Bainitic Needle Spacing
The characteristic distance between carbide needles within the bainitic microstructure, which is largely determined by the isothermal transformation temperature. Lower transformation temperatures result in finer, harder bainitic structures.
The term 'bainitic needle spacing' is not commonly used in practice – what is meant is probably the spacing of carbide needles in bainite. This forms during the transformation of austenite in the intermediate stage. Depending on cooling and temperature control, the structure becomes finer or coarser, and this determines whether the part is rather tough or hard. When welding, this means: if you don't have the cooling under control, the structure will tip in an undesired direction – either too soft and prone to cracking, or too hard and brittle. So: run a cleanly controlled cooling, then the structure will be right and the weld will hold.