Microstructure and Mechanical Properties of the Ternary Gas Shielded Narrow-Gap GMA Welded Joint of High-Strength Steel

: An 80%Ar-10%CO 2 -10%He ternary gas mixture was used as the shielding gas during the narrow-gap welding of thick Q690E high-strength steel plates. Complete and defect-free welded joints were obtained, and the microstructure and mechanical properties of the welded joint were investigated. The weld zone consists of a solidiﬁcation area and interlayer zone, and the heat-affected zone consists of a coarse-grain heat-affected zone (CG-HAZ) and a ﬁne-grain heat-affected zone (FG-HAZ). The microstructures of the weld zone are mainly lath bainite (LB), acicular ferrite (AF), and granular bainite (GB). The microstructure of the CG-HAZ is lath martensite (LM) and the microstructure of FG-HAZ is GB. Methods with different heat inputs were used to study their effects on the mechanical properties of the welded joint. It was found that the microstructure and mechanical properties of the welded joints are better with lower heat input. With tandem wire narrow-gap GMA welding, the tensile strength of the joints declined from 795.3 to 718.3 MPa and the impact toughness at − 40 ◦ C resulted in a weak position in the weld zone, which declined from 76~81 J to 55~69 J, when the welding speed reduced from 350 to 250 mm/min. With oscillating-arc narrow-gap GMA welding, the tensile strength achieved 853.4 MPa and the impact toughness at − 40 ◦ C was around 69~87 J. The results indicated that, under the appropriate heat input, the tensile strength of the joint exceeds 770 MPa and the low temperature impact toughness at − 40 ◦ C exceeds 69 J. A 155 mm-thick Q690E steel welded joint was obtained and the mechanical properties of the welded joint meets the requirements of the offshore drilling platforms.


Introduction
As the main load-bearing structure of the offshore drilling platform, the pile legs not only have to bear the gravity of the main structure of the drilling platform above the sea surface, but also have to withstand the corrosion of sea water, the impact of ocean currents, and the strong external force of seabed earthquakes for a long time during the whole service period [1][2][3][4]. Therefore, in the construction of the entire offshore platform, the construction quality of the pile legs is of great significance to the safety and service life of the entire offshore platform. The tensile strength of the welded joints in the pile legs is required to be greater than 770 MPa, and the low-temperature impact energy at −40 • C is required to be greater than 69 J [5]. Q690E high-strength steel is widely used as the structural material of pile legs because of its high strength and low ductile-brittle transition temperature [6].
Q690 grade steel is a kind of high-strength steel, whose microstructures consist of bainite and martensite. Its strength level is around 700 MPa. In recent years, two developed methods for 690 MPa grade have been commonly used: quenching and tempering, and the thermomechanical control process [7,8]. Q690 steels perform well as a structural material. However, some problems exist when Q690 steels are treated as a welding structure. Q690 steels have a tendency to cold crack, and thus need preheating during the welding 2 of 9 process [9,10]. At present, the thickness and length of Q690E steel for pile legs of drilling platforms have reached 178 mm and 167 m, respectively. The current welding methods for Q690E thick plates are shielded metal arc welding (SMAW), gas metal arc welding (GMAW), and submerged arc welding (SAW), and a wide-open groove is used to ensure the welding quality. SMAW and conventional GMAW cost a large sum in terms of welding materials and processing time, while SAW causes a significant decrease in the mechanical properties in CG-HAZs because of its high heat input [11]. An efficient welding method with high quality is needed.
In narrow-gap welding of thick plates, a deep and narrow gap is used as the groove instead of the traditional wide-open groove. Therefore, narrow-gap welding has a higher welding efficiency, and it also requires less filler metal [12][13][14][15][16]. However, the lack of sidewall fusion is the most frequent defect in narrow-gap welding due to the small included angle between the electrode and the sidewall [14]. Some advanced welding techniques were developed to expand the arc heating area and increase heat transfer to the sidewall, which can ensure the sufficient sidewall penetration, such as rotating-arc welding [17][18][19][20], swinging-arc welding [21][22][23][24], and tandem welding [25]. During the welding process, the shielding gas is ionized to form the arc. Therefore, the physical properties of the shielding gas will directly affect the heat transfer in the arc. Helium has special physical properties, such as a higher thermal conductivity and ionization energy. Hence, more heat could be transferred to the edge of the arc, which is beneficial to the sidewall penetration in narrow-gap welding. In our previous studies, it was found that the sidewall penetration can be increased by up to 40% with the addition of 10% helium to the Ar-CO 2 gas [26].
In this study, the 80%Ar-10%CO 2 -10%He ternary shielding gas mixture was used as the shielding gas to weld thick Q690E high-strength steel. The microstructure and mechanical properties of the welded joint under different welding heat inputs were investigated, and a welded joint with excellent properties was obtained with appropriate heat input.

