Bainitic structural stages
The differentiation into upper bainite with globular cementite particles and lower bainite with fine carbide precipitates based on the transformation temperature.
Bainite forms when austenite transforms in an intermediate temperature range. The structure is a mixture of ferrite and fine carbides, offering a good compromise between strength and toughness. When welding, you have to watch the cooling rate. If you cool too fast, you get martensite, which is hard and brittle. If you cool too slowly, soft ferrite or pearlite forms. The exact temperature control depends on the material. In practice, this means: keep the interpass temperature within limits and don't let the weld cool too quickly if you want a bainitic structure. When in doubt, post-weld heat treatment helps.