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Q235B plate using DT4 interlayer
Author’s Accepted Manuscript Mechanical properties and interfacial structure of hot-roll bonding TA2/Q235B plate using DT4 interlayer C. Yu, H. Xiao, H. Yu, Z.C. Qi, C. Xu www.elsevier.com/locate/msea
PII: DOI: Reference:
S0921-5093(17)30441-0 http://dx.doi.org/10.1016/j.msea.2017.03.118 MSA34901
To appear in: Materials Science & Engineering A Received date: 24 December 2016 Revised date: 30 March 2017 Accepted date: 31 March 2017 Cite this article as: C. Yu, H. Xiao, H. Yu, Z.C. Qi and C. Xu, Mechanical properties and interfacial structure of hot-roll bonding TA2/Q235B plate using DT4 interlayer, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.03.118 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Mechanical properties and interfacial structure of hot-roll bonding TA2/Q235B plate using DT4 interlayer C. Yu, H. Xiao*, H. Yu, Z.C. Qi, C. Xu National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao, Hebei 066004, China
*
Corresponding author at: National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao, Hebei 066004, China. [email protected] (H. Xiao)
Abstract In this paper, a TA2/Q235B plate was bonded with a DT4 interlayer by hot-rolling in a vacuum. The resulting interfacial structure and mechanical properties were analyzed. The results show that when the reduction is 25%, the rolling speed is 50 mm/s, and the bonding temperature is between 750 and 850°C, the shear strength of the bonding interface increases with temperature. It is difficult to bond TA2 and DT4 at a bonding temperature of 700°C. Brittle compounds, i.e., TiC, FeTi and Fe2Ti, are formed at the interface when the bonding temperature is 950°C, and the bonding strength significantly decreases. When the bonding temperature is 850°C, the shear strength of the interface reaches a maximum of approximately 237.6 MPa, and the fracture surface presents ductile fracture characteristics. Keywords: titanium clad steel plate; DT4; temperature; shear strength
1 Introduction Titanium is an ideal material for many process applications due to its superior corrosion resistance. It has been used in petroleum pipes, chemical reactors and heat exchangers. However, when pressure, temperature and/or size demand very thick plates, titanium equipment can become considerably expensive [1,2]. Titanium clad steel plates are not only resistant to corrosion but also have high strength and reduced costs. However, at high temperature, debonding is possible because of lots of intermetallic compounds generating on the titanium clad steel plates interface, the plates are only suitable for low temperature working. Due to these characteristics, titanium clad steel plates are widely used in the petrochemical, shipbuilding, and marine industries, among others [3-6]. Currently, the main methods of manufacturing titanium clad steel plates are explosive welding, explosive-rolling bonding and rolling bonding [7-12]. However, with explosive welding and explosive-rolling bonding, the shock wave of the explosion results in partial melting in a region of the composite interface that quickly solidifies, which can easily cause solidification defects such as shrinkage, cracks and pores at the interface[8]. Meanwhile, the environmental pollution caused by the explosion and the lower production efficiency greatly limits the application of explosive composite technology. Compared with the explosive 1
cladding and explosive-rolling bonding methods, the rolling bonding method can produce large-sized and thin titanium clad steel plates with high production efficiency, low pollution and low energy consumption[11,12]. The formation of brittle compounds such as TiC, FeTi and Fe2Ti on the interface may cause the degradation of mechanical properties[13,14]. The coexisting carbide and intermetallic compounds in the interlayer have more detrimental effects on the bonding strength than those individually formed. Compared with Fe, C has a stronger binding capacity with Ti; therefore, TiC is the easiest to form at the titanium steel interface[14]. To prevent the formation of brittle compounds such as TiC, FeTi and Fe2Ti that reduce the bonding strength, the method of using intermediate layer material is commonly adopted. Some studies have used niobium, copper, silver and nickel as the interlayer[13,15-17]; however, because of high costs, the application of these materials in the production of titanium clad steel plates has been limited. Therefore, the use of hot-rolling bonding between TA2 and Q235B using a DT4 interlayer was investigated to prevent the formation of TiC. The effect of the bonding temperature on the mechanical properties and microstructure of the bonding interface was also examined. 