In the manufacturing process of seamless steel pipes, it will be affected by various factors such as processes.When these factors disappear, if the above-mentioned effects and effects on the seamless steel pipe cannot be completely eliminated, and some of the effects and effects remain in the seamless steel pipe, such residual effects and effects are called residual stress.
Classification of residual stress:
1. According to the causes of stress, it is divided into:
(1) Thermal stress
(2) Phase transformation stress
(3) Shrinkage stress (also called mechanical resistance stress)
2. According to the different range of residual stress balance, it can be divided into:
(1) Macroscopic residual stress
(2) Microscopic residual stress
(3) Lattice distortion.
Removal of residual stress
1. Hammering method, using a steel hammer to hammer the part where the residual stress of the seamless steel pipe is concentrated, so that the metal surface receiving the hammering is subjected to the compressive stress of the hammering, and local plastic deformation occurs.Thereby reducing the peak value of residual stress, improving and balancing the distribution of the original residual stress of the workpiece, and avoiding brittle failure of the steel pipe.
2. The vibration aging method uses special equipment to make the seamless steel pipe resonate under the periodic external force of the special equipment, so that the microstructure and grains inside the steel pipe slip and twinkle.In this way, the peak value of residual stress is reduced, and the original residual stress distribution of the seamless steel pipe is improved and balanced. This method can remove about 50% of the residual stress or cut the peak value of the residual stress by about 50% in one hour.This is one of the most commonly used methods, with high processing efficiency and cost savings, but the biggest disadvantage is that the residual stress accumulated in the steel pipe cannot be completely eliminated.
3. Heat treatment aging is a traditional method to eliminate residual stress of seamless steel pipes, also known as artificial aging. With the help of heat treatment facilities, the seamless steel pipe is slowly and uniformly heated from room temperature to about 600 ° C, and kept at this temperature for 4-8 hours, and then the temperature is slowly cooled to below 120 ° C, and then cooled to room temperature. This method has a very good effect of eliminating residual stress, and the elimination speed is fast and sufficient. It is one of the most practical methods so far.
4. Natural aging is to place the seamless steel pipe outdoors to release and eliminate residual stress during the standing process. This method is not suitable for industrial mass-produced products. However, for the key components of high-value and high-precision equipment, the method of artificial aging + natural aging is more common
5. The welding stress is eliminated, and the temperature of the weld increases rapidly during welding, and the volume expands. The low temperature in the heat-affected zone hinders the expansion of the weld, resulting in compressive stress at the weld and tensile stress in the heat-affected zone. But at this time, the weld is in a plastic state, and the weld is thickened by the compressive stress, which relaxes the stress. When cooling after welding, the heat-affected zone has a fast cooling rate and quickly enters an elastic state, and the temperature at the weld is high and is in a plastic state. At this time, the weld shrinks, which is slower than that of the heat-affected zone. The weld hinders the shrinkage of the heat-affected zone. The weld is still under compressive stress, and the affected zone is under tensile stress. However, the weld is in a plastic state, and the plastic pier of the weld is thick, which relaxes this stress.
6. The method of stress relief in machining. After cutting the seamless steel pipe, some treatment measures can be taken, and the residual stress of the machined surface can also be adjusted. Surface strengthening treatment is one of the more commonly used methods. The surface strengthening treatment process is a process in which cold plastic deformation occurs on the surface of the part by cold extrusion, thereby improving its surface hardness and strength, and forming surface residual compressive stress. Commonly used surface strengthening processes are shot peening and rolling strengthening. Shot peening is to use a large number of beads in high-speed motion to impact the surface of the part, so that plastic deformation and plastic flow occur at the impact point, and a chilled layer and residual compressive stress are generated on the surface. Most of the bead shot is made of steel shot, which is sprayed by compressed air or centrifugal force.
7. Prestressed cutting is a method of generating residual compressive stress on the surface through the cutting process, that is, prestressing the steel pipe with a prestress within an elastic range before cutting.During the cutting process, the machined surface of the steel pipe will produce elastic deformation, and the prestress will be released after cutting. Due to the elastic recovery of the matrix, the machined surface will generate residual compressive stress.
