Heat treatment (heat treatment) before welding

Heat treatment (heat treatment) prior to welding is the process of heating the base metal covering all or part of the weld metal area, to a preheating temperature. It is possible to continue to use heating during the welding process, although the heat source temperature of the soldering iron is continuously connected to the desired temperature. The temperature between the welding layers is the temperature between the first and second welding layers. Finally, it can not be lowered under preheat temperature. Pre-healing heat can have many beneficial effects, but with complete ignorance can be a waste of money or, worse, weld damage.
Why heat (heat treatment) before welding?
There are four reasons to apply preheating:

1) Reducing the heat loss effect between the weld and the base metal. The metal structure is more malleable and can resist cracking.
2) The slow cooling rate gives the hydrogen time to diffuse out and not crack.
3) Reduce the shrinkage stress between the embankment and the base metal, which are especially important for high mechanical metals.
4) Reduce cracking during welding.
In addition, preheat heating can be used to assure specific mechanical properties, such as toughness.

When can heat (heat treatment) before welding be used?
Determination of whether or not a pre-heater is required requires a number of factors to be considered: Applicability criteria, base metal thickness, base metal chemical composition, ambient temperature cycles, stresses, hot cracking, and effects of hydrogen diffusion.
When standards are required, the minimum temperature required for heating with the base metal and the required thickness is given. This minimum value must be met with any change in the chemical composition and restriction encountered. And it can increase if necessary.
   Generally heating prior to welding is not required with low carbon steel thickness less than 25mm. However, as carbon content increases, poor hydrogen diffusion, increased thickness, the need for preheating is increased.
What is the preheat temperature?
- Preheating temperature is usually determined by the minimum value to prevent cracking in all cases. For example, with ASTM A572 Gr50-A36 with a thickness of 100mm, the initial heating temperature is AWS D1.1 107 ° C but according to AISC LRFD J2. 8, the preheating temperature of the required material is 175 ° C

How pre-heater application?
- Depending on the material thickness, the size of the welded structures and equipment available to consider a selection method. For example, with a small-sized soldering iron that can be used in a kiln, large-sized materials use a gas-heater or heat-treatment device.
The high accuracy of preheating temperature is not required.
When heated with welding joints, the AWS D1.1 standard specifies that the minimum heating temperature must be set at a distance equal to the maximum thickness and not less than 75 mm in all directions from the welding point. . And preheating is recommended to apply on the opposite side of the solder to perform the test on the solder surface.

Conclude:
Preheating heat can prevent hot cracking, ensuring the mechanical properties of joints such as toughness,
Apply preheating heat when required by standard, basic material, thickness, ..
The preheating temperature also applies to specific standards and in-depth calculations.
Heating can be done with a gas heater or with a heat treatment device.