Objective and method of post-weld heat treatment

Objective and method of post-weld heat treatment

The welding residual stress is caused by the uneven temperature distribution of welding parts and the thermal expansion and cold contraction of welding metal, so the welding construction will inevitably produce residual stress.

The most common method to eliminate the residual stress is high-temperature tempering, which is to heat the welding parts in the heat treatment furnace to a certain temperature and hold the heat for a certain period of time. By using the material to reduce the yield limit under high temperature, plastic flow is generated in the place with high internal stress, and the elastic deformation is gradually reduced, while the plastic deformation is gradually increased to reduce the stress.

The influence of post-weld heat treatment on tensile strength and creep limit of metal is related to the temperature and holding time of heat treatment.

The effect of post-weld heat treatment on the impact toughness of weld metal varies with different types of steel.

1. Purpose of post-welding heat treatment

1. Relaxation welding residual stress;

2. Stabilize the shape and size of the structure and reduce distortion;

3. Improve the performance of base material and welded joints;

4. Improve the ability to resist stress corrosion;

5. Further release harmful gases, especially hydrogen, in the weld metal to prevent the occurrence of delayed cracks.

2. Selection of heat treatment methods

Post-welding heat treatment generally adopts single high temperature tempering or normalizing and high temperature tempering treatment.

Normalizing and tempering heat treatment is used for gas welding joints.

This is because of the coarse grain of welding seam and heat-affected zone of gas welding, which needs to be refined, so normalizing treatment is adopted.

However, single normalizing cannot eliminate the residual stress, so high temperature tempering is needed to eliminate the stress.

The single medium tempering is only suitable for the assembly and welding of large common low carbon steel containers assembled on the site for the purpose of partially eliminating residual stress and dehydrogenation.

The vast majority of occasions is the use of a single high temperature tempering.

Heat treatment of heating and cooling should not be too fast, strive for internal and external wall uniformity.