Welding Metallurgy and Heat Treatment
This domain tests phase transformations in the weld and HAZ, and the correct design of post-weld heat treatment (PWHT).
Thermal cycles and microstructure
Cooling time from 800 to 500 °C (t8/5) controls whether you get ferrite-pearlite, bainite or martensite. High heat input lengthens t8/5 and softens; low heat input and thick sections shorten it and harden. Continuous cooling transformation (CCT) diagrams link steel composition and cooling rate to final microstructure and hardness.
Post-weld heat treatment
PWHT serves several purposes: tempering martensite, relieving residual stress, and diffusing hydrogen. For C–Mn and low-alloy steels, stress relief is typically 580–650 °C, with controlled heating and cooling above about 300–400 °C. Holding time is commonly about 1 hour per 25 mm of thickness with a minimum, and heating rate is limited (a common rule is 5500/t °C/h, t in mm). Austenitic stainless steels are not stress-relieved in this range because of sensitisation and sigma formation; solution annealing is a different treatment.
Worked example
A 40 mm C–Mn pressure part requires PWHT. Holding time ≈ 1 h per 25 mm → 40/25 = 1.6 h (about 96 min), with a minimum of 30 min. Maximum heating rate above 400 °C ≈ 5500/40 = 137 °C/h. Reasoning: thicker sections need slower heating to avoid steep through-thickness gradients and distortion, and longer holding to allow the whole section to reach temperature.
Traps
Applying a standard stress relief to 304 stainless and causing sensitisation. Cooling too fast after PWHT and re-forming martensite. Ignoring cladding or dissimilar joints when selecting temperature. Confusing normalising, annealing and stress relief.