Improve The Groove-like Corrosion Of Welds By Straight Seam Welded Pipes

The edge of the erw steel pipe blank is heated and pressed and butted together to form a weld seam. In this process, the high-frequency current with skin effect and proximity effect is highly concentrated, the current penetration depth and The affected areas are small and limited to phase transitions within a small heat affected zone. The phase change of the heat affected zone and the coolant of the unit exist at the same time.

The weld of the steel pipe undergoes a rapid heating and rapid cooling process before and after welding, which creates an undesired heat affected zone in the weld zone. A microstructure different from the base material (ferrite + pearlite), the tissue portion of which contains untempered martensite or bainite structure. This is an undesirable organization that affects the strength and mechanical properties of steel pipes. Therefore most steel pipe standards (such as API standards) require the removal of untempered martensite structure.

Generally, the steel for the lsaw steel pipe is basically 6.Mn-based and M1.8. hot-rolled steel, and the alloying elements have good comprehensive performance under all solid solution conditions. The Mn V system, the MIINb system, and the Mn Ti system belong to normalized steel. In addition to solid solution strengthening, there is precipitation strengthening effect, and it is necessary to undergo normalizing treatment to obtain the best comprehensive performance. The weld zone with higher carbon content hardens, the ferrite structure is less, and the main body of the intermediate structure is martensite. After normalizing, ferrite and pearlite structure can be obtained to improve toughness.

The high-frequency welding of straight seam welded pipe will cause a certain degree of damage to the tensile properties and corrosion resistance of the welded joint zone of the steel pipe. Mainly manifested in: uneven microstructure of the weld zone of the weld, segregation of impurity elements and alloying elements such as S and P; the precipitation and precipitation of carbides and nitrides in the heat-affected zone make stress corrosion cracking and the like easily induced in the weld zone. Post-weld heat treatment can largely eliminate the above-mentioned harmful factors. Heat treatment of high-frequency straight seam welded pipe can also improve the groove-like corrosion performance of the weld, which is one of the important means to prevent groove-like corrosion.

2. At present, the heat treatment condition of domestic steel tubes is such that the welded oil well pipes and boiler tubes are prone to hardening phase and microstructure stress during high-frequency welding, which makes the weld brittleness increase and the comprehensive mechanical properties decrease. Therefore, welds are to be welded after welding these welded pipes. The heat treatment allows the weld and its heat affected zone to have a strength match consistent with the parent and eliminate weld residual stress.

The in-line heat treatment process for straight seam welded pipe welds has normalizing, stress relief annealing, quenching-tempering, normalizing-tempering, etc. The heat affected zone formed by welding is narrow. Because the weld is not filled with metal, it is easy to be combined with heat after heat treatment. Material = Zhi. Therefore, the welded steel pipe suitable for various heat treatments is basically a high-frequency straight seam welded pipe. The internal stress generated by the straight seam welded pipe reduces the hardness and improves the ductility and toughness. This process is generally used for steel pipes with lower steel grades.

Online or offline: The steel tube is heated to 30~50 °C above the Ac3 phase transition point. After an air cooling condition similar to the static state, the steel tube is slowly cooled to below the critical temperature. Removes internal stress due to molding.

A worker from a steel pipe company, which is located in China.