Post Weld Heat Treatment

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, post-weld heat treatment is costly since it demands extra equipment, time, and handling. 

Post-weld heat treatment should only be undertaken after carefully considering its advantages. Post-weld 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 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 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. This can be 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 closely.

Reasons for Post-Weld Heat Treatment

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.

Post Weld Heat Treatment Process

Post-weld heat treatment is a controlled heating and cooling process. It happens after welding to: 

• Change the microstructure of the weld and base metal
• Relieve residual stresses
• Improve the mechanical properties of the welded joint

The specific process can vary depending on the: 

• Material
• Welding method
• Desired outcome

Here is a general overview of the typical steps involved in the process:

Preparation

Before starting, it’s crucial to clean the weld and the surrounding area thoroughly. You need to remove any contaminants like oil, grease, or debris. Proper cleaning ensures the effectiveness of the treatment.

Temperature Selection

The first step is to select the appropriate temperature. The temperature chosen depends on factors such as the: 

• Material type
• Thickness
• Desired outcome

Common temperature ranges for post-weld heat treatment are typically between 600°C (1112°F) and 760°C (1400°F).

Heating

Once you choose the temperature, the weld heats to the specified temperature. The heating rate should be controlled to prevent thermal shock and minimize the risk of cracking.

Soaking

After reaching the target temperature, the weldment is held at that temperature for a specified period of time. This is known as the “soaking” or “holding” time. The duration varies based on factors like: 

• Material
• Thickness
• Desired microstructural changes

Soaking allows for the diffusion of atoms within the material. This helps relieve residual stresses.

Cooling

After the soaking period, the weldment cools down gradually. The cooling rate is controlled to prevent rapid temperature changes. These changes could lead to thermal stress or cracking. Depending on the material and requirements different cooling methods can be used, such as: 

• Air cooling
• Furnace cooling
• Water quenching

Cooling Rate Control

For certain materials and applications, it’s essential to control the cooling rate. This will help to achieve specific microstructural changes. This may involve using specialized cooling equipment or techniques.

Inspection and Testing

After, the welded component is inspected and tested to ensure that it was successful. Common inspection methods include: 

• Hardness testing
• Non-destructive testing (NDT) like ultrasonic or radiographic examination
• Visual inspection

These tests confirm that the changes have occurred, and the weld meets the quality standards.

Documentation

Proper documentation of the process is essential for traceability and quality control. Records should include details such as the: 

• Temperature-time cycle
• Equipment calibration records
• Inspection results

Quality Assurance

Throughout the process, quality assurance measures should be in place. This will help to ensure that the treatment goes according to plan. This includes monitoring and control of: 

• Temperature
• Heating and cooling rates
• Other critical parameters

Post-Post Welding Heat Treatment Processes

Depending on the specific application, additional processes may be required, like: 

• Stress relieving
• Tempering
• Other heat treatments  

Safety Precautions

Safety precautions must be strictly adhered to in order to protect personnel and prevent accidents., such as: 

• Ensuring proper ventilation
• Personal protective equipment (PPE)
• Safe handling of hot materials

Documentation and Reporting

Details should be documented and reported for quality control and regulatory compliance, including: 

• Temperature profiles
• Holding times
• Cooling rates
• Inspection results
• Any deviations from the procedures

Post-weld heat treatment is a critical process in the welding industry. Its success relies on precise control and adherence to established procedures. When properly executed it can enhance the integrity of welded structures and components.

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 in some cases. A complete solution of post-weld 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. 

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