**1. Introduction**

Welded structures of thick plate are broadly applied to pipelines to transmit gas and oil in large ships, offshore oil drilling platforms, pressure vessels, shipbuilding [1,2], bridges and storage tanks, railway components, nuclear power plants, pressure, and naval vessels [3,4]. HSLA steels have gained widespread use because they exhibit higher strength and better weldability than conventional high alloy steels [5].

The weldability of HSLA steel is reasonably good due to its reduced carbon equivalent [6]. The equipment used for these processes is quite large, and for medium and heavy plate thicknesses, workpieces can only be jointed piece by piece using fusion welding processes, such as submerged arc welding (SAW) [7,8], gas tungsten arc welding (GTAW), gas metal arc welding (GMAW) [9,10], and SMAW [11]. Arc welding has productivity limits due to its low penetration depths. Because of this, multi-pass welding is required for these thick plate structures, which are cut with large-sized single V grooves and require multi-layer multi-pass arc welding, potentially presenting some disadvantages, such as gas porosity, slag inclusion, large levels of distortion and residual stress, poor welding quality defects, and low productivity [12,13]. In addition, other welding processing methods of HSLA steels joints include electron beam welding (EBW), LBW, and friction stir welding (FSW). However, the size of parts for EBW is still a challenge because the weld environment needs to be set in a vacuum chamber, the FSW of thick plates is constrained due to the size of the rotating tool, and the required rigidity of the welding machine [14–16]. In contrast, LBW is particularly attractive for innovative and cost-effective applications because of its high precision, low heat input, narrow weld pool, small welding deformation, and the narrow width of the heat-affected zone (HAZ) [17,18]. Nevertheless, the application of

**Citation:** Chen, Y.; Xu, X.; Liu, Y.; Cui, H. A Comparative Study on Microstructural Characterization of Thick High Strength Low Alloy Steel Weld by Arc Welding and Laser Welding. *Materials* **2023**, *16*, 2212. https://doi.org/10.3390/ ma16062212

Academic Editor: Antonino Squillace

Received: 12 February 2023 Revised: 26 February 2023 Accepted: 8 March 2023 Published: 9 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

LBW to HSLA steels has not been widely adopted for practical applications due to the short cooling times after welding [19–21].

Published comparative studies on SMAW and LBW of HSLA steel plates with higher thicknesses are very scant. The behavior of weldments has been related to microstructural features [22,23]. Therefore, studying the effect of different welding techniques and their parameters on thick Q890 HSLA steel is essential. The present study was carried out to compare the microstructural characteristics and mechanical properties of 16 mm thick Q890 HSLA steel weldment fabricated by SMAW and LBW techniques.

#### **2. Experimental Details**

The 16-mm-thick HSLA Q890 steel used in this investigation was provided by Baoshan Iron and Steel Co., Ltd., (Shanghai, China). The main chemical compositions (in wt.%) of the steel and X90 filling wire are given in Table 1. The equivalent carbon content (CE) for this steel grade amounted to about 0.55 according to SS-EN1011-2 [24], indicating a relatively high hardenability. Its parent microstructure was of a predominantly high temperature tempering structure of martensite (M). In addition, 200 mm × 200 mm welded blanks were fabricated by butt welding. LBW equipment consisted of an IPG Photonics 10 kW fiber laser system (IPG-10000, Burbach, Germany) mounted on a Fronius Transpuls Synergic 5000 wire feeding machine (Fronius, Pettenbach, Austria), and a system of shielding gas 99.99% pure Argon was used as the protective gas, which had a flow rate of 15 L/min. The laser beam was perpendicular to the steel sheet being welded. The weld geometries and dimensions are presented in Figure 1a,b. Prior to welding, mechanical polishing of the BM was performed and the oil on the BM surface was cleaned using acetone.

**Table 1.** Composition and content of chemical elements in HSLA Q890 and X90 filling wire (wt.%).


**Figure 1.** Dimensions of grooves shape (**a**) LBW joint (**b**) SMAW joint (**c**) Tensile specimen (**d**) Charpy specimen (Unit: mm).

