How welded steel pipes pretend to be seamless steel pipes and inspection methods

Many people cannot distinguish seamless steel pipes from welded steel pipes and even think that the two can be used together or replaced by each other. In fact, this view is not quite right.


Welded steel pipes, also known as welded steel pipes, are steel pipes made by welding steel plates or strips after curling and forming, and are generally fixed in length of 6 meters. The production process of welded steel pipes is simple, the production efficiency is high, there are many varieties and specifications, and the equipment investment is small. Q345B welded steel pipes are generally lower in strength than seamless steel pipes.


Most people distinguish them by whether there are welds or not, which is too one-sided. Seamless steel pipes do not have welds because the construction process is different from that of welded steel pipes. Seamless steel pipes are manufactured at a high temperature of 2200°F and are hot rolled. Using continuous casting slabs or initial rolled slabs as raw materials, a hole is punched out in the billet, so that the wall thickness of the pipe is uneven and the eccentricity is high. Therefore, the wall thickness difference of seamless steel pipe is larger than that of seamed pipe, and there is no inner wall weld rib of welded steel pipe in appearance.


Welded steel pipe is generally completed by cold rolling, high frequency welding, forming, and other processes. This seamed pipe is made of cold rolled plate and hot rolled steel coil as raw material. Usually, the tolerance of wall thickness is very small, and the wall thickness of the entire circumference is very uniform. In addition, welded steel pipes are divided into straight seam welded steel pipes and spiral welded steel pipes.


Because welded pipes have welds and poor pressure-bearing capacity, they are mostly used in low-pressure pipelines, heating pipelines, and water supply and drainage pipelines. Seamless steel pipes are relatively uniform have good pressure bearing capacity, and are usually used in pressure pipelines. Therefore, seamless steel pipes are better. Although welded steel pipes are cheap, their service life is relatively short, especially in the north.


In general, seamless steel pipes are directly pulled out of steel ingots, while welded steel pipes are welded after steel plates are rolled. As for whether the two can be used to replace each other if they are used for trusses, they can be replaced as long as the material and wall thickness are the same. In essence, seamless steel pipes and welded steel pipes have similar performance. However, compared with welded pipes, seamless steel pipes can be produced in more specifications, with higher process technology requirements and relatively higher costs.


First, the feasible areas for welded steel pipes to replace seamless steel pipes
1. At present, welded steel pipes can replace seamless steel pipes in more and more fields. First, because welded steel pipes are more uniform in thickness, they can play a role in decorative pipes, and can also make up for the defects of large-diameter seamless steel pipes (sizes above 219*6) that are difficult to make. Secondly, they can replace some structural pipes. In addition, with the development of high-strength welded steel pipe technology, some high-strength welded steel pipes can replace high-end seamless steel pipes;
2. There is a big gap between my country's welded steel pipes and foreign countries in replacing high-end seamless steel pipes. As far as oil well pipes are concerned, my country is mainly produced by long-process steel mills. The characteristics of small specifications and large output are not suitable for oil well supply and marketing, while other high-end pipe varieties (such as boiler pipes, petrochemical pipes, mechanical pipes, etc.) lack relevant core technologies;
3. The development of new energy vehicles and hydrogen energy industries in the future will bring more possibilities for welded steel pipes to replace seamless steel pipes. The replacement of high-end seamless steel pipes not only requires leading enterprises to study related technologies but also requires policies to play a role in it;
- Decoration: Since welded steel pipes are usually welded with strip steel, generally speaking, the wall thickness of welded steel pipes is more uniform, and they can replace the door and window steel, furniture, scaffolding, wire threading pipes, high-rise shelves, etc. used in light structures for buildings.
- Size: According to different welding processes, welded steel pipes are divided into straight seam welded steel pipes and spiral welded steel pipes. The strength of spiral welded steel pipes is generally higher than that of straight seam welded steel pipes. They can produce welded steel pipes with larger diameters with narrower billets, while seamless steel pipes with large diameters are usually not easy to make. Spiral welded steel pipes can make up for this defect.
- Structure: Spiral welded steel pipes can also be used as pile pipes, bridges, docks, roads, building structure pipes, etc.
- High-strength welded steel pipes: Unlike ordinary welded steel pipes, high-strength welded steel pipes can meet some fields with higher precision requirements, such as structural pipes for automobiles, oil pipes, boiler pipes, etc.


