Materials Park, OH, 2005, pp. 251–262. [2] H.K.D.H. Bhadeshia, Prog. Mater. Sci. 57 (2012) 268–435. [3] L. Ratke, P.W. Voorhees, Growth and Coarsening – Ostwald Ripening in Material Processing, Springer, New York, NY, 2002, pp. 117–166. [4] A.M. Cree, R.G. Faulkner, A.T. Lyne, Mater. Sci. Technol. 11 (1995) 566–571. [5] B.C. De Cooman, J.G. Speer, Fundamentals of Steel Product Physical Metallurgy, AIST, Warrendale, PA, 2011, pp. 116–122. [6] R.U. Joung, I.S. Chung, J. Kor. Soc. Heat Treat 8 (1995) 302–317. [7] K.G. Ata, S.A. Meisam, J. Iron Steel Res. Int. 17 (2010) 45–52. [8] M. Jahazi, G. Ebrahimi, J. Mater. Proc. Technol. 103 (2000) 362–366. [9] T.J. Koppenaal, Metall. Trans. 1 (1970) 3371–3376. [10] P.K. Liaw, M.G. Peck, G.E. Rudd, T.K. Lai, Eng. Fract. Mech. 43 (1992) 379–400. [11] L.W. Tsay, C.S. Chung, C. Chen, Int. J. Fatigue 19 (1997) 25–31. [12] S.C. Wu, H.C. Wen, M.J. Wu, C.P. Chou, Vacuum 86 (2012) 1828–1833. [13] D. Lian, Appl. Surf. Sci. 264 (2013) 100–104. [14] T. Kunitake, Y. Okada, J. Iron Steel Inst. Jpn. 84 (1998) 137–141. [15] D.A. Porter, K.E. Easterling, Phase Transformations in Metals and Alloys, third ed., CRC Press, Taylor & Francis Group, Finland399–408. [16] K. Narita, Trans. ISIJ 15 (1975) 145–151. [17] Y. Xiong, T. He, Z. Guo, H. He, F. Ren, A.A Volinsky, Mater. Sci. Eng. A 563 (2013) 163–167. [18] Z.L. Zhang, Y.N. Liu, J.W. Zhu, G. Yu, Mater. Sci. Eng. A483–484 (2008) 64–66.
Contents lists available at ScienceDirect
Materials Science & Engineering A journal homepage: www.elsevier.com/locate/msea
Microstructure and mechanical properties of spheroidized D6AC steel Young-Won Lee a, Young-Il Son a, Seok-Jae Lee b,n a b
Advanced Propulsion Technology Center, Agency for Defense Development, Yuseong, PO Box 35-16, Daejeon 305-600, Republic of Korea Division of Advanced Materials Engineering, Chonbuk National University, Deokjin-gu, Jeonju 561-756, Republic of Korea
art ic l e i nf o
a b s t r a c t
Article history: Received 7 May 2013 Received in revised form 18 July 2013 Accepted 22 July 2013 Available online 30 July 2013
The influence of spheroidization on the microstructure and mechanical properties of D6AC steel was investigated by means of dilatometry, micrograph analysis, and tensile testing. The continuous cooling transformation diagram was presented, and the critical cooling rate to obtain a fully martensitic microstructure was determined. Undissolved Mo carbide particles in the martensite matrix were observed after the solid solution treatment at 860 1C. The kinetics of the spheroidized cementite particles accelerated due to the carbide particles that were undissolved during the solid solution treatment. As the holding time for the spheroidization was increased, the total elongation increased, while the tensile strength and hardness were reduced according to a proportional relationship. & 2013 Elsevier B.V. All rights reserved.
