Seamless pipes made of low-alloy steel ASTM A335 Grade P11 can be used for power piping (ASME B31.1) or process piping (ASME B31.3) at elevated temperatures.
When two ASTM A335 P11 pipes are joined by welding, postweld heat treatment (PWHT) is required. The PWHT is performed to prevent or relieve the detrimental effects of high temperatures, steep temperature gradients, and residual stresses created during welding.
This heat treatment must be included in the Welding Procedure Specification (WPS) and carried out according to the essential variables of ASME BPVC Section IX.
According to QW/QB-420 of ASME BPVC Section IX, ASTM A335 Grade P11 (UNS K11597) is assigned a P-number of 4 and a group number of 1. The PWHT zone extends from the weld in all directions by the larger of 75 mm or 1.5 times the greater nominal wall thickness.
The zone must be heated to a temperature between 650°C (1200°F) and 705°C (1300°F) and maintained within this range for a specified time. The minimum holding time (t, in hours) depends on the pipe wall thickness (T, in mm): if T ≤ 50 mm, t = T/25 with a minimum of 0.25 hours; if T > 50 mm, t = 2 + 0.25 × (T – 50)/25.
At the construction site, PWHT is usually carried out using electric-resistance methods, such as flexible ceramic pad heaters. During the process, the heating and cooling rate must not exceed 335°C/hour (600°F/hour) divided by one-half the maximum material thickness in inches at the weld, and in any case, shall not exceed 335°C/hour (600°F/hour).