Second, the replacement of seamless steel pipes with welded steel pipes in China is far behind that in foreign countries.
The Princeton Steel Pipe Report in the United States lists the proportion of welded steel pipes and seamless steel pipes in seven aspects of oil-specific pipes, pipeline pipes, standard pipes, mechanical pipes, structural pipes, pressure pipes, and stainless steel pipes in the United States at that time. The proportion of welded steel pipes greatly exceeds that of seamless steel pipes, and even high-pressure boiler pipes use welded steel pipes.


Unlike the situation in foreign countries where welded steel pipes replace high-end seamless steel pipes, my country only has high-end welded steel pipes for oil that can replace seamless steel pipes. However, it is worth noting that although the strength can be replaced, the proportion of replacement is not large.


The reason for the small replacement ratio is mainly related to the steel production process. Most European and American countries are mainly short-process production. The consumption characteristics of oil well pipes vary with the conditions of oil wells in various regions. They have the characteristics of small batches and multiple varieties, which is very similar to the production method of electric furnace steel. My country's production is mainly long-process, and its advantages are few specifications and varieties and large batches, which are not suitable for the supply and marketing model of oil well pipes.


The reason why petroleum-welded steel pipes can be substituted and become the world's advanced level is closely related to the supporting industry-university-research cooperation. My country's petroleum system has its own welded steel pipe enterprises, and supporting petroleum universities, research institutes, and testing institutes. This is the result of the synergy of various aspects. However, the strength of welded steel pipes in other fields in my country, such as mechanical pipes, boiler pipes, petrochemical pipes, structural pipes, etc., is far from reaching the level of replacing high-end seamless steel pipes.


Third, the possible direction of welded steel pipes replacing seamless steel pipes in the future
1. The development of new energy vehicles provides more possibilities for welded steel pipes to replace seamless steel pipes.
The requirements of new energy vehicles for high strength and lightweight have doubled. Even a one-kilogram weight reduction is very important for improving the endurance of new energy vehicles. At present, leading domestic companies can use special alloy steel materials plus unique ERW (straight seam resistance) technology, so that welded steel pipes can replace seamless steel pipes for automobile stabilizer bars. Not only can they meet the strength requirements, but they can also achieve lightweight and have price advantages. However, it is worth noting that the special alloy materials mentioned are not common, and only a few companies can replace seamless steel pipes with such welded steel pipes.
2. With the rapid development of the hydrogen energy industry, welded steel pipe products may usher in further development.
Previously, domestic pipes were mainly used in low-pressure high-proportion hydrogen blending or high-pressure low-proportion hydrogen blending transportation. However, on September 4 this year, Baoji Steel Pipe Company successfully completed the first domestic full-size spiral welded steel pipe long-term high-pressure high-proportion hydrogen blending test, which provides the possibility for petroleum hydrogen blending high-pressure high-proportion welded steel pipe transportation, which can further reduce costs and improve efficiency. Referring to the use of high-strength welded steel pipes abroad, is also one of the directions for my country's future welded steel pipes to replace seamless steel pipes.
3. Policies may play a role in the replacement of welded steel pipes for high-end seamless steel pipes.
Referring to the development model of petroleum welded steel pipes, core technology may become the key. If welded steel pipes in other downstream fields are to replace related high-end seamless steel pipes, they also need to concentrate on the joint efforts of production, learning, and research. In addition to the development of some leading enterprises, policies also need to play an important role in it.


In short, from the perspective of cost-saving and reducing carbon emissions, the replacement of seamless steel pipes with welded steel pipes is an inevitable trend. Compared with foreign development experience, domestic welded steel pipes can only replace some seamless steel pipes in oil well pipes, while oil well pipes are still mainly seamless steel pipes in exports. The strength of other high-end pipes (boiler pipes, petrochemical pipes, etc.) is far from reaching that of welded steel pipes. In the future, technology will still be the core of development. In addition to the development of leading enterprises, policies should also play a role in promoting the coordinated progress of production, learning, and research.


