According to the manufacturing method, steel pipes can be divided into two categories: seamless steel pipes and welded steel pipes. Among them, ERW steel pipe is the main variety of welded steel pipe. Today, we mainly talk about two kinds of steel pipes used as raw materials for oil casing: seamless casing and ERW casing.
Seamless casing - casing made from seamless steel pipe
Seamless pipe refers to the steel pipe manufactured by four methods of hot rolling, cold rolling, hot drawing and cold drawing. The tube body itself does not have any welds.
ERW casing - casing made from electric welded pipe as raw material
ERW steel pipe (Electric Resistant Weld) refers to a straight seam welded pipe manufactured by a high frequency resistance welding process. The raw steel plates (coils) of ERW welded pipes are low-carbon microalloyed steels rolled by TMCP (Thermo-Mechanical Control Process).
1. Outer diameter tolerance
Seamless steel pipe: The hot-rolled forming process is adopted, and the sizing is completed at about 8000C. The composition of the raw material of the steel pipe, the cooling conditions and the cooling state of the roll have a great influence on its outer diameter, so the outer diameter control is difficult to be accurate and fluctuate. Larger range.
ERW steel pipe : It is formed by cold bending, and the diameter is reduced by 0.6% to complete the sizing. The process temperature is basically constant at room temperature, so the outer diameter is accurately controlled and the fluctuation range is small, which is conducive to eliminating black leather buckles;
2. Wall thickness tolerance
Seamless steel pipe: It is produced by round steel perforation, and the wall thickness deviation is large. The subsequent hot rolling can partially eliminate the uneven wall thickness, but the current most advanced units can only be controlled within ±5~10%t.
ERW steel pipe: using hot-rolled coil as raw material, the thickness tolerance of modern hot continuous rolling can be controlled within 0.05mm.
The defects on the outer surface of the billet used in the seamless steel pipe cannot be eliminated by the hot rolling process, and the defects can only be polished off after the finished product is completed; the spiral path left after perforation can only be partially eliminated during the wall reduction process.
ERW steel pipes use hot-rolled coils as raw materials. The surface quality of the coils is the surface quality of ERW steel pipes. The surface quality of hot-rolled coils is easy to control and has high quality. Therefore, the surface quality of ERW steel pipes is much better than that of seamless steel pipes.
Seamless steel pipe: The hot-rolled forming process is adopted. The composition of the raw material of the steel pipe, the cooling conditions and the cooling state of the roll have a great influence on its outer diameter, so the outer diameter is difficult to control accurately, and the fluctuation range is large.
ERW steel pipe: It is formed by cold bending, so the outer diameter is controlled accurately and the fluctuation range is small.
5. Tensile test
The tensile properties of the seamless steel pipe and the ERW steel pipe are in line with the API standard, but the strength of the seamless steel pipe is generally at the upper limit and the plasticity is at the lower limit. In comparison, the strength index of the ERW steel pipe is in the best state, and the plasticity index is 33.3% higher than the standard. The reason is that the performance of the hot-rolled coil, the raw material of ERW steel pipe, is guaranteed by means of microalloying smelting, out-of-furnace refining, and controlled cooling and rolling; a reasonable match.
The raw material of ERW steel pipe, the hot-rolled coil, has extremely high precision of controlled cooling and rolling during the rolling process, which can ensure the uniform performance of each part of the coil.
7. Grain size
The raw material of ERW steel pipe, the hot-rolled coil, is a wide and thick continuous casting billet, with a thick fine-grained surface solidification layer, no columnar crystal region, shrinkage cavity and looseness, small composition deviation and dense structure; in the subsequent rolling process Among them, the application of controlled cooling and controlled rolling technology further ensures the grain size of raw materials.
8. Anti-collapse test
ERW steel pipes are characterized by their raw materials and pipe-making processes. Its wall thickness uniformity and ovality are far superior to those of seamless steel pipes, which are the main reasons why the collapse resistance is higher than that of seamless steel pipes.
9. Impact test
Since the impact toughness of the base metal of ERW steel pipe is several times that of seamless steel pipe, the impact toughness of the weld is the key to ERW steel pipe. By controlling the impurity content of the raw material, the height and direction of the slitting burr, the shape of the forming edge, the welding angle, and the welding speed , Heating power and frequency, welding extrusion volume, intermediate frequency withdrawal temperature and depth, length of air cooling section and other process parameters ensure that the impact energy of the weld reaches more than 60% of the base metal. material for seamless performance.
10. Blasting test
The blasting test performance of ERW steel pipe is much higher than the standard requirements, mainly due to the high uniformity of wall thickness and uniform outer diameter of ERW steel pipe.
The seamless steel pipe is formed in a plastic state, and the straightness of the pipe end is relatively difficult to control with a single ruler (continuous rolling is 3 to 4 times the ruler);
ERW steel pipe is cold-worked, and it has on-line straightening in the state of reduced diameter, plus it is infinitely multiplied, so the straightness is better.