2 Experimental Commercial pure titanium TA2 and Q235B steel were used as clad metals, and commercial pure iron DT4 was used as the intermediate layer. The chemical compositions of these materials are presented in Table 1. The thicknesses of the TA2, the Q235 and the DT4 were 3.7 mm, 3 mm and 1 mm, respectively. The assembly pattern is shown in Figure 1. Before assembling the billet, the oxides on the surfaces of the metals were removed via grinding machine, and then, the surfaces were repeatedly cleaned with acetone and alcohol and immediately dried. While the exterior of the assembled billet was welded, argon gas was blown into the billet through a pipe to prevent internal oxidation at high temperature caused by the welding. Next, the pressure inside of the billet was brought to 0.1 Pa through the pipe using a vacuum pump, at which point, the pipe was sealed. The size of the assembled billet was 100 mm×50 mm×11.7 mm. Restricted by the size of the billet, the temperature of the billet rapidly dropped during the rolling process, and reheating is necessary for multi-pass rolling. However, the increased time the billet spent in the high temperature had an impact on the bonding strength. Some scholars have researched the roll bonding of titanium alloys and stainless steel using an interlayer, and they achieved decent bonding strength using a single pass rolling reduction of 20% and 25%[18,19]. Therefore, to investigate the effect of bonding temperature on bonding strength, to exclude reheating effects, and to obtain a good bonding condition, we chose a single pass rolling reduction of 25%. The billets were heated in a furnace for 2 h at temperature of 700°C, 750°C, 800°C, 850°C, 900°C and 950°C; the rolling speed was 50 mm/s; and the billets were cooled to room temperature in the air. To exclude the influence of air entry and other accidental factors on the experimental results, three billets were rolled at each temperature, and the experiments were repeated to ensure that at least two experimental datasets were reliable. Billets without 2
a DT4 interlayer were also prepared to provide comparable results under the same rolling process.
3
Table 1 Chemical composition of the row materials Composition(ω/%) Materials C
Si
Mn
P
S
O
N
H
Ti
Fe
TA2
0.012
-
-
-
-
0.07
0.01
0.008
Bal.
0.062
Q235B
0.15
0.19
0.38
0.017
0.009
-
-
0.017
0.009
Bal.
DT4
0.002
0.02
0.05
0.007
0.003
-
-
-
-
Bal.
Fig. 1 Assembly pattern
The samples for the tension-shear test were prepared according to the specifications set in GB/T 8547−2006 (Titanium clad steel plate) and GB/T 6396−2008 (Clad steel plates−Mechanical and technological test). Four samples of each plate were cut along the rolling direction and were tested to obtain an average shear strength. A tension-shear test was carried out using an INSPEKT Table 100 kN instrument. The stretching speed was 1 mm/min. The tension-shear test sample is shown in Figure 2.
Fig. 2 Schematic of the tension-shear sample
Specimens for microstructure examination were extracted along the rolling direction. The surfaces of the specimens were ground with emery papers up to No. 4000 and polished with an SiO2 suspension with a particle size of 0.04 μm, and then, the specimens were etched with a 4% nitric acid alcohol solution. The microstructure at the DT4 side near the interface was examined using optical microscopy (OM, ZEISS Scope A1). The morphology of the fracture surface was investigated using scanning electron microscopy (SEM, ZEISS Sigma 500). The interfacial compounds were analyzed using an X-ray diffractometer (XRD, D-Max 2500). The distribution of elements across the interface was examined by SEM equipped with an energy dispersive spectrometer (EDS). The microhardness near the interface was obtained using a microhardness tester (FM-700). 3 Results and discussion Figure 3 shows the shear strength of the interface with and without the DT4 interlayer at different bonding temperatures. In the presence of a DT4 interlayer and below 850°C，the shear strength of the interface increases with temperature; above 4
850°C，it decreases with temperature; and at 950°C, it significantly decreases. Between 750 and 900°C, the shear strength of the clad plates reaches the standard value of 140 MPa from the GB/T 8547-2006 type 1 titanium clad steel plate. Between 800°C and 900°C, the shear strength reaches the standard value of 196 MPa from the type 0 titanium clad steel plate. The maximum shear strength is ~237.6 MPa when the bonding temperature is 850°C. At temperatures between 750 and 900°C, the shear strength is lower without the DT4 interlayer than with it, and the maximum shear strength is ~197.9 MPa at 850°C. However, at 950°C, the shear strength is higher without the DT4 interlayer than with it because without the DT4 interlayer, a layer of TiC forms at the bonding interface, which acts as a barrier against the interdiffusion of Fe and Ti across the bonding interface[14]. With the DT4 interlayer, TiC does not form at the bonding interface, preventing the formation of the FeTi and Fe2Ti. The existence of intermetallic compounds decreases the bonding strength significantly.
