shop inspection for PWHT and after PWHT Postweld Heat Treatment
This operation can be defined as the heating of weldments immediately after welding in order to temper, stress relieve, or provide controlled cooling. It prevents the formation of hard or brittle structures prone to cracking in services. Postweld heat treatment is mandatory for certain alloys, combinations or alloys, and material thicknesses as outlined in the various codes and standards, such as ASME Section VIII Pressure Vessel Code. Consequently, welding procedures for such materials and/or thickness will contain the provisions for heat treatment.
Fabricators may have difficulty with prostweld heat treatment due to shop practices rather than the treatment itself. Cracking can occur in vessels that are held for heat treating until a full furnaces load is ready.
In general, at normal heat treating temperatures the time at temperature is specified as one hour per 1 in. (25 mm) of maximum thickness for vessels up to 2 in. (50 mm) thickness fabricators may suggest that since the component is being heated form both sides half-an-hour at temperature for each 1 in. (25) thickness is sufficient – but the one hour requirement should be maintained.
For vessels over 2 in (50 mm) in thickness, the usual holding time is two hour plus 15 minutes for each additional 1 in. (25 mm) over the 2 in. (50 mm) thickness. Again, some fabricators may want to cut this time by half since the component is being heated form all sides.
Holding time may be an accumulation of multiple heat-treat cycles. It dose not necessarily have to occur all at one time. Care must be taken to ensure that the accumulated time at the correct temperature equals that required to effect the desired treatment.
There is a maximum furnace temperature for introduction of the vessel. This varies with the alloy but is generally 800º F (426º C) for carbon steels. Heating rate vary with alloy as well but are usually in the 200 to 400º F (93 to 204º C) range. Some manufactures will try to heat as rapidly as possible in order to reduce the time required.
Cooling is normally performed in the furnace at a controlled rate unless the component si to be quenched. If quenching is performed then the component must be removed form the furnace and quenched as quickly as possible. Delays will cause such problems as carbide precipitation in austenitic alloy and reheat treatment will be necessary.