12. Steel consumption of casing with 10,000 meter footage
The wall thickness of ERW steel pipe is uniform, and its wall thickness tolerance can be ignored, while the control accuracy limit of the wall thickness difference of seamless steel pipe is ±5%t, which is generally controlled at ±5~10%t. In order to ensure that the minimum wall thickness can meet the standard requirements and performance, it can only be solved by appropriately increasing the wall thickness. Therefore, in the casing of the same specification and the same weight, the ERW steel pipe is 5-10% longer than the seamless steel pipe, or even more, which reduces the steel consumption of the 10,000-meter footage casing by 5-10%. Even at the same price, ERW bushing virtually saves 5-10% of the purchase cost for the user.
The difference between seamless steel pipe and welde d steel pipe :
1) Different production process
Seamless steel pipe: A steel pipe that is perforated from a whole round steel and has no welds on the surface.
Welded steel pipe: a steel pipe with seams on the surface that is formed by bending and deforming a steel strip or steel plate into a circular, square or other shape and then welded.
2) Different feature
Seamless steel pipe: the maximum diameter is 650mm, and the minimum diameter is 0.3mm. Depending on the application, there are thick-walled and thin-walled pipes.
Welded steel pipe: When the T-shaped welded steel pipe contains Ni, it has strong corrosion resistance in an acidic environment. In an environment containing sulfuric acid or hydrochloric acid, the higher the Ni content in the T-shaped welded steel pipe, the stronger the corrosion resistance. Under normal circumstances, only adding Cr to the T-shaped welded steel pipe can prevent the phenomenon of corrosion.
3) Different price
The price of seamless steel pipe is generally higher than that of welded steel pipe.
4) Different pressure bearing capacity
The pressure resistance of seamless steel pipes is higher than that of welded steel pipes. Welded pipes are generally around 10MPa. Now welded pipes are seamless.
5) Different uses
Seamless steel pipe: Seamless steel pipe is mainly used as petroleum geological drilling pipe, cracking pipe for petrochemical industry, boiler pipe, bearing pipe and high-precision structural steel pipe for automobile, tractor and aviation.
Welded steel pipes: Electric welded steel pipes are used in oil drilling and machinery manufacturing, etc.; furnace welded pipes can be used as water and gas pipes, etc., large-diameter straight seam welded pipes are used for high-pressure oil and gas transportation, etc.; spiral welded pipes are used for oil and gas transportation, pipe piles, bridge piers, etc.
The uneven wall thickness of the thick-walled seamless steel pipe is mainly reflected in the phenomenon of uneven spiral wall, uneven linear wall, and thick and thin walls at the head and tail.
The cause of the uneven thickness of the spiral thick-walled seamless steel pipe is the uneven wall thickness caused by the adjustment of the center line of the piercing machine, the inclination angle of the two rolls, or the reduction amount in front of the plug is too small, generally along the full length of the steel pipe. Helical distribution. The main measure is to adjust the rolling center line of the piercing machine to make the inclination angles of the two rolls equal, and adjust the pipe rolling mill according to the parameters given in the rolling table.
The reason for the uneven thickness of the linear wall is that the height adjustment of the saddle for the pre-piercing of the mandrel is not appropriate. When the mandrel is pre-pierced, it contacts the capillary on a certain side, which causes the temperature of the capillary to drop too fast on the contact surface, resulting in uneven wall thickness or even pulling. concave defects. The roll gap of continuous rolling is too small or too large. Deviation of the centerline of the tube rolling mill. Uneven reduction of single and double racks will result in linear symmetry deviation of ultra-thin (ultra-thick) steel pipes in the direction of single racks and ultra-thickness (ultra-thin) in the direction of double racks. The main measures are to adjust the height of the mandrel pre-piercing saddle and ensure that the mandrel and the capillary are centered. When changing the pass and rolling specifications, the roll gap should be measured to keep the actual roll gap consistent with the rolling table. Adjust the rolling center line with the optical centering device, and the center line of the pipe mill must be corrected during the annual overhaul.
The reason for the uneven wall thickness of the head and tail is that the front end of the tube blank is cut inclination, the bending is too large, and the centering hole of the tube blank is not correct, which is easy to cause the uneven wall thickness of the steel tube head. When piercing, the elongation coefficient is too large, the roll speed is too high, and the rolling is unstable. The unstable steel throwing of the piercer can easily cause uneven wall thickness at the end of the capillary tube. The measure is to check the quality of the tube blank to prevent the front end of the tube blank from being cut inclination and the amount of reduction is large, and the centering hole should be corrected when changing the hole pattern or overhauling. A lower piercing speed is used to ensure rolling stability and uniformity of capillary wall thickness. When the roll speed is adjusted, the matching guide plate is also adjusted accordingly.
Features of thick-walled seamless steel pipe production:
(1) Pipes with various outer diameters can be produced from strips of the same width
(2) The straightness of the pipe is good, the size is more accurate, and the inner and outer spiral welds increase the rigidity of the pipe body, so there is no need for sizing and straightening after welding.
