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Post weld heat treatment (PWHT) reduces residual stresses and improves toughness for both parent metal and weld material, while simultaneously clearing away hydrogen build-up in the weld area.

Most current piping and pressure vessel codes stipulate PWHT when material thickness surpasses an arbitrary limit, usually determined by both Charpy energy absorption and hardness requirements.

Prevent Hydrogen Induced Cracking

Hydrogen incorperation corrosion (HIC) occurs when hydrogen seeps into welded areas from sources such as moisture, contaminants and shielding gases during welding. This release of hydrogen causes embrittlement that leads to cracking in weld metal and the weld heat-affected zone (HAZ).

Hydrogen bake-out is a procedure utilized in PWHT welding that helps prevent hydrogen-induced cracking of welds. It involves heating them to an exact temperature for a specified duration before holding onto that state for some period of time.

Baking-out temperatures and soak periods depend on material type and welding technique; however, several codes such as ASME Section VIII Div. 1 stipulate a post weld heat treatment is mandatory if component thickness surpasses an established limit value.

Assuring this requirement is met is best achieved using electrodes with low hydrogen levels and preheats that meet minimum code requirements. Also make sure your wire spool is securely enclosed by using suitcase-style feeders, never touching it with dirty hands as this could introduce extra hydrogen during welding.

Strengthen the Weld

PWHT can reduce residual stresses present during welding to help avoid brittle fracture and distortions that might arise from machining or stress corrosion, thus helping prevent brittle fracture and eliminating distortions that arise from these processes.

PWHT involves heating the weld and heat-affected zones to temperatures that cause a decrease in their lower critical transformation temperature, thus decreasing internal stresses and improving mechanical properties.

Consistent, precise temperature control is crucial to providing effective post weld heat treatments. Failing to do so could weaken the weld by decreasing its strength and hardness; additionally, uneven stress relief or distortion could occur as a result.

Working with large and bulky pressure equipment can present unique challenges. All components must be placed into a furnace at once to be exposed to the same temperature cycle. Variations in thermal expansion could create issues that necessitate additional treatment or repairs.

Prevent Stress Corrosion Cracking

PWHT tempers hard, potentially brittle weld microstructural areas to lower peak residual stresses and thus decrease fatigue crack growth rates; however, this does not guarantee fatigue will never happen.

Each metal has unique properties that determine what kind and duration of postweld heat treatment it requires; for instance, carbon steels require stress relieving in order to avoid hidden hydrogen-induced cracking in high-strength pipeline piping; on the other hand, stainless steels need solution annealing in order to dissolve chromium carbides that form on their surfaces and restore their superior corrosion resistance.

PWHT relies heavily on furnace type, heating cycle, and soak time as variables which affect residual stresses to be relaxed during PWHT. Furthermore, applying local heat can have an impact on results: for instance, localized heating can create thermal gradients between heated and unheated areas, potentially shifting stress between areas and decreasing overall relaxation. To mitigate this effect effectively welds should be positioned so as not to come in contact with other components or adjacent areas when being heated up.

Prevent Distortion

Temperature and cooling speeds have an enormous effect on component distortion. This is especially true if they contain multiple sections connected together by joints.

By carefully orchestrating welding sequences, it is possible to avoid excessive distortion. Jigs and fixtures should also be utilized where applicable to prevent movement of components. It may also be beneficial to employ tack welds where applicable.

Post Weld Heat Treatment (PWHT) can reduce distortion by redistributing residual stresses. It involves heating the weld metal at an elevated temperature for an extended period, then slowly cooling it. The slow cooling allows uniform expansion and reduces stress on internal materials. Some fabrication codes mandate PWHT for weldments composed of high carbon steels to help reduce levels of welding residual stress which combine with load stresses to exceed material’s design limits and lead to distortions and distortion.