Transformation product formed when austenite is held or cooled in roughly the 250 to 550 °C range, below the pearlite range but above the Ms temperature. Upper bainite forms at the hot end and is feathery and less tough. Lower bainite forms near Ms and gives around 45 to 55 HRC with better toughness and less distortion than quenched-and-tempered martensite at the same hardness. Austempering is the deliberate route to a bainitic part.
Spring clips go to the salt line, the customer wants lower bainite, not tempered mart.
Also heard as
- upper bainite
- lower bainite
Related terms
Austempering
Quenching steel into a salt bath above Ms and holding it until the austenite fully transforms to bainite.
Martensite
The hard, needle-like phase that forms when austenite is quenched fast enough to trap its carbon.
Pearlite
A layered mix of ferrite and cementite that forms when austenite cools slowly through the eutectoid range.
TTT diagram
Time-temperature-transformation chart showing how long austenite takes to transform when held at a fixed temperature.
Ms temperature
The temperature at which martensite starts to form as austenite cools during a quench.