Keywords: Spheroidization Microstructure Mechanical properties Continuous-cooling-transformation (CCT) diagram D6AC steel
1. Introduction Spheroidization treatment in steels is carried out to obtain spheroidized carbide particles that are uniformly spread in a ferrite matrix [1,2]. A long isothermal holding below the Ae1 temperature is a typical spheroidizing process, which could be influenced by initial microstructure of steel. It is known that the microstructure with spheroidized carbide particles in the ferrite matrix shows a high ductility among various microstructures of steels. The high ductility of the spheroidized microstructure is correlated with a ductile ferrite matrix. The spheroidized microstructure has a lower hardness and higher ductility than the pearlite microstructure, which consists of a plate-type lamellar cementite and ferrite matrix. The superior ductility and low hardness of the spheroidized microstructure play an important role in the cold rolling and cold forming of low and medium carbon steels and in the machinability of high carbon steels before the final hardening processes. The relationship between the spheroidizing treatment and cold formability is briefly summarized as follows: First, spheroidized carbide particles are formed, and coarse particles are observed after a long holding time, resulting in a yield strength (YS) and ultimate tensile strength (UTS) reduction, and elongation increases as spheroidizing continues. Finally, the cold formability is improved.
n
Corresponding author. Tel.: +82 63 270 2298; fax: +82 63 270 2305. E-mail address: [email protected] (S.-J. Lee).
0921-5093/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2013.07.040
The spheroidized microstructure is the most stable due to the stress relief in the ferrite matrix and the minimized interfacial surface area per unit volume between the cementite particle and the ferrite matrix. The high ratio of the interfacial surface area to the unit volume for lamellar cementite in pearlite results in high surface energy. The tendency to reduce the surface energy is the driving force for dividing cementite lamellae into small spherical particles. The small cementite particles grow or merge each other to minimize the total interfacial surface area of a system as the equilibrium volume fraction of cementite is preserved at a given temperature, resulting in coarse particles. This growing process is referred to as coarsening or Ostwald ripening [3]. The rate of spheroidization is directly influenced by the carbon diffusion in ferrite and is gradually decreased as the average size of the spheroidized particles is increased. The addition of an alloying element, especially a strong carbide forming element such as Ti, V, Nb, etc. which could be as a solute or precipitate, decreases the diffusivity of carbon in ferrite, so the spheroidization reaction becomes slow. The effect of carbon diffusivity on the coarsening rate of spheroidized cementite particle is described in the following equation [4]: r 3 r 30 ¼
8γDC CV m t 9RT
ð1Þ
where r is the particle radius, r0 is the initial particle radius, γ is the interfacial energy between ferrite and cementite, DC is the diffusion coefficient of carbon, C is the equilibrium carbon concentration in ferrite, Vm is the molar volume of particle, R is the gas constant, T is temperature, and t is time. In fact, the coarsening of
Y.-W. Lee et al. / Materials Science & Engineering A 585 (2013) 94–99
particle means the cementite/ferrite boundary movement. The addition of alloying element influences the cementite/ferrite boundary movement not only by varying the carbon diffusivity but also by changing the thermodynamic conditions [5]. For instance, Si can dissolve in cementite and increase the solubility of carbon in ferrite, resulting in the retardance of cementite formation and growth in para-equilibrium condition. The spheroidized microstructure is formed at a proper temperature with a sufficiently long holding time for a carbon diffusion reaction. As a result, various spheroidizing treatment methods have been proposed to obtain an optimal spheroidized microstructure for steels. Joung and Chung investigated several heat treatment process conditions for spheroidized AISI 9260 (Fe– 0.62%C–0.93%Mn–1.87%Si) steel [6]. They found that martensite as the initial microstructure and a repeated cycling heat treatment around the Ae1 temperature could maximize the rate of spheroidized carbide particle formation. Hundreds of hours may be Table 1 Chemical composition of D6AC steel used in the present study (in wt%). C
Mn
Si
P
S
Cr
Mo
V
Fe
0.46
0.75
0.25
0.01
0.003
1.1
1.0
0.1
Bal.
