Heating the base metal to a specific desired temperature before welding is called as preheating. The temperature at which the base metal is heated is called as the preheat temperature. Preheat temperature can be determined by the applicable code. Please see table 1 to know how to find the preheat temperature in the respective code:
In absence of code, the following factors shall be considered for determining the preheat temperature; these are:
- Base metal thickness
- Surrounding temperature
- Base metal composition
- Hydrogen content of electrode
- Operational requirements
However, in general preheat is required above 25 mm thickness, but other factors shall always be considered before determining the preheat temperature.
Why preheating is required?
Preheating is carried out for the following reasons;
- It slows down the cooling rate of weld metal, HAZ (heat affected zone) and adjacent base metals, which yields a good microstructure to the metal, prevents martensite formation at microstructural level and prevents from cracking of the weld metal and HAZ.
- Preheating removes the diffusible Hydrogen from the base metal and hence prevents the chances of Hydrogen induced cracking (HIC).
- It helps in reducing the expansion and contraction rate.
- It Burns the unwanted material or impurities (if any) present on the joint surface.
- Preheating also helps in achieving better mechanical properties such as notch toughness.
Oxy-fuel heating torches, Induction heaters, Infrared radiant pane heaters and electrical heaters are commonly used for preaheating a joint. It is recommended to preheat the joint from back side (other side), to ensure that the entire volume of metal, surrounding the joint, has been heated.
Digital infrared pyrometers (Figure 1) or thermal chalks (Figure 2) can be used for checking the temperature of the preheated item.
Once preheating is done, welding shall be started immediately and if metal thickness is very high or if surrounding temperature is very less or otherwise, temperature should be checked during the welding also. The interpass temperature shall also be maintained, in order to continue the subsequent weld passes.
Please watch the video (given below) for a better understanding on Preheating;
Also read: Welding Symbols
Also read: Visual inspection of weld joint
Also read: CSWIP preparatory questions