We receive a lot of questions regarding post weld heat treatment (PWHT). Confusion seems to stem from a lack of metallurgical understanding, and how heat alters the microstructure of iron at given temperatures. So firstly, some background…
The atomic arrangement of iron—when it remains under transformation temperature— is called body centered cubic (BCC). The BCC structure is unwelcoming to carbon, accepting very little carbon into it’s microstructure, severely limiting BCC’s strength ratings.
However, when iron is brought to a specific temperature and mixed with carbon (and/or other alloys), iron’s BCC structure has a change of heart. BCC becomes face centered cubic (FCC), an atomic structure that accepts carbon with open arms. The result is called austenitic steel, and this transformation, and the subsequent rate of cooling, greatly alters the properties of the metal.
When steel is rolled, the grain structure is subjected to significant stress, detrimentally distorting the microstructure. PWHT is effective in relieving the internal stress, refining the grains and improving overall strength. PWHT is also very useful for ‘baking out’ excess hydrogen that may exist in weld metal, a defect that can cause hydrogen induced cracking. And finally, in addition to tempering the parent metal, PWHT tempers the heat affected zones (HAZ) caused by the welding arc, adding ductility to all areas of brittleness.
Ultimately, PWHT is highly effective if conducted properly. Excessive temperatures or holding times can adversely effect properties, so extreme care must be taken.