Fri, 03 Aug 2018 02:10:31 +0000

Key points of heat treatment process control for welded members

In order to ensure the performance and quality of welded parts, improve the plasticity of weld metal, eliminate or reduce welding residual stress and prevent cracks, heat treatment of welded parts is required when necessary. According to the sequence of heat treatment process and welding process, heat treatment of welding components is usually divided into preheat treatment, post-heat treatment and post-welding heat treatment.
1. Preheating treatment
The preheating treatment is to heat the structural parts in whole or in part before welding. The aim is to slow down the temperature gradient and cooling speed of the welding joint when it is heated. ℃ to 500? ℃, cooling time, to facilitate the escape of hydrogen in the weld, reduce the hardening organization, prevent cold cracks. In addition, the integral preheating of the component can improve the uniformity of temperature distribution and reduce internal stress.
The preheating temperature can be determined by empirical formula or by referring to relevant experimental data. It should be noted that these calculation formulas or experimental data are established under certain test conditions according to the minimum preheating temperature without cracks. Therefore, special attention should be paid to the application range when selecting them to avoid unnecessary losses. The high preheating temperature not only increases the cost, but also aggravates the working conditions of the welder. Therefore, the lower preheating temperature should be adopted as far as possible under the condition that the process requirements can be met. In the process of interlayer heating of multilayer welded members, the interlayer temperature should not be lower than the preheating temperature in order to prevent the occurrence of delayed cracks.
2. Post-heat treatment
The post-heat treatment is to use the preheating device to heat the welding parts or the welding area immediately after the welding stops and keep it for a certain time. The aim is to make the hydrogen in the weld metal escape fully and prevent the crack. In practical application, the effect of post-heat treatment on crack prevention is more obvious for low alloy steel with higher strength grade and welded members with larger thickness.
Choosing a reasonable post-heat treatment temperature can reduce the preheating temperature to a certain extent (generally by 50? Preheat ℃), or even cancelled. Reducing the preheating temperature can improve the working conditions of the workers, improve the production efficiency and reduce the energy consumption, and reduce the chance of thermal cracks caused by excessive preheating temperature.
Generally, the higher the post-heat treatment temperature, the shorter the thermal insulation time required. To avoid some common domestic low alloy high strength steel (including tensile strength at 800? Cracks are generated during post-heat treatment of tempered steel below MPa, and appropriate temperature and time of post-heat treatment should be selected (refer to the test data in the welding manual).
3. Heat treatment after welding
The post-welding heat treatment is to weld the component after the completion of welding, the whole or partial heat treatment. The purpose of this method is to improve the structure and properties of metal, reduce the residual stress of welding, soften the hardening structure of the affected area of welding heat, and improve the toughness and geometric stability of welding joints. According to different purposes of post-welding heat treatment, it can be divided into stress-removing heat treatment, post-welding annealing, normalizing, quenching, low-temperature stress removal, etc.
Heat treatment after welding can improve the toughness of low carbon steel, but it may reduce the toughness of some high strength steel. For high strength steel, if the heat treatment temperature exceeds the tempering temperature, the material will lose its original tempering effect, which will lead to the reduction of steel strength and toughness.
In addition, it is necessary to ensure the uniformity of heating temperature and strictly control the process of heat treatment when the welding components are heat treated as a whole. Otherwise, not only the residual stress cannot be reduced, but also the stress may be increased due to the uneven temperature.
Although the heat treatment process of welded components is complicated, qualified products can be produced as long as the purpose is clear, the method is appropriate and the adverse influencing factors are prevented.