SOJ Materials Science & Engineering

High-Strength Low-Alloy (HSLA) steel exhibits excellent tensile strength and ductility, and it also possesses good welding performance due to its low carbon equivalent. However, welding defects always inevitably appear during the fusion weldingof HSLA steel. In this paper, the previous investigations on the microstructure of the joined HSLA steel by different fusion welding processes arereviewed, and the mechanical properties of the fabricated joint are analyzed. Also, the practicability of different fusion welding processes on HSLA steel has been systematically analyzed. Finally, the prospect of the welding of HSLA steel is expected according to the research status.

Keywords: High-Strength Low-Alloy Steel; Fusion Welding; Welding Defect; Microstructure; Mechanical Properties

HSLA steels are classified as low-carbon steels, and their high strength is achieved by the addition of a low percentage of alloy elements that results in precipitation hardening and grain refinement strengthening through thermo mechanical processing [1]. HSLA steels are widely used for transportation pipelines of natural gas and oil, pressure vessels, offshore platforms, nuclear power plants, and bridges[2-5], in which welding process is one of the most critical manufacturing processes for the production of the above related products. HSLA steels can be welded by various fusion welding processes, including Arc Welding (AW) [6-9], Plasma Arc Welding (PAW) [10,11], Laser Welding (LW) [12-14] and Electron Beam Welding (EBW) [15,16].

Research status of fusion welding of HSLA steels
Arc welding Arc welding uses arc as a heat source to convert electrical energy into heat energy and melt the work piece, thereby achieving the purpose of joining metals. Aliporomirabad et al. [6] joined X70 pipeline steel by Tungsten Inert Gas (TIG) welding with a low heat input, and found that there was a high level of residual stress in the Weld Metal (WM) and Heat Affected Zone (HAZ), which was related to the microstructure of bainite and Widmanstätten ferrite. Lee et al. [7] investigated the microstructure and mechanical properties of LW, TIG and Metal Active Gas (MAG) welded DP780 joint, and found that the sizes of the weld seam and HAZ increased with the increasing welding heat input (MAG > TIG > LW). The joints welded by LW and TIG have higher strengths than those welded by MAG, because the microstructure of fusion zone of LW and TIG is composed of martensite. However, the cooling rate of MAG welding is relatively slow, thus the microstructure is mainly ferrite, as shown in (Figure 1) Guo et al. [8] investigated the loss of strength of S960 high strength steel produced by GMAW, the strength of the fabricated joint was 100 MPa lower than that of the Base Metal (BM). The authors also found that the failure zone was located in the HAZ owing to the softening effect. Lan et al. [9] conducted multiple submerged arc welding of HSLA steel with multiple micro alloyed electrodes. The results showed that the weld seam had good toughness, and the main fracture mode was ductile fracture, which was mainly due to the formation of acicular ferrite. However, the toughness of HAZ decreased significantly with the increasing heat input, which was attributed to the change of microstructure from lath Bainite/ martensite to coarse-grained bainite. It can be found from the above literature survey that the softened HAZ was usually one of the weakest areas of the joint because of the large heat input of arc welding. Meanwhile, cold cracks always tended to be produced in HAZ, which also seriously degraded the performance of the arc-welded joint.

Plasma arc welding
Linet al. [10] prepared the D6AC welded joint by PAW, and then tempered the joint at 300oc, 450oc,and 600oc for 1000 min. Results showed that the v-shaped tensile strength (V-notched tensile strength: NTS) of the joint without ant post-treatments increased significantly. However, brittleness of thejoint led to a significant reduction in NTS due to grain boundary sliding after the tempering treatment, especially at higher tempering temperatures (i.e. 450 oc and 600 oc). Ahialeet al. [11] studied the microstructure and high cycle fatigue properties of DP590 joint fabricated by GMAW and PAW. The results show that PAW sample had a higher fatigue life, a relatively flat S-N slope and a higher fatigue limitas compared to the GMAW sample (as shownin Figure 2). The longer fatigue life and higher fatigue limits of the PAW single lap joint specimens could be attributed to the reduced notch effect at the weld toe as compared with the GMAW single lap joint specimens.

Laser welding
In order to avoid the softening of HAZ, welding cracks and some other welding defects resulted from the high heat input during conventional fusion welding, LW with more concentrated energy density and lower heat input are also applied for the welding of HSLA steel [12-14]. [12] studied the difference in microstructure and mechanical properties of D406A steel welded by LW and TIG, and found that the deformation of the LW joint after welding was about 21% of the deformation of the TIG joint (Figure 3). Compared with LW welded joint, TIG welded jointshowed a wider longitudinal tensile stress area. Dissimilar metals of maraging steel 250 containing 8% Ni and AISI 4130 steel were successfully joined by TIG and LW according to the research of Joshiet al. [13]. The results showed that theLW joint showed a higher joint efficiency of 97%, while the efficiency of theTIG joint welded with continuous current only reached 62%. The width of the HAZ of AISI 4130 steel welded by LW was also reduced due to the rapid heating and cooling proceduresduring LW. Li et al. [14] fabricated two kinds of dual-phase steels (DP600 and DP980) by LW. The results showed that the average hardness of FZ (400 ± 11 HV) and BM (340 ± 13 HV) of DP980 steel were higher than that of DP600 steel. In addition, there was a significant drop in hardness curves of welded DP980 steel,indicatingthe severe softening occurring in HAZ, while the HAZ was no obvious in welded DP600 steel. The reason for the different softening degree within these two welded steels is that the DP600 steel used in this study possessedlower martensite content (26%) while the DP980 steel hadhigher martensite content (54%).

Electron beam welding
During EBW, the accelerated and focused electron beam is applied to bombard the work piece, and the kinetic energy of the electron is converted into thermal energy to melt the work piece and realize the joining. Vacuum Electron Beam Welding (VEBW) is currently the most widely-used EBW technology. [15] compared and studied the differences of microstructure and performance between the as-welded state and tempered state of 300M ultrahigh-strength steel produced by EBW. The results showed that the microstructure of the weld zone consisted of lower bainite, residual austenite, and proeutectoid ferrite, while the microstructure of the tempered weld zone contained tempered martensite. The tensile strength of the joint after the tempering treatment reached up to 1900 MPa. Zhanget al. [16] studied the effect of post-weld heat treatment on the microstructure and

mechanical properties of 300M ultrahigh-strength steel joined by EBW, and compared the joint with the conventional quenched and tempered BM sample. The results showed that heat treatment processes cannot eliminate the columnar dendrites within the weld seam, and the strength and impact toughness of the W-N2QT sample are better than that of the BMat tempered state. Although EBW also has the advantage of high energy density, most of EBW needs to be operated in a vacuum chamber. Because the vacuum degree is highly related to the size of the chamber, EBW has higher requirements for the size of the workpiece (Figure 4)

Summary and prospect
Although HSLA steel was successfully joined by different fusion welding processes, the welding defects such as welding cracking, welding porosity, and softeningof HAZ were also easy to be generated during the processing. At present, the researchers primarily focuses on improving the tensile properties of the fusion welded joint of HSLA steel, while there is lesserstudiespay attention to their corrosion and fatigue properties. At the same time, constructing the model of the relationship between the microstructure evolution and stress distribution during the welding procedure can provide guidance for performance prediction of the obtained joints of HSLA steel.

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

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