Weld Microstructure Analysis of Carbon Steel DSAW Steel Pipe

The microstructure of weld seam of high frequency DSAW steel pipe reflects the welding process performance and weld quality of the steel pipe.

High-frequency straight seam welding production is mainly the use of high-frequency current with the skin effect and the proximity effect of the two characteristics, the current is highly concentrated in the tube side welding surface, the tube side quickly heated to the welding temperature, Extrusion welding to complete. 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 transition (solid phase transition) after cooling to a lower temperature, Carbon steel pipe, mainly formed by fusion of melting material into crystalline structure of the fusion zone, and from the fusion line to the organization does not occur between the base metal heat affected zone. The transformation of the weld zone and the hot zone during the welding and continuous cooling processes is a continuous gradient of the organization formed. As the welding temperature is 1350 ~ 1500 ℃, the metal is heated in the peritectic range of welding, when the condensation occurs to 1493 ℃ peritectic reaction of the formation of austenite, in the continuous cooling to below the GS line will occur austenite Ferrite transformation, cooled to slightly lower than 723 ° c eutectoid reaction, the remaining austenite into pearlite, in the fusion zone eventually the formation of ferrite and pearlite structure. In the heat affected zone, a series of unbalanced microstructures are formed by the effect of welding heat, which is different from the original microstructure and properties of the base metal. When the peak value of welding thermal cycle is lower than Ac1, the metallographic structure is still primitive The ferrite and pearlite of the base metal structure, when heated to Ac1 or more, the organization will be significant changes. Steel in about 750 ℃ tissue changes occur, the first austenite pearlite formation of small austenite, austenite transformation in the ferrite has not yet occurred, because the welding heating speed, and soon cooled down by the pearlite transformation Into austenite in the subsequent cooling process to recrystallize into fine ferrite and pearlite. This area consists of fine ferrite and pearlite recrystallization and no phase transformation of coarse ferrite structure. In the region where the peak temperature is higher than this region, the pearlite and ferrite are fully austenitized at a temperature ranging from Ac3 to the grain coarsening initiation temperature of the steel at 900 to 1100C, but the grain size is not drastically long Large, in the continuous cooling to get small and uniform ferrite and pearlite structural steel pipe. When the heating temperature is above 1100 ℃, the austenite grains are coarse and coarse, and the grains are coarsest near the fusion zone. After cooling, the coarse ferrite and pearlite structure are formed,

Summary:

(1) The microstructures of high-frequency straight seam welds are composed of unbalanced continuous gradient microstructures, and the peak temperatures of different welding heat cycles form different microstructures.

(2) Welding speed too fast, too much extrusion pressure lead to unqualified weld fusion zone is too narrow, metal flow line rising angle steep, and affect the quality of the weld. Therefore, the welding speed should be properly controlled to reduce the pressure, so that the weld fusion completely, in order to improve the quality of the weld.

(3) The existence of more non-metallic inclusions in the weld, weakening the continuity of the metal, so that the weld strength, crushed unqualified. Welding temperature and extrusion pressure should not be too high, to minimize the inclusion content, improve the weld strength.

(4) Weld's associated coarse-grained structural steel pipe can make mechanical properties of welds decreased, especially the plasticity and toughness decreased significantly, the same cooling conditions in the welding, the heating temperature should not be too high, the cooling rate should not be too Fast, as far as possible to reduce the weld in the Wechsler organization.