What is Post-Welding Heat Treatment (PWHT)?
Post-welding heat treatment can have several effects on the HAZ and weld metal, depending on the welded material, used temperature, and the time the material is kept at the elevated temperature. But for the most part, PWHT is used to relieve the internal stresses caused by welding, reduce the brittleness of the HAZ, and allow the hydrogen to escape safely.
PWHT is a critical final step when welding materials that require it. Welding codes can also require PWHT in industries where pipes, pressure vessels, and structural steels are welded. These are mostly petrochemical, oil and gas, shipbuilding, construction, and offshore industries, where it’s critical for welds to be as strong as possible. PWHT can also be a required step by the insurers and authorized inspectors.
PWHT consists of bringing the welded workpiece to a specified temperature and maintaining that temperature precisely as required by the material and the application. Sometimes, the whole part needs to be treated. But, treating the local welded area often meets the PWHT requirements when it’s impractical to heat the entire workpiece.
Post-Heating For Hydrogen Diffusion
Post-heating is used to reduce the chances of hydrogen-induced cracking. Hydrogen cracking occurs in steels at ambient temperatures. The chances are higher when the weld has a sensitive microstructure, high internal stress, and when the joint is restrained.
Unlike other PWHT types, you should use post-heating to remove hydrogen immediately upon welding while the part is still hot from preheating. Post-heating works in tandem with preheating by keeping the part at an elevated temperature after the welding takes place. Once welding is done, the temperature must be elevated from the interpass temperature to the post-heating temperature as specified.
Keeping the workpiece at an elevated temperature allows the hydrogen to diffuse out of the weld and HAZ safely and not get trapped in the workpiece. It’s essential to follow specific welding codes or standards for detailed timing requirements, as they may vary depending on the material and application.
Stress Relief
Welding induces tensile and compressive stresses in the joint, especially if it’s highly restrained. When you weld, the molten filler metal expands in the joint. But, as soon as it cools, the weld metal starts pulling on the surrounding base metal, causing internal tensile stresses.
This internal stress does not go away after welding. It can cause distortion or weld failure upon being loaded in service conditions. Likewise, the internal stress can cause the workpiece to deform after being machined if the machining process removes the material “holding it together.” Internal stresses are like an invisible loaded spring in the material. They can’t wait to be released, which is almost always bad news for fabrication quality.
PWHT for stress relief requires bringing the part to the specified temperature and keeping it for a prolonged period of time. The exact temperature and time frames primarily depend on the material and its application.
Bringing the material to a high temperature makes it more ductile, allowing the microstructure to shift and relieve its internal stresses. This makes the weld and HAZ much less likely to suffer from brittle cracking, cracking from service loads, and hydrogen-induced cracking.
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