Experimental Materials, and Methods
During the tandem narrow-gap welding process, the two wires were controlled independently by two CLOOS 503 power sources. As shown in Figure 1, the ends of the two contact tips were bent to direct the two wires toward the opposite sidewalls to ensure sufficient sidewall penetration, and the bending angle was 7 • . The distance between the two wires was 8 mm, and the two arcs were attached to one single molten pool. The composition of Q690 base metal and the wire used in our work are shown in Tables 1 and 2.  During the swing-arc narrow-gap welding process, the arc and wire swing in the narrow-gap groove as the contact tip rotates back and forth. As given in Figure 1, the contact tip is screwed into the lower part of conducting rod, which is bent to form an angle of 7 • . The wire is fed through the inner hole of the conducting rod and stretches out from the contact tip so an angle of 7 • is formed between the wire and the axis of the conducting rod. The conducting rod is driven by a motor.
All power sources were operated in pulsed mode, in which the peak voltage and the base current were both held constant. ESAB OK AristoRod 69 welding wire with a diameter of 1.2 mm was used as the filler metal. No preheating was used before welding. The 80%Ar-10%CO 2 -10%He (vol.%) gas mixture with a flow of 40 L/min was used as the shielding gas.
The composition of Q690 base metal and the wire used in our work are shown in Tables 1 and 2.  The cross-sectional and metallographic samples were ground and polished, and finally etched using saturated FeCl 3 aqueous solution. The welded joint morphology and microstructure were examined with a Keyence VHX-1000E optical microscope (OM). The room temperature tensile strengths of the welded joint were tested on an Instron-5569 electronic universal testing machine, and the loading speed was 2.0 mm/min. Charpy impact tests were carried out to study the low-temperature impact toughness of the welded joint. During the test, the sample was placed in a liquid nitrogen refrigerator. When the temperature of the test sample reached −40 • C, the sample was quickly taken out and placed in the sample slot of the impact tester; then, the suspended pendulum was released to impact the sample to obtain the impact energy. A V-notch was made at the weld center, fusion line, and heat-affected zone, 2 mm outside from the fusion line. For thick plates, in order to make the test results more comprehensive, the tensile strength and the impact toughness at different positions along the weld thickness direction should be investigated. The sampling positions are shown in Figure 2. Tandem narrow-gap welding was used first, and the effects of heat input were studied. The welding parameters are given in Table 3, and three different welding speeds were chosen to achieve different magnitudes of heat inputs.  Tandem narrow-gap welding was used first, and the effects of heat input were studied. The welding parameters are given in Table 3, and three different welding speeds were chosen to achieve different magnitudes of heat inputs. A double-side narrow-gap groove was also used. During the welding process, when two layers at one side were completed, two layers on the other side were welded. The parameters are given in Table 4.

