What is the difference between seamless steel pipes and welded steel pipes? What is the difference between hot rolling and cold rolling?

Seamless pipes and welded pipes have similar properties in terms of steel tubes. However, compared to welded pipes, seamless pipes can be produced in a wider range of specifications; the manufacturing process is more complex, the technical requirements are higher, and the cost is relatively higher. The specific differences mainly depend on the area of use; since welded pipes have welds, heat absorption is affected, whereas seamless pipes are relatively more uniform ； Welded pipes also have a certain impact on fluidity. Hot-rolled and cold-rolled steel pipes are essentially the same, but cold-rolled pipes have higher precision, a more compact composition, and better grain structure.

There are two types of stainless nickel alloy steel pipes: seamless pipes and welded pipes. The prevalence of seamless and welded pipes varies around the world: in the United States, welded pipes account for 95% of stainless steel pipes, while in Europe seamless pipes make up 60%, and in Japan they account for 75%. Currently, sales of welded pipes in Europe are rising sharply, while sales of seamless pipes are declining significantly. In Asia, Taiwan and Indonesia have significantly increased the production of welded pipes, resulting in a decline in the sales of seamless pipes in Japan. The development of seamless pipes began in the United States during World War II. Since almost all seamless pipe equipment at that time came from Germany, and new equipment could not be obtained with the outbreak of war, the United States used its own technology to produce pipes. Trent and Carpenter were pioneers in the development of seamless pipe manufacturing technology, thereby giving rise to a new industry. After the war, Germany and Japan rebuilt their industries for seamless tube production technology, but they lagged behind the United States in terms of welded tube technology by several decades. Over the past twenty years, the production technology for seamed pipes has been gradually introduced to Italy, France, the Netherlands, England, Sweden, Taiwan, Indonesia, India, and other smaller ** and regions. Around the world, the use of sewn pipes has seen rapid development. Differences between welded pipes and seamless pipes: Concentricity: The manufacturing process for seamless pipes involves punching a hole in stainless steel billets at a temperature of 2200°F; at such high temperatures, the tool steel becomes soft as a result of stamping and drawing, allowing it to be formed into a spiral shape through the hole. As a result, the wall thickness of such pipes is uneven and the degree of eccentricity is high. Therefore, ASTM allows a greater wall thickness variation for seamless pipes than for welded pipes. Seamless pipes are made from precisely rolled steel sheets (with a width of 4-5 feet per roll). These cold-rolled sheets typically have a maximum wall thickness variation of 0.002 inches. The steel plate is cut to a width of πd, where d is the outer diameter of the pipe. The wall thickness tolerance of the welded pipe is very small, and the wall thickness is extremely uniform across the entire circumference. Welding performance: Generally, there are certain differences in the chemical composition between welded pipes and seamless pipes. The steel composition used for producing seamless pipes merely meets the basic requirements of ASTM. The steel used to produce welded pipes contains chemical compositions suitable for welding. For example, elements such as silicon, sulfur, manganese, and oxygen, combined in certain proportions with elements like tritectite, can produce a weld pool that facilitates heat transfer during welding, thereby ensuring full penetration of the entire weld seam. Steel pipes that lack these chemical components, such as seamless pipes, will experience various instability issues during welding; they are not easy to weld firmly nor can they be welded thoroughly. Grain size: Generally, the grain size of a metal is related to the heat treatment temperature and the time spent at that temperature. Therefore, the grain size of both annealed welded pipes and seamless pipes is the same. If the pipe to be welded undergoes minimum cold treatment, the grain size of the weld will be smaller than that of the base metal; otherwise, the grain sizes will be the same. Strength: The strength of a pipe depends on its alloy composition; therefore, seamless pipes and welded pipes made from the same alloy and subjected to the same heat treatment have essentially the same strength. After tensile testing and three-dimensional vibration testing, the tears in the grooved pipes occurred almost always at locations far from the welding points or heated areas. This is because there are fewer impurities at the weld site and the nitrogen content is slightly higher, resulting in better strength at the welded area compared to other parts. However, the ASME Boiler and Pressure Vessel Code holds that welded tubes can only withstand 85% of the allowable pressure, mainly because the data was collected prior to the improved welding equipment in use today. ASME specifies that welded pipes that pass 100% ultrasonic testing can fully withstand the permitted pressure. Similarly, in Europe and Asia, it is stipulated that welded pipes that pass the eddy current test can ensure high-quality welding properties, and such eddy current testing must be carried out by institutions following legitimate procedures and holding relevant licenses. Trent’s eddy current testing has been approved by the Swedish Ministry of Power. ASME believes that the low current