(3) It is easy to realize mechanization, automation and continuous biochemical market
(4) Compared with other equipment of similar scale, it has smaller external dimensions, less land occupation and investment, and faster construction.
(5) Compared with the straight seam welded pipe of the same size, the welding seam per unit length of the pipe is longer, so the productivity is lower
If the seamless steel pipe is corroded, it will affect the application of the steel pipe and increase the economic cost. The corrosion of the seamless steel pipe can be divided into four types, which will be introduced in detail next.
1. Intergranular corrosion. It refers to corrosion along dislocations, which destroys connections between crystals. This type of corrosion is very harmful, it will make the seamless steel pipe brittle or lose its compressive strength, which is very easy to cause safety accidents. Intergranular corrosion is the key corrosion method of martensitic seamless steel pipe. It is because the dislocation area and the crystalline composition or ground stress are different, resulting in a significant reduction in the electrode potential in the dislocation area, which in turn results in a difference in electrode potential.
2. Uniform corrosion. This means that all seamless steel pipe piping exposed to corrosive environments will undergo chemical changes and suffer uniform corrosion. This type of corrosion can accurately measure its corrosion rate and predict its corrosion level. Based on precise measurements, its safety performance and lifetime can be set. Therefore, in many types of corrosion, uniform corrosion is not a risk, and the corrosion level of uniform corrosion is generally accurately measured and predicted according to the reduction of seamless steel pipe quality and seamless steel pipe thickness. Outside the unique natural environment, the uniform corrosion rate of seamless steel pipes is very low, the service life is long, and the maintenance is low.
3. Pitting corrosion. It refers to a form of corrosion damage that occurs in part of the metal surface of the seamless steel pipe. After crevice corrosion occurs, it can rapidly develop into a deep, through seamless steel pipe. Pitting corrosion is very dangerous, especially for various high pressure vessels. After corrosion occurs, it should be polished or painted immediately to prevent further corrosion.
4. Fatigue corrosion. Refers to the damage of seamless steel pipes under the action of corrosive substances and alternating ground stress, characterized by corrosion pits and many cracks. Significantly reduces the fatigue limit of seamless steel pipes, resulting in premature rupture. Corrosion fatigue of seamless steel pipe does not mean fatigue of mechanical equipment, it does not have a certain fatigue strength. With the increase in the frequency of the circulatory system, the fatigue limit is reduced.
The content of several common elements in the seamless steel pipe will give you a brief introduction:
(1) Carbon; the higher the carbon content, the higher the hardness of the steel, but the worse its plasticity and toughness.
(2) Sulfur: It is a harmful contaminant in steel. When steel with high sulfur content is subjected to pressure processing at high temperature, it is easy to crack, which is usually called hot brittleness.
(3) Phosphorus: It can significantly reduce the plasticity and toughness of steel, especially at low temperature. This phenomenon is called cold brittleness. In high-quality steel, sulfur and phosphorus are strictly controlled. But on the other hand, the high content of sulfur and phosphorus in low carbon steel can make it easy to cut, which is beneficial to improve the machinability of steel.
(4) Manganese: It can improve the strength of steel, weaken and eliminate the adverse effects of sulfur, and improve the hardenability of steel. High-alloy steel (high-manganese steel) with high manganese content has good wear resistance. and other physical properties.
(5) Silicon; it can improve the hardness of steel, but the plasticity and toughness decrease. The steel used for electrician contains a certain amount of silicon, which can improve the soft magnetic properties.
(6) Tungsten: It can improve the red hardness and thermal strength of steel, and can improve the wear resistance of steel.
(7) Chromium; it can improve the hardenability and wear resistance of steel, and can improve the corrosion resistance and oxidation resistance of steel.
(8) Vanadium: It can refine the grain structure of steel and improve the strength, toughness and wear resistance of steel. When it melts into austenite at high temperature, it can increase the hardenability of steel; on the contrary, when it exists in carbide form, it will reduce its hardenability.
(9) Molybdenum: It can significantly improve the hardenability and thermal strength of steel, prevent temper brittleness, and improve remanence and coercive force.
(10) Titanium; it can refine the grain structure of steel, thereby improving the strength and toughness of steel. In stainless steel, titanium can eliminate or reduce the intergranular corrosion phenomenon of steel.
(11) Nickel: It can improve the strength and toughness of steel and improve hardenability. When the content is high, some physical properties of steel and alloys can be significantly changed, and the corrosion resistance of steel can be improved.
(12) Boron: When the steel contains a small amount of (0.001-0.005%) boron, the hardenability of the steel can be doubled.
(13) Aluminum; it can refine the grain structure of steel and inhibit the aging of low carbon steel. Improve the toughness of steel at low temperature, but also improve the oxidation resistance of steel, improve the wear resistance and fatigue strength of large-diameter seamless pipes.
(14) Copper; its prominent role is to improve the atmospheric corrosion resistance of ordinary low-alloy steels, especially when used in conjunction with phosphorus.