The welding of
ERW steel pipes is based on the high-frequency current skin and proximity effect, using high-frequency current or resistance heat to induce high-frequency current to heat and melt the butt edges of the tube blanks, and then extrusion and welding. . The biggest feature of ERW welds is that there is no external filler metal, fast heating speed, and high production efficiency. This is its advantage and its disadvantage. Without filler metal, it is impossible to improve the performance of the weld by filling alloy elements; the high heating temperature, short time tone, and large welding temperature gradient make it easy to produce hardened phases and structures for certain steel types during the welding process. Stress increases the brittleness of the weld and reduces the overall mechanical properties. Therefore, post-weld remedial measures - weld heat treatment - must be carried out on ERW welds to improve the structure, eliminate stress and improve performance.
1) Weld composition and performance characteristics of ERW steel pipes
According to the different peak values of the welding thermal cycle, the ERW weld can be divided into five zones: melting zone (T>T melting); overheating zone (1100 degrees Celsius)
Performance characteristics of ERW welds:
In recent years, the development of cold areas and offshore oil fields has provided broader development prospects for ERW steel pipes, which requires ERW steel pipes to have good low temperature and corrosion resistance properties. However, at present, the ERW steel pipe welds produced by many manufacturers have poor low-temperature toughness and high FATT values, which greatly limits their scope of use. The low-temperature toughness of ERW steel pipe welds has become an important indicator of the technical level of this steel pipe.
2) Welding seam online heat treatment process
1. Types of online heat treatment processes
Since the emergence of the welding seam online heat treatment process in the 1960s, the process and technology have become increasingly perfect so far. Generally speaking, the heating method is generally induction heating; in order to ensure the accuracy of the sensor heating the weld, most advanced units are equipped with a photoelectric weld automatic tracking system: in order to ensure the stability of the heating temperature, it is equipped with automatic temperature control System and process types In addition to the weld normalizing and weld annealing processes that are mostly used, some manufacturers use a quenching and tempering heat treatment process for high-strength steel pipes in order to improve the impact toughness of the weld. The quenching and tempering process makes the weld grains Refinement improves the impact toughness of the weld. In addition, there are normalizing plus tempering, quenching plus tempering, and the newly developed deformation heat treatment process.
2. Ordinary heat treatment process-weld seam normalizing
Currently, worldwide, the weld normalizing process is the most widely used in weld heat treatment. The weld normalizing process generally uses medium frequency (1-10KHz) power supply, but considering the noise generated by oscillation and other factors, it is best to use 1-3KHz first. Weld seam normalizing can be performed on steel pipes with a diameter of 50.8mm or more. Normalizing treatment can significantly improve the flattening performance of the weld, and its process parameters include: maximum heating temperature, heating speed, holding time and water cooling starting temperature, etc.
a. Maximum heating temperature
According to the thermal simulation test results, without considering the temperature difference between inside and outside, the maximum normalizing temperature can vary within a wide temperature range (860-1010 degrees Celsius), but it is best to choose 920-950 degrees Celsius. If the temperature is too high, the FATT of the weld will increase and the toughness of the weld will decrease. In addition, there is a corrosion phenomenon called "grooving crrosion" in the welds of ERW steel pipes in use. The heat treatment temperature of the weld determines the size of the groove corrosion system.
b. Heating speed
Generally speaking, a sudden change in heating rate will cause the phase to bend at the phase transformation point of the steel. Fast heating speed will increase the phase transformation point of steel and cause bending of phase transformation dynamics. However, judging from the thermal simulation results of the weld, changing the bending heating time between 650-950 degrees Celsius from 12S to 37S has little impact on the toughness of the weld (FATT and impact energy).
c. Keeping time
Due to the limitations of welding seam online heat treatment equipment and processes, there are not many online studies in this area. However, there are literature reports on the research results on the holding time and weld performance of small-sized carbon steel pipes using online heat treatment furnaces. The results show that heating the carbon steel welded pipe to above the Ac3 point in a short time can make the comprehensive properties of the weld and the base material consistent. Extending the holding time (0, 5, 10, 15, 30 min) does not cause much change in the mechanical properties of the weld, nor does it cause much change in the mechanical properties of the weld. But for alloy steel pipes, the longer the heat preservation time, the better the flattening performance. This may be related to the homogenization process of alloy elements in the weld. These researchers did not conduct research on the relationship between holding time and weld toughness.
d. Cooling rate and water cooling starting temperature
The cooling medium of the normalizing process specification is still air, but since the online weld induction heat treatment is performed on the production line, the air cooling process is completed while the steel pipe is running on the air-cooled roller. Therefore, in view of this difference, the API standard From the 39th edition onwards, the term "Similiar Normalising Heat-treatment" is used. In order to reduce the length of the air-cooled roller and improve the cooling efficiency, online normalizing heat treatment often uses a process that combines air cooling and water cooling.
