Welding Preheat ASME B31.1
- According to ASME B31.1:2018, table 131.4.1-1
In general, the use of preheat will tend to reduce the cooling rate in the weld and HAZ resulting in improved ductility. When no preheat is used, the heataffected zone is relatively narrow and exhibits its highest hardness. In some cases, depending on alloy content, martensite may be formed. However, when preheat is included, the heat-affected zone is wider and the resulting hardness is significantly lower due to the slower cooling rate which allows for the formation of ferrite, pearlite, and possibly bainite instead of martensite. Therefore, the welding inspector may be asked to monitor the preheat required for a particular welding operation. This requirement is primarily concerned with slowing down the cooling rate in the heat-affected zone to produce microstructures having desirable properties.
Base metal thickness also has an effect on the cooling rate; generally, welds on thicker base metal cool more rapidly than welds on thin sections. The larger heat capacity, or heat sink, associated with the thicker sections produces faster cooling of the weld bead. So, when thicker sections are being welded, various welding requirements, such as preheat, may be specified to reduce the cooling rate to improve the resulting mechanical properties of the heat-affected zone. Therefore, when welding heavy sections, preheat and interpass requirements are normally increased to aid in slowing down the resulting cooling rate.
Preheating the base metals is also effective in eliminating hydrogen pickup because hydrogen will diffuse out of most metals at temperatures of 200°F–450°F.