loss is due to the excellent properties of the seamed pipe. Corrosion resistance: The level of corrosion resistance also depends on the composition of the alloy. The corrosion resistance of seamless pipes with the same chemical composition is identical to that of welded pipes that have undergone complete heat treatment. The supplementary tests provided by ASTM demonstrate that the corrosion resistance at the weld is equal to or better than that of the base metal. In an environment of acidic chlorides, corrosion at the welds of poorly heat-treated welded pipes accelerates, but this is only required for corrosion testing; in reality, the environment is not that harsh. Flexibility and ductility: The ductility of the welded joint can be verified through the following tests specified by ASTM: bending at 45°, then bending further to 90°, and then straightening along the weld seam ； Then reverse the grooved tube and repeat the above steps to bend the weld inner diameter to 180°. The standard for acceptable weld quality is that no tearing or intergranular separation is allowed when viewed under 40x magnification. The pipe bending radius is determined by the alloy composition; generally, the minimum bending radius is 2D. The ideal welding condition is for the weld to be in a neutral or compressed state. Furthermore, the pipes should be annealed to reduce their hardness, thereby improving their bending performance. Wall thickness/diameter: Thin-walled pipes with small thickness/diameter values are preferably manufactured by welding. Thick-walled pipes with large thickness/diameter values are preferably manufactured by stamping. Overall quality: average. The quality of welded pipes is better than that of seamless pipes, as welded pipes are made from precision cold-rolled sheets that have passed inspection, allowing any defects to be confined to the welding areas. Seamless pipes are formed by stamping stainless steel billets with punches, which results in many tears in the pipe wall that is created through extrusion. Eddy current testing shows that the defect rate of welded pipes is generally lower than that of seamless pipes. Using ultrasonic testing, the background noise of seamless pipes is so high that it is difficult to detect their defects. The background noise of the slotted tube is very low, making it easy to locate defects. Hot-rolled sheets have low hardness, are easy to process, and exhibit good ductility. Cold-rolled sheets have high hardness, making them relatively difficult to process, but they are not prone to deformation and possess high strength. Hot-rolled sheets have relatively low strength and poor surface quality (with oxidation and low finish), but they possess good ductility; they are generally medium to thick sheets. Cold-rolled sheets, on the other hand, have high strength and hardness as well as a high surface finish; they are usually thin sheets and can be used for stamping purposes. Hot-rolled steel plates have mechanical properties that are far inferior to those of cold-worked steel, and also lower than those of forged steel; however, they possess good toughness and ductility. Cold-rolled steel sheets have low toughness due to a certain degree of work hardening, but they achieve a good yield-to-tensile strength ratio; they are used for manufacturing parts such as cold-formed spring sheets. Moreover, since the yield point is close to the tensile strength, there is no early warning of potential hazards during use, and accidents can easily occur when the load exceeds the allowable limit. By definition, ingots or billets of steel are difficult to deform at room temperature and not easy to process; they are generally heated to 1100–1250°C for rolling, and this rolling process is known as hot rolling. Most steel is rolled using the hot rolling method. However, since iron oxide scale tends to form on the surface of steel at high temperatures, causing the surface of hot-rolled steel to become rough and leading to large size variations, steel with a smooth surface, precise dimensions, and good mechanical properties is required. In such cases, hot-rolled semi-finished products or finished products are used as raw materials and then processed through cold rolling. Rolling at room temperature is generally understood as cold rolling; from a metallurgical perspective, the boundary between cold rolling and hot rolling should be defined by the recrystallization temperature. That is, rolling below the recrystallization temperature is called cold rolling, while rolling above the recrystallization temperature is called hot rolling. The recrystallization temperature of steel is 450–600°C. As the name implies, in hot rolling, the temperature of the rolled material is high; therefore, the resistance to deformation is low, allowing for large amounts of deformation. Taking steel plate rolling as an example, the thickness of the continuous cast billet is generally around 230 mm, while after rough rolling and finish rolling, the final thickness is 1–20 mm. At the same time, due to the low width-to-thickness ratio of the steel plate, the requirements for dimensional accuracy are relatively low, and shape defects are less likely to occur; control of convexity is the main focus. For organizations with specific requirements, this is generally achieved through controlled rolling and cooling, that is, by controlling the starting temperature, final rolling temperature, and coiling temperature during precision rolling in order to regulate the microstructure and mechanical properties of the steel strip. In cold rolling, there is generally no heating step before starting the rolling process. However, due to the thin thickness of the steel