How to improve the impact toughness of
ERW steel pipe (Electric Resistance Welded) weld seams is a major problem in pipe making technology. On the basis of mass production practice, various factors affecting the impact toughness of ERW straight seam resistance welded steel pipe weld seams were analyzed. Research points out that in order to further improve the impact toughness of welds and meet the high toughness requirements of long-distance pipelines for ERW steel pipes, it is necessary to start by controlling the quality of raw steel coils, combined with the pipe making process, to strengthen the quality of forming, welding and post-weld online heat treatment control.
Characteristics of impact toughness of ERW steel pipe welds:
The welding process of ERW steel pipe uses the skin effect principle generated by high-frequency current to heat the edge of the hot-rolled coil to a molten state, and then welds it through mechanical extrusion. The steel pipe produced through this process will have a white fusion line in the center of the weld, and the heat-affected zones on both sides of the fusion line will produce metal streamlines extending from the middle to the inner and outer surfaces. Since the ERW steel pipe uses the base metal as the welding material, the performance of the weld is closely related to the performance of the base metal. Among ordinary materials, the difference in the energy absorbed by the Charpy impact test between the weld and the base material is very small. On the contrary, there is a big difference in high-toughness materials. The impact toughness of the weld is significantly worse than that of the base material, but significantly better than that of the weld of ordinary materials. It can be seen from this that the impact toughness of the weld center of the ERW steel pipe is improved. Even if high toughness materials are used, although the toughness of the weld is improved, the impact toughness of the weld center cannot be completely improved.
Factors affecting the impact toughness of welds:
The main factors affecting the impact toughness of welds are:
(1) Physical and chemical properties of raw materials
(2) Grain size of raw materials and non-metallic inclusions
(3) Weld heat treatment conditions
(4) Molding conditions
(5) Welding conditions
1. Physical and chemical properties of raw materials
ERW steel pipe uses the base metal as the welding material, so the physical and chemical properties of the raw materials directly determine the physical and chemical properties of the weld. In order to study the impact properties of materials with different chemical compositions and the impact properties of their base metal and welds that are affected by changes in chemical composition, two materials with different chemical compositions were used to make steel pipes and then Charpy impact tests were conducted. In order to eliminate the influence of the pipe making process on the test results, all samples were normalized.
2. Grain size of raw materials and non-metallic inclusions
During the welding process of ERW steel pipes, due to mechanical pressure, metal streamlines extending from the middle of the steel plate to the inner and outer surfaces will be produced in the heat-affected zones on both sides of the weld. The direction of the metal streamlines near the center of the weld is basically the same as that of fusion. The lines are parallel and coincident with the fusion line. Metal streamlines are band-like structures and inclusions that extend along the rolling direction during hot rolling of steel plates. The greater the thickness of the metal streamlines and the grain size of the raw material or the more non-metallic inclusions, the thicker and more obvious the metal streamlines will be. . When there are many non-metallic inclusions and they are unevenly distributed, the base metal will segregate seriously. During high-frequency welding, under the action of the extrusion force of the squeeze roller, a large number of non-metallic inclusions in the segregation zone will enter the welding fusion zone along the metal streamlines. The segregation lines can be clearly seen from the low-magnification hot acid sample. Intersect at the fusion line at a certain angle. In this case, the impact toughness of the weld will be seriously reduced and the impact absorbed energy will be greatly reduced. Therefore, during the smelting and rolling stages of raw steel coils, measures should be taken to reduce inclusions and refine the grains. During the pipe making process, the rising angle of the metal streamlines should also be reasonably controlled. Generally, the clamping between the metal streamlines and the fusion line is required. The angle is controlled between 25 and 45 degrees, and it is best to be basically symmetrical up, down, left, and right.
