Purpose and method of heat treatment of welded steel pipe

The welding residual stress is caused by the uneven temperature distribution of the welded steel tube caused by welding, the thermal expansion and contraction of the weld metal, etc., so the residual stress will inevitably be generated along with the welding construction. The most common method of relieving residual stress is high temperature tempering. It is to heat the welded pipe in a heat treatment furnace to a certain temperature and hold it for a certain period of time, and use the reduction of the yield limit of the material at high temperature to cause plastic flow in places with high internal stress, gradually reduce elastic deformation, and gradually increase plastic deformation to reduce stress. .

The effect of post-weld heat treatment on the tensile strength and creep limit of metal 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 types.

1. Purpose of post-weld heat treatment

(1) Relax welding residual stress;
(2) Stabilize the shape and size of the structure and reduce distortion;
(3) Improve the properties of base metal and welded joints;
(4) Improve the ability to resist stress corrosion;
(5) Further release harmful gases in the weld metal, especially hydrogen, to prevent the occurrence of delayed cracks.

2. Selection of heat treatment method

Post-weld heat treatment generally adopts single high-temperature tempering or normalizing plus high-temperature tempering. For gas welding joints, normalizing and high temperature tempering are adopted. This is because the grains of the gas welding seam and the heat-affected zone 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 and welding of large-scale ordinary low-carbon steel containers assembled on site, and its purpose is to achieve partial elimination of residual stress and dehydrogenation. In most cases, a single high temperature tempering is used. The heating and cooling of the heat treatment should not be too fast, and the inner and outer walls should be uniform.

Article source: https://article-realm.com/article/Finance/33450-Purpose-and-method-of-heat-treatment-of-welded-steel-pipe.html