Fourth, do you think it is feasible to replace seamless steel pipes with welded steel pipes?
Hot-rolled welded steel pipes have the following advantages over ordinary cold-formed welded steel pipes:
1. More uniform microstructure;
2. No internal stress, reducing cracking of the pipe;
3. Reliable and consistent mechanical properties;
4. Greatly improved structural integrity and ductility;
5. Improved toughness of the steel pipe;
6. Higher pressure integrity;
7. Higher safety;
8. No strength loss during additional welding and heating;
9. Better corrosion resistance;
10. Better thread processing, grooving, and bending capabilities, even for small pipes, it is not easy to crack, indent, or flatten;
11. It can replace seamless steel pipes in many applications and is more economical than seamless steel pipes;
12. The thickness and roundness are more uniform and can better match the connecting pipe.


Fifth, can straight seam steel pipes be used as seamless steel pipes? How to distinguish?
Nowadays, the competition in various industries is really fierce. No matter which industry has competition, the steel pipe industry is no exception. In order to increase profits, many manufacturers will use straight seam steel pipes as seamless steel pipes. This behavior is completely undesirable.


Straight seam steel pipes are made by submerged arc welding or resistance welding. This kind of steel pipe is connected to the weld. These enterprises use polishing machines to grind and polish the welds. After replacement and polishing, the welds will be worn and the surface will be smooth. The overall view is seamless steel pipes. No insiders can see it. This kind of fraudulent equipment cannot detect whether it is a straight seam steel pipe or a seamless steel pipe. Only experts can see that some projects need to be painted with anti-rust paint on the outside, making it more difficult to distinguish, which makes many buyers very uneasy.


So, how to distinguish between straight seam steel pipes and seamless steel pipes? Seamless steel pipes are directly customized in steel mills. Straight steel pipe manufacturers cannot produce them at all. Generally speaking, they are all external cooperation. When we purchase, we try to let suppliers provide guarantees and certificates for steel pipes. Even if they do a good job, there are traces. Generally, the width of the weld of the straight steel pipe is about 2 cm. When we buy seamless steel pipes, if there are traces of about two centimeters in the entire steel pipe, we can verify whether there is a problem. Whether the seamless steel pipe has a problem with the anti-rust paint, we can use a simple and rough method to verify, that is, the steel pipe is verified during the cutting process. The seamless steel pipe has no butt joint, and the straight seam steel pipe is a steel plate folded in half with a joint.


Due to different production processes, seamless steel pipes and welded steel pipes are produced. Seamless steel pipes include rare cold extrusions such as hot rolling, cold rolling, and cold drawing. Welded steel pipes are composed of crimped and welded steel plates. Small ones are usually straight seam welds, while large ones are usually spiral welds. Due to the presence of welds in welded steel pipes, the mechanical properties of welded joints are poor, and the application range is limited, but the price is low. The difference is that welded steel pipes have welds and seamless steel pipes have no welds. Of course, if the welded steel pipe is well-treated, the layman will not see the difference.


If the welded steel pipe is not treated, there will be obvious welds and protrusions, such as traces. The better point is to wipe off the weld, but there is still a big difference to the naked eye. There are also a small number of cold-rolled welded steel pipes with high surface roughness and high precision, which can be sold as fake seamless steel pipes. A careful observation will reveal that there will be a continuous spiral of metallic luster on the pipe wall, that is, the metallographic structure of the original weld is different from that of other parts. To be honest, it may be a bit difficult for a layman to observe this.


Sixth, seamless steel pipe geometric dimensions and appearance inspection:
① Seamless steel pipe wall thickness inspection: micrometer, ultrasonic thickness gauge, no less than 8 points at both ends, and record.
② Seamless steel pipe outer diameter and ovality inspection: caliper, vernier caliper, ring gauge, measure large and small points.
③ Seamless steel pipe length inspection: steel tape measure, manual, automatic length measurement.
④ Seamless steel pipe curvature inspection: ruler, level ruler (1m), feeler gauge, fine wire to measure curvature per meter and full-length curvature.
⑤ Seamless steel pipe end face groove angle and blunt edge inspection: angle ruler, pallet.