95
required to complete the spheroidization from pearlite, and separate small carbide particles in bainite or martensite can reduce the holding time. In the case of the initial pearlite microstructure, a smaller interlamellar spacing between the cementite and pearlitic ferrite results in a faster spheroidizing rate due to the diffusion distance of carbon [7]. The spheroidization of martensite is occasionally carried out in high alloy tool steels, which easily form martensite during slow cooling such as natural air cooling. Undissolved carbide particles accelerate the formation of the spherodized microstructure, so it is important to restrict the solid solution temperature in order to maintain some undissolved carbide particles. When the size of a spheroidized cementite particle is greater than 4 μm, the particle acts as a nucleation site for microcracks [8]. D6AC steel, a medium carbon steel with low alloy content and high strength, has been widely used in aerospace and defense components due to the high yield to tensile strength ratio and the superior ductility [9]. A typical heat treatment process of D6AC steel is direct quenching from a high austenitic temperature to room temperature, followed by conventional tempering. The chemical composition of D6AC steel is similar to AISI 4340 and AISI 4140, but D6AC steel contains additional V and Cu, which are a strong carbide forming element and a solid solution strengthening element, respectively. Several studies on the fracture of D6AC steel have been reported [10–12]. Recently, Lian investigated the effects of tempering condition on microstructure and mechanical properties of D6AC steel [13]. Even though a typical microstructure of spheroidized cementite with a ferrite matrix improves the cold formability, few studies about the spheroidization of D6AC steel related to the microstructure and mechanical properties have been reported. Therefore, this paper addresses the influence of spheroidization on the microstructure and mechanical properties of D6AC steel to improve the cold formability during cold working.
2. Experimental procedure
Fig. 1. CCT diagram of D6AC steel austenitized at 860 1C for 10 min.
The chemical composition of the D6AC steel used in this study is listed in Table 1. The material is a medium carbon low alloy steel containing carbide-forming alloying elements such as Cr, Mo, and V. The solid solution treatment was carried out in a tube furnace filled with Ar gas to prevent oxidation. Two different solid solution temperatures were adopted: 860 1C and 1000 1C. The holding time for the solid solution treatment was 10 min. The sample was directly quenched in room temperature oil to achieve a full martensite microstructure. The spheroidizing temperature was 700 1C, which is below the Ae1 temperature, and the
Fig. 2. (a) Optical micrograph and (b) SEM micrographs of the as-received D6AC sample.
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maximum holding time was 10 h, which was followed by air cooling. The transformation kinetics of the austenite decomposition in D6AC steel was investigated with various cooling rates using a dilatometer. The sample was heated to 860 1C with a heating rate of 3 1C/s, and was then held for 10 min. The dilatometer sample dimensions were 3 1 10 mm3. The microstructure of the heattreated sample was observed by means of an optical microscope (OM) and a scanning electron microscope (SEM). The mechanical properties were evaluated using a room temperature tensile test and a microhardness measurement. An ASTM E8 tensile sample was used, and the tensile rate was 1 10 3 s 1.
3. Results and discussion 3.1. Dilatometry analysis Fig. 1 shows the continuous cooling transformation (CCT) diagram of D6AC steel cooled from 860 1C with different cooling rates ranging from 0.1 to 100 1C/s. The martensite transformation without any other diffusional transformations only occurs when the cooling rate is greater than 1 1C/s because there is enough hardenability due to the addition of the alloying elements to retard the diffusional ferrite and bainite transformations. The observed martensite start (Ms) temperature was about 320 1C regardless of the cooling rate, since the martensite transformation is an athermal transformation that only relies on the transformation temperature. The transformation strain observed around 508 1C at the cooling rate of 0.1 1C/s represents the bainite start (Bs) temperature. According to the empirical equation for the Bs temperature prediction proposed by Kunitake and Okada [14], the predicted Bs temperature for the chemical composition in Table 1, except for the amount of V, is 539 1C. This calculation value indicates the highest temperature at which the upper bainite transformation occurs, and the actual Bs temperature is decreased as the cooling rate increases since the bainite transformation is diffusion controlled. As the cooling rate increases to 0.5 1C/s, the observed Bs temperature is about 407 1C, while the Ms temperature is also around 320 1C. The austenite formation kinetics of D6AC steel was investigated using heating dilation curves. The start temperature of the austenite formation (Ac1) was 745 1C, and the finish temperature of the austenite formation (Ac3) was 813 1C with a heating rate of 0.3 1C/s. However, as the heating rate increased to 30 1C/s, the Ac1 temperature was 768 1C and the Ac3 temperature was 834 1C. The increases in the transformation temperatures reveal that the austenite formation kinetics involves a diffusion-controlled transformation.
