Quenched and tempered high-strength steels are desirable for use in many industrial applications because they contain low carbon composition and preferable thermomechanical properties. High-strength quenched and tempered low alloy steels derive their strength from quenching, a process by which the steel is rapidly cooled to room temperature and temper heat treatment where the steel is heated to a temperature well below the melting point at a suitable rate, to increase the steel ductility, toughness and the grain size of the matrix. The quenching and tempering processes produce a microstructure largely made up of tempered martensite with little amounts of lower bainite [1] [2] [3].

These steels have the capability to meet the demand for higher-strength construction grade material. Due to their ability to protect products from environmental degradation, their reduced weight, high-strength and high impact values, quenched and tempered high-strength steels are widely used in the construction of numerous critical applications including bridges, ships, large-scale petroleum storage tanks and containers [4] [5] [6].

The welding of high-strength steels has had major drawbacks such as cold cracking, residual stresses and distortion where conventional fusion arc welding processes such as SMAW, GMAW, SAW have been employed [7] [8] [9] [10] [11] and in most cases, precautions such as preheating and post-weld heat treatment (PWHT) are employed [12] [13] [14] [15] [16]. These treatments are however costly and associated with some hazards. To maintain the mechanical properties as well as minimize distortion of arc welded joints which are a great challenge because of the associated high heat input, there is a need to explore new solutions in the production process on improving the weldability of these steels. The use of high energy density and low heat input process in Laser Beam Welding ensures narrow welds, a smaller heat-affected zone (HAZ), which cools very rapidly with very little distortion, and a high depth-to-width ratio [17] [18] [19] [20]. Previous investigations have demonstrated that the weld metal’s microstructure and the HAZ of steel are influenced by the welding speed, defocusing distance, and shielding gas flow rate [21] - [27]. In addition, the stability of the keyhole and the escape of hydrogen in the molten pool are largely influenced by defocusing distance and gas shielding which often leads to the formation of undesired porosities during welding [28] [29] [30]. Inevitably, it is expected that the microstructure and the mechanical properties of welded joints of high-strength quenched and tempered low alloy SM570 (JIS) steel are affected by the welding process parameters.

To achieve the best product quality and high production rates, the present work was conducted to explore the effect of laser power, welding speed, defocusing distance and shielding gas flow rate on the microstructure and mechanical properties of welded joints of quenched and tempered high-strength steel. Since welding is widely used in the fabrication of various engineering components, understanding the factors affecting the quality of welded joints is critical for maintaining their mechanical properties.

The 7.0 mm thick plates of SM570 low alloy high-strength steel with the highest strength and toughness among the steel grades under JIS G3106 welded structure steel, were chosen for laser welding experiments. Their chemical composition and mechanical properties are given in Table 1 and Table 2 respectively.

Both bead-on-plate (Figure 1) and autogenous square butt-welded joints (Figure 2) were produced using a CO₂ laser system with 5 kW maximum output power (CW) and a CNC welding head. Figure 2 shows a schematic illustration of the butt joints of 7 mm thickness. The steel plates were machined to geometry of 150 mm × 100 mm × 7 mm and then welded perpendicular to the rolling direction.

Table 3 shows the investigated laser welding parameters were laser power (P), welding speed (S), defocusing distance (Dd), and shielding gas flow rate. At a constant laser power of 5.0 kW, the welding speed varied between 0.5 and 3.0 m/min, the defocusing distance varied between 0.0 mm and 1.0 mm. To protect the weld bead from contamination and to reduce the formation of absorbing plasma, argon gas shielded both the weld pool top and bottom at a flow rate between 15 and 30 L/min.

Visual examinations of laser beam welded joints showed neither internal nor external welding defects. Then, various specimens were cut in the cross-sections of the welds and prepared through grinding with grinding papers of various grit sizing from coarse to fine in the order 120, 240, 320, 600, 800 and 1000 and surface polishing with alumina paste. Natal solution was used for etching 2% Nitric acid (HNO₃) and 98% Alcohol (CH₃OH). A low-magnification stereoscope was used to determine the weld structure and an optical microscope was used for microstructure analysis. To investigate the hardness distributions through the weld, heat-affected zone and base metal, microhardness was measured at a load of 300 g by a Vickers testing machine (Matsuzawa, Akita, Japan). Tensile testing was performed by a universal tensile testing machine (Sinofound CO., Beijing China) integrated with computerized control and data acquisition system. Flat tensile specimens were prepared from the laser beam butt welds (Figure 3(a)) according to ASTM A370 [31]. Impact test specimens were prepared by etching (2% nital) to indicate the weld metal (WM), HAZ and the base metal (BM). A flaw or notch was introduced on the impact test specimens in the 3 different weld zones in order to examine the weld toughness at these locations (Figure 3(b)). The amount of energy absorbed by the material during fracture was determined at −20˚C.

In this study, autogenous laser welding of both BOP and butt joints of JIS G3106 SM570 steel was investigated. From the study, it was found that the difficulty related to conventional fusion arc welding of high-strength quenched and tempered steel can be avoided using the non-conventional laser beam welding process. The selected optimal welding conditions were a laser power of 5.0 kW, a 500 mm/min welding speed, an argon gas shielding flow rate of 30 L/min, and a defocusing distance of −0.5 mm. An optimum butt weld with a narrow weld zone, a small heat-affected zone (HAZ), very little distortion, and a high depth-to-width ratio was made. Subjected to examination and tests, a good combination of mechanical properties including hardness (355 HV), tensile strength (580 MPa) and toughness (1.4 J/mm²) of welded joints was observed. Laser welded joints characterised by minimum distortion as well as microstructure and mechanical properties; close to those of the base metal will enhance the application of high-strength quenched and tempered low alloy steels in bridges, ships, large-scale petroleum storage tanks, etc.

This research is funded by the Pan African University through the Institute of Basic Sciences, Technology & Innovation (PAUSTI), Kenya. The authors gratefully acknowledge the help of the Laser-X Co. Ltd.-Japan in conducting the laser welding experiments and the Central Metallurgical Research and Development Institute (CMRDI), Cairo for their support in material testing, examinations, workshops and related industrial visits. The research work is jointly supervised by both PAUSTI and CMRDI. Assistance provided by Ahmed Saiyah, CMRDI during this study is highly appreciated.

The authors declare no conflicts of interest regarding the publication of this paper.

Marozva, P.E., Mose, B.R., El-Batahgy, A.-M. and Mbuya, T.O. (2022) Laser Beam Welding of 600 MPa Quenched and Tempered High-Strength Steel. World Journal of Engineering and Technology, 10, 241-253. https://doi.org/10.4236/wjet.2022.102015