Knowledge of the Application of Pre-Heat Treatment in Welding

Pre-heat treatment in welding is a process measure in which the entire or partial welded piece is heated to a specified temperature before welding. Important component welding, alloy steel welding, and thick component welding all require preheating before welding.

Effects of Pre-Heat Treatment in Welding

• Slows down the cooling rate after welding, effectively preventing cracks. Properly extending the cooling time within the 800~500℃ range facilitates the escape of diffusible hydrogen from the weld metal, avoiding hydrogen-induced cracks. It also reduces the hardness of the weld and heat-affected zone, improving the crack resistance of the welded joint.

  
  

• Reduces welding stress. Uniform local preheating or overall preheating can reduce the temperature difference (also known as the temperature gradient) between various parts of the welded workpiece. This not only lowers welding stress but also reduces the welding strain rate, which helps to prevent welding cracks.

  
  

• Reduces the constraint degree of the welding structure. Preheating is particularly effective in reducing the constraint degree of fillet weld joints. As the preheating temperature increases, the incidence of cracks decreases.

Requirements for Pre-Heat Treatment in Welding

Temperature of Pre-Heat Treatment in Welding

The preheating temperature should be determined based on the chemical composition of the base material, welding performance, thickness, the constraint degree of the welded joint, welding methods, welding environment, and relevant product technical standards. For important structures, the minimum preheating temperature at which no cracks occur should be determined through crack tests. 

The higher the preheating temperature, the better the effect of preventing cracks. However, exceeding the necessary preheating temperature can coarsen the metal grains near the fusion zone, reducing the quality of the welded joint and worsening working conditions.

Methods of Pre-Heat Treatment in Welding

Overall pre-heat treatment in welding is typically performed using various furnaces. However, in practical welding production processes, it is often difficult or even impossible to use overall preheating for many large structures, such as large spherical tanks and pipelines. Therefore, localized preheating is often used to prevent cracks. Localized heating typically uses gas flame heating or infrared heating. The preheating temperature is usually measured with a surface thermometer.

Precautions for Pre-Heat Treatment in Welding

• When welding different grades of steel together, the preheating temperature should be selected according to the steel grade requiring the higher preheating temperature.

  
  

• When adopting localized pre-heat treatment in welding, care should be taken to prevent excessive local stress. The preheating range should be at least three times the thickness of the welded piece on each side of the weld, and not less than 100mm.

  
  

• The temperature of the welded pieces that require preheating should not be lower than the preheating temperature throughout the welding process.

  
  

• When cutting, beveling, cleaning roots, grooving, or welding temporary seams using thermal processing methods, preheating requirements should also be considered.