Generation of residual stress
The manufacture of welded steel tubes generally undergoes major processes such as uncoiling, straightening, trimming, forming, and welding, as well as a series of auxiliary processes such as flaw detection, flattening, and pressure testing. A part of the residual stress has been generated in the plate during the rolling process, that is, the residual stress already exists in the plate before the pipe is made. Processes such as decoiling, straightening, and trimming redistribute the existing residual stress in the sheet and regenerate a small part of the residual stress.
Influence of Residual Stress on Steel Pipe Properties
1. Reduce the load capacity of the steel pipe
2. Affect the fatigue strength of the steel pipe
3. Residual stress leads to stress corrosion cracking
Method for Relief of Residual Stress
In addition to minimizing the generation of residual stress during the welded pipe manufacturing process, various methods can be used to eliminate the generated residual stress. The method to be used should be selected according to the requirements of the welded pipe through economic and technical comparison.
1. Natural aging to eliminate residual stress
This method is to use the effect of nature, mainly the effect of temperature difference, to slowly release the residual stress in the welded pipe. Its advantage is that it is simple and easy to operate, and the cost is low. Its disadvantage is that it takes a long time, takes up product funds for a long time, and its effect is not obvious in a short period of time.
2. Overall high temperature tempering
This is generally to heat the welded pipe to a certain temperature and then keep it warm for a period of time and then gradually cool it down. The effect of eliminating residual stress depends on the heating temperature, holding time, cooling speed, etc. If the tempering parameters are selected properly in this method, the effect of eliminating residual stress is more significant. However, the overall high-temperature tempering is expensive, and it is difficult to realize on the production line. Unless there are special requirements, it is generally not suitable for use.
3. Local heating
The effect of this method is not as ideal as that of overall heating, it can only reduce the peak value of residual stress, but not completely eliminate it. The effect of relieving residual stress is related to the temperature gradient, which in turn is related to the heating range.
4. Mechanical stretching method (overload method)
Each steel pipe must be subjected to a hydrostatic test. Generally, the pressure is maintained for a certain period of time after being pressurized. The purpose is to test the quality of the welded pipe, and on the other hand, it can also eliminate part of the residual stress.
5. Other methods
Some welded pipe manufacturers use internal control forming instead of traditional external control forming, and the inner wall of the welded pipe changes from tensile stress to compressive stress. The overall residual stress level is reduced. In addition, when performing anticorrosion treatment on welded pipes, sandblasting is used to remove rust, and part of the residual stress can also be eliminated while removing rust.
The manufacture of welded steel tubes generally undergoes major processes such as uncoiling, straightening, trimming, forming, and welding, as well as a series of auxiliary processes such as flaw detection, flattening, and pressure testing. A part of the residual stress has been generated in the plate during the rolling process, that is, the residual stress already exists in the plate before the pipe is made. Processes such as decoiling, straightening, and trimming redistribute the existing residual stress in the sheet and regenerate a small part of the residual stress.
Influence of Residual Stress on Steel Pipe Properties
1. Reduce the load capacity of the steel pipe
2. Affect the fatigue strength of the steel pipe
3. Residual stress leads to stress corrosion cracking
Method for Relief of Residual Stress
In addition to minimizing the generation of residual stress during the welded pipe manufacturing process, various methods can be used to eliminate the generated residual stress. The method to be used should be selected according to the requirements of the welded pipe through economic and technical comparison.
1. Natural aging to eliminate residual stress
This method is to use the effect of nature, mainly the effect of temperature difference, to slowly release the residual stress in the welded pipe. Its advantage is that it is simple and easy to operate, and the cost is low. Its disadvantage is that it takes a long time, takes up product funds for a long time, and its effect is not obvious in a short period of time.
2. Overall high temperature tempering
This is generally to heat the welded pipe to a certain temperature and then keep it warm for a period of time and then gradually cool it down. The effect of eliminating residual stress depends on the heating temperature, holding time, cooling speed, etc. If the tempering parameters are selected properly in this method, the effect of eliminating residual stress is more significant. However, the overall high-temperature tempering is expensive, and it is difficult to realize on the production line. Unless there are special requirements, it is generally not suitable for use.
3. Local heating
The effect of this method is not as ideal as that of overall heating, it can only reduce the peak value of residual stress, but not completely eliminate it. The effect of relieving residual stress is related to the temperature gradient, which in turn is related to the heating range.
4. Mechanical stretching method (overload method)
Each steel pipe must be subjected to a hydrostatic test. Generally, the pressure is maintained for a certain period of time after being pressurized. The purpose is to test the quality of the welded pipe, and on the other hand, it can also eliminate part of the residual stress.
5. Other methods
Some welded pipe manufacturers use internal control forming instead of traditional external control forming, and the inner wall of the welded pipe changes from tensile stress to compressive stress. The overall residual stress level is reduced. In addition, when performing anticorrosion treatment on welded pipes, sandblasting is used to remove rust, and part of the residual stress can also be eliminated while removing rust.
