Importance of post weld heat treatment for welded joints

Post weld heat treatment can eliminate residual stress, prevent deformation, improve the performance of base metal and welded components, soften heat affected zone, increase weld ductility, improve toughness, discharge harmful hydrogen, improve creep performance and fatigue strength.

However, improper selection of post weld heat treatment process will reduce the performance of welded joints. Therefore, post weld heat treatment has become an important link in pressure vessel manufacturing. High temperature tempering, normalizing and solid melting treatment are widely used heat treatment methods for post weld welded joints. High temperature tempering can solve the adverse impact of welding and deformation on the quality of pressure vessels.

1.Post weld heat treatment can eliminate welding residual stress

With the increase of heat treatment temperature and holding time, the residual stress is reduced. When the temperature rises above 550 ℃, the residual stress can be considered to be completely eliminated, but the effect of holding time is not as obvious as that of temperature rise

2.Hardening and softening of heat affected zone of welded joint

Due to the reduction of residual stress, the plasticity and toughness are improved, the hardenability is reduced, and the hardened area of the joint is softened.

3.Hydrogen reduction in welded joints

During heat treatment, the temperature of the welded joint increases and hydrogen escapes. When it is heated below 300 ℃ and maintained for 2-4 hours, hydrogen can be removed. When it is heated to 550 ℃ - 650 ℃, hydrogen can be removed.

4.Influence on tensile strength of weld metal

The influence of weld metal tensile degree is related to heat treatment temperature and holding time. The higher the temperature and holding time, the lower the tensile strength, the higher the alloy content and the greater the strength reduction rate.

5.Impact toughness of weld metal

Excessive heat treatment will reduce the impact value of steel. Appropriate post weld heat treatment can improve the impact toughness. For some high-strength steels, the impact value will decrease after heat treatment. For carbon steel and Mn NB Ni steel, the impact value will change little after post weld heat treatment.

6. Influence on decarburization layer width

The higher the heat treatment temperature, the longer the holding time, and the greater the degree of decarburization, especially for dissimilar steel joints.

The microstructure obtained by tempering has good strength, elasticity, plasticity and toughness. Normalizing is to heat the weldment to 30 ℃ - 50 ℃, take it out after heat preservation and cool it in the air, so as to improve the microstructure and refine the grain. Single normalizing can not eliminate the residual stress after welding.

Solid solution heat treatment is to heat the steel to 920 ℃ - 1150 ℃, then cool it quickly, precipitate carbides at the grain boundary within 450 ℃ - 850 ℃, and then re melt into austenite to obtain uniform solid melt, eliminate intergranular corrosion and improve the corrosion resistance of the joint. The solid solution heat treatment does not adopt local heating method, but should be uniformly heated as a whole.

Choosing appropriate post weld heat treatment process is conducive to achieve the expected effect of post weld heat treatment.