The air cooling speed depends on various factors such as the maximum heating temperature, rolling line speed and steel pipe wall thickness. The air cooling process must wait until the austenite turns into fine ferrite and pearlite structures before it can be terminated. The starting temperature of water cooling affects the toughness of the weld. The starting temperature of water cooling for normalizing heat treatment of ERW steel pipe welds must be lower than 500 degrees Celsius in order to make the welds have good toughness. Some people also believe that the lower critical temperature of carbon steel is about 371 degrees Celsius, and it must be lower than this temperature before water cooling can be implemented.
3. Normalizing + tempering and quenching + tempering
During the normalizing treatment of some low-alloy high-strength steel (HSLA) welded pipes, the hardening phenomenon produced during air cooling will reduce the plasticity of the weld; after normalizing treatment of ordinary carbon steel pipes, the comprehensive performance of the weld is also lower than The base material is low. In order to overcome these defects, some manufacturers use normalizing + tempering and quenching + tempering processes.
The normalizing + tempering process is: after austenitization, without heat preservation, it is directly cooled to waste, and then tempered without heat preservation. In this way, for some high-strength welded pipes, the hardened phase produced during air cooling can be tempered to form a ferrite + tempered troostite structure, which reduces the strength of the weld and improves the toughness.
For some special-purpose ERW steel pipes, in order to further improve their strength, some manufacturers adopt the strengthening treatment process of quenching + tempering, with good results. The general specifications of this process are: spray quenching the weld after austenitization, and then tempering. After reaching the tempering temperature, immediately transfer to water cooling and forced cooling, which can expand the use range of ERW steel pipes.
Compared with normalizing, this strengthening process has the following advantages:
(1) The martensite obtained after quenching has high strength, and the carbides obtained after high-temperature tempering are granular distribution. Therefore, the steel has strength properties equal to or higher than those of normalizing, and its plasticity and impact toughness Much better than normalizing;
(2) The ratio of yield to strength is high;
(3) The performance of steel pipes becomes more uniform and more accurately controlled.
4. Online deformation heat treatment process of ERW steel pipe
For low carbon (0.04%C), low carbon, phosphorus (0.001%s, 0.003%P) Nb, V microalloyed rolled steel, primary normalizing, secondary normalizing, quenching + tempering, quenching and other processes It has little effect on improving its toughness. In order to give full play to the potential advantages of high toughness of controlled rolling of steel strips and achieve "iso-toughness matching" between the ERW steel pipe welds and the base metal, some Japanese manufacturers have applied deformation heat treatment technology to the online heat treatment of ERW steel pipe welds, which has greatly improved the ERW steel pipe weld toughness, especially low temperature toughness. The characteristics of this process are: after ordinary normalizing treatment, the heat softening of the heated weld area is used, and the bending resistance is significantly lower than that of other areas of the pipe body. The U&R (Upsetting and Rolling) method is used to make the weld area produce Shape bending with a certain compression rate and then air cooling to improve the toughness of the weld.
a. Control of deformation temperature
The deformation temperature cannot be too high. Otherwise, it is easy for the dynamic recrystallization of austenite to fully develop during the bending process, resulting in large grains. Therefore, the deformation temperature should be controlled below the recrystallization temperature of steel. Below the recrystallization temperature, as the deformation amount increases, the weld grains are refined and its turning temperature decreases.
b. Range of deformation amount
Although the ERW steel pipe weld seam has its own thermal properties after online deformation heat treatment: when the deformation temperature is 850-790 degrees Celsius and the deformation amount is 6%, the absorbed energy of the weld seam at -60 degrees Celsius is 7J. However, since the inner roller is installed on a mandrel about 20m long, the rolling force is adjusted by oil pressure. Based on existing experience, the author believes that this kind of device has poor operability and is difficult to control accurately. Therefore, the performance of the deformed weld is not easy to be stable. Other ways to improve the toughness of ERW steel pipe welds must be found.
3) Performance inspection of ERW steel pipe weld seams after heat treatment
The quality inspection of welds after heat treatment of ERW steel pipes, in addition to process inspections such as flattening and weld stretching, as well as Charpy impact fracture toughness test and DWTT test, this article would like to discuss the metallographic inspection and weld hardness inspection.
Metallographic examination of normalized welds:
After the heat treatment of the ERW steel pipe weld seam, its metallographic examination includes both macro and micro parts. The macroscopic inspection should measure the width of the complete normalizing zone and the incomplete normalizing zone in the weld area, as well as the left and right symmetry of the weld fusion line. The normal morphology should be: the heat treatment affected zones on both sides of the fusion line are well symmetrical, and the complete normalizing zone after heat treatment completely encompasses the welding heat affected zone. Only in this way can it be shown that during the heat treatment process, the heating sensor and the weld are well aligned and the heat treatment temperature is appropriate. If the heat-affected zone is asymmetrically distributed around the fusion line, it means that the heating sensor is deviated from the center of the weld during heat treatment. The metallographic inspector should promptly feedback this information to the on-site operator and make adjustments quickly.
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