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What Is Stress Relieving? | Piping Engineering Glossary
What Is Stress Relieving?
In piping engineering and process engineering, stress relieving is a heat treatment that reduces the residual stresses left in metal after welding, forming, or machining. In piping, it is usually performed as post-weld heat treatment (PWHT), in which the welded joint is heated to a controlled temperature, held for a set time, and then cooled slowly.
How Stress Relieving Works
Welding leaves high residual stresses around each butt weld because the weld metal and nearby metal heat and cool unevenly. Heating the joint to a temperature below the material’s transformation range, typically about 595°C to 650°C (1100°F to 1200°F) for carbon steel, lets the metal relax and redistribute those stresses. The hold time is usually based on wall thickness, often around one hour per 25 mm of thickness.
Beyond reducing residual stress, PWHT also tempers hard microstructures in the heat-affected zone next to the weld junction, improving toughness and lowering hardness. This makes welds more resistant to cracking in demanding services.
Applications
Thick-Walled Carbon Steel
Codes commonly require PWHT for carbon steel joints above a certain wall thickness, often around 19 mm (3/4 inch). Thick sections retain the highest residual stresses after welding.
Alloy Steels
Chromium-molybdenum and other alloy steels often need PWHT regardless of thickness, as defined in the welding procedure specification. Skipping it can leave brittle, crack-prone welds.
Cracking-Prone Services
Owners often specify PWHT for caustic, amine, and wet H2S services to reduce the risk of stress corrosion cracking. These requirements may apply even where the code itself does not require heat treatment.
Benefits
Reduces Cracking Risk
Lower residual stress and hardness reduce the risk of stress corrosion cracking and brittle fracture. This is especially important in thick or high-strength welds.
Improves Toughness
Tempering the heat-affected zone restores toughness lost during welding. Joints become more tolerant of small flaws.
Dimensional Stability
Relieving stresses reduces distortion during later machining or service. This helps keep flanges and fittings aligned.
Limitations
Added Time and Cost
PWHT requires heating equipment, temperature recording, and skilled technicians, adding cost to both shop welds and field welding. Field PWHT can also extend construction schedules.
Risk of Improper Treatment
Too low a temperature fails to relieve stresses, while too high a temperature can damage material properties. Accurate thermocouple placement and records are essential.
Not Suitable for All Materials
Some materials, such as many austenitic stainless steels, generally do not need PWHT and can be harmed by unnecessary heat treatment.
FAQ
What is the difference between stress relieving and PWHT?
In piping, the terms are often used interchangeably. PWHT is the general term for any heat treatment after welding, and stress relieving is its most common purpose.
How is PWHT performed in the field?
Local PWHT uses electrical resistance heating pads or induction coils wrapped around the joint, with insulation and thermocouples to control and record the temperature cycle.
How do I know PWHT was done correctly?
Temperature charts are reviewed against the procedure, and hardness testing of the weld and heat-affected zone confirms that hardness is within the specified limits.
Can a weld be repaired after PWHT?
Yes, but repairs usually require the joint to be heat treated again, because new welding reintroduces residual stresses.
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