The welding process will generate welding thermal cycles, and the microstructure and mechanical properties of the heat affected zone will be uneven. The different micro zones of the welded joint can be divided into three categories based on the highest temperature experienced during the welding thermal cycle: weld zone (>1500 ℃), coarse grain heat affected zone (approximately 1100-1490 ℃), fine grain heat affected zone (900-1100 ℃), and partial phase transformation recrystallization heat affected zone (750-900 ℃). The microstructural changes in different parts of the heat affected zone cause changes in the mechanical properties of the welded joint, especially the impact toughness. The experiment adopts two microalloy design experimental schemes with the same process system to observe the performance of low-temperature impact toughness of different specimens.

Experimental materials and processes

The welding base material adopts 20mm thick hot-rolled steel plate, and its main chemical composition is shown in Table 1. The main mechanical properties of the welding base material are as follows: yield strength of 630 MPa, tensile strength of 760 MPa, elongation of 19%, impact absorption energy of 155 J at -20 ℃, and a dual phase structure of bainite ferrite and granular ferrite. At the center of the welded joint perpendicular to the direction of the weld seam, a metallographic specimen was cut using wire cutting. After rough grinding and mechanical polishing, it was corroded with 3% nitric acid alcohol. The microstructure of the welded joint was observed under a LEICA DMIRM metallographic electron microscope, and the changes in Vickers microhardness values in different heat affected zones were detected using an FM 700 microhardness tester. Multiple automatic welding experiments were conducted on laboratory hot-rolled steel plates using a Lincoln double wire submerged arc welding machine, with a focus on studying the changes in microstructure and impact toughness of the heat affected zone of the welded joints of the experimental steel plates. According to the national standard GB/T 12470, low strength matching is used for the selection of welding wire and flux. The chemical composition of the welding base material, as well as the matching welding wire and flux, is shown in Tables 1-3. The experimental steel samples with component numbers A (including Ni) are numbered 1 #~3 #, and the experimental steel samples with component numbers B (including B) are numbered 4 #~6 #. According to the requirements of the national standard GB/T 985, the welding test plate should have a V-shaped groove with the notch position shown in Figure 1. The V-shaped notches should be opened in the CGHAZ area and FGHA area respectively.

The welding process is shown in Table 4. Using 5 passes of welding, in order to reduce the coarsening of the microstructure in the HAZ area, the temperature interval between passes is controlled at 180-200 ℃. The calculation formula for welding heat input is shown in equation (1).

In the formula, E is the welding line energy, kJ/cm； U is the welding voltage, V; I is the welding current, A; v is the welding speed, mm/min

Experimental results and analysis

Microstructure and microhardness of welded joints

The microstructure of the heat affected zone of the welded joint is shown in Figure 2. Figure 2 (a) shows the microstructure of the fusion line zone of the base material, with added schematic lines representing the fusion line. Figure 2 (b) shows the microstructure of the weld zone. From the figure, it can be seen that the microstructure of the weld zone is composed of needle like ferrite and pre eutectoid ferrite. The needle like ferrite microstructure is very fine, and the needle like ferrite mainly nucleates in the inclusions in the weld. The pre eutectoid ferrite grows perpendicular to the fusion line as a whole. Figure 2 (c) shows the microstructure of the coarse-grained heat affected zone, which is a granular bainite structure with coarse primary austenite grain boundaries and numerous elongated or granular M/A islands distributed inside. Each coarse granular bainite has a similar crystal orientation, and the primary austenite grain boundaries are the effective grain boundaries (>15 °) of the granular bainite. The crystal contains a large number of subgrain boundaries [2], which may reduce the impact toughness of the welded joint. Figure 2 (d) shows the microstructure of the fine-grained region, mainly consisting of fine granular ferrite with a small amount of carbides distributed at the grain boundaries.