Fig. 3 Effect of the bonding temperature on the shear strength of titanium clad steel plates
Figure 4 shows the microstructures on the DT4 side at bonding temperatures of 700°C，850°C and 950°C. At 700°C，the ferrite on the iron side was deformed and tended to form a banded structure under the rolling pressure. At 850°C, the ferrite on the iron side near the interface increased in size because trace elements had diffused toward the interface. At 950°C，the ferrite on the iron side recovered, recrystallized and grew, and the ferrite near the interface transformed into columnar ferrite. The black belt adjacent to the interface on the iron side indicated that compounds had formed there.
5
Fig. 4 Microstructures of the DT4 side at (a) 700°C, (b) 850°C, and (c) 950°C
Figure 5 shows secondary electron images of the fracture surface on the DT4 side at bonding temperatures of 700°C，850°C and 950°C. As shown in figure 5(a), the fracture surface is flat when the bonding temperature is 700°C, and some grinding traces were present. This result shows that TA2 and DT4 had not bonded well at 700°C. As shown in figure 5(b), when the bonding temperature is 850°C, the fracture surface presents obvious ductile fracture features with many dimples. As shown in figure 5(c), at 950°C, the fracture surface presents brittle fracture characteristics .
Fig. 5 SEM of the fracture surface at (a) 700°C, (b) 850°C, and (c) 950°C
Figure 6 shows the XRD analysis results of the fracture surfaces when the bonding temperature is 850°C. Figures 6(a) and 6(b) show that Ti and Fe are on the fracture of the TA2 side and that Fe and some Ti are on the DT4 side; no TiC, FeTi or Fe2Ti were found on either fracture surfaces. Combined with the ductile fracture features shown in figure 5(b), the results indicate that TA2 and DT4 bonded well, and fracture of the bonding interface occurs on the DT4 side. Figure 7 shows the XRD analysis results of the fracture surfaces when the bonding temperature is 950°C. 6
Mainly FeTi and Fe2Ti, with some TiC, are on the fracture surface of the TA2 side, and Fe with some FeTi, Fe2Ti and TiC are on the DT4 side. Thus, the formation of TiC, FeTi and Fe2Ti compounds at the bonding interface significantly decreases the bonding strength, at a shear strength of only 36.2 MPa. The XRD analysis results of the fracture surfaces when there is no DT4 interlayer and the bonding temperature is 850°C are shown in figure 8. TiC is found on both sides of TA2 and Q235B, which makes the bonding strength (~197.9 MPa) lower than that in the presence of the DT4 interlayer (~237.6 MPa).
Fig. 6 XRD patterns of fracture surfaces bonded at 850°C: (a) the TA2 side and (b) the DT4 side
Fig. 7 XRD patterns of fracture surfaces bonded at 950°C: (a) the TA2 side and (b) the DT4 side
Fig. 8 XRD patterns of fracture surfaces bonded at 850°C: (a) the TA2 side and (b) the Q235B side
Figure 9 shows the microhardness results near the interface at bonding temperatures of 700°C, 850°C and 950°C. It was found that peak hardness was 7
achieved at the interfaces of TA2 and DT4 at different temperatures. At 700°C, the hardness is high on the TA2, DT4 and Q235B sides due to work hardening at low rolling temperature. At 950°C, the microhardness reaches 286.9HV, and it is also high on the TA2 side. However, on the DT4 side, the microhardness is low. At 850°C, the peak hardness is 157.3HV, which is the lowest among the three peak hardnesses, similar to that of the substrates of TA2 and DT4. This finding indicates that few intermetallic compounds formed on the interface. According to the three peak hardnesses at the interfaces of TA2 and DT4 in figure 9 and the secondary electron images of the interfaces in figure 10, it indicates that intermetallic formed on the interface at all different temperatures. As shown in figure 10(a) and figure 10(b), at 700°C and 850°C, no obvious thickness intermetallic compounds were found on the bonding interfaces. As shown in figure 10(c), at 950°C, the intermetallic compounds approximately 5 μm formed on the interface.