The LBW experiments were carried out with a laser power of 8 kW, a welding velocity of 0.3 m/min, and a negative defocusing length of 4 mm.

The welding process of SMAW consisted of six layers and twenty-one passes. The basic parameters of SMAW were set as: a welding current of 250 A; a welding velocity of 0.11 m/min; a welding voltage of 28 V. To prevent cracks, the preheating temperature before welding was 120 ◦C, the inter-pass temperature was strictly controlled below 200 ◦C, and post-weld heat treatments were adopted. After welding, the weldment was cooled in air for 20 min, heated to 360 ◦C in 60 min, and kept constant for 30 min in a heat treatment furnace before being cooled in air.

Nine replications of each level of two joints were made to achieve measurement accuracy. Out of those, three were selected for tensile tests, four were employed in impact tests, and two were used for microhardness and microstructure observations. Following GB/T 228-2002 [25], tensile test specimens were sectioned from the welded blanks, as presented in Figure 1a,b. Tensile tests were performed on a Zwick/Roell Z100 (Zwick Roell Group, Ulm, Germany) universal testing facility with a constant strain rate of 1 mm/min at room temperature. The tensile test recorded for each joint was the average of three trials. The schematic of tensile test specimens is presented in Figure 1c.

The Charpy V-notch impact tests were performed at −40 ◦C (according to GB/T 229-2007 [26]). Four different test sampling locations in each joint were designed in view of the effect of the sampling position on the low temperature impact property, as shown in Figure 1a,b. The absorbed energy value recorded for each joint was the average of four trials. The schematic of the unstandardized 2.5 × 10 × 55 mm specimen with a V-notch is presented in Figure 1d.

The metallographic specimens were prepared by cutting along the cross section of the weld followed by a conventional procedure, such as mounting, grinding, polishing, or etching. Thereafter, the etched samples were observed using OM (Leica DM4000 M LED, Leica Camera, Wetzlar, Germany). Microstructures of the unetched cross sections were characterized by using SEM (JEOL JSM-7800F Prime, JEOL Ltd., Tokyo, Japan) with EDX.

Vickers microhardness tests (Zwick/Roell ZHμ, Zwick Roell Group) were utilized at the mid-thickness of the weld cross section to evaluate the microhardness of the weld metal (WM) and HAZ. According to ASTM:E384, an indentation load of 500 g with a dwell time of 15 s was employed for each test.

#### **3. Results and Discussion**

#### *3.1. LBW Weld Microstructure*

The HAZ can be described by four subzones at a fine-grained level: the coarse-grained heat affected zone (CGHAZ); the fine-grained heat affected zone (FGHAZ); the two-phase zone (TPZ); and the over-tempering zone (OTZ) in detail. The definition of each zone is as follows: the fusion line is used as the boundary between the weld metal (WM) and HAZ; the boundary between the CGHAZ and the FGHAZ is about 10 μm in width with obvious grain size growth; the boundary between the FGHAZ and the TPZ is where the tempered sorbite of the base metal (BM) disappears; the boundary between the TPZ and the OTZ is based on the disappearance of white striped microstructures and the appearance of obvious granular carbides. Each zone width was calculated using the average width method that was equal to the zone area divided by its thickness using Image-Pro Plus software: IPP 7.0.

Figure 2a shows the optical morphology of the LBW joint. It can be seen that the weld morphology is good without any obvious defects. Although the carbon equivalent (0.55%) of the Q890 HSLA steel is a little higher, the LBW performance of the welded joint had been greatly improved by the preheating treatment before welding. It can also be seen that the maximum and minimum bead width of LBW are about 9.5 mm and 3.6 mm, respectively. The maximum and minimum bead width of the HAZ are 4 mm and 2.2 mm, respectively, and its average bead width is 2.9 mm. The weld penetration is about 13 mm. The LBW weld can be divided by <sup>1</sup> WM, <sup>2</sup> CGHAZ, <sup>3</sup> FGHAZ, <sup>4</sup> TPZ, <sup>5</sup> OTZ, and <sup>6</sup> BM.

**Figure 2.** Optical morphology of joints (**a**) the LBW (**b**) the SMAW.
