According to EN 13480-4 (2024 Edition), post-weld heat treatment (PWHT) of piping spools is mandatory for certain material thicknesses and service conditions. The standard defines three heat-treatment categories in Annex I: normalizing, stress relieving, and hardening plus tempering. Each has specific holding temperature ranges and minimum holding times.
Section 8.3 of EN 13480-4 specifies the soaking band: the temperature must be uniform within +/-20°C across the heated zone. Thermocouples must be placed at the hottest and coolest points, typically within 50 mm of the weld edge. For local PWHT, the heated band width must be at least five times the wall thickness on each side of the weld.
Hardness limits are given in EN 13445-4 for pressure vessels, which also applies to piping components in many European projects. The maximum allowable hardness for carbon steel is 250 HV10, and for low-alloy steels it is 300 HV10. Exceeding these limits may require additional tempering or a revised procedure.
An audit-ready PWHT record sheet should include: heat-treatment category, furnace or local heating method, thermocouple locations and calibration, actual time-temperature chart with soaking band overlay, and hardness test results. The chart must show the complete cycle from preheat to cool-down, with the soaking period clearly marked.
What is the most common nonconformity you see on PWHT charts during audits?
Source: EN 13480-4, 2024