The Significance of Studying the Heat Affected Zone of Welded Pipe Welding

Welding heat affected zone (HAZ) is different from weld. The performance of weld is guaranteed by chemical composition adjustment, redistribution and proper welding process. However, the performance of HAZ can not be adjusted by chemical composition. This is due to the uneven distribution of microstructure under the effect of thermal cycle. For general welding structure, the hardening, embrittlement, toughening and softening of HAZ are mainly considered to improve the performance The comprehensive mechanical properties, corrosion resistance and fatigue properties should be determined according to the specific application requirements of the welded structure.

A complete weld of high frequency welded pipe consists of fusion zone and heat affected zone. Fusion zone is a narrow sense weld. Welding theory and practice point out that the welding quality depends not only on the weld, but also on the heat affected zone. Sometimes the problem of heat affected zone is much more complex than that of weld, which is especially obvious when welding high strength alloy steel pipe. There are many cases of failure of oil pipe and high strength alloy steel pipe Examples show that the problem often lies in the heat affected zone.

Therefore, it is important to study the microstructure and properties of heat affected pipe blank for welding strength and quality. The microstructure and morphology of heat affected zone of low carbon equivalent welded pipe are closely related to carbon equivalent of steel. The welding heat affected zone of high frequency straight welded pipe belongs to the category of low carbon equivalent, which consists of coarse grain zone, phase transformation recrystallization zone, incomplete recrystallization zone and recrystallization zone It is composed of crystallization zone and aging embrittlement zone.

In the superheated zone, the temperature is between 1400 ° C and 1100 ° C. The closer to the fusion zone, the higher the temperature is. Some refractory particles, such as carbides and oxides, melt into austenite in the solidus. The austenite grains are coarse, and the coarse austenite is easy to form overheated widmanstatten structure during cooling, which leads to the decrease of toughness.

The temperature of phase transformation recrystallization zone is between 1100 ~ 850 ° C. when heated, recrystallization phase transformation occurs, which makes the grain refined. It is equivalent to the structure of low carbon steel after normalizing and has good comprehensive mechanical properties. The temperature of incomplete recrystallization zone is 850 ℃~ Between 700 ° C, only part of the metals in this region recrystallize, which is the coexistence of coarse and fine grains of original ferrite, and the mixed mechanical properties are poor.

Recrystallization zone, when the temperature is 700 ℃~ When the temperature is between 500 ° C and room temperature, the internal structure of the metal does not change, only the shape of the grain changes. It is an equiaxed ferrite grain with lower strength and hardness than the base metal, and higher plasticity and toughness. It is the softening zone and embrittlement zone of the weld. When the temperature is between 500 ° C and room temperature, the metal structure is close to the base metal. The longer the storage time is, the closer the structure and properties are to the base metal By analyzing the microstructure of HAZ, not only the welding process can be improved, but also the formation and distribution of welding stress of welded pipe can be understood.