How to calculate PWHT holding time
Post-weld heat treatment relieves residual stress and softens the heat-affected zone by soaking the part at temperature for a set time. Under ASME Section VIII, Division 1 (UCS-56) the soak is one hour per inch of thickness with a 15 minute minimum, and above 2 inches it becomes 2 hours plus 15 minutes for every additional inch. A 1-1/2 inch carbon steel weld therefore holds for 1 hour 30 minutes.
Carbon steels in P-No. 1 are stress relieved at a minimum of 1,100 °F, with 1,100 to 1,250 °F the usual working band, while low-alloy Cr-Mo materials in P-No. 4 and P-No. 5 start at 1,300 °F. Above 800 °F the heating and cooling rate is limited to 400 °F per hour divided by the thickness in inches, never required to be below 100 °F per hour and never above 400 °F per hour. On thick sections the ramp time often exceeds the soak time itself.
These are the general stress-relief rules; the governing code, the design specification and the owner's requirements can all change the temperature and time actually applied. Thermocouple placement, the heated band width for local PWHT and the choice between furnace and field heating also need to be defined, so have the final procedure reviewed by a welding engineer and the authorized inspector.
Frequently Asked Questions
ASME Boiler and Pressure Vessel Code Section VIII, Division 1 (UCS-56) calls for one hour per inch of thickness with a 15 minute minimum. Above 2 inches the rule becomes 2 hours plus 15 minutes for each additional inch over 2 inches.
A steep temperature gradient creates thermal stress between the surface and the core, which can distort or crack the part. Above 800 °F the rate is capped at 400 °F per hour divided by the thickness in inches, and it need not be less than 100 °F per hour nor more than 400 °F per hour.
No. Depending on thickness, material and service conditions it may be exempted or mandatory. Check the governing code, the design specification and the owner's requirements, and confirm the final decision with a welding engineer or the authorized inspector.