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Pre Heat
Preheating minimizes the temperature difference between the welding arc and the base material. This benefits the weldment in several ways.
1.it helps to lessen shrinkage stresses that can lead to cracking and distortion.
2. Proper preheating helps to slow the cooling rate of the finished weld and reduce hardness in the heat-affected zone (HAZ), which creates a weld that is less brittle and more ductile.
3.Preheating introduces the necessary heat into the weld area to ensure proper penetration. This benefits thick materials and those that conduct heat quickly. By preheating, you can use less heat in the welding arc and still achieve optimal penetration, because the base material starts out at an elevated temperature.
To assist in the cooling rate of the area that will be welded and be under extreme tempertures. This may be required to avoid cracking in the weld area.
Preheating increases depending on the schedule of the material (EG: Pipe & fittings). Distortion due to extreme heat can occur, so preheating required to avoid or minimize accordingly.
It also increases the carboncontent of the steel and diffusable content of th hydrogen in the welded metal.
preheating for welding depend on material type and thickness;
such steels are susceptible to crack because of creating thermal stress. note to thermal expansion coefficient of materials.
another case is the thickness that is more important. if steel thickness is great, the difference in temperature between center and surface caused thermal stress. if thermal stress is greater than UTS strength, the crack would be created and would fail in the material.
There are two mains reasons for Preheating.
1-To remove moisture from the weld areas which can avoid welding defects like porosity & cracking.Since hydrogen is introduced during the welding process.
2- To reduce the thermal gradient which is caused due to sudden contact of a localised heat source with the cool base metal which is being welded.
because sometimes the welding rod are wet thats way ned to pre heat , then start to weld. or they ned oben to put their the welding rod andthey ready to used they are all heat.
1.preheat was done to remove the moisture inthe base material.
2.Preheat was doing to slow down the cooling rate so hydrogen will be less escaped so hydrogen cracking can be avoided.and and reduce the hardness rate in haz
Preheating is the process applied to raise the temperature of the parent steel before welding. It is used for the following main reasons: The slower cooling rate encourages hydrogen diffusion from the weld area by extending the time period over which it is at elevated temperature (particularly the time at temperatures above approximately 100°C) at which temperatures hydrogen diffusion rates are significantly higher than at ambient temperature. The reduction in hydrogen reduces the risk of cracking.
To slow the cooling rate of the weld and the base material, potentially resulting in softer weld metal and heat affected zone microstructures with a greater resistance to fabrication hydrogen cracking.
to avoid cold cracking and slow down the cooling rate. preheat process is related to the thickness of weld metal and the material type itself.
Because the base material starts out at an elevated temperature.
By preheating , you can use less heat in the welding are and still achieve optimal penetration, becouse the base metarial starts out at an elevated temperature.
to slower the cooling rate of the base materials and the weld.
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