99 Diseases of Pressure Equipment: Inadequate Postweld Heat Treatment (PWHT)

Inadequate PWHT is one of our pressure equipment nemeses. We normally specify PWHT for a variety of pressure equipment integrity reasons including when we need to lower residual stresses, increase resistance to cracking or soften weld hardness. All for the purpose of prolonging the service life of our equipment and preventing unexpected failures. But this issue is clearly where those old sayings of "Buyer beware" and "You don't get what you expect, you get what you inspect" apply quite often. Too often, buyers don't get uniform temperatures, sufficient soak times, specified minimum temperatures, etc., unless we monitor the PWHT operation and carefully specify where to place the thermocouples. Sometimes buyers don't even specify sufficient temperatures to allow adequate stress relief. This is especially true for some low alloy equipment and equipment that may need a bit higher PWHT temperatures for environmental cracking resistance than is specified for "normal code construction". I once saw a tower that was being PWHT'd in the field with a fired burner in the bottom end up with a slight banana shape because of inadequate temperature monitoring. I've also witnessed a nozzle on a component being PWHT'd in the field with electric resistance mats severely oxidized because the PWHT thermocouple was placed only on the opposite side of a very heavy section into which the nozzle was welded. The mats kept on heating until they burned the nozzle up and the heavy section finally came up to temperature.

Do all your PWHT standards and specifications include the appropriate QA/QC that will ensure that you receive what you expect?