Before formal welding work begins, the welded joint area of the thick steel plate must be preheated.
During welding, cracks may occur in the welding area due to rapid local heating and cooling.
Preheating is a process to slow down the rapid heating and cooling process in the welding area.
Joints with high restraint force can reduce the shrinkage stress after preheating, and preheating can also eliminate moisture in the welding area, eliminating the source of hydrogen gas.
When welding, the appropriate preheating temperature should be selected according to the ambient temperature of the workplace, the material and thickness of the steel, and preheat the welding.
When there are no special requirements, the preheating temperature can be selected according to Table 1.
Table 1 Commonly used preheating temperature.
| Steel Classification | room temperature | Plate thickness (mm) | Preheating and interlayer temperature control (℃) |
| Common Carbon Structural Steel | Above 0℃ | ≥50 | 70~100 |
| Low Alloy Structural Steel | Above 0℃ | ≥36 | 70~100 |
For components that require preheating, preheating must be carried out evenly within a range of 100 mm on both sides of the weld line before welding. The preheating temperature measurement must be made 50 mm from the weld line.
Table 2 Required preheating conditions for different thicknesses of steel materials.
| Low carbon steel components | Low Carbon Steel Pipe Components | Components Q345,16Mnq 15MnV, 15MnVg | |||
| Steel thickness (mm) |
Temperature below (℃) |
Steel tube wall thickness (mm) |
Temperature below (℃) |
Steel thickness (mm) |
Temperature below (℃) |
| ≤30 | -30 | ≤16 | -30 | ≤10 | -26 |
| 31~50 | -10 | 17~30 | -20 | 10~16 | 10 |
| 0 | 16~24 | -5 | |||
| 51~70 | 0 | 31~40 | -10 | 25~40 | 0 |
| 41~50 | 0 | Over 40 | Any temperature | ||
When the ambient temperature in the workplace is below 0°C, the preheating temperature of the welding component must be determined by testing.
Table 3 Preheating temperature required for different steel materials.
| Carbon Content in Carbon Steel (%) |
Preheating temperature (℃) |
light alloy steel | Preheating temperature (℃) |
| <0.20 | Not preheating | ||
| 0.20~0.30 | <100 | ||
| Carbon Content in Carbon Steel (%) |
Preheating temperature (℃) |
light alloy steel | Preheating temperature (℃) |
| 0.30~0.45 | 100~200 | 100~150 | |
| 0.45~0.80 | 200~400 |
When welding in an environment where the temperature is below 0°C, low carbon steel must also be preheated.
Table 2 shows the conditions for preheating thick steel sheets. Table 3 is the temperature that needs to be preheated.
