Preheat 1 Purpose of preheating Reduce the cooling rate […]
1 Purpose of preheating
Reduce the cooling rate after welding.
Reduce the tendency of hardening to prevent cracks.
Reducing the temperature difference in the heat-affected zone is conducive to reducing welding stress.
2 Scope of preheating application
When welding steels with a greater tendency to harden, preheating is required before welding; preheating cannot be performed when welding chromium-nickel austenitic stainless steels.
The selection of preheating should be based on comprehensive consideration of steel composition, thickness, structural rigidity, joint form, welding material, welding method and environmental factors, and determined through weldability experiments.
Heating range: Generally, a soaking area should be maintained within the range of 75～100mm on both sides of the groove. The temperature measurement point should be taken at the edge of the hot zone. The heating width on each side of the butt joint shall not be less than 5 times the plate thickness.
Post-heating (after welding, heat preservation and slow cooling of the weldment can slow down the cooling rate of the weld and heat-affected zone, and play the same role as preheating).
1. H elimination treatment: heat the weldment to 250-350 immediately after welding. C range, heat preservation for 2-6 hours, air cooling afterwards.
2. Purpose: To accelerate the escape of H in the weld metal, greatly reduce the H content in the weld and heat-affected zone, and prevent the occurrence of cold cracks.
3. Scope of application: If the weldment cannot be heat treated immediately and the weldment needs to be removed in time, the H must be eliminated.
Post-weld heat treatment
Meaning: a treatment method to heat the weldment as a whole or partly, and then furnace or air cool
Reduce welding residual stress.
Soften hardened parts.
Optimize the structure and performance of welds and heat effects.
Improve the plasticity and toughness of the joint.
Stabilize the size of the structure.
Commonly used post-weld heat treatment methods
Overall heating treatment: the welding parts are placed in a heating furnace for overall heating treatment, and satisfactory treatment results can be obtained. The temperature of the weldment should be 300 when entering and exiting the furnace. Below C, 300. The heating and cooling rates below C are related to the plate thickness. Should meet the following requirements:
For thick-walled vessels, the heating and cooling rate is 50～150℃/h, and the temperature difference at the top of the furnace should not exceed 50℃ during the overall treatment. If the weldment is too long and needs to be divided into two treatments, the overlapped heating part should be more than 1.5m.
Local heat treatment: For simple cylindrical containers and pipe fittings that are long in size and inconvenient for overall treatment, but relatively regular in shape, they can be treated locally. For local treatment, ensure that there is sufficient heating width on both sides of the weld. The heating width of the cylinder is related to the radius and wall thickness of the cylinder and is calculated by the following formula.