3.2. Microstructure Fig. 2 shows the optical and SEM micrographs of the as-received D6AC sample before the solid solution treatment. The microstructure observed consisted of mainly ferrite and pearlite, a small amount of bainite, and some small particles like carbides. This sample was heated to the different solution temperatures (860 1C and 1000 1C), held for 10 min, and was then immediately quenched to room temperature at a cooling rate of 100 1C/s. Fig. 3 compares the microstructural changes affected by the solid solution temperature. The samples show similar martensitic microstructures, but some areas of spherical particles exist are observed in the sample annealed at 860 1C, as indicated in Fig. 3a. It is thought that the particles are Mo- and V-carbides which are not fully dissolved during the solid solution treatment at the relatively low temperature. On the other hand, no spherical carbide particles are observed in the sample annealed at 1000 1C, as seen in Fig. 3b. This indicates that the carbide particles that existed in the initial condition have
Fig. 4. SEM-EDS analysis of carbide particles in the as-quenched D6AC sample after austenitizing at 860 1C for 10 min. The red line (bottom) indicates the counts for V, the green line (middle) indicates the counts for Cr, and the blue line (top) indicates the counts for Mo. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 3. SEM micrographs of the as-quenched D6AC samples austenitized at different temperatures: (a) 860 1C and (b) 1000 1C. The holding time is 10 min.
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been fully dissolved during the solid solution treatment at the relatively high temperature. The spherical particle that is expected as a carbide particle is line scanned using SEM-EDS (Energy Dispersive Spectrometry), and the result is shown in Fig. 4. The spherical particle is an undissolved Mo carbide, because the concentration of Mo in the particle is two times higher than that in a matrix. It is reported that the dissolution temperature of Mo carbide in austenite is between 900 and 1000 1C [15]. The dissolution temperature of VC in austenite can be predicted by using the solute product equation
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proposed by Narita [16]. The solubility product equation for VC is given by log10 ½V½C ¼ 9500=T þ 6:72
ð2Þ
where V and C are the alloy contents in wt%, and T is the dissolution temperature in Kelvin. Considering the chemical composition of D6AC steel in Table 1, the dissolution temperature of VC that is calculated from Eq. (1) is 999 1C. Therefore, most of the VC particles are dissolved during the solid solution treatment at 1000 1C, and a small amount of very fine VC particles remains.
Fig. 5. SEM micrographs of the spheroidized D6AC samples with different heat treatment conditions: (a) austenitized at 860 1C and spheroidized for 1 h, (b) austenitized at 860 1C and spheroidized for 5 h, (c) austenitized at 1000 1C and spheroidized for 1 h, and (d) austenitized at 1000 1C and spheroidized for 5 h. The austenitizing time is 10 min, and the spheroidizing temperature is 700 1C.
Fig. 6. Engineering stress–strain curves of D6AC steel with various heat treatment conditions. The time indicates the spheroidizing time at 700 1C. The austenitizing temperatures are (a) 860 1C and (b) 1000 1C.
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Fig. 5 shows the SEM micrographs of the spheroidized microstructures in the samples that were annealed at different solid solution temperatures. The average size and volume fraction of the carbide particles increase as the holding time increases to 5 h. The observed average particle size in the sample annealed at 860 1C seems relatively larger, which could be due to undissolved Mo carbides. Ata and Meisam have reported that undissolved carbide particles accelerate spheroidization [7]. Thus, the undissolved particles due to the low solid solution temperature brought about the faster kinetics of spheroidization in D6AC steel.
