Laser Beam Welding of 600 MPa Quenched and Tempered High-Strength Steel ()
1. Introduction
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.
2. Experimental Procedure
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.
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Figure 1. BOP image of the welded JIS G3106 SM570 plate.
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Figure 2. Experimental setup of laser beam welding.
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Table 1. Chemical composition (wt%) of the used base metal.
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Table 2. Mechanical properties of the used base metal.
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.
3. Results and Discussions
3.1. Macrostructure Investigation
3.1.1. Effect of Welding Speed
The influence of welding speed on the weld profile was investigated at the selected laser power (5 kW) and a defocusing distance of −0.5 mm. Initial results of the bead-on-plate laser welding showed incomplete penetration for the 10 mm thickness with variations in welding speed from 3 to 0.5 m/min, Figure 4.
However complete penetration was achieved on the 7 mm thickness (see Figure 5). Figure 6 shows the relationship between welding speed and fusion zone depth/width ratio. Increasing the welding speed from 0.6 to 1 m/min results in a sharp increase in the depth/width ratio from 1.23 to 1.38.
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Figure 3. Schematic illustration and dimensions of (a) transverse tensile test specimen of the welded joint, (b) V-notch location of the impact test specimen in the BM, HAZ and WM.
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Figure 6. Influence of welding speed on weld depth/width ratio of 7 mm thickness.
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Table 3. Welding parameters used for laser welding.
The results showed a significant increase in the fusion zone size at lower welding speeds and consequently a decrease in depth/width ratio. Complete penetration with a relatively acceptable fusion zone size for the 7 mm base metal thickness was achieved at a welding speed of 0.6 m/min as shown in Figure 5(d). At high welding speeds, penetration is usually low because of the drop in heat input. However, in this case, the rate of increase of the weld width was significant, leading to a low depth to ratio at lower speeds although it remained within an acceptable range. The fusion zone was found to be symmetrical about the axis of the laser beam (Figure 7). The butt joint is symmetrical in shape with complete fusion. This is the expected geometry of a laser-welded joint, without excess penetration from the root side or visible porosity in the joint.
3.1.2. Effect of Defocusing Distance
The focus position, distance between the optical focal point and specimen surface, also referred to as the defocusing distance, were investigated. Figure 8 shows the face sides of the 7 mm welds made at various defocusing distances. Visual inspection of the laser-welded joints made with a defocusing distance of 0.5 mm below the specimen surface gave sound weldments with complete penetration and without defects like porosities and undercuts, Figure 9.
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Figure 7. A cross-section of butt weld made using P = 5 kW, S = 0.5 m/min, Dd = −0.5 mm.
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Figure 8. Face side of 7.0 mm thick plates of JIS G3106 SM570 produced by laser power of 5 kW, welding speed of 0.5 m/min, shielding gas flow rate of 30 L/min and defocusing distance (a) 0.0 mm, (b) −0.5 mm, (c) −1.0 mm.
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Figure 9. Butt welds of 7.0 mm thick plates of JIS G3106 SM570 produced by laser power of 5 kW, welding speed of 0.5 m/min, shielding gas flow rate of 30 L/min and defocusing distance of −0.5 mm (a) face side, (b) root side.
3.1.3. Effect of Shielding Gas Flowrate
The influence of gas shielding flow rate is determined by increasing the Argon shielding flow rate from 15 to 30 L/min at constant laser welding parameters: Laser Power of 5 kW, welding speed of 0.8 m/min and defocusing distance of −0.5 mm. Increasing the argon gas flow rate, prevented excessive plasma formation, maintained the stability of the keyhole and in turn brought about full penetration of the weld. In addition to that, at 30 L/min the cross-section of the weld was smoother with a narrower weld face and the weld root wider, Figure 10.
3.2. Microstructure Investigation
The base metal (BM) has a microstructure consisting of martensite and a small proportion of ferrite grains. The butt joint in this case contains identical chemical constituents to the base metal. This is because of the autogenous laser welding process in which no filler metal is used meaning the joint microstructure is also made up of martensite. Since the heat input in the HAZ was slightly below its melting point, it was sufficient enough to cause microstructural changes to give finer grains. Solidification in the weld metal (WM) led to coarse columnar grains in the microstructure (Figure 11).
3.3. Mechanical Properties
Figure 12 shows tensile after testing and it was observed that failure occurred by ductile rupture in the base metal. The ultimate tensile strength was 580 MPa acceptable and within the tensile strength of the base metal of 570 - 720 MPa, indicating the correctness of welding parameters, particularly laser power, defocusing
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Figure 10. Macrographs showing weld quality produced by laser power of 5 kW, welding speed of 0.5 m/min, a defocusing distance of −0.5 mm and a shielding gas flowrate of (a) 15 L/min, (b) 30 L/min.
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Figure 11. Optical microscopic photographs at a magnification of 50 μm of the (a) used base metal used, (b) HAZ of full penetration square butt welded joint, (c) weld metal of full penetration square butt welded joint.
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Figure 12. Macrographs showing fractured location in the tensile specimen.
distance, and welding speed and gas flowrate. Visual inspection also showed no structural changes which have a significant influence in decreasing mechanical properties usually brought about by the laser welding thermal cycle. Metallographic examinations also support the retaining of the material structural characteristics.
Figure 13 gives the hardness distribution of the base metal, HAZ and weld metal of JIS G3106 SM570 steel. High hardness of between 300 - 355 HV in the WM and HAZ near the weld metal. However, the least hardness of 218 HV was observed as we approach the BM. This is expected because the mechanical properties of steel, in general, are characteristic of its microstructure in these different zones. Complete penetration of 7.0 mm thickness had a width of 4.0 mm and a minimized HAZ width of 1.5 mm.
To determine the impact resistance (toughness) of JIS G3106 SM570 steel, the amount of energy absorbed during fracture was ascertained by the Charpy-V notch test. The impact toughness of the laser-welded butt joint was investigated in three different positions. The results of the Charpy-V impact toughness in the base metal (BM), HAZ and weld metal (WM) at −20˚C are shown in Figure 14. There was an uneven distribution of toughness along the butt welded joint, with the lowest impact energy observed at WM while the highest impact energy was observed in the BM. However, all measured values in all the different zones exceed the minimum required absorbed energy.
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Figure 14. Impact absorbed energy distribution.
4. Conclusion
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.
Acknowledgements
This research is funded by the Pan African University through the Institute of Basic Sciences, Technology & Innovation (PAUSTI), Kenya. The authors acknowledge the Central Metallurgical Research and Development Institute (CMRDI), Cairo for their support in experimental work, 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.