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Purpose of preheating
1. Reduce the cooling rate after welding.
2. Reduce hardening tendency to prevent cracks.
3. Reduce the temperature difference in the heat affected zone, which is beneficial to reduce welding stress.
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Range of preheating applications in Metal Welding Service
When welding steels with high hardening tendency, preheat them before welding. Do not preheat chromium-nickel austenitic stainless steel when welding.
The SELECTION of preHEATING SHOULD be based on the composition, thickness, structural rigidity, joint form, welding material, welding method and environmental factors, and through the stability test to determine.
Heating range: generally, a heating area should be maintained in the range of 75 ~ 100㎜ on both sides of the groove. The temperature measuring point should be taken at the edge of the hot area. The heating width on each side of the butt joint shall not be less than 5 times the plate thickness.
After the heat
(After welding, heat preservation and slow cooling of welds can slow down the cooling speed of welds and heat-affected zones, and play the same role as preheating)
1. H-elimination treatment: immediately after welding, heat the weldment to 250 ~ 350. C range, heat preservation 2 ~ 6 hours, after air cooling.
2. Purpose: To accelerate the escape of H in the weld metal, greatly reduce the amount of H in the weld and heat affected zone, and prevent the generation of cold cracks.
3. Scope of application: if the weldment can not be heat-treated immediately and the weldment must be removed in time, it is necessary to eliminate H even if the treatment.
Post weld heat treatment
Meaning: Welding parts of the whole or local heating insulation, and then furnace cooling or air cooling a treatment method
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role
1. Reduce welding residual stress;
2. Softening and hardening parts;
3. Improve the microstructure and properties of welds and heat effects;
4. Improve the plasticity and toughness of the joint;
5. Size of stable structure.
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Post welding heat treatment is commonly used
Overall heating treatment: the weldment placed in the heating furnace overall heating treatment, can get a satisfactory treatment effect. The temperature of welding parts entering the furnace and coming out should be 300. C below 300. Heating and cooling rates below C depend on plate thickness. The following requirements shall be met:
For thick-walled containers, the heating and cooling rates are 50 ~ 150℃ / h, and the maximum temperature difference in the furnace shall not exceed 50℃ during overall treatment. If the weldment is too long and needs to be divided into secondary treatment, the overlapping heating part should be more than 1.5m.
Local heat treatment: for long size inconvenient overall treatment, but the shape of the relatively regular simple cylindrical container, pipe fitting, can be local treatment. Adequate heating width on both sides of the weld should be ensured for local arrangement. The heating width of the cylinder is related to the radius and wall thickness of the cylinder, which can be calculated according to the following formula.
For example, for the local heat treatment of the cylinder girth weld with a diameter of 1200mm and a wall thickness of 24mm, it should be heated to the specified treatment temperature within the range of 600mm centered on the weld.
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Generally, post-welding heat treatment should be considered in the following cases:
1. Common low-alloy steel with higher strength grade of base metal and greater tendency of delayed cracking.
2. Pressure vessels and other welded structures working at low temperature, especially pressure vessels used below brittle transition temperature.
3. Common low-alloy steel with higher strength grade of base metal and greater tendency of delayed cracking.
4. Pressure vessels and other welded structures working at low temperature, especially pressure vessels used below the brittle transition temperature.
5. Components that work under alternating loads and require fatigue strength.
6. Large pressurized container.
7. Welding structure with stress corrosion and stable geometric dimensions required after welding.