Diffusion-controlled bainite formation

Bainite forms during controlled cooling between ferrite and martensite formation. Carbon diffuses out of the ferrite and forms fine carbides – this gives the structure its good combination of strength and toughness. When welding, you have to be careful: if you cool too fast, you get hard, crack-sensitive martensite. If you cool too slowly, the grain becomes coarse and toughness suffers. For a bainitic structure, you need a targeted, medium cooling rate. It's in the range that is neither too hasty nor too sluggish – you have to develop a feel for it.