3.3. Mechanical properties
Fig. 7 shows the relationship between the ultimate tensile strength (UTS) and the total elongation depending on the heat treatment conditions. The mechanical properties of D6AC steel which is quenched and then tempered at various temperatures [13] are compared. The spheroidized microstructure shows better ductility than the tempered martensite microstructure in D6AC steel. This means that the spheroidized D6AC steel can have better cold formability than the D6AC steel after a conventional quenching and tempering treatment. Fig. 8 compares the microhardness values of the samples with different heat treatment conditions. The hardness of the as-received sample before the heat treatment was 447 HV, while that of the as-quenched sample was greater than 900 HV due to the martensite microstructure. A higher hardness value of the asquenched sample austenitized at 860 1C is observed, which is similar to the tendency of the strength change, as shown in Fig. 6.
Fig. 6 compares the engineering stress–strain curves obtained using different heat treatment conditions. The maximum tensile strength and total elongation of the as-received sample are 1133 MPa and 7.9%, respectively. The sample was fractured without plastic deformation in the as-quenched sample. The increase in strength was about 100 MPa in the sample annealed at 860 1C, which is due to the carbide precipitation strengthening effect. After the samples are spheroidized for 1 h, the sample annealed at 1000 1C shows relatively higher strength and lower ductility. The effect of the solid solution temperature on the strength and ductility disappear as the samples are held for 5 h. When the samples are spheroidized for 10 h, the sample annealed at 1000 1C shows good elongation greater than 25%, whereas the ductility of the sample annealed at 860 1C decreases slightly. Therefore, the ductility of the spheroidized D6AC steel can be improved by increasing the holding time after the carbide particles are fully dissolved in the high austenite temperature region. Also, the yield point elongation was observed in the sample with a long holding time after annealing at 860 1C. Xiong et al. observe the occurrence of the yield point elongation as the spheroidization of cementite [17]. Yield point elongation is also observed when the spheroidization of cementite is completed in hypereutectoid steel [18]. Thus, it could be supposed that the observation of yield point elongation in spheroidized D6AC steel results from the coarsening of spheroidized cementite particles. A more detailed analysis is required for the correlation between yield point elongation and particle coarsening.
Fig. 8. Variations of hardness for D6AC steel with different heat treatment conditions: (a) as-quenched, (b) spheroidized for 1 h, (c) spheroidized for 5 h, and (d) spheroidized for 10 h. The spheroidizing temperature is 700 1C.
Fig. 7. Combinations of ultimate tensile strength and total elongation for D6AC steel with various heat treatment conditions. The tempering temperature was varied from 250 1C to 650 1C [13].
Fig. 9. Linear relationship between ultimate tensile strength and hardness for D6AC steel regardless of heat treatment condition.
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These results according to the solid solution condition are related to undissolved carbide particles at the lower solid solution temperature. The microhardness values are significantly decreased by spheroidization, but the variation of hardness is insensitive to the solid solution temperature and holding time for spheroidization. In general, strength and hardness have a proportional relationship. The linear relationship between the UTS and hardness of D6AC steel is observed, as shown in Fig. 9. These mechanical properties are mainly influenced by the microstructure controlled by the heat treatment conditions. 4. Conclusions The effects of the spheroidizing treatment on the microstructure and mechanical properties in D6AC steel were investigated. The transformation temperatures and the phase transformations during cooling were investigated by means of dilatometer testing, and the CCT diagram for D6AC steel was presented. The critical cooling rate to obtain a fully martensitic microstructure was 1 1C/s, and the bainite transformation occurred with a cooling rate that was slower than the critical cooling rate. Undissolved carbide particles in the martensite matrix were observed in the sample quenched from 860 1C. These undissolved carbide particles accelerated the kinetics of the spheroidized cementite particles. The ductility of the spheroidized D6AC steel was improved, and the strength and hardness decreased together as the holding time
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increased for the spheroidization at 700 1C. The spheroidizing treatment in D6AC steel increased the elongation compared with the quenching and tempering treatment, so it is expected that the spheroidization of D6AC steel can improve the cold formability for cold working, such as flow forming.
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