Fig. 9 Microhardness near the bonding interface
Fig. 10 SEM of the bonding interface at (a) 700°C, (b) 850°C, and (c) 950°C
Figure 11 shows the element’s distribution profiles across the interface between TA2 and DT4 at a bonding temperature of 850°C. Ti and Fe diffusion is evident, and the diffusion distance is approximately 1 μm. The C distribution has no apparent change near the interface, which indicates that the diffusion of C from Q235B to TA2 is effectively suppressed by DT4.
8
Fig. 11 Elements distribution profiles of the interface between TA2 and DT4 at 850°C
4 Conclusions (1) The TA2/Q235B plate was bonded by hot-rolling in a vacuum with a DT4 interlayer. When the reduction is 25%, the rolling speed is 50 mm/s, and when the bonding temperature is between 700~850°C, the shear strength of the interface increases with temperature. When the bonding temperature is between 850~950°C, the shear strength of the interface decreases with temperature. The maximum shear strength was approximately 237.6 MPa at 850°C, and it was higher with the DT4 interlayer than without it (~197.9 MPa) under the same rolling process. (2) When the bonding temperature is 850°C and a DT4 interlayer is present, the formation of brittle compounds such as TiC, FeTi and Fe2Ti is effectively inhibited. The bonding strength is improved, and the fracture surface presents ductile fracture characteristics. (3) When the bonding temperature is 700°C, TA2 and DT4 do not bond well. At 950°C, TiC, FeTi and Fe2Ti are generated at the bonding interface of TA2 and DT4, significantly decreasing the bonding strength. Acknowledgments The authors would like to thank the support from the Natural Science Foundation of HeBei Province (E2015203311) and the National Natural Science Foundation of China (No. 51474190 ). References [1] H.T. Jiang, X.Q. Yan, J.X Liu, X.G. Duan, Effect of heat treatment on microstructure and mechanical property of Ti−steel explosive-rolling clad plate, T Nonferr Metal Soc. 24 (2014) 697-704. [2] S.A.A.A. Mousavi, P.F. Sartangi, Experimental investigation of explosive welding of cp-titanium/AISI 304 stainless steel, Mater. Des. 30 (2009) 459-468. [3] J.S. Ha, S.I. Hong, Deformation and fracture of Ti/439 stainless steel clad composite at intermediate temperatures, Mater. Sci. Eng. A 651 (2016) 805-809. [4] N. Kahraman, B. Gülenç, F. Findik. Joining of titanium/ stainless steel by explosive welding and effect on interface, J Mater Process Tech. 169 (2005) 127−133. [5] H. Su, X.B. Luo, F. Chai, J.C. Shen, X.J. Sun, F. Lu, Manufacturing technology and application trends of titanium clad steel plates, J. Iron Steel Res. Int. 22 (2015) 977-982. 9
[6] P. Manikandan, K. Hokamoto, M. Fujita, K. Raghukandan, R. Tomoshige, Control of energetic conditions by employing interlayer of different thickness for explosive welding of titanium/304 stainless steel, J Mater Process Tech. 195 (2008) 232-240. [7] P.Manikandan, K.Hokamoto, A.A. Deribas, K. Raghukandan, R. Tomoshige, Explosive Welding of Titanium/Stainless Steel by Controlling Energetic Conditions, Mater Trans. 47 (2006) 2049–2055. [8] J. Song, A. Kostka, M. Veehmayer, D. Raabe, Hierarchical microstructure of explosive joints: Example of titanium to steel cladding, Mater. Sci. Eng. A528 (2011) 2641-2647. [9] Y. Zheng, Hot-rolling of explosion-formed Ti-steel clad plate, Iron Steel 25 (1990) 28-32. (in Chinese) [10] D.S. Zhao, J.C. Yan, Y. Wang, S.Q. Yang, Relative slipping of interface of titanium alloy to stainless steel during vacuum hot roll bonding, Mater. Sci. Eng. A 499 (2009) 282-286. [11] Y.M. Hwang, H.H. Hsu and Y.L. Hwang, Analytical and Experimental Study on Bonding Behavior at the Roll Gap During Complex Rolling of Sandwich Sheets, Int J Mech Sci. 42 (2000) 2417-2437. [12] Z.A. Luo, G.L. Wang, G.M. Xie, Interfacial Microstructure and Properties of a Vacuum Hot Roll-bonded Titanium-Stainless Steel Clad Plate with a Niobium Interlayer, Acta Metall. Sin. 26 (2013) 754-760. [13] S. Kundu, S. Chatterjee, Evolution of interface microstructure and mechanical properties of titanium/304 stainless steel diffusion bonded joint using Nb interlayer, ISIJ Int. 50 (2010) 1460-1465. [14] T. Momono, T. Enjo, K. Ikeuchi, Effect of carbon content on the diffusion bonding of iron and steel to titanium, ISIJ Int. 30 (1990) 