How to Identify Seamless vs. Welded Steel Pipes?

Common steel pipes are mainly welded steel pipes and seamless steel pipes. In the process of engineering construction, the appropriate steel pipe will be selected according to the specific construction conditions, and the cost will be the primary factor. Therefore, this article distinguishes welded steel pipes and seamless steel pipes from the aspects of steel pipe performance, process and identification method, hoping to provide reference for the selection of steel pipes in the process of engineering construction.

With the continuous advancement of science and technology, steel pipe production process has also been significantly improved. At present, there are two main forms of common steel pipes, namely welded steel pipes and seamless steel pipes. Since the cost of welded steel pipes is significantly lower than that of seamless steel pipes, some businesses process welded steel pipes in order to seek more economic benefits, and then pretend to be seamless steel pipes, and then sell them in the market, causing market confusion. The processed welded steel pipes cannot be effectively identified from the appearance. Once in the process of engineering construction, the welded steel pipes should be used instead of seamless steel pipes, which will cause gas or liquid leakage, and in severe cases, it is easy to cause explosions. Therefore, it is necessary to distinguish welded steel pipes from seamless steel pipes to ensure the quality of the project.

Welded steel pipes are generally made by deforming steel strips or steel plates into round or square shapes under the action of external forces, and then welding the surface seams to form welded steel pipes. Commonly used processes include arc welding, high frequency or low frequency, cold rolling, etc., but these processes will result in welds on the steel pipes. At present, according to the shape of the weld, it can be divided into straight seam and spiral welded steel pipes, but the processes used by the two are different. Spiral steel pipes are welded by submerged arc welding, which has the advantages of high welding efficiency and less dust. For high-frequency or low-frequency welding, it is mainly based on the principle of electromagnetic induction. Since the AC charge will have adjacent and turbine thermal effects in the conductor, the steel at the edge of the weld can be molten and finally connected.

Seamless steel pipes are mainly made by perforating solid billets or ingots to make rough pipes, and then through hot rolling, cold rolling, cold drawing and other processing to form seamless steel pipes. Since seamless steel pipes contain a middle hole section, this hollow section can be used as a pipeline for transmitting fluids.

There are four main methods to distinguish welded steel pipes from seamless steel pipes:

1. Metallographic method

Metallographic method is one of the main methods to distinguish welded steel pipes from seamless steel pipes. High-frequency resistance welded steel pipes do not add welding materials, so the weld seam in the welded steel pipe is very narrow. If the method of rough grinding and then corrosion is used, the weld seam cannot be clearly seen. Once the high-frequency resistance welded steel pipe is welded and has not undergone heat treatment, the weld structure will be essentially different from the parent material of the steel pipe. At this time, the metallographic method can be used to distinguish welded steel pipes from seamless steel pipes. In the process of distinguishing the two steel pipes, it is necessary to cut a small sample with a length and width of 40mm at the welding point, and perform rough grinding, fine grinding and polishing on it, and then observe the structure under a metallographic microscope. When ferrite and widmanstatten, parent material and weld zone structure are observed, the welded steel pipe and seamless steel pipe can be accurately distinguished.

2. Corrosion method

In the process of using the corrosion method to distinguish welded steel pipes and seamless steel pipes, the weld of the processed welded steel pipe should be polished. After the polishing is completed, the polishing traces should be seen. Then, the end face is polished with sandpaper at the weld, and the end face is treated with 5% nitric acid alcohol solution. If an obvious weld appears, it can be proved that the steel pipe is a welded steel pipe. The end face of the seamless steel pipe has no obvious difference after corrosion.

Performance characteristics of welded steel pipes:

Welded steel pipes have the following properties because they are processed by high-frequency welding, cold rolling and other processes.

First, good thermal insulation function. The heat loss of welded steel pipes is relatively small, only 25%, which is not only conducive to transportation, but also reduces costs.

Second, it is waterproof and corrosion-resistant. During the construction process, there is no need to set up a separate trench, just bury the steel pipe directly underground or underwater, which reduces the difficulty of construction.

Third, it is impact-resistant. Even in a low temperature environment, the steel pipe will not be damaged, so its performance has certain advantages.

Performance characteristics of seamless steel pipes:

Because the metal material of seamless steel pipes has a large tensile strength, it has a stronger ability to resist damage, and it has a hollow channel, so it can effectively transport fluids. It is precisely because of its strong transport capacity that the corrosion resistance of seamless steel pipes is higher than that of welded steel pipes, and its stiffness is relatively large. Therefore, the more loads the seamless steel pipe carries, the more it can be widely used in projects with higher construction requirements.

3. Distinguish welded steel pipes and seamless steel pipes according to the process

In the process of distinguishing welded steel pipes and seamless steel pipes according to the process, all welded steel pipes are welded according to processes such as cold rolling and extrusion. In addition, when high-frequency, low-frequency arc welding pipes and resistance welding pipes are used to weld steel pipes, spiral pipe welding and straight seam pipe welding will be formed, which will form round steel pipes, square steel pipes, oval steel pipes, triangular steel pipes, hexagonal steel pipes, diamond steel pipes, octagonal steel pipes, and even more complex steel pipes. In short, different processes will form steel pipes of different shapes, so that welded steel pipes and seamless steel pipes can be clearly distinguished. However, in the process of distinguishing seamless steel pipes according to the process, they are mainly distinguished according to the hot rolling and cold rolling treatment methods, and seamless steel pipes also have two main forms, namely hot-rolled steel pipes without steel pipes and cold-rolled steel pipes without steel pipes. Hot-rolled seamless steel pipes are formed by perforation, rolling and other processes, especially large-diameter and thick seamless steel pipes are welded by this process; cold-drawn pipes are formed by cold drawing of tube billets, and the material strength is lower, but its outer and inner surfaces are smooth.