3. Weld heat treatment conditions
Metal flow lines in welds and heat-affected zones can be eliminated through heat treatment. In order to study the difference in impact toughness of two types of weld samples welded under the same conditions, with and without heat treatment, two groups of samples were taken. One group was heat treated and the metal streamlines basically disappeared, and the other group was not. Heat treatment, there are obvious metal streamlines. The weld heat treatment process is to reheat the weld to improve the hardened structure formed during welding into a proper structure and refine the grains. But how to control the heat treatment temperature can achieve the purpose of refining the grains and improving the toughness of the weld. According to relevant foreign statistics, when the weld is heated between Ac3+ 50 and Ac3+ 150 in the normalizing temperature range, it is expected to obtain a fine ferrite + pearlite structure and improve the bonding. Weld toughness. The relationship between weld heat treatment temperature and weld ductile-brittle transition temperature. One of the important factors regarding heat treatment conditions is the water cooling starting temperature. After the steel pipe weld is heated by heat treatment, it must undergo a long-distance air cooling. In order to shorten the distance of the air cooling section and prevent the steel pipe from deforming after sizing, the steel pipe must be water-cooled before entering the sizing.
One of the important factors regarding heat treatment conditions is the holding time after heating. Online weld heat treatment is different from overall heating in a heat treatment furnace. It has two characteristics: on the one hand, it uses medium frequency induction heating, with fast heating speed and uneven temperature in the direction of the pipe wall; on the other hand, it uses local heating , the cooling speed is fast, and there are few Y grains produced at the heat treatment temperature. These two problems can be solved by using two or more intermediate frequency heat treatment machines for continuous heating. This method can not only make the temperature in the direction of the pipe wall more uniform, but also extend the holding time. However, to increase equipment investment, we must also consider whether the air cooling section is long enough and whether it can meet the requirement that the starting temperature of water cooling should not be too high.
4. Welding conditions
High-frequency welding machines are generally automatic welding machines that can automatically control the output of welding power according to the welding temperature and welding speed. Therefore, as long as the extrusion amount, welding speed, V-shaped opening angle of the welding zone and other factors are well controlled during welding, it will be good. to control welding quality.
(1) Extrusion amount
The way to control the amount of extrusion is to adjust the extrusion force of the extrusion roller. Increasing the extrusion force of the extrusion roller can increase the extrusion amount, which is beneficial to the discharge of oxides and inclusions, and improves the impact toughness of the weld. But if the amount of extrusion is too large, it can also be harmful.First of all, excessive extrusion will cause the welding point to deviate from the center of the extrusion roller, and the two will contact in advance. By the time they reach the center of the extrusion roller, the temperature of the welding point has dropped, making it difficult to discharge oxides and inclusions. Secondly, when the extrusion amount is too large, the welding power that needs to be output will increase. If the welding power output is insufficient, the welding temperature will not be enough, resulting in cold welding. The extrusion amount should be controlled at 0.4~0.6t. On the other hand, the amount of extrusion can also be reflected from the angle between the metal streamline and the fusion line. A small angle between the metal streamline and the fusion line indicates a large extrusion amount; a large angle between the metal streamline and the fusion line indicates a small extrusion amount.
(2) Welding speed
Increasing the welding speed is beneficial to the stability of the welding temperature, but the welding speed is limited by the equipment capabilities, including the power of the forming machine and the power of the high-frequency welding machine. It cannot be increased infinitely, and the highest speed should be selected for welding within the allowable range.
(3) V-shaped opening angle of welding area
The selection of the V-shaped opening angle is related to the material strength, thickness and welding extrusion amount, generally 3 to 6. The thinner the material, the lower the strength and the greater the welding extrusion amount, a larger opening angle should be selected, otherwise a smaller opening angle should be selected. The ideal V-shaped opening angle should keep the welding point as close to the center of the extrusion roller as possible, which is conducive to the discharge of oxides and inclusions and improves the impact toughness of the weld.
Conclusion:
Through the analysis of the basic factors that affect the impact toughness of ERW steel pipe welds, it can be found that the ultimate goal of reasonably controlling various related factors in production practice is to reduce the oxides and inclusions in the welds, refine the grains, and thereby improve the impact toughness. When producing ERW steel pipes with high impact toughness requirements, raw materials, forming, welding and weld heat treatment should be considered, and all factors should be integrated to achieve the best welding quality. Through reasonable adjustments in the above aspects, the impact toughness, ductile-brittle horizontal transition temperature, etc. of the base metal and weld of the produced ERW steel pipe can roughly reach the same level.
Tips: In case ASTM A53 Grade B in ERW (electric resistance welded) pipe, the weld seam shall be done the heat treatment with a minimum 1000°F [540°C]. In this way the no untempered martensite remains.
In case ASTM A53 B pipe in cold expanded, then expansion should not exceed 1.5% of the required OD.
