Knowledge of post-weld heat treatment

1. The welding residual stress is caused by the uneven temperature distribution of the weldment caused by welding, the thermal expansion and contraction of the weld metal, etc., so residual stress will inevitably occur with the welding construction.

The most common method to eliminate residual stress is high temperature tempering, that is, the weldment is heated to a certain temperature and held for a certain period of time in a heat treatment furnace, and the yield limit of the material is reduced at high temperature to cause plastic flow in the place with high internal stress. The elastic deformation gradually decreases, and the plastic deformation gradually increases and the stress decreases.

The effect of post-weld heat treatment on the tensile strength and creep limit of metals is related to the heat treatment temperature and holding time. The effect of post-weld heat treatment on the impact toughness of weld metal varies with different steel grades.

2. The choice of heat treatment method

Post-weld heat treatment generally selects single high temperature tempering or normalizing plus high temperature tempering treatment. For gas welding joints, normalizing and high temperature tempering heat treatment are used. This is because the grains of the weld seam and heat-affected zone of gas welding are coarse, and the grains need to be refined, so normalizing treatment is adopted. However, single normalizing cannot eliminate residual stress, so high temperature tempering is required to eliminate stress. A single medium-temperature tempering is only suitable for the assembly welding of large-scale ordinary low-carbon steel containers assembled on the site, and its purpose is to achieve partial elimination of residual stress and hydrogen removal. In most cases, a single high temperature tempering is used. The heating and cooling of the heat treatment should not be too fast, and strive to make the inner and outer walls uniform.

3. Heating method of post-weld heat treatment

⑴Induction heating. The steel generates an induced electric potential in the alternating magnetic field, and the steel generates heat due to the effect of eddy current and hysteresis, that is, induction heating. Nowadays, power frequency induction heating with simple equipment is mostly used in engineering.

⑵Radiation heating. Radiant heating uses a heat source to radiate heat to the metal surface, and then conduct the heat to other directions from the metal surface. Therefore, the temperature difference between the inner and outer walls of the metal is large during radiant heating, and the heating effect is worse than that of induction heating. Radiation heating commonly used flame heating method, resistance furnace heating method, infrared heating method.