Identification Method of Seamless Steel Pipe and Welded Steel Pipe
Today, a client sent me some pictures to help him see if the carbon steel pipe is seamless or welded. The size of the steel pipe is 133 *4.5mm, and the outside diameter of the pipe is 133mm and the thickness is 4.5mm. The contract he signed with the supplier is 133 *4.5mm seamless steel pipe, but he suspected that he met a traitor and gave him welded steel pipe (possibly seamless steel pipe, that is, seamless treatment of welded pipe in the relevant treatment industry, used to impersonate seamless steel pipe, called seamless steel pipe, in fact, it is a kind of deception. Behavior, welded pipe is welded pipe, no matter how seamless treatment of its essence or welded pipe. He was deceived by passing off goods as seamless steel pipes. The following picture is sent by this customer:
|Metallographic method||Corrosion method|
|Identify by process||Classification by use|
These pictures above can clearly show the traces of welded pipes. Ninety-nine out of ten are welded pipes.
However, sending these pictures is not the original intention of this article. The original intention of this article is to help you systematically sort out how to identify seamless steel pipe and welded steel pipe. Not much nonsense, let’s move on to the theme:
Welded steel pipe and seamless steel pipe are two common forms of steel pipe. In the process of Engineering construction, the appropriate steel pipe will be selected according to the specific construction situation, and the cost will be the primary factor. Therefore, this paper distinguishes welded steel pipe and seamless steel pipe from the performance, technology and identification method of steel pipe, hoping to provide reference for the selection of steel pipe in the construction process.
With the continuous progress of science and technology, China’s steel pipe production process has also been significantly improved. At present, there are two main forms of common steel pipe, namely, welded steel pipe and seamless steel pipe. Because the cost of welded steel pipe is lower than that of seamless steel pipe, in order to obtain more economic benefits, some merchants process welded steel pipe, then pretend to be seamless steel pipe, and then sell it into the market, resulting in market chaos. From the appearance, the welded steel pipe processed can not be effectively distinguished. Once the welded steel pipe is used in the construction process, it will lead to gas or liquid leakage. In serious cases, it is easy to cause explosion. Therefore, it is necessary to distinguish the welded steel pipe from the seamless steel pipe, so as to ensure the quality of the project.
Welded steel pipe is usually made of steel strip or steel plate under the action of external force, which is deformed into a circle or square, and then welded the surface seams, thus forming a welded steel pipe. Usually, arc welding, high frequency or low frequency, cold rolling and so on are adopted, but welding with these processes will lead to welds in steel pipes. At present, according to the weld shape, it can be divided into straight seam and spiral welded steel pipe, but the technology used by them is different. Spiral steel pipe is 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. Because of the adjacent and turbine thermal effects of alternating current charge in conductor, the steel at the edge of the weld can be melted and finally the welding can be completed.
Seamless steel pipe is mainly made of solid billet or ingot through perforation, capillary, and through hot rolling, cold rolling, cold drawing and other processing, and then formed seamless steel pipe. Because the seamless steel tube contains a hollow section, the hollow section can be used as a pipeline for conveying fluid.
There are three main methods to distinguish welded steel pipe from seamless steel pipe:
1. Metallographic method
Metallographic method is one of the main methods to distinguish welded steel pipe from seamless steel pipe. High frequency resistance welded steel pipe does not add welding material, so the weld of welded steel pipe is very narrow, if the method of rough grinding and corrosion is adopted, the weld can not be clearly seen. Once the high frequency resistance welded steel pipe is welded and not heat treated, the weld structure will be fundamentally different from the steel pipe parent material. At this time, metallographic method can be used to distinguish welded steel pipe and seamless steel pipe. In the process of distinguishing the two kinds of steel pipes, it is necessary to cut small samples with length and width of 40mm at the weld, roughly grind, fine grind and polish them, and then put them under metallographic microscope to observe the structure. When ferrite and widmanstatten, base metal and weld zone structure are observed, welded steel pipe and seamless steel pipe can be distinguished accurately.
