Bainitic intermediate microstructure
A microstructure of acicular ferrite and finely dispersed carbides formed via isothermal transformation above the martensite start temperature, offering a balanced combination of strength and toughness.
Bainite forms when you don't cool the steel too quickly after welding, but hold it in a certain temperature range. You then get a structure that is harder and stronger than normal ferrite-pearlite, but not as brittle as martensite. It's the middle ground between strength and toughness. When welding, you have to be careful not to accidentally get into this range if you don't want it. With higher-strength steels, this can lead to cracks if the heat-affected zone stays in the critical window too long. The exact temperature depends on the material, you have to look it up in the data sheet. In practice, this means: if you want to deliberately produce bainite, you need controlled cooling or a post-weld heat treatment. If you're welding normally, you have to keep an eye on the cooling rate so you don't slip into an unwanted intermediate structure.