Objective Ultra-narrow gap laser welding (ultra-NGLW) is a type of advanced welding technology for high-strength thick steel plates that use a laser as the heat source in an ultranarrow groove. It has the advantages of high welding accessibility and efficiency, low heat input and residual stress, and low deformation. Ultra-NGLW is suitable for welding thick high-strength steel plates in pressure vessels, ships, pipes, and hydropower equipment. Different from the common arc welded joints, in the process of multilayer filler wire welding, the complex thermal process differently changes the microstructure of each filling layer, resulting in different mechanical properties of each micro-zone. Therefore, the residual stress distribution in the ultranarrow gap laser welded high-strength steel joint is related to its special microstructure.

Methods In this paper, the B950CF bainitic high-strength steel and XK-01 wire with diameter of 1.2 mm were considered the research objects. Ultra-NGLW joints with three different thicknesses of 20, 50, and 70 mm were welded using the laser welding system comprising a TRUMPF-10002 high power laser and a Fronius automatic wire feeder.

Considering the complex microstructure and highly uneven mechanical properties of the ultra-NGLW joint, the micro-shear test was adopted to study the relationship between the mechanical properties and microstructures of the joint. The micro-shear specimen with a dimension of 1.5 mm×1.5 mm×40 mm, including weld metal (WM), heat-affected zone (HAZ), and base metal (BM), was cut from the different filling layers of the joint.

The surface residual stress of the joints with different thicknesses was measured using the μX360n X-ray residual stress tester. Considering the variance of the thermal-physical and mechanical properties of the B950CF high-strength steel with temperature, the properties of the B950CF steel at different temperatures were calculated using the JMat Pro simulation software. Based on the ABAQUS simulation software, a simulation model of the 70-mm-thick ultraNGLW joint was established and the residual stress of this joint was calculated.

Results and Discussions The microstructure of the ultra-NGLW joint is considerably more complicated than that of the arc welded joint. The specimen 2 and specimen 3 mainly comprise acicular ferrite and granular bainite. Moreover, the microstructure of the cover layer mainly comprises ferrite and upper bainite, with a small amount of columnar crystals, and low-carbon martensite. The micro-shear test results indicate that the shear stress distribution of the three specimens is “M” type. Approximately 150--200 μm lath martensite is observed at the fusion line, which is brittle and hard with high shear strength. The maximum shear strength is 658.8 MPa. It can be seen from the shear power in the micro-zone of the ultra-NGLW joint that the specimen 2 and specimen 3 exhibit the highest toughness, and the toughness of the cover layer is poor. The toughness of all specimens sharply decreases at the fusion line, which is attributed to the existence of martensite.

The residual stress of the ultra-NGLW joints is studied using X-ray nondestructive test and simulation. The results show that the residual stress distribution on the upper surface of the joints with different thicknesses is “W” shaped. The formation of martensite is the main reason for the high compressive stress(-214---476 MPa) at the fusion line. The tensile stress increases with an increase in the joint thickness. The highest residual tensile stress (measured value of 362 MPa and calculated value of 662 MPa) is located in the weld subsurface filling layer. Because the martensite decomposes in the filling layer, the remelting and high temperature tempering cause the volume shrinkage and thus induce the large residual tensile stress. It interrupts the uniformity of the residual stress distribution.

Conclusions Results show that the specimen 2 and specimen 3 mainly comprise acicular ferrite and granular bainite. The micro-shear strength (600--630 MPa) is higher than that of the BM (570 MPa), and the weld fusion line comprises lath martensite with the highest micro-shear strength of 658.8 MPa. The toughness of the middle and bottom filling layer is higher than that of the cover layer, which mainly comprises columnar crystals and low-carbon martensite.

The residual stress distribution on the upper surface of the ultra-NGLW joints with different thicknesses is “W” type. Owing to the existence of martensite, the compressive stress at the fusion line is -476 MPa (20 mm joint) and the highest tensile stress is 166 MPa (70 mm joint) in the fine-grain zone of HAZ. The tensile stress increases with an increase in the joint thickness.

The simulation results of the residual stress distribution of the 70-mm-thick ultra-NGLW joint matches the experimental results. The highest residual tensile stress (measured value of 362 MPa and calculated value of 662 MPa) is located in the weld subsurface filling layer. Because the martensite decomposes in the filling layer, the remelting and high temperature tempering cause the volume shrinkage and thus induce the large residual tensile stress.