Post Weld Heat Treatment

Post Weld Heat Treatment (PWHT) is a controlled process of reheating and cooling a welded material to improve its properties, relieve residual stresses, and ensure compliance with engineering codes and standards. It is commonly used in industries such as oil and gas, power generation, and structural fabrication.

Purpose of PWHT

Stress Relief:

1. Welding generates residual stresses due to rapid heating and cooling.
2. PWHT reduces these stresses, preventing deformation and failure.

Improved Mechanical Properties:

1. Enhances toughness and ductility.
2. Reduces brittleness in heat-affected zones (HAZ).

Reduced Risk of Cracking:

1. Minimizes the risk of hydrogen-induced cracking and stress corrosion cracking.

Microstructure Refinement:

1. Helps refine grain structures altered by welding, restoring material properties.

Compliance with Standards:

1. Many codes (e.g., ASME, AWS) mandate PWHT for certain materials and applications.

Key Steps in PWHT

Preparation:

1. Cleaning the weld area to remove contaminants.
2. Ensuring proper support and fixtures to prevent warping.

Heating:

1. Heating the welded component to a specific temperature (e.g., 600–750°C for carbon steels).
2. Rate of heating must be controlled to avoid thermal shock.

Soaking:

1. Maintaining the target temperature for a specific duration (e.g., 1 hour per inch of material thickness).
2. Ensures uniform heat distribution and stress relief.

Cooling:

1. Controlled cooling to avoid rapid thermal contraction.
2. Methods: air cooling or furnace cooling.

Common PWHT Methods

Furnace Heating:

1. Large components are placed in an industrial furnace.
2. Offers uniform heating for bulk stress relief.

Local Heating:

1. Focused heat application using electrical resistance or induction coils.
2. Used for specific weld joints or repairs.

Torch Heating:

1. Used for field applications where furnaces are unavailable.
2. Requires skilled operators for temperature control.

Factors Influencing PWHT

Material Type:

1. Carbon steels, low-alloy steels, and stainless steels require different PWHT parameters.
2. Some materials (e.g., austenitic stainless steels) may not need PWHT due to inherent properties.

Welding Code Requirements:

1. Standards like ASME Section VIII, API 650, and AWS D1.1 specify PWHT for certain applications.

Thickness and Joint Design:

1. Thicker materials often require longer soaking times and controlled cooling rates.

Service Conditions:

1. Components exposed to high pressures, temperatures, or corrosive environments benefit significantly from PWHT.

Inspection After PWHT

Hardness Testing:

1. Ensures the material is within acceptable hardness limits.

Non-Destructive Testing (NDT):

1. Methods like radiographic or ultrasonic testing to check for cracks or defects.

Dimensional Checks:

1. Verifying that the component remains within tolerances post-treatment.