Intermediate Post Weld Heat Treatment for ASTM A508 Grade 3 Steel

1. Introduction

ASTM A508 Grade 3 is a manganese, nickel, molybdenum low alloy steel supplied in the quenched and tempered condition. It is extensively used in nuclear pressure retaining components such as reactor pressure vessels and pressurisers.

Due to its high hardenability and thick section application, strict control of welding thermal cycles is required. One such control measure is intermediate post weld heat treatment, typically applied in the range of 593 °C to 621 °C.

This article explains the metallurgical basis, application conditions, and code relevance of this practice.

2. Definition of Intermediate PWHT

Intermediate post weld heat treatment refers to a controlled heat treatment applied after welding but before final stress relief heat treatment. It is not intended to replace final PWHT. Its purpose is to condition the weldment by reducing hydrogen content, lowering residual stresses, and stabilising the microstructure prior to further welding or final heat treatment.

3. Metallurgical Basis

3.1 Hydrogen Induced Cracking Control

During welding, diffusible hydrogen may be introduced from consumables, fluxes, or environmental contamination.

In ASTM A508 Grade 3, the heat affected zone can transform to martensite due to rapid cooling. This microstructure is highly susceptible to hydrogen induced cracking.

Holding the weldment at 593 °C to 621 °C promotes hydrogen diffusion out of the material, thereby reducing the risk of delayed cracking.

This process is commonly referred to as hydrogen removal heat treatment.

3.2 Residual Stress Reduction

Welding generates high residual stresses due to localised thermal expansion and contraction.
Intermediate PWHT provides partial stress relief by allowing stress relaxation at elevated temperature. This reduces the likelihood of crack initiation and improves dimensional stability before further processing.

3.3 Microstructure Modification in the Heat Affected Zone

The heat affected zone in quenched and tempered steels may contain untempered martensite after welding.

This condition is characterised by:

High hardness
Low toughness
Increased crack susceptibility

Intermediate PWHT tempers this microstructure, resulting in:

Reduced hardness
Improved toughness
More uniform microstructural distribution

3.4 Reheat Cracking Mitigation

ASTM A508 Grade 3 is susceptible to reheat cracking during subsequent heat treatment, particularly in coarse grained heat affected zones.

Intermediate PWHT reduces this risk by:

Lowering residual stress levels
Reducing hardness gradients
Stabilising grain boundary behaviour

3.5 Retention of Mechanical Properties

The selected temperature range is below the original tempering temperature of the base material, typically between 650 °C and 700 °C.

This ensures that:

Over tempering does not occur
Strength and toughness are maintained
The quenched and tempered condition is preserved

4. Application Conditions

Intermediate PWHT is typically required under the following conditions:

Thick section weldments with high restraint
Multi pass welding with high heat input
Fabrication of nuclear pressure boundary components
Weld repairs on in service components
Situations where final PWHT is delayed

5. Code and Standard Practice

The application of intermediate PWHT and hydrogen removal heat treatment is supported by industry codes and recommended practices, including:

ASME Boiler and Pressure Vessel Code Section III
ASME Boiler and Pressure Vessel Code Section VIII
API 934
AWS D10.10

These documents recognise the importance of hydrogen control and stress management in low alloy steels used in critical service.

6. Summary

Function	Effect
Hydrogen removal	Reduces risk of hydrogen induced cracking
Stress relaxation	Lowers residual welding stresses
Microstructure tempering	Improves toughness and reduces hardness
Reheat cracking control	Enhances resistance during subsequent heat treatment
Property preservation	Maintains base material strength and toughness

7. Conclusion

Intermediate post weld heat treatment at 593 °C to 621 °C is a necessary control measure in the welding of ASTM A508 Grade 3 steel.

It provides a controlled approach to hydrogen removal, stress reduction, and microstructural stabilisation without compromising the mechanical properties of the quenched and tempered material.

Its correct application is essential in ensuring the integrity and long term performance of safety critical components, particularly in nuclear pressure vessel service.