Effects of Heat Input on the Microstructure and Mechanical Properties
As shown in Figure 3, three complete and defect-free welded joints were obtained. The sidewalls had sufficient penetrations. When the welding speed was 250 mm/min, seven layers were needed to fill the gap; when the welding speed increased to 300 or 350 mm/min, eight filling layers were needed. This indicates that preheating is not required in narrowgap welding of Q690E steel thick plates. The narrow and deep groove can confine the welding heat in the groove and reduce the cooling rate of the weld bead, which can prevent the cold cracks.  Generally, martensite, bainite, and ferrite exist in the low carbon micro-alloyed system, and Q690 grade TMCP steels belong to this alloy system [27]. In addition, these kinds of phases could be distinguished through morphological features by optical microscope [28]. Figures 4-6 show the microstructure of the typical zones in the welded joints. Welded joints mainly include the base metal zone, heat-affected zone (HAZ), and weld zone (WZ). The heat-affected zone contains the coarse-grain zone (CG-HAZ) and fine-grain zone (FG-HAZ). The weld zone also contains the interlayer zone. Different regions present different microstructures. The microstructure of CG-HAZ is mainly lath martensite (LM); the microstructure of FG-HAZ is finer granular bainite (GB); the microstructure of WZ is acicular ferrite (AF) and a small amount of granular bainite (GB); the microstructure of the interlayer in the WZ contains acicular ferrite (AF), lath bainite (LB), and granular bainite (GB) in the original austenite grain, and fine secondary Generally, martensite, bainite, and ferrite exist in the low carbon micro-alloyed system, and Q690 grade TMCP steels belong to this alloy system [27]. In addition, these kinds of phases could be distinguished through morphological features by optical microscope [28]. Generally, martensite, bainite, and ferrite exist in the low carbon micro-alloyed system, and Q690 grade TMCP steels belong to this alloy system [27]. In addition, these kinds of phases could be distinguished through morphological features by optical microscope [28]. Figures 4-6 show the microstructure of the typical zones in the welded joints. Welded joints mainly include the base metal zone, heat-affected zone (HAZ), and weld zone (WZ). The heat-affected zone contains the coarse-grain zone (CG-HAZ) and fine-grain zone (FG-HAZ). The weld zone also contains the interlayer zone. Different regions present different microstructures. The microstructure of CG-HAZ is mainly lath martensite (LM); the microstructure of FG-HAZ is finer granular bainite (GB); the microstructure of WZ is acicular ferrite (AF) and a small amount of granular bainite (GB); the microstructure of the interlayer in the WZ contains acicular ferrite (AF), lath bainite (LB), and granular bainite (GB) in the original austenite grain, and fine secondary austenite decompositions along the grain boundaries of the original austenite grain.   Generally, martensite, bainite, and ferrite exist in the low carbon micro-alloyed system, and Q690 grade TMCP steels belong to this alloy system [27]. In addition, these kinds of phases could be distinguished through morphological features by optical microscope [28]. Figures 4-6 show the microstructure of the typical zones in the welded joints. Welded joints mainly include the base metal zone, heat-affected zone (HAZ), and weld zone (WZ). The heat-affected zone contains the coarse-grain zone (CG-HAZ) and fine-grain zone (FG-HAZ). The weld zone also contains the interlayer zone. Different regions present different microstructures. The microstructure of CG-HAZ is mainly lath martensite (LM); the microstructure of FG-HAZ is finer granular bainite (GB); the microstructure of WZ is acicular ferrite (AF) and a small amount of granular bainite (GB); the microstructure of the interlayer in the WZ contains acicular ferrite (AF), lath bainite (LB), and granular bainite (GB) in the original austenite grain, and fine secondary austenite decompositions along the grain boundaries of the original austenite grain.   Generally, martensite, bainite, and ferrite exist in the low carbon micro-alloyed system, and Q690 grade TMCP steels belong to this alloy system [27]. In addition, these kinds of phases could be distinguished through morphological features by optical microscope [28]. Figures 4-6 show the microstructure of the typical zones in the welded joints. Welded joints mainly include the base metal zone, heat-affected zone (HAZ), and weld zone (WZ). The heat-affected zone contains the coarse-grain zone (CG-HAZ) and fine-grain zone (FG-HAZ). The weld zone also contains the interlayer zone. Different regions present different microstructures. The microstructure of CG-HAZ is mainly lath martensite (LM); the microstructure of FG-HAZ is finer granular bainite (GB); the microstructure of WZ is acicular ferrite (AF) and a small amount of granular bainite (GB); the microstructure of the interlayer in the WZ contains acicular ferrite (AF), lath bainite (LB), and granular bainite (GB) in the original austenite grain, and fine secondary austenite decompositions along the grain boundaries of the original austenite grain.   With the increase in the welding heat input, the grain size in the CG-HAZ gradually increases. In the WZ, the morphology of bainitic ferrite changes from AF to LB with the increase in heat input. When the welding speed decreases to 250 mm/min, there is little AF content in the WZ, and the microstructure is dominated by LB. In the interlayer, the content of fine secondary austenite decompositions increases as the heat input increases.
The tensile strengths of the welded joints are given in Table 5. It can be seen that the tensile strength increases as the heat input decreases; when the welding speed reaches 350 mm/min, the tensile strength of the three samples exceeds 770 MPa. It was found that all the samples were broken at the weld zone, which indicates that the weld zone is the weak part of the welded joint. The tensile strengths were approximately similar to the strengths of Xu et al. [29], while the strengths under proper welding speeds were higher in our work. The impact toughness of the welded joints is given in Table 6. The upper (U), middle (M), and bottom (B) positions were tested. The HAZ has the highest impact toughness, and the weld zone has the lowest impact toughness. When the welding speed was 250 mm/min, the impact toughness of the upper and middle part of the welded joint did not reach 69 J. When the welding speed increased to 300 and 350 mm/min, the impact toughness of all the samples exceeded 69 J. The impact toughness of the fusion zone and the HAZ decrease as the heat input increases. The grain size of the CG-HAZ increases as the heat input increases. Therefore, the impact toughness decreases. However, it can be seen that the impact toughness of the fusion zone and HAZ is much greater than 69 J, and the weak part is still the weld zone.
The weld zone contains GB, AF, and LB, and as the heat input increases, the content of the LB increases. The formation temperature of LB is above 600 • C, while the formation temperature of AF is around 500 • C. When the welding heat input is too large, the hightemperature residence time of the metal increases, which promotes the formation of LB and thus causes a significant reduction in the AF content. It is generally believed that a larger grain orientation difference tends to more significantly prevent cracking. Commonly, interfaces of lath bainite are large angle grain boundaries [30][31][32]. Nevertheless, the effective grain size of LB depends on the size of the lath bundle, while the effective grain of AF is the AF grain itself. In addition, interfaces of acicular ferrite are all large angle grain boundaries [33,34]. Thus, AF grains appear to be more effective for preventing cracking than LB lath bundles. A higher heat input causes the reduction in the AF content, which leads to the reduction in tensile strength of the welded joint and the impact toughness of the weld zone.

