The Microstructure and Property of a Titanium-Carbon Steel Clad Plate Prepared Using Explosive Welding

Titanium (Ti) and its alloy have excellent corrosion resistance, low density, high specific strength, and heat resistance; thus, they are widely used in the aviation, aerospace, and petrochemical industries. Especially, titanium exhibits excellent resistance to corrosion attack in many aggressive media and is deserving of close attention as a structural material in the design of chemical processing machinery. Thus, lots of equipment manufacturers in petrochemical industry pay more attention to titanium-based materials. However, the price of titanium materials is high, which increases product cost of the equipment [ 1 3 ]. It limits the wide application of titanium. One of the most common methods to reduce the production cost is to reduce titanium content in the equipment. Thus, the technique to produce a reliable joint of titanium-based material to other metals is of great importance. However, it is well known that the welding of titanium with other metals is difficult due to the formation of intermetallic compounds during welding. Lots of brittle Fe-Ti series intermetallic compounds would form with traditional welding method (such as fusion welding, diffusion bonding, and brazing methods). These brittle Fe-Ti intermetallic compounds weaken the joint strength [ 4 6 ]. The most effective method to join Ti with other metal is explosive welding, which can produce different clad materials such as titanium-steel clad materials, titanium-copper clad materials, and titanium-nickel clad materials [ 7 9 ]. Titanium-steel clad materials possess both titanium and steel advantages (high strength and toughness, reduce costs as well as high corrosion resistance). Thus, the demand for titanium-steel clad materials increases in pipes and vessels (such as tanks, piping, autoclaves, and heat exchangers) [ 10 12 ]. Recently, although the interface microstructure and mechanical properties of titanium clad materials have been investigated by some studies [ 10 14 ], few works have been carried out to investigate the corrosion performance of the titanium materials before and after explosive welding. As well, the microstructure and mechanical properties of this clad material still need further investigation. For this purpose, a clad plate of titanium and carbon steel was obtained using explosive welding. After welding, the microstructure, mechanical properties, and corrosion resistance of this clad material were investigated.

Tow specimens for shearing test and one specimen for tensile test were prepared according to the ASTM B898 standard [ 18 ]. The tolerance value of the specimens sizes should meet the requirement of the standard. Shearing and tensile tests were carried out on a testing machine (DLY-10A) at room temperature. The shearing speed was 0.2 mm/min. A three-electrode system attached to an electrochemical workstation (CHI660E) was used for the electrochemical measurement. The original Ti material, carbon steel and the clad plate specimens were used as the working electrode, while the saturated calomel electrode and platinum electrode were used as reference electrode and counter electrode, respectively. The corrosive medium was 3.5 wt.% NaCl solution at room temperature. The polarization curve was scanned from −2.0 to 2.0 Vat a scan rate of 0.5 mV/s. The corrosion potential () and corrosion current density () for each specimen were obtained from the Tafel curve using CorrView software.

Parallel arrangement was employed for the experiment. Figure 1 shows the assemble scheme of explosive welding. The Ti and carbon steel plates were placed in the parallel direction. The spacers (made of pure titanium) were placed on the surface of the carbon steel base plate to support the Ti plate. The height of the spaces was about 8 mm. The ANFO explosive layer was spread on the surface of the cladding plate. In addition, the explosives were packed in the wooden box with the same dimension as the cladding plate. The thickness of the explosive was 20 mm, the density was 0.72 g/cm, and the velocity was approximately 2140 m/s. Based on our previous research [ 15 ], the collision point velocity (V) and dynamic collision angle (θ) were obtained. They were 2089 m/s and 14.8°, respectively. Thus, the Ti plate impact velocity (V) could be calculated by following Equation (1) with the Vand θ.

3. Results and Discussion

10,

During the explosive welding process, sufficient joining impact pressure is produced at the interface, ensuring the successful preparation of the clad plate [ 14 ]. It could be seen that the Ti and carbon steel are tightly joined, and nonbounding defects are observed at the clad plate surface or side (shown in Figure 2 a). During cladding, the high-speed collision produces large plastic at the interface of the metals, yielding the increase in the hardness values of the clad plate at both sides [ 4 11 ]. Thus, to remove the work-hardening effect [ 12 ], the Ti-carbon steel clad plates are generally annealed at 550–650 °C (atmosphere, holding 1 h), to release the residual stress produced during welding. After heat treating, the clad plate is inspected using the ultrasonic C-scanning imaging technology. The inspect results show that the bonding interface has the typical waves ( Figure 2 b). Obviously, the interface waves are uniform without any defect at the bounding interface, suggesting that the bounding interface is great. There are 7–8 waves in 1 cm length, also indicating that the bonding quantity of the clad plate is good [ 12 19 ]. Figure 2 c shows the optical microstructure at the interface of clad plate. It shows that the typical wavy interface has been formed, and no visible defects such as pores or cracks are found. The reason is that during the drastic collision process, and the kinetic energy of Ti plate is partly transformed into potential energy, leading to the plastic deformation of both plates at the interface. In addition, it should be mentioned that some melt blocks in the interface wave vortices are also found.

