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The importance of preheating and postwelding heat treatment
Add Date:2022/1/17 Views: 98
1. Preheating before welding
Preheating and post-welding heat treatment are very important to ensure the welding quality. Welding of important components, alloy steel and thick parts requires preheating before welding.
The main functions of preheating before welding are as follows:
1. Preheating can slow down the cooling rate after welding, which is conducive to the escape of diffusible hydrogen in the weld metal and avoids hydrogen-induced cracks. At the same time, the degree of hardening of the weld and the heat-affected zone is reduced, and the crack resistance of the welded joint is improved.
2. Preheating can reduce welding stress. Uniform local preheating or overall preheating can reduce the temperature difference (also known as temperature gradient) between the workpieces to be welded in the welding area. In this way, on the one hand, the welding stress is reduced, and on the other hand, the welding strain rate is reduced, which is beneficial to avoid welding cracks.
3. Preheating can reduce the restraint of the welded structure, especially the restraint of the fillet joint. With the increase of the preheating temperature, the incidence of cracks decreases. The selection of preheating temperature and interpass temperature is not only related to the chemical composition of the steel and electrode, but also to the rigidity of the welded structure, welding method, ambient temperature, etc., which should be determined after comprehensive consideration of these factors. In addition, the uniformity of the preheating temperature in the thickness direction of the steel sheet and the weld zone has an important influence on reducing the welding stress. The width of local preheating should be determined according to the restraint of the workpiece to be welded. Generally, it should be three times the wall thickness around the weld area, and should not be less than 150-200 mm. If the preheating is not uniform, instead of reducing the welding stress, it will increase the welding stress.
2. Post-weld heat treatment
Post-weld heat treatment refers to a heat treatment method in which the whole or part of the weldment is heated and kept warm, and then furnace cooled or air cooled. Its purpose is to eliminate hydrogen, eliminate welding stress, improve weld structure and comprehensive performance.
1. Post-weld hydrogen elimination treatment refers to the low-temperature heat treatment performed after the welding is completed and the weld has not been cooled to below 100 °C. The general specification is to heat to 200～350℃ and keep it for 2-6 hours. The main function of post-weld hydrogen elimination treatment is to accelerate the escape of hydrogen in the weld and heat-affected zone, which is extremely effective in preventing welding cracks during welding of low-alloy steels.
2. During the welding process, due to the non-uniformity of heating and cooling, and the restraint or external restraint of the component itself, welding stress will always be generated in the component after the welding work is completed. The existence of welding stress in the component will reduce the actual bearing capacity of the welded joint area, cause plastic deformation, and even lead to the damage of the component in severe cases.
Stress relief heat treatment is to reduce the yield strength of the welded workpiece at high temperature to achieve the purpose of relaxing the welding stress. There are two commonly used methods: one is the overall high temperature tempering, that is, the whole weldment is put into the heating furnace, slowly heated to a certain temperature, then kept for a period of time, and finally cooled in the air or in the furnace. In this way, 80%-90% of welding stress can be eliminated. Another method is local high-temperature tempering, that is, only heating the weld and its surrounding area, and then slowly cooling, reducing the peak value of the welding stress, making the stress distribution relatively flat, and partially eliminating the welding stress.
3. After some alloy steel materials are welded, the welded joints will appear hardened structure, which will deteriorate the mechanical properties of the material.
In addition, this hardened structure may lead to the destruction of the joint under the action of welding stress and hydrogen. After heat treatment, the metallographic structure of the joint is improved, the plasticity and toughness of the welded joint are improved, and the comprehensive mechanical properties of the welded joint are improved.
Preheating and post-welding heat treatment are very important to ensure the welding quality. Welding of important components, alloy steel and thick parts requires preheating before welding.
The main functions of preheating before welding are as follows:
1. Preheating can slow down the cooling rate after welding, which is conducive to the escape of diffusible hydrogen in the weld metal and avoids hydrogen-induced cracks. At the same time, the degree of hardening of the weld and the heat-affected zone is reduced, and the crack resistance of the welded joint is improved.
2. Preheating can reduce welding stress. Uniform local preheating or overall preheating can reduce the temperature difference (also known as temperature gradient) between the workpieces to be welded in the welding area. In this way, on the one hand, the welding stress is reduced, and on the other hand, the welding strain rate is reduced, which is beneficial to avoid welding cracks.
3. Preheating can reduce the restraint of the welded structure, especially the restraint of the fillet joint. With the increase of the preheating temperature, the incidence of cracks decreases. The selection of preheating temperature and interpass temperature is not only related to the chemical composition of the steel and electrode, but also to the rigidity of the welded structure, welding method, ambient temperature, etc., which should be determined after comprehensive consideration of these factors. In addition, the uniformity of the preheating temperature in the thickness direction of the steel sheet and the weld zone has an important influence on reducing the welding stress. The width of local preheating should be determined according to the restraint of the workpiece to be welded. Generally, it should be three times the wall thickness around the weld area, and should not be less than 150-200 mm. If the preheating is not uniform, instead of reducing the welding stress, it will increase the welding stress.
2. Post-weld heat treatment
Post-weld heat treatment refers to a heat treatment method in which the whole or part of the weldment is heated and kept warm, and then furnace cooled or air cooled. Its purpose is to eliminate hydrogen, eliminate welding stress, improve weld structure and comprehensive performance.
1. Post-weld hydrogen elimination treatment refers to the low-temperature heat treatment performed after the welding is completed and the weld has not been cooled to below 100 °C. The general specification is to heat to 200～350℃ and keep it for 2-6 hours. The main function of post-weld hydrogen elimination treatment is to accelerate the escape of hydrogen in the weld and heat-affected zone, which is extremely effective in preventing welding cracks during welding of low-alloy steels.
2. During the welding process, due to the non-uniformity of heating and cooling, and the restraint or external restraint of the component itself, welding stress will always be generated in the component after the welding work is completed. The existence of welding stress in the component will reduce the actual bearing capacity of the welded joint area, cause plastic deformation, and even lead to the damage of the component in severe cases.
Stress relief heat treatment is to reduce the yield strength of the welded workpiece at high temperature to achieve the purpose of relaxing the welding stress. There are two commonly used methods: one is the overall high temperature tempering, that is, the whole weldment is put into the heating furnace, slowly heated to a certain temperature, then kept for a period of time, and finally cooled in the air or in the furnace. In this way, 80%-90% of welding stress can be eliminated. Another method is local high-temperature tempering, that is, only heating the weld and its surrounding area, and then slowly cooling, reducing the peak value of the welding stress, making the stress distribution relatively flat, and partially eliminating the welding stress.
3. After some alloy steel materials are welded, the welded joints will appear hardened structure, which will deteriorate the mechanical properties of the material.
In addition, this hardened structure may lead to the destruction of the joint under the action of welding stress and hydrogen. After heat treatment, the metallographic structure of the joint is improved, the plasticity and toughness of the welded joint are improved, and the comprehensive mechanical properties of the welded joint are improved.
