What is the difference between ERW pipe and spiral welded pipe?
Performance comparison between carbon ERW steel pipe and spiral steel pipe:
I. Overview
The welding of ERW steel pipe is the skin effect and proximity effect of high frequency current. The process of using high-frequency current and induction high-frequency current (emerged at the end of the 20th century) to resistively heat and melt the edge of the tube blank, and apply extrusion force to weld them together.
The welding of SSAW steel pipe is a process in which the weldment, welding wire and flux are melted by an arc under the flux layer, and the molten flux forms a slag to isolate the air from contacting the molten pool, thereby forming a joint process.
Erw pipe and spiral steel pipe
There are two main differences:
A) SSAW has metal fillers such as welding wire, while ERW does not.
b) SSAW forms a molten pool, ERW does not form a molten pool, but only forms a molten state at the edge of the billet.
2. Advantages of ERW steel pipe
a) The weld length is short and the HAZ is smaller than SSAW;
B) High geometric accuracy;
c) If the deburring device is used, the weld reinforcement is lower than that of SSAW, and the conveying resistance is small;
d) High production efficiency, the welding speed is 15-20m/min, more than 10 times that of SSAW.
3. Advantages of SSAW steel pipe
a) The direction of the weld avoids the main stress direction when the pipeline is under pressure;
b) A single defect on the weld should be less framed;
c) Steel strips with the same width can be used to produce steel pipes with different outer diameters, or steel strips with different widths can be used to produce steel pipes with the same outer diameter.
4. Welding performance comparison
A) ERW has fast heating speed, highly concentrated heat, and no filler metal. It is not possible to add alloying elements to the welding wire flux to compensate for the burning of alloying elements during the welding process like SSAW, and it cannot effectively improve the structure and properties of welded joints like SSAW (microalloying is the main way to refine grains. Fine grains is the only way to simultaneously increase strength and toughness).
B) ERW does not form a molten pool and therefore has poor slag capability. 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 weld pool, which is conducive to the floating of inclusions in the slag.
c) There is no slag/gas protection during ERW welding (SSAW has slag protection formed by flux), and the structure of the welded joint is easily oxidized at high temperature, which makes the welded joint tend to be brittle.
d) ERW has the characteristics of high local heating temperature and short cooling time. The gradient of the welding temperature field is large, it is easy to produce hardening phase, and the welding stress is large. The plasticity and toughness of welded joints are not ideal. Therefore, all domestic ERW steel pipes 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 welding speed of ERW is too fast, the edge quality of raw materials cannot pass NDT inspection, and the NDT inspection of weld seam after welding is also difficult to guarantee the quality.
f) Since ERW is welded by pressing molten metal, it is difficult to detect effectively using NDT. Therefore, incomplete fusion and gray spots are problems that eRw steel pipes have been difficult to effectively solve for decades. Groove corrosion in ERW pipeline failure is mainly caused by lack of fusion.
g) ERW will inevitably form more “D” type joints during pipeline construction, while SSAW will not. It is well known that the “D” joint is the most concentrated part of the stress, which will reduce the fatigue life of the welded joint.
h) The residual stress of traditional SSAW pipe is relatively large, and the dimensional accuracy is not as high as that of ERW. (In fact, the welding residual stress of ERW is relatively large, of course, if proper heat treatment can be carried out, part of it can be eliminated). Nowadays, the production of spiral submerged arc welded pipes generally adopts advanced technologies such as low residual stress forming and mechanical expansion of pipe ends, and the quality is almost the same as that of straight seam submerged arc welded pipes.
For the spiral submerged arc welded pipe produced by Henggang, the residual stress is controlled below 10% of the yield strength of the raw material, and the resilience is controlled below 20mm. The hydrostatic test will further reduce the residual stress and make the residual stress tend to be evenly distributed. After the pipe end is mechanically expanded, the dimensional accuracy can reach the level of straight seam submerged arc welding or straight seam submerged arc welding steel pipe, which greatly reduces the amount of girth weld and the misalignment in field welding.
