Austempering

When bainitizing, you cool the steel quickly from the austenite region to an intermediate temperature level and hold it there until transformation is complete. This produces a fine structure of ferrite needles and carbides instead of martensite or pearlite, offering a good combination of hardness and toughness. In welding, you get bainite when the cooling time is in the right window. If you cool too fast, it becomes martensitic and crack-prone; too slow, and you get soft, coarse structure. If you want to post-weld treat, you bring the part back to austenitizing temperature and carry out the isothermal transformation deliberately. Make sure the transformation runs to completion. If too much retained austenite remains, you'll have problems with embrittlement later. And don't overdo the tempering, otherwise the bainitic structure breaks down into coarse pearlite again.