978-984. [15] M. Eroglu, T.I. Khan, N. Orhan, Diffusion bonding between Ti-6Al-4V alloy and micro duplex stainless steel with copper interlayer, Mater. Sci. Tech-lond. 18 (2002) 68-72. [16] A. Laik, A.A. Shirzadi, G. Sharma, R. Tewari, T. Jayakumar, G.K. Dey, Microstructure and Interfacial Reactions During Vacuum Brazing of Stainless Steel to Titanium Using Ag-28 pct Cu Alloy, Metall. Mater. Trans. A 46 (2015) 771-782. [17] S. Kundu, S. Chatterjee, Effect of bonding temperature on interface microstructure and properties of titanium– 304 stainless steel diffusion bonded joints with Ni interlayer, Mater. Sci. Tech-lond. 22 (2006) 1201-1207. [18] J.C. Yan, D.S. Zhao, C.W. Wang, S.Q. Yang, Vacuum hot roll bonding of titanium alloy and stainless steel using nickel interlayer, Mater. Sci. Tech-lond. 25 (2009) 914-918. [19] D.S. Zhao, J.C. Yan, Y.J. Liu, Z.S. Ji, Interfacial structure and mechanical properties of hot-roll bonded joints between titanium alloy and stainless steel using niobium interlayer, T. Nonferr. Metal Soc. 24 (2014) 2839-2844.
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PII: DOI: Reference:
S0921-5093(17)30441-0 http://dx.doi.org/10.1016/j.msea.2017.03.118 MSA34901
To appear in: Materials Science & Engineering A Received date: 24 December 2016 Revised date: 30 March 2017 Accepted date: 31 March 2017 Cite this article as: C. Yu, H. Xiao, H. Yu, Z.C. Qi and C. Xu, Mechanical properties and interfacial structure of hot-roll bonding TA2/Q235B plate using DT4 interlayer, Materials Science & Engineering A, http://dx.doi.org/10.1016/j.msea.2017.03.118 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Mechanical properties and interfacial structure of hot-roll bonding TA2/Q235B plate using DT4 interlayer C. Yu, H. Xiao*, H. Yu, Z.C. Qi, C. Xu National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao, Hebei 066004, China
*
Corresponding author at: National Engineering Research Center for Equipment and Technology of Cold Strip Rolling, Yanshan University, Qinhuangdao, Hebei 066004, China. [email protected] (H. Xiao)
Abstract In this paper, a TA2/Q235B plate was bonded with a DT4 interlayer by hot-rolling in a vacuum. The resulting interfacial structure and mechanical properties were analyzed. The results show that when the reduction is 25%, the rolling speed is 50 mm/s, and the bonding temperature is between 750 and 850°C, the shear strength of the bonding interface increases with temperature. It is difficult to bond TA2 and DT4 at a bonding temperature of 700°C. Brittle compounds, i.e., TiC, FeTi and Fe2Ti, are formed at the interface when the bonding temperature is 950°C, and the bonding strength significantly decreases. When the bonding temperature is 850°C, the shear strength of the interface reaches a maximum of approximately 237.6 MPa, and the fracture surface presents ductile fracture characteristics. Keywords: titanium clad steel plate; DT4; temperature; shear strength
1 Introduction Titanium is an ideal material for many process applications due to its superior corrosion resistance. It has been used in petroleum pipes, chemical reactors and heat exchangers. However, when pressure, temperature and/or size demand very thick plates, titanium equipment can become considerably expensive [1,2]. Titanium clad steel plates are not only resistant to corrosion but also have high strength and reduced costs. However, at high temperature, debonding is possible because of lots of intermetallic compounds generating on the titanium clad steel plates interface, the plates are only suitable for low temperature working. Due to these characteristics, titanium clad steel plates are widely used in the petrochemical, shipbuilding, and marine industries, among others [3-6]. Currently, the main methods of manufacturing titanium clad steel plates are explosive welding, explosive-rolling bonding and rolling bonding [7-12]. However, with explosive welding and explosive-rolling bonding, the shock wave of the explosion results in partial melting in a region of the composite interface that quickly solidifies, which can easily cause solidification defects such as shrinkage, cracks and pores at the interface[8]. Meanwhile, the environmental pollution caused by the explosion and the lower production efficiency greatly limits the application of explosive composite technology. Compared with the explosive 1
