Effect of heat input on microstructure and properties of hybrid fiber laser-arc weld joints of the 800 MPa hot-rolled Nb-Ti-Mo microalloyed steels

Highlights

• The geometry of fusion zone was changed with heat input decreased.

• The microstructure in fusion zone was different with heat input increased.

• The softening did not influence tensile properties with heat input increased.

• Impact toughness of fusion zone and coarse-grained HAZ was deteriorate.

• Heat input only affected microstructure of fusion zone and coarse-grained HAZ.

Abstract

Hybrid fiber laser-arc welding (HLAW) process was applied to a novel hot-rolled Nb-Ti-Mo microalloyed steels of 8 mm thickness. The steel is primarily used to manufacture automotive and construction machinery components, etc. To elucidate the effect of heat input on geometry, microstructure and mechanical properties, different heat inputs (3.90, 5.20 and 7.75 kJ/cm) were used by changing the welding speeds. With increased heat input, the depth/width of penetration was decreased, and the geometry of fusion zone (FZ) changed to “wine cup-like” shape. In regard to the microstructural constituents, the martensite content was decreased, but granular bainite (GB) content was increased. The main microstructural difference was in the FZ cross-section at 7.75 kJ/cm because of the effect of thermal source on the top and bottom. The microstructure of the top part consisted of GB, grain boundary ferrite, and acicular ferrite, while the bottom part was primarily lath martensite. The hardness distribution was similar for different heat inputs. Hardness in FZ, coarse-grained HAZ and mixed-grained HAZ was higher than the base metal (BM), but for the fine-grained HAZ was similar or marginally less than the base metal (BM). Tensile strain was concentrated in the BM such that the fracture occurred in this region. In summary, the geometry, microstructure, and mechanical properties of weld joints were superior at heat input of 5.20 kJ/cm.

Introduction

Fusion welding is the most common and effective method for the joining of steels. During steel welding, the fine microstructure obtained by controlled rolling and controlled cooling is not retained, leading to deterioration in the performance of fusion weld joints. Thus, welding method is an aspect of concern in designing steels, especially in controlling the microstructure and properties of weld joints of high strength and ultra-high strength steels [1]. Conventional arc welding, such as gas metal arc welding (MAG/MIG), welding rod arc welding, etc., which have low heat source density and large welding heat input such that the performance of weld joints does not satisfy the operation requirements, in spite of its low production cost and equipment. The extensive application of high energy laser beam provides a new heat source for welding of high strength steels and stainless steel, etc. [2], [3], [4]. However, the application to thicker steel plates is limited because of the low tolerance of group gap from small laser spot diameter, together with the shielding of plasma. Thus, hybrid fiber laser-arc welding (HLAW) that combines both laser and gas metal arc welding (MAG/MIG) heat source, has deeper welding penetration, superior welding stability and welding quality, provides flexibility [5], [6], are aspects of significant interest in the control of microstructure and properties of ultra-high strength thick steel plates during the welding process [7].

Gao et al. [8] studied the evolution of welding process parameters on weld appearance, microstructure and hardness of medium-carbon steel weld joints (carbon equivalent was 0.15%) using 5 kW CO₂ laser MIG hybrid welding. While Huang et al. [9] studied the effect of preheat temperature on cold cracking, microstructure, microhardness, tensile and impact properties of weld joints using high power 15 kW CO₂ laser MIG hybrid welding. They observed that the best combination of microstructure and properties can be obtained at a preheat temperature of 120 °C. They also concluded that the strength and toughness of fusion zone of microalloyed steel welded by HLAW was superior to the base metal because of fine bainitic ferrite, dispersed carbides and high dislocation density [10]. The welding of 30 mm thick high strength steels was carried out using novel double-sided hybrid fiber laser-arc welding by Chen et al. [11]. The toughness of weld joints was less than the base metal but the strength was high. While, higher strength in weld joints was obtained compared to the base metal, but toughness was 80% of base metal, on welding 16 mm thick weathering steel S355J2W using hybrid laser-MAG multi pass welding [12]. Ren et al. [13] observed that hybrid laser-arc welding (HLAW) had lower residual stresses compared to conventional arc welding because of single pass using HLAW for 10–20 mm thick steel plates, whereas 3–4 passes were required in conventional arc welding. The welding of HSLA-65 ship building steels of 9 mm thickness was carried out using HLAW by Munro et al. [14], [15], who pointed that the minimum weld deformation, desired microstructure and mechanical properties were obtained in the weld joint at a heat input of 4 kJ/cm. Laitinen et al. [16] studied the microstructure and properties of two types of 700 MPa grade high strength steel weld joints by Disk HLAW. The results showed that reduced softening, increased strength and toughness, and coarsening of austenite grains were obtained through the use of increased welding speed, which led to lower heat input. Inferior toughness was observed in fusion zone, with only 20 J impact energy at −40 ℃. The effect of welding speed and preheat temperature on the microstructure and properties of X80 pipeline steel weld joints of 14 mm thickness using high power HLAW was studied by Turichin et al. [17]. The comparative study between laser welding, HLAW, and gas shielded arc welding on microstructure and properties of quenched 960 MPa grade ultra-high strength steel weld joints was studied by Siltanen et al. [18]. Compared to the three welding methods, superior strength and toughness of weld joints was obtained by laser welding and HLAW. The study suggested that the geometry of weld joints was the main factor to determine the fatigue performance. Wahba et al. [19] invented a new HLAW to realize the welding of 25 mm thick SM490A steel plates using a single pass through full penetration and double penetration for 50 mm thick plates. The welding properties of hot-rolled S460ML steels of 10 mm thickness was assessed by Chaussé et al. [20] using hybrid laser MAG welding. The welding of UTS 780 MPa grade high strength steels of 11 mm thickness was carried out using high power disk hybrid laser MAG welding by Pan et al. [21]. They studied the effect of shielding gas on penetration, defects and mechanical properties of weld joints. In summary, HLAW has been widely used in ship building and pipeline industry [22], with satisfactory welding quality and efficiency.

Microalloyed steels are based on traditional C-Mn steel, with the addition of microalloying elements (Nb, Ti, Mo, V, B and Cu) to obtain fine ferrite, low carbon bainite and acicular ferrite microstructure through controlled rolling and controlled cooling. A high fraction of microalloyed carbides (<10 nm) are dispersed in the ferrite matrix, which increases the strength of steels [23], [24], [25], [26]. At present, microalloyed steels are used for automotive components and mechanical sector etc. [27], [28]. With increase in strength and requirement of superior quality weld joints involving microalloyed steels, it is important to study the application of HLAW technology to weld microalloyed steels.

In the study described here, welding of a novel hot-rolled 800 MPa tensile strength Nb-Ti-Mo microalloyed steels of 8 mm thickness was carried out by HLAW. The effect of heat input on geometry, microstructure, hardness, strength and impact toughness of weld joints was studied.