When the used roll forming machine is used to make pipes, a complete high-frequency weld will appear, and the formation of a complete high-frequency weld is composed of two areas, the fusion zone and the heat-affected zone. The fusion zone is a narrow weld. Welding theory and practice point out that the quality of welding depends not only on the weld, but also on the welding heat-affected zone; sometimes there are more problems in the heat-affected zone than the weld, which is more complicated. It is especially obvious when the strength of alloy steel pipe is used. Therefore, it is of great significance to study the changes in the structure and properties of the heat-affected zone tube blank for the strength of the weld and the quality of the welded pipe.

The structure and morphology of the welding heat-affected zone is closely related to the carbon equivalent of the steel. The welding heat-affected zone of the used roll forming machine belongs to the low-carbon equivalent category. Crystallization zone, recrystallization zone and aging embrittlement zone.

(1) Overheating zone. The temperature is between 1400 and 1100 °C, and the closer to the fusion zone (weld), the higher the temperature; it is manifested that some refractory points such as carbides and oxides dissolve into austenite at the solidus, and austenite grains thick. Coarse austenite is easy to form superheated Widmanite structure during cooling, resulting in reduced toughness.

(2) Phase change recrystallization zone. The temperature is between 1100~850°C, above the Ac3 line; the recrystallization phase transition occurs when heated, so that the grains are refined; it is equivalent to the normalized structure of low-carbon steel, and has good comprehensive mechanical properties.

(3) Incomplete recrystallization zone. When the temperature is between 850 and 700 °C, only part of the metal in this region undergoes recrystallization phase transformation; it is the coexistence and mixing of the original ferrite coarse grain and fine grain region, and the mechanical properties are poor.

(4) Recrystallization zone. When the temperature is between 700 and 500°C, the internal structure of the metal does not change after heating, only the shape of the grain changes; it is an equiaxed ferrite grain, the strength and hardness are lower than that of the base metal, and the plasticity and toughness are improved. is the softening zone of the weld.

(5)Aging embrittlement zone. When the temperature is between 500 and room temperature, the metal structure is close to the base metal; the longer the placement time, the closer the structure and properties are to the base metal.

Through the analysis of the welding heat affected zone of the used roll forming machine, not only can the welding process be better improved, but also it is helpful to understand the formation and distribution of welding stress on the welded pipe.