Basics of Reading a PWHT Chart

Most materials used in mechanical construction industries undergo heat treatment to enhance their mechanical properties. PWHT charts are graphs which display temperature variations during post weld heat treatment (PWHT) processes, such as post weld heat treatment (PWHT). This article will review how to read a PWHT chart; its X-Axis represents Temperature while its Y-Axis represents Time.

X-Axis

This axis represents the rate of heating, soak and cooldown and is symbolized by four curves: AB, BC, CD and AE. Each represents different temperature curves for specific pipe size and wall thickness combinations. You’ll note that as pipe diameter increases for any given wall thickness combination the value “th” decreases as geometrical factors influence weldability which should reduce residual stresses that would require PWHT treatment otherwise. It also suggests that keeping constant diameter may lower heat requirements thus speeding up processing speeds overall.

Temperature

The chart includes several lines (AB, BC, CD and AE) representing different aspects of welding; Rate of Heating Curve 2) Soaking Temperature Curve 3) Rate of Cooling Curve and 4) Room Temperature Curve are among them. Each curve takes different amounts of time to complete; their exact timings depend on the thickness of weld. For instance, repair welds that meet minimum base metal thickness requirements may not require restricted heating at all.

As the yield strength of welds diminish at PWHT temperatures, it is critical that components being treated be adequately supported to prevent unnecessary distortion. This is usually accomplished using trestles shaped specifically to fit their item which are arranged at regular intervals across its length. When treating large items such as pressure vessels, internal supports made from similar material should also be employed; automatic temperature recorders with calibrated controls should be utilized during this process.