We manufacture spherical tanks made of low-carbon-steel shells ASME SA-537 Class-2, containing pressurized Liquefied Petroleum Gas (LPG). The tank is formed from 1.97-inch (50-mm) thick steel plates having a composition of: 0.24% C, 0.64-1.46% Mn, 0.035 % P, 0.035 % S, 0.13-0.55% Si, 0.38% Cu, 0.28% Ni, 0.29% Cr and 0.09% Mo.
After erection of the spherical tank, the steel plates will be welded together and subjected to a post-weld heat treatment. The minimum stated post-weld heat treatment for ASME SA-537 Class 2 is 1105°F (595°C). What will happen if I heat treat at 1155°F (625°C) for 123 minutes?
High-level residual stresses can occur in weldments due to restraint of the parent metal during weld solidification. The stresses may be as high as the yield strength of material itself. And when combined with normal load stresses, these may exceed the design stresses.
The removal of residual stresses takes place due to the fact that the thermal energy received by the metal allows for grain-boundary sliding and removal of metallurgical defects like dislocations, vacancies and slip planes.
A most important aspect of post-weld heat treatment (PWHT) is the prevention of brittle fracture. PWHT softens the hardened zones and improves machinability. Removal of residual stresses also becomes necessary where dimensional stability is required. In this case, the heat treatment consists of the stress relief, annealing or solution annealing, depending upon the requirements.
Residual stresses resulting from welding are reduced by a post-weld thermal stress-relief heat treatment. The residual stress remaining in a material after stress-relief treatment will depend on rate of cooling. The percentage relief of internal stress is independent of steel type, composition or yield strength.
Based on data available to me, 123 minutes (2 hours) at 1155°F (625°C) is equivalent to a temper of about 9.5 hours at 1105°F (595°C). Since you will be performing a stress relief on the post-weld assembly, 9.5 hours at temperature appears to be more than adequate.
The 123 minutes should be time at temperature (excluding heat up time). With 1.97 inch (50 mm) thickness, there will be some appreciable heating time. Knowing the amount of time at temperature in order to judge whether to temperature/time combination will provide the stress relief you are seeking is essential. I believe you should be at a temperature for a period of time such that your soak/temperature combination is equivalent to at least 2 hours at 1105°F (595 °C) or about 0.5 hour or more at a temperature of 1155°F (625°C).
Contacting the steelmaker to determine if there is any precipitation or other property-altering effects that occur during this tempering operation is also advised.