2. Corrosion method
In the process of distinguishing welded steel pipe from seamless steel pipe by corrosion method, the weld seam of welded steel pipe should be polished. After polishing, the polished trace should be seen. Then the end face should be polished with sandpaper at the weld seam, and the end face should be treated with 5% nitric acid alcohol solution. If there is an obvious weld seam, the end face should be polished with sandpaper at the weld seam. It can be proved that this steel pipe is welded steel pipe. There is no significant difference in the end face of seamless steel tube after corrosion.
Performance of Welded Steel Pipe
Welded steel pipe has the following properties because it is processed by high frequency welding and cold rolling.
First, the thermal insulation function is good. The heat loss of welded steel pipe is relatively small, only 25%, which is not only conducive to transportation, but also reduces the cost.
Secondly, it is waterproof and corrosion-resistant. In the process of Engineering construction, it is not necessary to install pipe trenches alone, but to bury the pipe directly underground or underwater, which reduces the construction difficulty of the project.
Thirdly, it has impact resistance. Even at low temperature, the steel pipe will not be damaged, so its performance has certain advantages.
Performance of Seamless Steel Pipe
Because of the high tensile strength of the seamless steel tube metal material, its anti-destruction ability is stronger, and it has hollow passage, so it can effectively transport fluid, it is its strong transport ability, so seamless steel tube has higher corrosion resistance than welded steel tube, and its stiffness is relatively large. Therefore, the more load the seamless steel tube bears, so it can be widely used in projects with higher construction requirements.
3. Identify welded and seamless steel tubes according to process
In the process of distinguishing welded steel pipe and seamless steel pipe according to technology, all welded steel pipe is welded according to cold rolling and extrusion. In addition, spiral pipe welding and straight seam pipe welding will be formed when high frequency and low frequency arc welding pipe and resistance welding pipe are used to weld steel pipe, and circular and square pipe will be formed. Elliptical steel pipe, triangular steel pipe, hexagonal steel pipe, rhombic steel pipe, octagonal steel pipe, and even more complex steel pipe. In a word, different shapes of steel pipes will be formed by different processes, so that welded steel pipes and seamless steel pipes can be distinguished clearly. However, in the process of identifying seamless steel pipes according to technology, it is mainly based on hot rolling and cold rolling treatment, and seamless steel pipes also have two main forms, namely hot-rolled seamless steel pipes and cold-rolled seamless steel pipes. Hot-rolled seamless steel pipes are formed by piercing and rolling, especially large diameter and thickness seamless steel pipes are welded by this process; cold-drawn pipes are cold-drawn by tube blanks, which have lower strength, but smooth external and internal control surfaces.
4. Distinguishing welded and seamless steel pipes by use
Welded steel pipe has higher flexural and torsional strength and more load-bearing capacity, so it is widely used in the manufacture of mechanical parts. For example, oil drill pipe, automobile transmission shaft, bicycle frame and steel scaffolding used in construction are all made of welded steel pipe. However, seamless steel pipe can be used as a pipeline for conveying fluids because of its hollow section and long steel without joints around it. For example, it can be used as a pipeline to transport oil, natural gas, gas, water and so on. In addition, the bending strength of seamless steel tube is relatively small, so it is widely used in superheated steam pipes of low and medium pressure boilers, boiling pipes and superheated steam pipes for locomotive boilers. In a word, welded steel pipe and seamless steel pipe can be clearly distinguished by the classification of their uses.
In order to ensure the higher quality of construction in China, it is necessary to distinguish welded steel pipe from seamless steel pipe, and then use appropriate steel pipe for construction. In the process of distinguishing welded steel pipe from seamless steel pipe, metallographic method and corrosion method can be used to observe the structure and weld seam, so as to check the weld seam and finally identify it accurately. At the same time, it can also be distinguished according to the performance and use of the two kinds of steel pipes, and then choose reasonable steel pipes for construction.