Microstructure and Mechanical Properties of the Welded Joint of Extra-Thick Q690E Steel Plate
It can be seen from the above results that the microstructure of the welded joint is seriously affected by the welding heat input. The mechanical properties of the welded joints reduce as the heat input increases. Therefore, relatively low welding heat input should be used in order to obtain better microstructure and mechanical properties. Xu et al. [24] found that the effective heat input in swing-arc narrow-gap welding is reduced as compared to non-oscillating welding with a V-type groove. Therefore, swing-arc narrow-gap welding was used in 155 mm-thick Q690E steel.
The welded joint was obtained by filling 34 layers. As presented in Figure 7, the joint was defect-free, and the weld appearance and weld profile were both good. Although the welding heat input is lower using a swing arc, the addition of helium promotes the heat transfer to the sidewall and the penetration is ensured.
The microstructure of the typical zone of the welded joint is shown in Figure 8. It can be seen that the weld zone is composed of AF; the microstructure of CG-HAZ is still mainly lath martensite (LM), but the grain size significantly decreases compared with that of tandem welding.
al. [24]found that the effective heat input in swing-arc narrow-gap welding is reduced as compared to non-oscillating welding with a V-type groove. Therefore, swing-arc narrow-gap welding was used in 155 mm-thick Q690E steel.
The welded joint was obtained by filling 34 layers. As presented in Figure 7, the joint was defect-free, and the weld appearance and weld profile were both good. Although the welding heat input is lower using a swing arc, the addition of helium promotes the heat transfer to the sidewall and the penetration is ensured. The microstructure of the typical zone of the welded joint is shown in Figure 8. It can be seen that the weld zone is composed of AF; the microstructure of CG-HAZ is still mainly lath martensite (LM), but the grain size significantly decreases compared with that of tandem welding. The mechanical properties of the welded joint were also tested. Table 7 shows the tensile strength of the welded joints from the upper position to the bottom position. It was found the average tensile strength of each sample exceeds 835.4 MPa, and the value is higher than that of the tandem welded joint. The tensile strength of the swing-arc narrowgap welding joints in our work was similar to the results of Cai et al. [35]. Table 8 shows the impact toughness of the welded joints at −40 °C, and indicates that the lowtemperature impact toughness of each position of the welded joint was above 69 J. was defect-free, and the weld appearance and weld profile were both good. Although the welding heat input is lower using a swing arc, the addition of helium promotes the heat transfer to the sidewall and the penetration is ensured. The microstructure of the typical zone of the welded joint is shown in Figure 8. It can be seen that the weld zone is composed of AF; the microstructure of CG-HAZ is still mainly lath martensite (LM), but the grain size significantly decreases compared with that of tandem welding. The mechanical properties of the welded joint were also tested. Table 7 shows the tensile strength of the welded joints from the upper position to the bottom position. It was found the average tensile strength of each sample exceeds 835.4 MPa, and the value is higher than that of the tandem welded joint. The tensile strength of the swing-arc narrowgap welding joints in our work was similar to the results of Cai et al. [35]. Table 8 shows the impact toughness of the welded joints at −40 °C, and indicates that the lowtemperature impact toughness of each position of the welded joint was above 69 J. The mechanical properties of the welded joint were also tested. Table 7 shows the tensile strength of the welded joints from the upper position to the bottom position. It was found the average tensile strength of each sample exceeds 835.4 MPa, and the value is higher than that of the tandem welded joint. The tensile strength of the swing-arc narrowgap welding joints in our work was similar to the results of Cai et al. [35]. Table 8 shows the impact toughness of the welded joints at −40 • C, and indicates that the low-temperature impact toughness of each position of the welded joint was above 69 J.

1.
The completed narrow-gap welded joint of Q690E high-strength steel with sufficient sidewall penetration can be obtained using 80%Ar-10%CO 2 -10%He ternary shielding gas. Preheating before welding was not used.

2.
The welding heat input affects the microstructure and mechanical properties of the welded joint. As the heat input increases, the morphology of bainitic ferrite changes from AF to LB, which leads to the decrease in the tensile strength and low-temperature impact toughness of the welded joint. The grain size of the heat-affected zone increases with the increase in the heat input.