cladding and explosive-rolling bonding methods, the rolling bonding method can produce large-sized and thin titanium clad steel plates with high production efficiency, low pollution and low energy consumption[11,12]. The formation of brittle compounds such as TiC, FeTi and Fe2Ti on the interface may cause the degradation of mechanical properties[13,14]. The coexisting carbide and intermetallic compounds in the interlayer have more detrimental effects on the bonding strength than those individually formed. Compared with Fe, C has a stronger binding capacity with Ti; therefore, TiC is the easiest to form at the titanium steel interface[14]. To prevent the formation of brittle compounds such as TiC, FeTi and Fe2Ti that reduce the bonding strength, the method of using intermediate layer material is commonly adopted. Some studies have used niobium, copper, silver and nickel as the interlayer[13,15-17]; however, because of high costs, the application of these materials in the production of titanium clad steel plates has been limited. Therefore, the use of hot-rolling bonding between TA2 and Q235B using a DT4 interlayer was investigated to prevent the formation of TiC. The effect of the bonding temperature on the mechanical properties and microstructure of the bonding interface was also examined. 2 Experimental Commercial pure titanium TA2 and Q235B steel were used as clad metals, and commercial pure iron DT4 was used as the intermediate layer. The chemical compositions of these materials are presented in Table 1. The thicknesses of the TA2, the Q235 and the DT4 were 3.7 mm, 3 mm and 1 mm, respectively. The assembly pattern is shown in Figure 1. Before assembling the billet, the oxides on the surfaces of the metals were removed via grinding machine, and then, the surfaces were repeatedly cleaned with acetone and alcohol and immediately dried. While the exterior of the assembled billet was welded, argon gas was blown into the billet through a pipe to prevent internal oxidation at high temperature caused by the welding. Next, the pressure inside of the billet was brought to 0.1 Pa through the pipe using a vacuum pump, at which point, the pipe was sealed. The size of the assembled billet was 100 mm×50 mm×11.7 mm. Restricted by the size of the billet, the temperature of the billet rapidly dropped during the rolling process, and reheating is necessary for multi-pass rolling. However, the increased time the billet spent in the high temperature had an impact on the bonding strength. Some scholars have researched the roll bonding of titanium alloys and stainless steel using an interlayer, and they achieved decent bonding strength using a single pass rolling reduction of 20% and 25%[18,19]. Therefore, to investigate the effect of bonding temperature on bonding strength, to exclude reheating effects, and to obtain a good bonding condition, we chose a single pass rolling reduction of 25%. The billets were heated in a furnace for 2 h at temperature of 700°C, 750°C, 800°C, 850°C, 900°C and 950°C; the rolling speed was 50 mm/s; and the billets were cooled to room temperature in the air. To exclude the influence of air entry and other accidental factors on the experimental results, three billets were rolled at each temperature, and the experiments were repeated to ensure that at least two experimental datasets were reliable. Billets without 2
a DT4 interlayer were also prepared to provide comparable results under the same rolling process.
3
Table 1 Chemical composition of the row materials Composition(ω/%) Materials C
Si
Mn
P
S
O
N
H
Ti
Fe
TA2
0.012
-
-
-
-
0.07
0.01
0.008
Bal.
0.062
Q235B
0.15
0.19
0.38
0.017
0.009
-
-
0.017
0.009
Bal.
DT4
0.002
0.02
0.05
0.007
0.003
-
-
-
-
Bal.
Fig. 1 Assembly pattern
The samples for the tension-shear test were prepared according to the specifications set in GB/T 8547−2006 (Titanium clad steel plate) and GB/T 6396−2008 (Clad steel plates−Mechanical and technological test). Four samples of each plate were cut along the rolling direction and were tested to obtain an average shear strength. A tension-shear test was carried out using an INSPEKT Table 100 kN instrument. The stretching speed was 1 mm/min. The tension-shear test sample is shown in Figure 2.
