Bainite Start
Is Bs a single universal temperature for all steels?
No. Bs is composition-dependent and also varies with the definition used: microstructural Bs, kinetic Bs, or discrete-heat-treatment Bs are all described in the literature.
How is Bs typically identified in practice?
By dilatometry, metallography, and comparison with TTT/CCT behavior; empirical equations are also used to estimate Bs from steel chemistry when test data are not available.
Does bainite always form above martensite start, Ms?
No. Some bainite transformations can occur below Ms in certain steels, and the literature notes overlap between bainite transformed below Ms and martensitic transformation behavior.
Bainite start (Bs) is the highest temperature at which bainite begins to form from austenite during cooling or isothermal holding; above Bs bainite is not observed, and below it the bainitic transformation can proceed. In steel metallurgy, Bs is a critical transformation temperature used to predict whether austenite transforms into bainite rather than pearlite or martensite, generally within 125–550 °C depending on composition.
On the shop floor, Bs sets the thermal target for austempering and bainitic transformation cycles. A heat-treater holds the part in the bainite window long enough to develop the desired microstructure instead of letting the cooling curve drift into pearlite or martensite territory. Because Bs depends strongly on carbon, manganese, chromium, nickel, and molybdenum, empirical equations are used to estimate it from chemistry; two steels with different alloy contents require different soak temperatures and hold times even when the part geometry is identical. This matters when components need a balance of hardness, toughness, and dimensional stability. Bainitic structures offer high strength with less quench distortion than a full martensitic route, but only if the thermal profile enters and sustains the bainite-start regime. In process control, Bs is used with TTT/CCT diagrams and dilatometry data to verify the cycle crosses into the bainitic region rather than skipping directly to martensite start.
Wrong steel chemistry assumption: A heat cycle copied from one alloy is applied to another without recalculating Bs; the result is either no bainite formation or an unintended mixed structure because Bs shifts with carbon and alloying additions.
Holding in the wrong temperature band: A part transferred too hot or cooled too slowly spends time above bainite start, allowing pearlite or ferrite to form first; too cold and the part moves toward martensite before bainite develops.
Ignoring section size and thermal lag: Thick sections or stacked loads do not reach the same core temperature as the surface, so the outside transforms near Bs while the core misses the bainitic window, creating mixed hardness and warpage.