Figure 3 shows the variation trend of microhardness of the welded joint of sample 5 from the centerline of the weld seam. Using low strength matching welding metal, the microhardness value in the weld seam area is lower than that of the base metal and heat affected zone. Starting from the center of the weld seam, perform hardness testing in sequence along the direction perpendicular to the weld seam. Due to the influence of welding thermal cycles, the microstructure of the weld joint changes significantly. The testing distance of adjacent hardness points is set to 200 μ m, and the rest of the areas are set to 500 μ m. From the figure, it can be seen that there is a significant difference in the microhardness values of the welded joint, with the hardness of the welding heat affected zone (HAZ) being higher than that of the base metal and weld zone. In the welding heat affected zone, there are also significant variations among different heat affected zones. The hardness value of the fine grain heat affected zone is the smallest, about HV 215, the hardness value of the coarse grain heat affected zone is the largest, with an average value of about HV 244, and the average microhardness value of the base metal is about HV 215

Mechanical performance results of welded joints

Due to the fact that the strength of the welded deposited metal is much lower than that of the base metal, the tensile fracture location of the welding is always in the weld zone. Table 5 shows the impact absorption energy of the coarse grain heat affected zone near the welding fusion line. The impact absorption energy of the CGHAZ zone in samples numbered 1 #~3 # is between 40~76 J, while the impact absorption energy of samples numbered 4 #~6 # is lower, between 8~45 J. Comparing the impact toughness of CGHAZ with different compositions, it can be seen that B element segregates in the original austenite grains under high-temperature thermal cycling, forming BN during cooling and causing grain boundary embrittlement [3], while Ni element is beneficial for improving toughness.

Table 6 shows the impact absorption energy of the fine grain heat affected zone in the welding heat affected zone. From Table 6, it can be seen that the impact toughness of the fine-grained heat affected zone is comparable to that of the base material. This indicates that the sudden changes in the microstructure of the welding heat affected zone result in significant differences in the mechanical properties of different parts. The high impact toughness in this area is mainly attributed to the small effective grain boundary size. According to the experimental results of Pick et al., the refinement of grain size can not only improve the strength of the material, but also reduce the ductile brittle transition temperature (DBTT) of the material and increase the impact absorption energy of the plateau on the ductile brittle transition curve [3].

Impact fracture morphology of welding heat affected zone

Figure 4 shows the impact fracture morphology of the coarse grain heat affected zone in samples 2 # and 5 #. From the overall morphology, it can be seen that most of the fractures belong to cleavage fracture surfaces, and the shape of the fractures has not undergone significant plastic deformation. The microscopic fracture morphology of sample 2 # shows that the size of the unit cleavage plane of the cleavage plane is very large, with the maximum unit cleavage plane size exceeding 100 μ m, indicating that the cleavage crack extends very far along the nearly straight line on this cleavage plane, and the crack propagation work is very small. A small area of ductile dimples appeared near the notch position in sample 5, greatly increasing the initiation energy of cracks and improving the relative toughness.

Figure 5 shows the impact fracture morphology of the fine grain heat affected zone of samples 2 # and 5 #. Comparing the overall fracture morphology of the coarse grain heat affected zone, it can be seen that about 50% of the fiber section appears in the V-shaped root of the fine grain heat affected zone fracture. In the high magnification morphology image, a large number of small dimples can be seen on the fiber section, which is an important reason for improving the impact toughness of the fine grain heat affected zone. Due to the high impact absorption energy of the fine-grained heat affected zone, the resistance to crack propagation is relatively high, and crack propagation often deviates towards the coarse-grained heat affected zone or even the weld direction. From the overall view, it can also be seen that there is a large angle inclined surface at the lower end of the fracture [4], which significantly reduces the crack propagation absorption energy of the coarse-grained heat affected zone to a certain extent.

Closing remarks

(1) The microstructure of the welding base metal is mainly composed of bainite ferrite and polygonal ferrite. The microstructure of the weld zone is mainly needle shaped ferrite, while the welding heat affected zone is mainly granular bainite. The changes in the microstructure of the welding heat affected zone are very significant.

(2) The microstructure of the coarse grain heat affected zone is granular bainite due to its close proximity to the fusion line. Granular bainite has coarse primary austenite grain boundaries, which can deteriorate impact toughness. The fracture surface of the coarse grain heat affected zone is a cleavage fracture.

(3) The impact absorption energy of the fine-grained heat affected zone is similar to that of the base metal, and the macroscopic fracture surface exhibits plastic deformation, showing ductile fracture. There are numerous small ductile dimples distributed on the fracture surface.

(4) Under high-temperature thermal cycling, element B segregates in the original austenite grains and forms BN during cooling, leading to grain boundary embrittlement. Ni is beneficial to improving toughness to a certain extent.