The welding of SSAW steel pipe is a process in which the weldment, welding wire and flux are melted by the arc under the flux layer, and the molten flux forms a slag to isolate the air and the molten pool from contact, thereby forming a welding joint.
B) SSAW forms a weld pool, ERW does not form a weld pool, only a molten state is formed at the edge of the billet.
2. Advantages of ERW steel pipe
3. Advantages of SSAW steel pipe
4. Welding performance comparison
A) ERW has fast heating speed, highly concentrated heat, and no filler metal. It is not possible to use wire flux to add alloying elements to compensate for the burning of alloy elements in the welding process like SSAW, and it cannot effectively improve the structure and performance of welded joints like SSAW (microalloying is the main way to refine grains. Grain refinement is the only way to increase strength and toughness at the same time).
B) ERW does not form a welding pool, so the welding dross ability is poor. If there is slag inclusion on the edge of the raw material, it can only remain in the weld and become a quality hazard. However, SSAW forms a welding pool, which is beneficial for slag inclusions to float in the slag.
C) There is no slag/gas protection during ERW welding (SSAW has slag protection formed by flux), and the welded joint structure at high temperature is easily oxidized, which makes the welded joint have a tendency to embrittlement.
D) ERW has the characteristics of high local heating temperature and short cooling time. The welding temperature field has a large gradient, which is prone to hardening phase and large welding stress. The plasticity and toughness of the welded joint are not ideal. Therefore, all ERW steel pipes in the country require post-weld heat treatment (Q+T or Q+N), but the domestic post-weld heat treatment process is not yet mature.
E) Compared with SSAW, the ERW welding speed is too fast, the edge quality of the raw material cannot be tested by NDT, and the NDT test of the weld after welding is also difficult to ensure the quality.
F) Since ERW is welded by pressing molten metal, it is difficult to use NDT for effective inspection. Therefore, incomplete fusion and gray spots are the problems of eRw steel pipes, which have been difficult to effectively solve for decades. Groove corrosion in ERW pipeline failure is mainly caused by unfusion.
H) The residual stress of traditional SSAW pipe is relatively large, and the dimensional accuracy is not as high as ERW. (In fact, the residual welding stress of ERW is also relatively large, of course, a part of it can be eliminated if proper heat treatment can be performed). Nowadays, advanced technologies such as low residual stress forming and mechanical expansion of pipe ends are commonly used for the production of SSAW pipes, and the quality is not much different from LSAW.