Bainitic microstructure

Bainite forms during transformation between pearlite and martensite. When welding, you get this structure if cooling is controlled so the steel doesn't drop too fast through the critical range. If cooling is too slow, you get pearlite or ferrite; too fast and you end up with martensite, which is hard and brittle. Bainite is tougher than martensite and handles stresses better, so it's what you want for many weld joints. The right cooling time depends on the steel grade and plate thickness – only the data sheet or experience from last time helps there.