Vickers hardness tests are performed to inspect original Ti, carbon steel and the clad plate. The hardness values of original Ti and carbon steel are 127 and 163 HV, respectively. In addition, Figure 6 shows Vickers hardness variation with the different distances from the interface of the clad plate. At the titanium layer, the hardness value is 120 HV close to the interface. Towards the thickness center of the titanium layer, similar hardness values (122–135 HV) were measured at 50, 100, 150, 200, 250, 300, 350 and 500 μm to the interface. In addition, at the same distance in the carbon steel layer (0, 50, 100, 150, 200, 250, 300, 350 and 500 μm, hardness values are measured respectively, yielding the values of 212, 246, 233, 231, 215, 220, 209, 169 and 171 HV. Most of them are higher than original carbon steel. In general, due to the deformation of the plates resulting from the collision at high velocity, the hardness values near the joint interface of the clad plate should increase. After heat treating, the values of the clad plate would decrease to that of original materials. In this paper, the hardness values of titanium layer are similar to that of the original Ti material. It indicates that heat treatment releases residual stress at the titanium layer. However, at the layer of carbon steel, the increased hardness values are found from the interface to 300 μm away. Over 300 μm, the hardness values decrease and approach to that of the original carbon steel. It indicates that heat treatment could not completely release the residual stress at the carbon steel layer.

τ

b) values of two samples are 175 and 200 MPa, respectively (

τ

b ≥ 137.9 MPa is acceptable) [

Rp

0.2) and ultimate tensile strength (

Rm

) of the clad plate are 344 and 524 MPa, respectively. In addition, the elongation (

A

) is 25%. Moreover, these values meet the requirement of ASTM B898 specification (

R

p0.2 ≥ 260 MPa,

R

m ≥ 485 MPa,

A

≥ 17%). According to the recent research [Figure 7 and Figure 8 show the results of shear and tensile tests to characterize mechanical properties of the clad plate, in which the shearing test is conducted to evaluate the bonding quality of the clad plate. The shearing strength () values of two samples are 175 and 200 MPa, respectively ( Figure 7 ). They are all higher than that of ASTM B898 standard requirement (≥ 137.9 MPa is acceptable) [ 18 ]. Thus, the bonding quality of the clad plate meets the application requirement. In addition, the interfaces of the shearing specimens are uniform and wavy, consisting with the results of C-scanning ( Figure 2 b). The typical tensile curves, specimen and test result of the clad plate are shown in Figure 8 a,b. The clad plate exhibits typical elastic deformation firstly and then plastic deformation ( Figure 8 a). The yield strength () and ultimate tensile strength () of the clad plate are 344 and 524 MPa, respectively. In addition, the elongation () is 25%. Moreover, these values meet the requirement of ASTM B898 specification (≥ 260 MPa,≥ 485 MPa,≥ 17%). According to the recent research [ 21 22 ], the mechanical properties of the explosively welded clad plate would be mainly influenced by the component with a high strength. In addition, the explosive welding mainly influences the region adjacent to the clad plate interface whose thickness is no more than 300 μm. It means the tensile properties of the clad plate would be minor influenced by the interface microstructure change. Thus, the tensile strength of the clad plate is similar to that of carbon steel plate.

Ecorr

values of three specimens are different. The

Ecorr

values of carbon steel, original Ti material and the clad plate are −977.6, −746.7 and −782.4 mVSCE, respectively. The

Ecorr

value of clad plate is more positive than that of carbon steel material, which increases about 20%. In addition, from the polarization curves parameters (

icorr

) of original Ti material, carbon steel and the clad plate are 1.13 × 10−7, 8.79 × 10−9 and 2.11 × 10−8 A/cm2, respectively. The

icorr

value of the clad plate is 1 order of magnitude lower than that of carbon steel material. These results indicate the corrosion resistance of the clad plate is much better than carbon steel material [

Ecorr

value of original Ti material is more negative, and

icorr

is 1 order of magnitude lower compared with the clad plate. In other words, the corrosion resistance of titanium side in clad plate decreases. The phenomenon needs to be investigated in the future.

Although it is well known that titanium-based material has excellent corrosion resistance, a tremendous amount of heat and pressure released by explosives on the surface of the Ti plate during the explosive welding could decrease the corrosion resistance of titanium material. To investigate this, the corrosion performance of Ti materials before and after explosive welding is evaluated using polarization curves. Figure 9 shows the polarization curves of original Ti material, carbon steel and the clad plate. It can be seen that the carbon steel possesses active dissolution, and its anodic current density increases rapidly with the increase in anodic overpotential. The original Ti material and clad plate behave similarly, with a wide potential range of passive region. The polarization curves are fitted using CorrView software, and the corrosion potential and corrosion current density are obtained, as shown in Table 2 . It shows that thevalues of three specimens are different. Thevalues of carbon steel, original Ti material and the clad plate are −977.6, −746.7 and −782.4 mV, respectively. Thevalue of clad plate is more positive than that of carbon steel material, which increases about 20%. In addition, from the polarization curves parameters ( Table 2 ), the corrosion current density values () of original Ti material, carbon steel and the clad plate are 1.13 × 10, 8.79 × 10and 2.11 × 10A/cm, respectively. Thevalue of the clad plate is 1 order of magnitude lower than that of carbon steel material. These results indicate the corrosion resistance of the clad plate is much better than carbon steel material [ 23 ]. However, the differences are noted between original Ti material and the clad plate. Thevalue of original Ti material is more negative, andis 1 order of magnitude lower compared with the clad plate. In other words, the corrosion resistance of titanium side in clad plate decreases. The phenomenon needs to be investigated in the future.