Precipitation Hardening

Post-weld heat treatment means: after welding, the part is deliberately kept at temperature so that the structure stabilizes and stresses relax. If you skip it or do it too short, the material gets hard and brittle, and the part can crack quickly. But if you leave it too long or too hot, the grain coarsens and the steel loses toughness. So stick to the specified temperature and time for the material, then it's fine.