Things Welding Inspectors Should Know

When welding base materials, preheating and/or post-weld heat treatment may be necessary. These thermal treatments are generally required to ensure suitable weld integrity. They will typically prevent or remove undesirable characteristics in the completed weld. However, heat treatment is costly since it demands extra equipment, time, and handling. Heat treatment should only be undertaken after carefully considering its advantages. Heat treatment will be mandatory in some instances, as with heavy sections of low alloy steel. It will be a justifiable precaution against early failure in service in other cases.

There are several reasons for incorporating these thermal treatments within the welding procedure. Here are some of the most common reasons to consider.

Preheating

The heat applied to the base metal or substrate to meet and maintain preheat temperatures.” This is how the AWS Standard Welding Terms and Definition define preheating. “The temperature of the base metal in the volume surrounding the welding point immediately before starting.” This is how the AWS Standard Welding Terms and Definition define preheat temperatures.

You can perform preheating using:

  • Gas burners
  • Oxy-gas flames
  • Electric blankets
  • Induction heating
  • Heating in a furnace

For good results, it is essential for the heating to be uniform around the joint area. Intense, non-uniform heating is of little use in retarding cooling. It may be detrimental in causing: 

  • Higher residual stresses
  • Distortion
  • Undesirable metallurgical changes in the base material

The inspector should heat the weld joint evenly through the material thickness. It should heat to the desired minimum temperature specified when preheating. It is desirable to apply the heating sources to one side of the material surface and measure the temperature on the opposite side. To obtain a uniform temperature through the material thickness. The inspector should conduct the heating and temperature measurements from the same surface. They must ensure that more than just the material surface has had heat. It is crucial to ensure that the entire material thickness has heated to a uniform temperature. 

To establish a preheat temperature, they may need to consider an interpass temperature limitation. This information should be shown in the welding procedure specification. When they specify an interpass temperature, they must inspect the weld area before depositing the next weld bead. Welding may not continue if the measured temperature exceeds the maximum interpass conditions. The weldment must have time to cool down to the specified upper limit of the interpass temperature before continuing with the weld.

The welded component preheat and interpass temperature should go through an evaluation. Especially when it is dependent on the metallurgical properties of the material. Or if it is dependent on the desired mechanical properties.

 A procedure for welding mild steel, which has:

  • A low carbon content
  • Relatively low hardenability
  • Used in an application with no special service requirements

You may consider a minimum preheat and interpass temperature based on the material thickness. There will be specific minimum and maximum requirements for preheating and interpass temperatures. Especially when welding for heat-treatable low-alloy steel and chromium-molybdenum steel. These low alloy materials can have high hardenability and are susceptible to hydrogen cracking. Letting these materials cool too quickly or overheat can affect their performance requirements.

When welding nickel alloys, high heat input during the welding operation is concerning. The heat input of the welding process and the preheat and interpass temperature can seriously affect these materials. High heat input can result in:

  • Excessive constitutional liquation
  • Carbide precipitation
  • Other harmful metallurgical phenomena

In addition, these metallurgical changes may promote cracking or loss of corrosion resistance. Procedures for welding some aluminum alloys, such as: 

  • Heat-treatable 2xxx series
  • Heat-treatable 6xxx series
  • Heat-treatable 7xxx series

are often concerned with overall heat input reduction. 

With these materials, the maximum preheat and interpass temperature is in control. This is to minimize its annealing and over-aging influence on the heat-affected zone.


In critical applications, the preheat temperature must be precisely controlled. They can use controllable heating systems, and attach thermocouples to monitor the heated part. These thermocouples provide a signal to the controlling unit that can regulate the power source required for heating. Using this equipment, they can control the heated part extremely close.

Some of the reasons for preheating are

To Drive Away Moisture From the Weld Area

This is by heating the material’s surface to a low temperature, above the boiling point of water. This will dry the plate surface and remove the undesirable contaminants that may otherwise cause: 

  • Porosity
  • Hydrogen embrittlement
  • Cracking by introducing hydrogen during the welding process

To Lower the Thermal Gradient

All arc welding processes use a high-temperature heat source. A steep temperature differential occurs between the heat source and the welded cool base. This temperature difference causes: 

  • Differential thermal expansion
  • Contraction
  • High stresses around the welded area

They can reduce the temperature differential by preheating the base material. This will minimize distortion and excessive residual stress problems. If preheating is not carried out, a significant differential in temperature can occur. It will occur between the weld area and the parent material. This would lead to martensite formation and probable cracking when welding with high hardenability.

The Post weld Heat Treatment

They use several types of post-weld heat treatments for various reasons.

a) Post-weld heat treatment is most generally used for stress relief. The purpose of stress-relieving is to remove any internal or residual stresses that may be present from the welding operation. Stress relief after welding may be necessary. This will:

  • Reduce the risk of brittle fracture
  • Avoid subsequent distortion on machining
  • Eradicate the risk of stress corrosion

b) For some alloy steels, a thermal tempering treatment may be necessary to obtain a suitable metallurgical structure. This treatment is generally performed after the weld has cooled. Under certain circumstances, it may require completing it before it cooled to prevent cracking.

c) Coarse weld structures in steel, obtained with electro-slag welding, may require normalizing after welding. This treatment will:

  • Refine the coarse grain structure
  • Reduce stresses after welding
  • Remove any hard zones in the heat-affected zone

d) The precipitation-hardening alloys, are sometimes required to undergo post-weld heat treatment. This is to regain their original properties. They only use an aging treatment is used in some cases. A complete solution of heat treatment and artificial aging treatment will recover properties.

It is essential that the welding inspector understands the requirements. This is to ensure they are being conducted correctly in terms of relevant welding procedure specifications. Especially when the welding operations involve preheating and/or post-weld heat treatment. 

To learn more about the things welding inspectors should know, contact us today.