Bainite is a metallurgical structure/phase, which is caused by cooling from an appropriate austenitizing temperature to a preselected temperature above the Ms line of the Time-Temperature-Transformation (TTT) diagram.

The phase will begin to transform at temperatures below 1020°F into the constituent of bainite, which will begin to precipitate out of the austenite when cooled from the appropriate hardening temperature.

As the temperature begins to cool below the level of 1020°F, bainite will begin to form (upper bainite and progressively down to lower bainite) within the surrounding austenite grains. When etched with a standard 3-5% etchant, it will be difficult to observe from other surrounding phase constituents. This is because it will change how it appears under the microscope according to the temperature of transformation.

In alloy steels, the transformation temperature will be different than those of plain-carbon steels due to the influence of alloying elements such as:
  • Carbon
  • Chromium
  • Molybdenum
  • Manganese
  • Boron
  • Silicon
  • Tungsten
These elements have the ability to manipulate the critical cooling curve on the TTT diagram. Typically, the nose of the curve will shift to the right.

The resulting bainite is a tough phase. The cooling rate is stopped short of the appropriate Ms line according to the steel chemistry. In other words, martensite is not allowed to form. The result is a very tough structure that does not necessitate any tempering. The resulting hardness will be determined by the temperature to which the steel is cooled.

The cooling method from the austenitizing temperature will be accomplished in a molten salt bath comprised of potassium nitrate and sodium nitrite. As long as the temperature is higher than the Ms transformation temperature (TTT diagram) for that particular steel, bainite will form.

Depending on the type of bainite (upper or lower) that is required in relation to the final mechanical properties for a particular steel, this will determine the salt-bath temperature for quenching. Quenching into oil is not usually good practice to form bainite, simply because of the flashpoint temperature of oil. It then becomes a safety issue.