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The influence of boron and nickel elements on the welding performance of steel
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.
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