How to distinguish stainless steel seamless pipes from stainless steel welded pipes?

Untreated stainless steel seamless pipes and welded pipes are easy to distinguish, but stainless steel pipes that have been seamlessly treated may be difficult to distinguish with the naked eye, so how do we distinguish between seamless stainless steel pipes and welded stainless steel pipes?

1. Differences in production processes

Stainless steel welded pipes: Steel plates or steel strips are welded after being crimped by the unit and the die. The welding methods are divided into straight seam welded pipes (ERW pipe) and spiral welded pipes, and the more common ones are straight seam welded pipes.

Stainless steel seamless pipes: The outer skin is cut off from round steel, and hot-rolled and perforated after centering to reach the required size.

Difference in concentricity: The seamless pipe manufacturing process is carried out at 2200 degrees. Holes are formed by punching in stainless steel billets. Under the action of this high temperature, the tool steel softens and then spirals out of the hole. The wall thickness of this pipe is not uniform at all, and the eccentricity is relatively high. Stainless steel welded pipes are made of precision cold-rolled plates. The wall thickness difference of these cold-rolled plates is generally around 0.002 inches, so the wall thickness difference is very small, and the wall thickness of the entire pipe is very uniform.

2. Difference in appearance

Stainless steel welded pipe: high precision, uniform wall thickness, high brightness inside and outside the pipe (the brightness of the steel pipe surface is determined by the surface grade of the steel plate), can be arbitrarily scaled; can be made into thin-walled pipe.
Stainless steel seamless pipe: low precision, uneven wall thickness, low brightness inside and outside the pipe, high cost of cutting to length, and pitting and black spots inside and outside the pipe are not easy to remove. Usually the wall is thicker.

The difference between stainless steel seamless pipe and stainless steel welded pipe also includes corrosion resistance. The quality of corrosion resistance also depends on the composition of the alloy. Seamless pipes with the same chemical composition have the same corrosion resistance as heat-treated stainless steel welded pipes. The welded parts of seamed steel pipes that have not been fully heat-treated are easily corroded.

3. Difference in performance

The corrosion resistance, pressure resistance and high temperature resistance of seamless pipes are much higher than those of stainless steel welded pipes. With the improvement of the production process of stainless steel welded pipes, the mechanical properties and mechanical properties are gradually approaching those of seamless pipes, and welded pipes can replace seamless pipes in some aspects.

In general, the chemical composition of stainless steel welded pipes and seamless pipes is somewhat different. The steel composition of seamless pipes just meets the basic requirements, while stainless steel welded pipes contain chemical compositions suitable for welding, such as silicon, sulfur, manganese, oxygen and triangular ferrite. If these elements are missing, the welding process will be very difficult.

4. Difference in price

Stainless steel seamless pipes are more complicated in the production process and are more expensive than welded pipes.

5. Difference in use

Stainless steel welded pipes are often used as decorative pipes, support pipes, and product pipes. The pressure-bearing performance is general, and it is mainly used to transport water, oil, gas, air and general low-pressure fluids such as hot water or steam.

Stainless steel seamless pipes: pipes used for conveying fluids in engineering and large equipment, and can also be used for high-temperature and high-pressure conveying fluid mains and pipelines in power plants and nuclear power plant boilers.

In fact, if the two are compared together, seamed steel pipes have more advantages than seamless steel pipes. Since the seamed steel pipe has passed a series of tests and is made of cold-rolled plates, all the disadvantages are limited to welding. Seamless steel pipes are stamped from stainless steel billets, so the pipe wall will be torn during the kneading process. Moreover, through testing, the defective qualified rate of seamed steel pipes is mostly lower than that of seamless steel pipes, and under ultrasonic testing, it can be found that the background noise of seamless steel pipes is relatively large, so it is difficult to find defects. On the contrary, seamed steel pipes are easier to find faults.

Read more: 304 vs. 316 Stainless Steel: What's the Difference? or 7 Things to consider when purchasing stainless steel ERW pipes