Fig. 2 Schematic of the tension-shear sample
Specimens for microstructure examination were extracted along the rolling direction. The surfaces of the specimens were ground with emery papers up to No. 4000 and polished with an SiO2 suspension with a particle size of 0.04 μm, and then, the specimens were etched with a 4% nitric acid alcohol solution. The microstructure at the DT4 side near the interface was examined using optical microscopy (OM, ZEISS Scope A1). The morphology of the fracture surface was investigated using scanning electron microscopy (SEM, ZEISS Sigma 500). The interfacial compounds were analyzed using an X-ray diffractometer (XRD, D-Max 2500). The distribution of elements across the interface was examined by SEM equipped with an energy dispersive spectrometer (EDS). The microhardness near the interface was obtained using a microhardness tester (FM-700). 3 Results and discussion Figure 3 shows the shear strength of the interface with and without the DT4 interlayer at different bonding temperatures. In the presence of a DT4 interlayer and below 850°C，the shear strength of the interface increases with temperature; above 4
850°C，it decreases with temperature; and at 950°C, it significantly decreases. Between 750 and 900°C, the shear strength of the clad plates reaches the standard value of 140 MPa from the GB/T 8547-2006 type 1 titanium clad steel plate. Between 800°C and 900°C, the shear strength reaches the standard value of 196 MPa from the type 0 titanium clad steel plate. The maximum shear strength is ~237.6 MPa when the bonding temperature is 850°C. At temperatures between 750 and 900°C, the shear strength is lower without the DT4 interlayer than with it, and the maximum shear strength is ~197.9 MPa at 850°C. However, at 950°C, the shear strength is higher without the DT4 interlayer than with it because without the DT4 interlayer, a layer of TiC forms at the bonding interface, which acts as a barrier against the interdiffusion of Fe and Ti across the bonding interface[14]. With the DT4 interlayer, TiC does not form at the bonding interface, preventing the formation of the FeTi and Fe2Ti. The existence of intermetallic compounds decreases the bonding strength significantly.
Fig. 3 Effect of the bonding temperature on the shear strength of titanium clad steel plates
Figure 4 shows the microstructures on the DT4 side at bonding temperatures of 700°C，850°C and 950°C. At 700°C，the ferrite on the iron side was deformed and tended to form a banded structure under the rolling pressure. At 850°C, the ferrite on the iron side near the interface increased in size because trace elements had diffused toward the interface. At 950°C，the ferrite on the iron side recovered, recrystallized and grew, and the ferrite near the interface transformed into columnar ferrite. The black belt adjacent to the interface on the iron side indicated that compounds had formed there.
5
Fig. 4 Microstructures of the DT4 side at (a) 700°C, (b) 850°C, and (c) 950°C
Figure 5 shows secondary electron images of the fracture surface on the DT4 side at bonding temperatures of 700°C，850°C and 950°C. As shown in figure 5(a), the fracture surface is flat when the bonding temperature is 700°C, and some grinding traces were present. This result shows that TA2 and DT4 had not bonded well at 700°C. As shown in figure 5(b), when the bonding temperature is 850°C, the fracture surface presents obvious ductile fracture features with many dimples. As shown in figure 5(c), at 950°C, the fracture surface presents brittle fracture characteristics .
Fig. 5 SEM of the fracture surface at (a) 700°C, (b) 850°C, and (c) 950°C
Figure 6 shows the XRD analysis results of the fracture surfaces when the bonding temperature is 850°C. Figures 6(a) and 6(b) show that Ti and Fe are on the fracture of the TA2 side and that Fe and some Ti are on the DT4 side; no TiC, FeTi or Fe2Ti were found on either fracture surfaces. Combined with the ductile fracture features shown in figure 5(b), the results indicate that TA2 and DT4 bonded well, and fracture of the bonding interface occurs on the DT4 side. Figure 7 shows the XRD analysis results of the fracture surfaces when the bonding temperature is 950°C. 6
Mainly FeTi and Fe2Ti, with some TiC, are on the fracture surface of the TA2 side, and Fe with some FeTi, Fe2Ti and TiC are on the DT4 side. Thus, the formation of TiC, FeTi and Fe2Ti compounds at the bonding interface significantly decreases the bonding strength, at a shear strength of only 36.2 MPa. The XRD analysis results of the fracture surfaces when there is no DT4 interlayer and the bonding temperature is 850°C are shown in figure 8. TiC is found on both sides of TA2 and Q235B, which makes the bonding strength (~197.9 MPa) lower than that in the presence of the DT4 interlayer (~237.6 MPa).