Intermediate Post Weld Heat Treatment for ASTM A508 Grade 3 Steel

Frequently Asked Questions

What is intermediate PWHT in welding?
Intermediate post weld heat treatment is a controlled heating cycle applied after welding and before final PWHT.
Its primary functions are to reduce diffusible hydrogen, partially relieve residual stresses, and stabilise the weld and heat affected zone microstructure.

Why is intermediate PWHT required for ASTM A508 Grade 3 steel?
ASTM A508 Grade 3 has high hardenability and forms martensite in the heat affected zone during welding.
This makes it susceptible to:
• Hydrogen induced cracking
• Reheat cracking
• High residual stress accumulation
Intermediate PWHT reduces these risks before final heat treatment.

What is the typical temperature range for intermediate PWHT?
The typical temperature range is:
593 °C to 621 °C
This range is selected to allow hydrogen diffusion and stress relaxation while avoiding over tempering of the quenched and tempered base material.

How does intermediate PWHT prevent hydrogen induced cracking?
At elevated temperature, diffusible hydrogen becomes mobile and escapes from the weld metal and heat affected zone.
This prevents hydrogen accumulation at stress concentration sites, which is the primary mechanism for hydrogen induced cracking.

Is intermediate PWHT the same as hydrogen bake out?
They are closely related but not always identical.
Hydrogen bake out specifically targets hydrogen removal, while intermediate PWHT includes:
• Hydrogen diffusion
• Stress relaxation
• Microstructure tempering
In practice, intermediate PWHT often performs the function of hydrogen bake out.

Does intermediate PWHT replace final PWHT?
No.
Intermediate PWHT is a preparatory step. Final PWHT is still required to achieve full stress relief and meet code requirements for pressure retaining components.

What happens to the microstructure during intermediate PWHT?
Untempered martensite formed in the heat affected zone is partially tempered.
This results in:
• Reduced hardness
• Improved toughness
• Lower crack susceptibility

Why is the temperature kept below the final tempering temperature?
ASTM A508 Grade 3 is originally tempered at approximately 650 °C to 700 °C.
Keeping intermediate PWHT below this range ensures:
• No significant loss of strength
• No over tempering
• Retention of fracture toughness

When should intermediate PWHT be applied?
It is typically required in:
• Thick section weldments
• Multi pass welding operations
• Nuclear pressure vessel fabrication
• Weld repairs on critical components
• Situations where final PWHT cannot be performed immediately

What is reheat cracking and how does intermediate PWHT help?
Reheat cracking occurs during subsequent heat treatment due to residual stresses and susceptible microstructures.
Intermediate PWHT reduces this risk by:
• Lowering residual stress levels
• Tempering brittle structures
• Improving grain boundary stability

Which standards support intermediate PWHT practice?
Relevant industry standards include:
• ASME Boiler and Pressure Vessel Code Section III
• ASME Boiler and Pressure Vessel Code Section VIII
• API 934
• AWS D10.10
These documents recognise hydrogen control and stress management requirements for low alloy steels.

Can intermediate PWHT be applied locally?
Yes.
Local heating methods may be used where full furnace treatment is not practical, provided that:
• Temperature control is uniform
• Heating and cooling rates are controlled
• Thermal gradients are minimised
Guidance is provided in AWS D10.10.

What are the risks of skipping intermediate PWHT?
Failure to apply intermediate PWHT can lead to:
• Hydrogen induced cracking
• Delayed cracking after welding
• Increased residual stresses
• Higher risk of reheat cracking during final PWHT

How long should intermediate PWHT be held?
Holding time depends on thickness and procedure requirements, but must be sufficient to allow:
• Hydrogen diffusion
• Uniform temperature distribution
Typical durations are defined in welding procedure specifications and project codes.

Is intermediate PWHT mandatory for all A508 Grade 3 welds?
No.
It is not universally mandatory, but it becomes necessary when:
• Section thickness is high
• Hydrogen risk is elevated
• Restraint is significant
• Code or project specification requires it