High pressure boiler tube tempering method

Tempering is a heat treatment process in which a high-pressure boiler tube quenched into martensite is heated to a temperature below the critical point, held for an appropriate time, and then cooled to room temperature. 

After normalizing or quenching in the production process, high-pressure boiler tubes usually need to be tempered to improve their comprehensive mechanical properties, increase toughness and reduce residual stress. The following describes several common tempering methods:

Batch tempering
After normalizing or quenching the boiler tube is usually loaded into the tempering furnace, the furnace temperature is determined according to the steel used and design requirements, generally between 260°C and 370°C.
Heat preservation in the furnace for a period of time, so that the internal temperature of the pipe is uniform, and the residual stress and work hardening effect caused by quenching are fully eliminated.
Then through air cooling or air cooling, the pipe is slowly cooled to room temperature, so as to achieve the purpose of improving toughness and plasticity and reducing hardness.

Continuous tempering
In modern automated production lines, quenched boiler tubes go directly into the continuous tempering furnace.
The pipe passes through a preset tempering area on the conveyor belt, and the temperature, holding time and cooling rate in the furnace are controlled by the computer to achieve fully automated continuous processing.
Continuous tempering process can improve production efficiency, ensure that each pipe can obtain the same heat treatment effect, while reducing the uncertainty caused by human operation, suitable for large quantities, high stability requirements of product production.

Local tempering
For some special parts (such as welded joints or local stress concentration areas), the induction heating technology is used for local tempering.
Rapid local heating to the target temperature through induction heating, rapid cooling after heat preservation, mainly used to improve the toughness of welding heat affected zone and reduce local brittleness.

The main purpose of tempering treatment

Residual stress relief
During the annealing process, the internal stress generated by cold processing such as drawing and quenching is released to reduce the risk of deformation or cracking caused by stress concentration during the use of the pipe.

Improve microstructure
Through tempering, the internal grain of the steel is recrystallized and homogenized, which significantly improves the toughness and ductility of the material, reduces brittleness, and ensures long-term stable operation under high temperature and pressure.

Optimized mechanical properties
After tempering, the hardness of the pipe is moderately reduced, but the plasticity and impact resistance are improved, which is conducive to subsequent processing and welding operations, while ensuring that the pipe will not be brittle fracture due to excessive hardness in long-term service.

Improve welding quality
For welded areas, tempering can improve the performance of the welded joint, making it more compatible with the mechanical properties of the base material and preventing problems at the weld due to reduced heat-affected zone performance.

Conclusion

The tempering treatment of high-pressure boiler tube is a key heat treatment process. Through batch tempering, continuous tempering or local tempering, the residual stress can be effectively eliminated, the grain structure can be improved, the hardness can be reduced and the toughness can be improved, so that the pipe can show excellent deformation resistance and crack resistance under high temperature and high pressure conditions. Different tempering methods have different focuses, enterprises can choose the appropriate tempering process according to the specific product requirements and production scale, to ensure that the product meets the design standards and long-term safe operation requirements.