strip, shape defects are likely to occur. Moreover, the product is obtained after cold rolling; therefore, many complex processes are employed to control the dimensional accuracy and surface quality of the steel strip. The cold rolling production line is long, has many pieces of equipment, and features a complex process. As users place higher demands on the dimensional accuracy, sheet shape, and surface quality of steel strips, the control models, L1 and L2 systems, as well as sheet shape control methods for cold rolling mills are more numerous compared to those used in hot rolling. Moreover, the temperatures of the rolls and the steel strip are also important control parameters. In terms of thin sheets, cold-rolled products and hot-rolled products represent the difference between preceding and subsequent processing steps. Hot-rolled products serve as raw materials for cold-rolled products. Cold rolling involves using roller mills to process hot-rolled steel coils that have been pickled; it is a form of cold working aimed at converting thick hot-rolled sheets into thinner cold-rolled sheets. For example, a hot-rolled sheet with a thickness of 3.0 mm can be processed to produce cold-rolled coils with a thickness of 0.3–0.7 mm. The underlying principle is the use of extrusion to force deformation. Difference: Simply put, 1. Cold-rolled steel sheets have a certain level of gloss on their surface and feel smooth to the touch, similar to those common steel water bottles used for drinking water. 2. If hot-rolled sheets are not pickled, their surface is similar to that of many ordinary steel sheets available on the market: the rusted areas appear red, while the unrusted areas are purplish-black (due to iron oxide scale). The performance advantages of cold-rolled and hot-rolled sheets are as follows: 1. Higher precision, with the thickness variation of cold-rolled strips being no more than 0.01~0.03 mm. 2. Thinner dimensions: cold rolling can produce steel strips as thin as 0.001 mm ； The thinnest thickness achievable for hot-rolled products is now 0.78 mm. 3. The surface quality is superior; cold-rolled steel sheets can even have a mirror-like surface ； The surface of hot-rolled sheets has defects such as iron oxide scale and pitting. 4. Cold-rolled sheets can have their mechanical properties such as tensile strength, as well as their process-related properties such as stamping performance, adjusted according to the customer’s requirements. Cold rolling and hot rolling are two different steel rolling techniques. As the name implies, cold rolling involves rolling steel at room temperature, resulting in steel that has high hardness. Hot rolling is the process of rolling steel at high temperatures. 1. In hot rolling, continuous casting slabs or primary rolling slabs are used as raw materials; they are heated in a step-type furnace, descaled using high-pressure water, and then fed into the roughing mill. After the material from the roughing mill has had its ends cut off, it enters the finishing mill, where rolling is carried out under computer control. Following final rolling, the steel undergoes laminar cooling (with the cooling rate controlled by a computer) and is then coiled by a coiler to form straight coils. The ends of straightened curled hair often have a tongue-shaped or fish-tail shape, with poor accuracy in terms of thickness and width; defects such as wavy edges, folded edges, and tower shapes are commonly present at the edges. Its coil weight is high, and the inner diameter of the steel coil is 760 mm. (It is commonly used in the pipe manufacturing industry. ) After the straight steel coils undergo processes such as end trimming, tail trimming, edge trimming, as well as multiple rounds of straightening and leveling on the finishing line, they are then cut into sheets or re-rolled to produce products such as hot-rolled steel sheets, leveled hot-rolled steel coils, and slitted strips. If hot-rolled finish coils are pickled to remove scale and coated with oil, they become hot-rolled pickled sheet coils. This product tends to partially replace cold-rolled steel sheets; it has a moderate price and is very popular among users. 2. Cold rolling: Using hot-rolled steel coils as raw material, acid washing is carried out to remove the oxide scale, followed by cold continuous rolling. The resulting product is a hard-rolled coil. Due to the cold work hardening caused by continuous cold deformation, the strength and hardness of this coil increase while its toughness and ductility decrease; as a result, its formability deteriorates, and it can only be used for parts that require simple deformation. Hard-rolled coils can be used as raw material for hot-dip galvanizing plants, as all hot-dip galvanizing lines are equipped with annealing lines. The weight of the hardened coil is generally between 6 and 13.5 tons, with an inner diameter of 610 mm for the steel coil. Generally, cold-rolled sheets and coils should undergo continuous annealing (using CAPL units) or bell-type furnace annealing to eliminate work hardening and rolling stresses, so as to meet the mechanical property requirements specified in the relevant standards. Cold-rolled steel sheets have superior surface quality, appearance, and dimensional accuracy compared to hot-rolled sheets. Moreover, their thickness can be reduced to around 0.18 mm, which makes them very popular among users. Deep processing of products using cold-rolled steel coils as a substrate results in high-value-added products. Such as electro-galvanizing, hot-dip galvanizing, fingerprint-resistant electro-galvanizing, colored coated steel coils, vibration-damping composite steel plates, and PVC-coated steel plates

