Factors affecting the impact toughness of ERW steel pipe welds
The initial impact property reflects the ability of the material to resist impact load. The practical significance of the impact toughness index is to reveal the brittle tendency of the material, which is generally measured by impact tests. Combined with the ERW welded pipe production process and its practical production process, the main factors affecting the impact toughness of ERW welded pipe welds are as follows:
1. Raw materials
Since the ERW steel pipe uses the base metal as the welding material, the performance of its weld is closely related to the performance of the base metal. The factors that affect the impact toughness of raw materials mainly include the following aspects:
①Ingredients
In addition to Fe, C and alloying elements that often appear in steel, there are also impurity elements such as S and P, among which C and S have the greatest impact on the initial impact. As the C content increases, the pearlite content in the steel increases correspondingly. Since pearlite contains a large amount of brittle lamellar cementite, the impact toughness is poor. S often exists in the form of compound FeS in steel, and the low-melting eutectic on the grain boundary melts at a higher temperature, so that the steel cracks along the grain boundary. P is partially dissolved in the ferrite in the steel, which increases the strength and hardness of the ferrite, and reduces the plasticity and plasticity;Most of Si dissolves in ferrite to strengthen it, thereby improving the strength and hardness of the steel and reducing the plasticity and toughness.
②Inclusions
The inclusions in the steel will reduce the impact toughness of the steel, and the gray spots at the weld contain a lot of oxides, which are mostly in the form of composite inclusions at high temperatures.
In the actual production of ERW welded pipes, it is found that the transverse impact fracture of the weld often appears gray, also known as "oxide inclusions". It has a great influence on the toughness of the weld. No matter whether it is heat treated or not, there are still such defects in the weld. Its existence is an important reason why the quality of ERW welded pipes is difficult to improve.
③Grain size
The finer the grains, the more uniformly dispersed the deformation caused by the external force is in the grains, which reduces the phenomenon of excessive stress concentration. At the same time, the finer the grains, the less conducive to the propagation of cracks, so the finer the grains, the better the impact toughness of the material itself.
Therefore, the raw materials should be kept as few inclusions as possible, and the grains should be refined, and the rising angle of the metal streamline should also be reasonably controlled in the process of making pipes.
2. Molding quality
During ERW welding, the shape of the pipe edge determines the stability of the weld. During the forming process, corrugations, wrinkles, bulges and work hardening are prone to occur, which will affect the toughness quality. Before entering the extrusion spoke, the two edges of the steel strip should be as parallel as possible to ensure that the two plate edges have high parallelism for high-frequency welding, which is convenient for the discharge of materials and inclusions during welding and reduces the formation of gray spots. The welding temperature of the local protruding points on the butt welding surface is high, which makes the edge of the plate melt unevenly, which increases the degree of local oxidation and is prone to defects.
If there is a burr on the edge of the strip, it will cause early contact and sparks during welding, resulting in local cold welding defects. The edge of the strip is bent or the thickness of the edge is reduced, which will make the edge appear wavy, resulting in the formation of gray spots and affecting the toughness.
3. Welding process
The welding process is closely related to the welding quality. The main parameters are: heat input, welding extrusion force, welding extrusion amount, welding speed, opening angle, and welding temperature.
The welding process parameters should be selected according to the wall thickness of the pipe, and the size of the opening angle should be reasonably set.Under the condition of ensuring sufficient welding power, gradually increase the welding speed and apply appropriate extrusion force, so that when the extrusion amount reaches 2/3 times the wall thickness, the impact toughness of the weld can be appropriately improved.
4. The influence of heat treatment process
The microstructure formed by ERW welding just after welding is relatively coarse and the grain size is small. After heat treatment, the grain size is refined, the microstructure distribution is more uniform, and the impact toughness of the weld is improved.At the same time, the metallographic structure of the weld also directly affects the impact toughness. After the weld is heated by heat treatment, it must go through a long-distance air cooling, and the steel pipe must be water-cooled before entering the sizing. If the starting temperature of water cooling is too high, a bainite structure will be formed, reducing the impact toughness of the weld.
Improvement measures to improve the impact toughness of ERW steel pipe welds
1. The raw materials should be steel with uniform and fine grains and high purity.
2. During the molding process, the edges of the plate should be kept parallel, and the extrusion force should be uniform.
3. Welding process control: the size of the opening angle is preferably 3-4, comprehensively adjust the welding power, welding speed and extrusion force,In order to make the extrusion amount reach 2/3 times the wall thickness, and form a reasonable metal streamline shape.
4. For different grades of steel, the heat treatment temperature should be determined according to the carbon content and wall thickness, corresponding to the iron-carbon phase diagram to determine Ac3 and heating temperature, and make appropriate adjustments to optimize the structure.
