Several factors contribute to pipe damage in pipeline systems, including corrosion, erosion, welding defects, material defects, vibration stress, and mechanical damage. In addition, damage occurring in the weld area is often related to weld quality in the heat-affected zone (HAZ) and residual stress. To enhance weld quality, one effective method is the implementation of Post Weld Heat Treatment (PWHT). PWHT is a critical component of the heat treatment process to alleviate residual stress generated during welding. The PWHT process can be carried out in two ways: full PWHT by subjecting the entire object to heat treatment in a furnace (typically applied to complete equipment such as pressure vessels or boilers) or localized heating near the welding area only (commonly used for pipes or high-pressure vessels).
PWHT is a heat treatment process frequently applied to materials after welding to reduce residual stress, achieve uniform grain structure, and enhance material toughness. The PWHT process involves the application of heat to the material after welding, intending to improve the mechanical properties and microstructure of the heat-affected and welded metal regions. Typically, PWHT involves reheating the material to a temperature below the lower critical temperature (A1), approximately 1,333°F. The carbon iron (Fe3C) phase diagram, specifically carbon steel with a carbon content below 0.8%, exhibits a ferrite and pearlite structure at room temperature. Carbon steel with a carbon content greater than 0.8% but less than 2% shows a pearlite and cementite structure. The post-weld heat treatment is generally performed at temperatures ranging from approximately 300°F to 1,125°F (150°C to 600°C), with the duration of treatment varying based on the material thickness. It is crucial to carefully control the cooling rate after welding to prevent the formation of a hard martensite structure, which can render the material brittle and susceptible to cracking.
The heat-affected zone (HAZ), situated adjacent to the weld metal, undergoes a thermal cycle of rapid heating and cooling during the welding process, making it the most critical area of the weld joint. Consequently, the region near the weld melt line exhibits increased roughness and high hardness values. After PWHT is performed in the HAZ and weld metal area, the hardness value decreases, indicating a reduction in residual stress. Residual stress significantly influences the mechanical properties of the material, including hardness. The decrease in residual stress is attributed to the relaxation of material subjected to PWHT at a temperature of 1,125°F (600°F), allowing the release of residual stress throughout all PWHT-treated parts.
Materials with higher carbon content typically require PWHT after welding, as do materials with higher alloy content and greater cross-sectional thickness. Generally, Post Weld Heat Treatment (PWHT) is performed on specific types of materials based on their thickness and grade, including:
- Carbon Steel and Low-Temperature Carbon Steel (LTCS) with a thickness of at least 0.79 inches (20MM) and above.
- Low Alloy Steel containing less than 0.5% Cr (Chromium) and a thickness greater than 0.79 inches (20MM).
- Low Alloy Steel containing more than 0.5% Cr (Chromium) and a thickness above 0.51 inches (13MM).
Conducting PWHT operations requires trained and experienced personnel who have obtained permission from a welding engineer. During the PWHT process, three critical factors warrant special attention: holding time, heating temperature, and cooling rate.
In addition to improving the quality of welded joints in the HAZ area, PWHT is sometimes employed by welding engineers for other reasons, such as corrosion prevention in pipes used for fluids susceptible to corrosion.