Weld Microstructure Analysis of Carbon Steel Straight Seam Welded Pipe

The microstructure of high-frequency straight seam welded pipe welds reflects the welding process performance and welding quality of the steel pipe.


The production of high-frequency straight seam welded pipe mainly uses high-frequency currents with two characteristics of skin effect and proximity effect. The current is highly concentrated on the welding side of the tube, and the tube side is quickly heated to the welding temperature, and the extrusion welding is completed. During the welding process, the metal undergoes a crystallization transition (solidification transition) from the liquid phase to the solid phase, and then undergoes a recrystallization transformation (solid phase transition) cooling to a lower temperature in the carbon steel, mainly by fusion zone of the molten material. With the fusion of the crystal structure, no heat-affected zone of the matrix metal occurs from the fusion line of the tissue. During welding and continuous cooling, the transition of the weld zone and hot zone is a continuous gradient of tissue formation. With the welding temperature of 1350 ~ 1500 °C, the metal is heated within the peritectic range, when the condensation occurs in the peritectic reaction of 1493 °C austenite formation, continuous cooling to below the GS line occurs austenite - iron The phase transformation of the element body, cooling to slightly lower than 723 °C eutectoid reaction, the rest of the austenite transform into pearlite, the final formation of ferrite and pearlite structure in the fusion zone. In the heat-affected zone, the effects of welding heat create a series of unbalanced microstructures that differ from the original structure and properties of the matrix. When the welding thermal cycle peaks below AC1, the metallurgical structure remains ferritic and pearlite of the original base metal structure, and when heated above Ac1, the structure will undergo major changes. The microstructure of the steel changes at 750°C. The first austenitic pearlite forms in the small Austrian. The ferrite-austenite transformation has not yet occurred. As the welding heats up quickly, the pearlite cooled down quickly becomes austenite. In the subsequent cooling process, fine ferrite and pearlite are crystallized. This zone is recrystallized with fine-grained ferrite and pearlite, and the coarse ferrite has no phase transition. In the peak temperature higher than this area, pearlite and ferrite complete austenitizing temperature at the starting temperature of the grain coarsening steel from AC3 at 900 1100, but the particle size did not grow up completely, in the continuous cooling to get small The uniform ferrite and pearlite structure. When the heating temperature is higher than 1100 °C, the austenite grains are coarse, and the grains are coarse, coarse and near the fusion zone. After cooling, coarse ferrite and pearlite are formed.


to sum up:
(1) The microstructure of high-frequency straight seam welds is composed of unbalanced and continuous gradients. The peak temperatures of different welding thermal cycles form different microstructures.


(2) The welding speed is too fast, the extrusion pressure is too large and the weld fusion zone is not qualified. The metal flow line has a steep rise angle, which affects the weld quality. Therefore, the welding speed should be properly controlled, the pressure should be reduced, and the welding seam should be completely fused to improve the weld quality.


(3) There are many non-metallic inclusions in the weld, which weakens the continuity of the metal and reduces the strength of the weld seam. Welding temperature and extrusion pressure should not be too high, minimize inclusion content, and increase weld strength.


(4) Welding-related coarse-grained structure can reduce the mechanical properties of the weld, especially the plasticity and toughness. Under the same cooling conditions, the heating temperature should not be too high, the cooling rate should not be too fast, and the weld should be minimized. Wei's organization.