Fig. 6 XRD patterns of fracture surfaces bonded at 850°C: (a) the TA2 side and (b) the DT4 side
Fig. 7 XRD patterns of fracture surfaces bonded at 950°C: (a) the TA2 side and (b) the DT4 side
Fig. 8 XRD patterns of fracture surfaces bonded at 850°C: (a) the TA2 side and (b) the Q235B side
Figure 9 shows the microhardness results near the interface at bonding temperatures of 700°C, 850°C and 950°C. It was found that peak hardness was 7
achieved at the interfaces of TA2 and DT4 at different temperatures. At 700°C, the hardness is high on the TA2, DT4 and Q235B sides due to work hardening at low rolling temperature. At 950°C, the microhardness reaches 286.9HV, and it is also high on the TA2 side. However, on the DT4 side, the microhardness is low. At 850°C, the peak hardness is 157.3HV, which is the lowest among the three peak hardnesses, similar to that of the substrates of TA2 and DT4. This finding indicates that few intermetallic compounds formed on the interface. According to the three peak hardnesses at the interfaces of TA2 and DT4 in figure 9 and the secondary electron images of the interfaces in figure 10, it indicates that intermetallic formed on the interface at all different temperatures. As shown in figure 10(a) and figure 10(b), at 700°C and 850°C, no obvious thickness intermetallic compounds were found on the bonding interfaces. As shown in figure 10(c), at 950°C, the intermetallic compounds approximately 5 μm formed on the interface.
Fig. 9 Microhardness near the bonding interface
Fig. 10 SEM of the bonding interface at (a) 700°C, (b) 850°C, and (c) 950°C
Figure 11 shows the element’s distribution profiles across the interface between TA2 and DT4 at a bonding temperature of 850°C. Ti and Fe diffusion is evident, and the diffusion distance is approximately 1 μm. The C distribution has no apparent change near the interface, which indicates that the diffusion of C from Q235B to TA2 is effectively suppressed by DT4.
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Fig. 11 Elements distribution profiles of the interface between TA2 and DT4 at 850°C
4 Conclusions (1) The TA2/Q235B plate was bonded by hot-rolling in a vacuum with a DT4 interlayer. When the reduction is 25%, the rolling speed is 50 mm/s, and when the bonding temperature is between 700~850°C, the shear strength of the interface increases with temperature. When the bonding temperature is between 850~950°C, the shear strength of the interface decreases with temperature. The maximum shear strength was approximately 237.6 MPa at 850°C, and it was higher with the DT4 interlayer than without it (~197.9 MPa) under the same rolling process. (2) When the bonding temperature is 850°C and a DT4 interlayer is present, the formation of brittle compounds such as TiC, FeTi and Fe2Ti is effectively inhibited. The bonding strength is improved, and the fracture surface presents ductile fracture characteristics. (3) When the bonding temperature is 700°C, TA2 and DT4 do not bond well. At 950°C, TiC, FeTi and Fe2Ti are generated at the bonding interface of TA2 and DT4, significantly decreasing the bonding strength. Acknowledgments The authors would like to thank the support from the Natural Science Foundation of HeBei Province (E2015203311) and the National Natural Science Foundation of China (No. 51474190 ). References [1] H.T. Jiang, X.Q. Yan, J.X Liu, X.G. Duan, Effect of heat treatment on microstructure and mechanical property of Ti−steel explosive-rolling clad plate, T Nonferr Metal Soc. 24 (2014) 697-704. [2] S.A.A.A. Mousavi, P.F. Sartangi, Experimental investigation of explosive welding of cp-titanium/AISI 304 stainless steel, Mater. Des. 30 (2009) 459-468. [3] J.S. Ha, S.I. Hong, Deformation and fracture of Ti/439 stainless steel clad composite at intermediate temperatures, Mater. Sci. Eng. A 651 (2016) 805-809. [4] N. Kahraman, B. Gülenç, F. Findik. Joining of titanium/ stainless steel by explosive welding and effect on interface, J Mater Process Tech. 169 (2005) 127−133. [5] H. Su, X.B. Luo, F. Chai, J.C. Shen, X.J. Sun, F. Lu, Manufacturing technology and application trends of titanium clad steel plates, J. Iron Steel Res. Int. 22 (2015) 977-982. 9
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