What are the changes of seamless steel pipe after heat treatment

The seamless steel pipe is mostly produced by hot rolling and cold drawing process. In the cold drawing process, the seamless steel pipe often needs to be annealed. Seamless steel pipe after annealing treatment, there will be significant changes in microstructure, mechanical properties and subsequent processing properties, as follows:

1. Release of residual stress
During cold processing such as drawing and rolling, large residual stresses will accumulate inside the seamless steel pipe, which may lead to product deformation, dimensional instability and cracks in subsequent welding and use. After annealing treatment, most of these residual stresses in the steel pipe can be released, thereby reducing the risk of local deformation and cracking of the seamless steel pipe caused by stress concentration, and improving the overall stability and reliability of the pipe.

2. Recrystallization and adjustment of microstructure
During the annealing process, the high density dislocation and non-uniform grain structure inside the seamless steel pipe will recrystallize, which is manifested as:

Grain refinement or recombination: under the appropriate temperature and holding time, the grains originally deformed and elongated by cold processing will re-grow into more uniform and fine grains. The fine grain structure can significantly improve the toughness and ductility of the material.

Removal of work hardening: in the process of cold working, the seamless steel pipe will appear work hardening phenomenon, resulting in increased hardness and decreased plasticity. Annealing eliminates work hardening, reducing hardness and improving plasticity for subsequent bending, welding and other machining operations.

3. Adjustment of mechanical properties
Because annealing changes the microstructure of the steel, the mechanical properties undergo the following changes:

Hardness reduction: The hardness of the annealed seamless steel pipe is significantly reduced, which helps to improve the workability and weldability of the material, but at the same time, it is necessary to pay attention to the tensile strength and yield strength may be reduced.

Improved toughness and ductility: The annealing process can make the grain structure become more uniform, thereby improving the impact toughness and ductility of the material. This is especially important when secondary processing (such as bending and welding) is required.

Enhanced stability: The annealed seamless steel pipe has reached a more stable state during the heat treatment process, and the size and shape of the product remain more stable during use, reducing the risk of deformation caused by temperature changes or mechanical loads.

4. Improvement of welding performance
After annealing, due to internal residual stress release and grain structure adjustment, the welding performance of the steel pipe has been significantly improved:

Improved weld quality: When annealed steel pipe is welded, the welded joint is heated evenly, reducing the risk of cracking due to the heat affected zone and ensuring that the mechanical properties of the welded joint match the base material.
Reduce post-welding deformation: the deformation problem is easy to occur in the welding process, because the annealing has made the material stress uniform distribution, post-welding shrinkage and stress concentration phenomenon will be significantly reduced.

5. Dimensional stability and subsequent processability improvement
Annealing not only improves the mechanical properties of seamless steel pipes, but also makes the product more stable in size and shape:

Dimensional stability: Due to the release of internal stress, the steel pipe is not prone to dimensional deviations caused by temperature changes or long-term service after annealing, and meets the strict engineering tolerances.
Subsequent processability enhancement: The annealed material has better ductility and toughness, which is convenient for subsequent processing operations such as bending, punching, and cutting, reducing the risk of fracture during processing and improving processing efficiency.

Summary

Annealing treatment is an important process in the production of seamless steel pipe. By eliminating residual stress, adjusting microstructure, improving mechanical properties and improving weldability, the seamless steel pipe shows higher stability and safety under high temperature and high pressure and long-term service environment. For engineering projects, the annealed seamless steel pipe not only facilitates subsequent processing and installation, but also significantly improves the overall service life and reliability of the system.