Written by Jody Muelaner
Typically, thicker components require pre-heating but it depends on the material type.
Pre-heating components before welding can reduce the rate of cooling and result in stronger and more reliable welds. The slower cooling process provides more time for hydrogen to defuse from the weld affected zone, which reduces the risk of cracking.
Additionally, slower cooling produces microstructures in the weld metal and the heat-affected zone, which are more ductile and provide greater resistance to cracking. Shrinkage stresses may also be reduced.
For thicker parts, it is possible to achieve the ideal outcome with less heat applied during welding. The thicker the material being welded, the more the surrounding material will act as a heat sink, causing the weld to cool rapidly after the energy source is removed. This means that thick components are more likely to require pre-heating, but this is not the only criteria.
However, Alloy contents, diffusible hydrogen in the weld metal, and restraint around the weld are also important considerations. It is difficult to provide simple rules for the material thickness at which pre-heating is required, although rules-of-thumb are available for specific materials.
Pre-heating can be achieved using a number of heat sources. For smaller components, a furnace or oven can be used and this may provide the best process control.
For larger components, options include:
- Arrays of torches
- Electrical resistance heaters
- Induction heaters
- Radiation heaters
Process control is extremely important when pre-heating, to ensure the material is heated to the correct temperature and that it remains within the required temperature tolerance throughout the welding process. This involves three key process parameters:
1. Pre-heat temperature
2. Maximum interpass temperature
3. Minimum interpass temperature
Pre-heat is the temperature of the surfaces to be welded before welding commences. Maximum interpass is the maximum temperature at which subsequent weld runs are deposited, and it affects microstructural development. Minimum interpass is the minimum temperature at which subsequent weld runs are deposited, and it affects hydrogen cracking and should generally be close to the pre-heat temperature.
The minimum interpass temperature may also need to be maintained for a specified period of time after the weld is deposited to ensure sufficient hydrogen diffusion. Temperatures can be monitored using temperature indicating crayons, thermocouples, or contact thermometers.
Filed Under: Welding • soldering