4. Classify welded steel pipes and seamless steel pipes by use

Welded steel pipes have higher bending and torsional strength and greater load-bearing capacity, so they are generally widely used in the manufacture of mechanical parts. For example, oil drill pipes, automobile transmission shafts, bicycle frames, and steel scaffolding used in construction are all made of welded steel pipes. However, seamless steel pipes have hollow cross-sections and are long strips of steel with no seams around them, so they can be used as pipelines for conveying fluids. For example, they can be used as pipelines for conveying oil, natural gas, coal gas, water, etc. In addition, the bending strength of seamless steel pipes is relatively small, so they are widely used in superheated steam pipes for low and medium-pressure boilers, boiling water pipes, and superheated steam pipes for locomotive boilers. In short, welded steel pipes and seamless steel pipes can be clearly distinguished by the classification of uses.

In order to ensure higher quality of engineering construction in China, it is necessary to distinguish welded steel pipes from seamless steel pipes, and then use appropriate steel pipes for construction. In the process of distinguishing welded steel pipes from seamless steel pipes, metallographic method and corrosion method can be used to observe the structure and welds, so as to inspect the welds and finally accurately distinguish them. At the same time, the two steel pipes can be distinguished according to their performance and use, and then reasonable steel pipes can be selected for construction.

Principle of identification of seamless steel pipes and welded steel pipes:

Macroscopic identification: Observe the prepared steel pipe sample with the naked eye or using a magnifying glass of ≤10 times. If there is welding phenomenon, that is, the complete welding morphological characteristics of the weld zone, fusion zone and parent material zone, it is identified as a welded steel pipe. If there is no welding phenomenon, it is a seamless pipe.

Microscopic identification: If it is difficult to identify macroscopically, use a metallographic microscope for microscopic identification, adjust the optical path and magnification of the metallographic microscope, start from a lower magnification, observe the entire cross section, and if it is difficult to confirm the signs of welding, use a higher magnification for further analysis and confirmation. According to the presence of welding structure characteristics, identify whether it is a welded steel pipe or a seamless steel pipe.

Refer to GB/T 13298, cold cut and cut the steel pipe on the complete cross section, and remove the heat-affected zone for hot cutting. Steel pipes should be ground and polished in full cross section as much as possible. If the diameter of the steel pipe is large and cannot be ground and polished in full cross section, it can be cut into several sections on the cross section. The sawing width is not more than 2 mm (wire cutting is recommended), and the angle between the sawing and the tangent of the point is not more than 45° to ensure that the weld is not missed. The schematic diagram of the sawing and the steel pipe is shown in Figure 1.

Figure 1 Schematic diagram of sawing angle

Organization display method to identify seamless steel pipes and welded steel pipes:

Etching method:

The etchant is recommended to use 3%~5% nitric acid-ethanol solution, and other metallographic structure display agents can also be used. In a ventilated place at room temperature, the prepared reagent is evenly immersed or applied on the surface to be inspected. The etching time depends on the type of steel and the reagent. After corrosion, rinse with water, rinse with ethanol, and blow dry quickly with hot air.

Chemical deposition film method:

When forming the film, immerse the sample in the reagent in a ventilated place, observe the color of the sample, and take out the sample immediately when the surface of the sample changes from yellow to blue, rinse with clean water, rinse with ethanol, and blow dry quickly with hot air.

Identification method

Magnification:

10x, 50x, 100x, 200x, 500x, etc. are all possible, depending on the size of the test sample and the clarity of the microstructure, so as to be able to clearly see the state, category and whether the organization has been welded.

Macro identification

After the above sample preparation, a magnifying glass with a magnification of ≤10 times can be used for observation, as shown in Figure 2, the middle of the figure is the macro welding morphology.

Figure 2 Macro welding morphology

• -base material area; 2--weld area; 3--fusion area

Microscopic identification Identification of seamless steel pipes and welded steel pipes

If no weld is found in the macro, a microscope can be used for detection. The microstructure that has not been welded, hot-rolled banded structure, Li crystal structure, equiaxed grains, etc. are continuous. The welded steel pipe has obvious welding structure morphology characteristics. Stainless steel generally has four areas: sensitization zone, weld zone, fusion zone, and parent material, see Figure 3.

Figure 3 Stainless steel welding zone

• -Sensitization zone; 2--Weld zone; 3--Fusion zone; 4--Parent material zone

Welding structure characteristics

Metallographic structure reference picture: The welded structure has an obvious fusion zone. The weld has a cooling crystallization structure, and the crystallization is generally directional. The heat-affected zone structure near the weld parent material has characteristics such as carbide precipitation or coarse structure.

Carbon steel welding structure

Magnification 100 times, refer to Figures 4~7;

1--Fusion zone; 2--Weld zone; 3--Heat-affected zone

Stainless steel welding structure

Magnification 50 times, refer to Figures 8~9

1--Weld zone: 2--Fusion zone; 3--Parent material zone