Classification of residual stress:
1. According to the causes of stress, it is divided into:
(1) Thermal stress
(2) Phase transformation stress
(3) Shrinkage stress (also called mechanical resistance stress)
2. According to the different range of residual stress balance, it can be divided into:
(1) Macroscopic residual stress
(2) Microscopic residual stress
(3) Lattice distortion.
Removal of residual stress
1. Hammering method, using a steel hammer to hammer the part where the residual stress of the seamless steel pipe is concentrated, so that the metal surface receiving the hammering is subjected to the compressive stress of the hammering, and local plastic deformation occurs.Thereby reducing the peak value of residual stress, improving and balancing the distribution of the original residual stress of the workpiece, and avoiding brittle failure of the steel pipe.
2. The vibration aging method uses special equipment to make the seamless steel pipe resonate under the periodic external force of the special equipment, so that the microstructure and grains inside the steel pipe slip and twinkle.In this way, the peak value of residual stress is reduced, and the original residual stress distribution of the seamless steel pipe is improved and balanced. This method can remove about 50% of the residual stress or cut the peak value of the residual stress by about 50% in one hour.This is one of the most commonly used methods, with high processing efficiency and cost savings, but the biggest disadvantage is that the residual stress accumulated in the steel pipe cannot be completely eliminated.
3. Heat treatment aging is a traditional method to eliminate residual stress of seamless steel pipes, also known as artificial aging. With the help of heat treatment facilities, the seamless steel pipe is slowly and uniformly heated from room temperature to about 600 ° C, and kept at this temperature for 4-8 hours, and then the temperature is slowly cooled to below 120 ° C, and then cooled to room temperature. This method has a very good effect of eliminating residual stress, and the elimination speed is fast and sufficient. It is one of the most practical methods so far.
4. Natural aging is to place the seamless steel pipe outdoors to release and eliminate residual stress during the standing process. This method is not suitable for industrial mass-produced products. However, for the key components of high-value and high-precision equipment, the method of artificial aging + natural aging is more common
5. The welding stress is eliminated, and the temperature of the weld increases rapidly during welding, and the volume expands. The low temperature in the heat-affected zone hinders the expansion of the weld, resulting in compressive stress at the weld and tensile stress in the heat-affected zone. But at this time, the weld is in a plastic state, and the weld is thickened by the compressive stress, which relaxes the stress. When cooling after welding, the heat-affected zone has a fast cooling rate and quickly enters an elastic state, and the temperature at the weld is high and is in a plastic state. At this time, the weld shrinks, which is slower than that of the heat-affected zone. The weld hinders the shrinkage of the heat-affected zone. The weld is still under compressive stress, and the affected zone is under tensile stress. However, the weld is in a plastic state, and the plastic pier of the weld is thick, which relaxes this stress.
6. The method of stress relief in machining. After cutting the seamless steel pipe, some treatment measures can be taken, and the residual stress of the machined surface can also be adjusted. Surface strengthening treatment is one of the more commonly used methods. The surface strengthening treatment process is a process in which cold plastic deformation occurs on the surface of the part by cold extrusion, thereby improving its surface hardness and strength, and forming surface residual compressive stress. Commonly used surface strengthening processes are shot peening and rolling strengthening. Shot peening is to use a large number of beads in high-speed motion to impact the surface of the part, so that plastic deformation and plastic flow occur at the impact point, and a chilled layer and residual compressive stress are generated on the surface. Most of the bead shot is made of steel shot, which is sprayed by compressed air or centrifugal force.
7. Prestressed cutting is a method of generating residual compressive stress on the surface through the cutting process, that is, prestressing the steel pipe with a prestress within an elastic range before cutting.During the cutting process, the machined surface of the steel pipe will produce elastic deformation, and the prestress will be released after cutting. Due to the elastic recovery of the matrix, the machined surface will generate residual compressive stress.
