ASTM A106/B is commonly used for pressure piping in alumina refineries. Due to the problem of caustic cracking in alumina refineries, piping must be stress relieved after welding, usually through a post-weld heat treatment (PWHT) process at a temperature of 635°C. However, multiple PWHT procedures tend to degrade the mechanical properties of steel and design standards have set limits on the length of time PWHT can be carried out before steel should be replaced. In this study, the effect of PWHT time on the properties of A106/B parent metal was examined, with particular emphasis on the effect on toughness. PWHT was carried out for 1, 8 and 40 hours and the results were compared with those for as-received material. Impact transition temperature and room-temperature tensile results were determined using standard tests. The reference temperature, To, was determined using ASTM E1921 with arc-shaped tension specimens. The results showed that the ductile-to-brittle transition temperature increased significantly as PWHT time increased and the implications of this to a fracture mechanics analysis of plant operations are discussed.