The initial impact property reflects the ability of the material to resist impact load. The practical significance of the impact toughness index is to reveal the brittle tendency of the material, which is generally measured by impact tests. Combined with the ERW welded pipe production process and its practical production process, the main factors affecting the impact toughness of ERW welded pipe welds are as follows:
1. Raw materials
Since the ERW steel pipe uses the base metal as the welding material, the performance of its weld is closely related to the performance of the base metal. The factors that affect the impact toughness of raw materials mainly include the following aspects:
①Ingredients
In addition to Fe, C and alloying elements that often appear in steel, there are also impurity elements such as S and P, among which C and S have the greatest impact on the initial impact. As the C content increases, the pearlite content in the steel increases correspondingly. Since pearlite contains a large amount of brittle lamellar cementite, the impact toughness is poor. S often exists in the form of compound FeS in steel, and the low-melting eutectic on the grain boundary melts at a higher temperature, so that the steel cracks along the grain boundary. P is partially dissolved in the ferrite in the steel, which increases the strength and hardness of the ferrite, and reduces the plasticity and plasticity;Most of Si dissolves in ferrite to strengthen it, thereby improving the strength and hardness of the steel and reducing the plasticity and toughness.
②Inclusions
The inclusions in the steel will reduce the impact toughness of the steel, and the gray spots at the weld contain a lot of oxides, which are mostly in the form of composite inclusions at high temperatures.
In the actual production of ERW welded pipes, it is found that the transverse impact fracture of the weld often appears gray, also known as "oxide inclusions". It has a great influence on the toughness of the weld. No matter whether it is heat treated or not, there are still such defects in the weld. Its existence is an important reason why the quality of ERW welded pipes is difficult to improve.
③Grain size
The finer the grains, the more uniformly dispersed the deformation caused by the external force is in the grains, which reduces the phenomenon of excessive stress concentration. At the same time, the finer the grains, the less conducive to the propagation of cracks, so the finer the grains, the better the impact toughness of the material itself.
Therefore, the raw materials should be kept as few inclusions as possible, and the grains should be refined, and the rising angle of the metal streamline should also be reasonably controlled in the process of making pipes.
2. Molding quality
During ERW welding, the shape of the pipe edge determines the stability of the weld. During the forming process, corrugations, wrinkles, bulges and work hardening are prone to occur, which will affect the toughness quality. Before entering the extrusion spoke, the two edges of the steel strip should be as parallel as possible to ensure that the two plate edges have high parallelism for high-frequency welding, which is convenient for the discharge of materials and inclusions during welding and reduces the formation of gray spots. The welding temperature of the local protruding points on the butt welding surface is high, which makes the edge of the plate melt unevenly, which increases the degree of local oxidation and is prone to defects.
If there is a burr on the edge of the strip, it will cause early contact and sparks during welding, resulting in local cold welding defects. The edge of the strip is bent or the thickness of the edge is reduced, which will make the edge appear wavy, resulting in the formation of gray spots and affecting the toughness.
3. Welding process
The welding process is closely related to the welding quality. The main parameters are: heat input, welding extrusion force, welding extrusion amount, welding speed, opening angle, and welding temperature.
The welding process parameters should be selected according to the wall thickness of the pipe, and the size of the opening angle should be reasonably set.Under the condition of ensuring sufficient welding power, gradually increase the welding speed and apply appropriate extrusion force, so that when the extrusion amount reaches 2/3 times the wall thickness, the impact toughness of the weld can be appropriately improved.
4. The influence of heat treatment process
The microstructure formed by ERW welding just after welding is relatively coarse and the grain size is small. After heat treatment, the grain size is refined, the microstructure distribution is more uniform, and the impact toughness of the weld is improved.At the same time, the metallographic structure of the weld also directly affects the impact toughness. After the weld is heated by heat treatment, it must go through a long-distance air cooling, and the steel pipe must be water-cooled before entering the sizing. If the starting temperature of water cooling is too high, a bainite structure will be formed, reducing the impact toughness of the weld.
Improvement measures to improve the impact toughness of ERW steel pipe welds
1. The raw materials should be steel with uniform and fine grains and high purity.
2. During the molding process, the edges of the plate should be kept parallel, and the extrusion force should be uniform.
3. Welding process control: the size of the opening angle is preferably 3-4, comprehensively adjust the welding power, welding speed and extrusion force,In order to make the extrusion amount reach 2/3 times the wall thickness, and form a reasonable metal streamline shape.
4. For different grades of steel, the heat treatment temperature should be determined according to the carbon content and wall thickness, corresponding to the iron-carbon phase diagram to determine Ac3 and heating temperature, and make appropriate adjustments to optimize the structure.
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