Phase Transformation Kinetics during Welding
The time- and temperature-dependent transformation of the austenitic microstructure into ferrite, bainite, or martensite during the cooling phase of the welded joint.
If you want to influence the grain structure when welding, you have to control the cooling. Depending on how fast the heat is removed, different microstructures form in the steel – from soft and tough to hard and brittle.
If you cool too fast, hard, crack-prone structure can form. If you cool too slowly, the structure becomes coarse and toughness suffers. The right cooling rate depends on the material – with some steels you have to preheat, with others normal air cooling is enough.
Make sure the heat doesn't escape too quickly, especially with thick parts. If in doubt, ask before you embrittle a weld.