Define post-weld heat treatment (PWHT).

Post-weld heat treatment (PWHT) refers to the procedure applied after welding to relieve residual stresses that may have developed during the welding process and to reduce hardness in the heat-affected zone of the welded material. This treatment is critical to improving the overall mechanical properties of the weldment, including ductility and toughness, and to ensuring that the welded structure can withstand operational stresses and loading conditions without risk of failure.

By carefully controlling the temperature and time of exposure during PWHT, metallurgical transformations can be achieved that alleviate stresses and modify the material structure, thus enhancing the performance and longevity of the weld. This treatment is especially important in applications where high strength and ductility are required, such as in pressure vessels, piping, and structural steelwork.

Other options do not accurately describe PWHT. The enhancement of appearance, checking weld integrity, or cooling methods do not encompass the purpose and benefits of post-weld heat treatment. Thus, the correct understanding of PWHT as a crucial step in welding processes highlights its importance in maintaining the integrity and safety of welded joints.