Bainite Hardening of Steel: Part One

Depending on the cooling rate and alloy composition, austenite can transform in to different phases and structures. Martensite is a hard and brittle phase, but can be tempered to have a high toughness due to reduced residual stresses and a decrease in dislocation density. Slower cooling rates often results in pearlite, a lamellar structure of ferrite and cementite, which is formed at relatively high temperatures. Ferrite and cementite grow side by side to form pearlite, by a so-called edge growth. Pearlite is a soft and ductile structure and is not desired in high pressure applications.

However, at lower temperatures (550°C down to martensite start temperature Ms), the mechanisms behind the formation of ferrite and cementite are different, resulting in a nonlamellar structure. The diffusion of carbon gets sluggish at lower temperatures, resulting in a fine and complex structure called bainite. During the transformation into bainite, ferrite is the leading growing phase, and the formation of cementite will occur when the carbon concentration, of the surrounding austenite or the ferrite phase, is high enough to allow precipitation of carbides. The growth of bainite and the mechanisms behind it have been a topic of debate for a long time. Two main theories are generally discussed: diffusionless and diffusional theory. As the names reveal, the theories explain independent mechanisms that depend on diffusion or diffusionless transformation.

Bainite hardening or austempering is a commonly used process especially in the bearing industry.

Traditionally bainite hardening was always combined with salt bath technology. About 25 years ago, when the old salt bath technology lost its’ importance because of dirt, safety and ecological reasons, the interest in bainite hardening was decreasing, too. Times have changed, modern salt bath installations are completely capsuled and safe. The composition of salts was adapted to fulfil the ecological requirements and recycling of the used salts can be carried out.

Metallurgical Fundamentals

The structure of bainite can vary in a wide range, depending on the temperature of the salt bath. Depending on the temperature, two different structures can be achieved: the “upper” and the “lower” bainite (Figure 1).

Figure 1: Structure of bainite needles, lower bainite, B) upper bainite structure

The hardness of bainite varies from about 40 HRC for upper bainite to about Rockwell 60 HRC for lower bainite. This increase in hardness, as with pearlite, is a reflection of the decrease in size and spacing of the carbide platelets as the transformation temperature decreases.

Cooling strategies for bainite hardening

Possible cooling rates are influenced by many parameters, such as quenching speed of the salt, the salt bath temperature, agitation of the quenching media, dimensions and weight of the load (or the parts), loading density, etc. Figure 2 shows a continuous (A) and an isothermal TTT-diagram (B) of SAE 52100 (= 100Cr6), the typical bearing steel. Depending on the steel grade, for bigger parts or higher wall thickness the formation of bainite cannot be avoided by continuous quenching, leading to an inhomogeneous structure (Figure 2A). This structure is caused by different types of bainite, which develop during continuous quenching.

Figure 2: Possible cooling strategies for bainite hardening: Continuous cooling (red), isothermal cooling with dwell time for desired hardness and structure (blue) and multi-stage process (green, orange)

To achieve a homogeneous structure, the material grade and the quenching ability of the quenching bath must fit to the load to allow the cooling curves to pass the pearlite and the bainite “nose” by isothermal quenching. As you can see from Figure 2B, the dwell time for fully finishing the bainitic structure could become very long. This time is increasing with increasing alloying content of the material, especially with Nickel or Molybdenum, and by lowering the salt bath temperature. To shorten the dwell time, two different strategies can be chosen: to increase the dwell temperature at the end of the bainitization or to quench the parts slightly below martensite start, create some martensite seeds and then follow the dwell time at bainitization temperature.

Table 1 gives an example for reducing the dwell time for bainite hardening of bearing steels, which were austenized at 845 - 860°C. The variation of the austempering temperature can shorten the dwell time for 60 up to 70%:

Table 1: Cooling strategies for bainite hardening of bearing steel