Steel pipes are generally divided into seamless steel pipes and black pipes (welded steel pipes). Seamless steel pipes are suitable for high-pressure and medium-pressure pipelines, while black pipes are typically used in low-pressure pipelines. The main difference between hot rolling and cold rolling is as follows: 1. Cold-rolled shaped steel allows for local buckling of the cross-section, thereby enabling full utilization of the load-bearing capacity of the member after buckling ； On the other hand, hot-rolled section steel does not allow local buckling of its cross-section. 2. The causes of residual stress in hot-rolled and cold-rolled sections are different, so there are also significant differences in their distribution across the cross-section. The residual stress distribution on the cross-sections of cold-formed thin-walled steel sections is of a bending type, whereas that on the cross-sections of hot-rolled or welded steel sections is of a thin-film type. 3. The free torsional stiffness of hot-rolled sections is higher than that of cold-rolled sections, so the torsional resistance of hot-rolled sections is better than that of cold-rolled sections

The main difference between seamless steel pipes and welded steel pipes is that seamless steel pipes have no welds, while welded steel pipes have welds: either straight seams or spiral seams. Seamless steel pipes have better pressure resistance and sealing performance than welded steel pipes, making them suitable for applications with harsh operating conditions.

When calculating the thickness of welded steel pipes, a weld coefficient of 0.6 to 1 is taken into account; the selection of this weld coefficient is specified in GB50316

Seamless steel pipes are generally used in pressure pipelines and can withstand pressure. Seamed steel pipes are used in platforms, supports, construction, and other applications.

In the use of pressure pipelines, it is difficult to produce seamless pipes with large diameters and thin walls; in such cases, welded pipes are considered as an alternative