Having said so much, many people may still not understand how to operate it. Next we will talk about the specific operation methods and steps:
Macroscopic identification: When the prepared steel tube sample is observed with naked eyes or magnifying glass less than 10 times, it is found that there are welding phenomena, i.e. complete welding morphological characteristics of weld zone, fusion zone and base metal zone, which are identified as welded steel pipe, and seamless steel pipe if there is no welding phenomena.
Microscopic identification: If it is difficult to identify macroscopically, use metallographic microscope to do microscopic identification, adjust the optical path and magnification of metallographic microscope, and observe the whole cross section from a lower multiple. It is found that there are signs of welding that are difficult to confirm, and use a higher multiple to further analyze and confirm. According to the existence of welded structure characteristics, the welded steel pipe or seamless steel pipe can be identified.
Referring to GB/T 13298, the steel pipe is cut on the complete cross section by cold cutting, and the heat affected zone is removed by hot cutting. The steel pipe should be polished by full section grinding as far as possible. If the diameter of the steel pipe is too large to be polished by full section grinding, it can be cut into several sections on the cross section. The width of the saw edge is not more than 2 mm (wire cutting is recommended). The angle between the saw edge and the tangent of the point is not more than 45 degrees, so as to ensure that the weld seam is not missed. The diagram of the saw edge and the steel pipe is shown in fig 1.
Fig. 1 Diagram of sawing angle
Organizational Display Method
The corrosive agent is recommended to use 3%~5% nitric acid-ethanol solution or other metallographic structure indicator. At room temperature and ventilation, the prepared reagent is evenly soaked or smeared on the surface to be inspected. The etching time depends on the type of steel and the reagent. After corrosion, rinse with clean water, rinse with ethanol and dry quickly with hot air.
Chemical Deposition Film Method
When forming film, the sample should be immersed in the reagent at the ventilated place, and the color of the sample should be observed. When the surface of the sample is yellow to blue, the sample should be taken out immediately, washed with clean water, washed with ethanol, and dried quickly by hot air.
The way to distinguish
10 x, 50 x, 100 x, 200 x, 500 x and so on, depending on the size of the sample and the clarity of the microstructures, to see the state of the microstructures, types and whether it has been welded or not.
After the above sample preparation, the magnifying glass with magnification factor less than 10 times can be used to observe. As shown in Fig. 2. the macroscopic welding morphology is shown in the middle of the figure.
1 – Base Material Area
2 – Weld Zone
3 – Fusion Zone
If no weld is found in macroscopical view, microscopic examination can be used. Without welded microstructure, hot rolled strip structure, twin structure and equiaxed grain have continuity. The welded steel pipe has obvious morphology characteristics. Stainless steel generally has four zones: sensitization zone, weld zone, fusion zone and base metal. See Figure 3.
Figure 3 Stainless steel welding zone
1 – Sensitization Zone
2 – Weld Zone
3 – Fusion Zone
4 – Base Material Area
Microstructure Characteristics of Welding
Metallographic structure reference picture: After welding, there is a clear fusion zone. There is a cooling crystallization structure at the weld seam, and the crystallization is generally directional. The structure of HAZ near the base metal of the weld is characterized by carbide precipitation or coarse structure.
Welding Microstructure of Carbon Steel
The magnification is 100 times, refer to Fig. 4-7.
1 – Fusion Zone
2 – Weld Zone
3 – HAZ
Welding Microstructure of Stainless Steel
The magnification is 50 times, refer to Fig. 7-8.
1 – Weld Zone
2 – Fusion Zone
3 – Base Material Area
The above article mainly carries on the identification analysis to the common carbon steel and stainless steel pipes on the market. The identification methods of other types of steel pipes will be uploaded when they are free. I hope it will be helpful to you.
Source: China Steel Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.pipelinedubai.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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 Hebei Provincial Local Standard DB13/T 1484-2011