A technician is tasked with welding a thick section (50 mm) of structural steel that will experience dynamic loading. To minimize residual stresses and ensure proper grain refinement after welding, what post-weld heat treatment (PWHT) is commonly applied, and what is its primary effect?

Question: A technician is tasked with welding a thick section (50 mm) of structural steel that will experience dynamic loading. To minimize residual stresses and ensure proper grain refinement after welding, what post-weld heat treatment (PWHT) is commonly applied, and what is its primary effect?

Answer options: ✅ Stress relieving: heating to a subcritical temperature and then slow cooling to reduce internal stresses.

• Normalizing: heating above the upper critical temperature and then air cooling to refine grain structure.
• Annealing: heating above the upper critical temperature and very slow furnace cooling to soften the material.
• Quenching and tempering: rapid cooling followed by reheating to increase hardness and toughness.

Correct answer: Stress relieving: heating to a subcritical temperature and then slow cooling to reduce internal stresses.

Explanation: For thick sections and dynamic loading applications, stress relieving (a type of PWHT) is crucial. It involves heating the welded component to a subcritical temperature (below critical transformation temperatures) and then slowly cooling it, which allows internal residual stresses accumulated during welding to relax without significantly altering the base metal's microstructure or mechanical properties through phase transformation.