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Bainite is a phase in steel that can be created by heating the steel up to an austenitic temperature followed by a designed cooling procedure. An interesting fact about bainite is that it was previously developed under the name troostite.

When heat treating a steel to make it hard or create alternative structures, there are three conditions that are necessary to accomplish the required phase change. They must be adhered to in order to accomplish the desired phase. These three conditions are carbon content, temperature and cooling rate.

Therefore, with sufficient carbon present coupled with the steel being at the appropriate temperature to ensure phase transformation to austenite followed by soaking at temperature to ensure complete austenite phase transformation, different cooling speeds will result in different metallurgical phase conditions.

The next required step will be the appropriate cooling rate to ensure the phase transformation into bainite. The phases that can be created include ferrite, ferrite plus pearlite, pearlite, upper bainite and martensite (Fig. 2). It should be noted that the bainite phase is formed at a higher process temperature than necessary to form martensite. Bainite is a microstructural phase that is tougher (higher impact strength) but also reasonably hard.

Lower bainite is somewhat more difficult to distinguish hardness in relation to that of martensite. It differs in hardness to that of pearlite.

*Click the image for greater detail

Fig. 1. Photomicrograph showing a lamellar structure with ferrite lathes

Understanding the Bainite Structures

Upper Bainite

Upper bainite is formed as a result of initially creating the austenite phase in a plain carbon steel, a tool steel or a martensitic stainless steel. It is created by cooling the steel down from the austenite range down to a temperature of approximately 950-1000°F (510-538°C). There is no “magic number” for the appropriate quench/cool-down temperature rate.

The final process selection temperature is dependent on the hardness value and other mechanical properties that are dependent on the final phase temperature.

Upper bainite structures in low-carbon/low-alloy steels and high-carbon/high-alloy steels will have a structure of what is known as lamellar pearlite with thin ferrite lathes (Fig. 1) dispersed throughout the observed microscopic grain. The accomplishment of a bainitic structure is determined by:

• Austenitizing temperature selection
• Cool down to a nominal and approximate intermediate temperature of 975°F (524°C)
• Holding time at the cool-down nominal temperature of 975°F (524°C)

The expected hardness value will be determined by the carbon content of the steel. If the steel is an alloy steel, a combination of the carbon and carbide-forming elements will determine the hardness value.

Lower Bainite

Lower bainite is formed as a result of the selection of both the austenitic temperature and final cooling-rate temperature, as well as cool down to a nominal temperature (I suggest down to below 150°F).

The resulting hardness value will be determined by the carbon content of the steel. If the steel is an alloy steel, then there will be a combination of both the carbon and carbide-forming elements.

Generally, the lower bainitic strength is determined by the structural fineness and the carbide-forming elements’ carbide dispersion. These properties will be determined by the temperature selection of the quench medium.

The unique property of either of the bainitic structures (upper and lower) is that the material does not require a tempering procedure. This feature is simply because we are not transforming the created austenite to martensite, which would necessitate a tempering procedure.

*Click the image for greater detail

Fig. 2. Formation of the phases that can be created

Laboratory Examination of a Treated Sample

The observation of upper bainite is observed microscopically by the following procedures after the temperature transformation and phase structure have stabilized. 

• Cut a sample.
• Mount the sample in a thermos-setting compound.
• Pre-grind the sample using silicon-carbide papers from 180 grit up to 600 or 800 grit.
• Rough polish the sample using 1-micron abrasive polishing compound (diamond or aluminum oxide slurry).
• Final polish the sample using a 0.5-micron abrasive polishing compound (diamond or aluminum oxide slurry). I prefer to use aluminum oxide because of its solubility in water and the abrasiveness of the aluminum.
• Rinse the sample in cold, running water.
• Spray the sample with de-natured alcohol.
• Blow-dry the sample with a warm-air blower.
• Observe the sample microscopically between 400x up to 1000x.

All figures/graphics provided by the author.