When bearing steels are in their soft (unhardened) state, metallurgists refer to their structure as being in the pearlite state. In order to harden the steel it must be heated to a very high temperature and then cooled very rapidly.
When heated in the heat treat furnace to 1,750°F, the structure transforms from pearlite to what is known as austenite. After quenching (very rapid cooling), the structure then transforms from austenite to martensite.
Once transformed to martensite, the steel becomes very hard. However, at this point it is not considered “thermally stabilized”. This is because not all of the austenite transforms into martensite during the quenching process. This phenomenon is called “retained “austenite”.
If the steel is not thermally stabilized, the retained austenite will over an extended period of time (possibly years) transform into martensite. This transformation is accompanied by an increase in volume that is called metallurgical growth (not to be confused with thermal growth). Metallurgical Growth will cause a change in dimension and form of any steel parts such as bearings’ even at room temperature.
While not a problem with low precision commodity type bearings, in high precision (ABEC 5P, 7P, 9P) miniature bearings this lack of dimensional stability can cause problems.
In order to eliminate this unwanted metallurgical growth, the steel must be subjected to thermal stabilization. This is accomplished by repeated cycles of chilling at -120 F and tempering to transform a large percentage of the retained austenite to martensite.
Area of failure in bearing heat treatment
Distortion is very important to bearing races. Not only do we want to preserve the case, or surface, properties given during surface treatments like carburizing and nitriding, but grinding or hard turning is a relatively expensive process. Final dimensional results are critical to bearing performance. Many manufacturers have elaborate schemes to try to control or minimize distortion, which is the holy grail of bearing heat treatment.
Factors include quench media, uniform media flow through the load, stacking, fixture quenching, and the number of times the part was cooled and how it was cooled. Other, less thought-of items include the temperature achieved, how long at temperature, furnace fixture maintenance, rate of heating/uneven heating of the parts, and cooling orientation.
Quenching phenomenon and method to avoid distortion
The heat transfer during immersion quenching is very complex and it is not very well understood by scientists even to date. Quenching results in boiling of the quenchant and during boiling, the quenchant vaporizes carrying away the heat from the surface of the component.
Thermal conductivity of the steel component, viscosity of the quenchant, chemical nature of the quenchant, temperature of the bath, agitation levels of the bath and a host of other factors affect the overall heat transfer rate. Of significant importance is the fact that the heat transfer mechanism in the quenchant can be distinctly different at different temperatures, giving rise to three important phenomena called (a) the vapor blanket (b) nucleate boiling and (c) convective phases.
Distortion or cracks are formed due to high temperature difference between surface and core of a component. Distortion and cracks can be minimized in a heat treated component by choosing the right quenchant for the component. One of the most important things is that the cooling rate must be just adequate; neither too high nor too low. The correct cooling rate is decided by the steel composition, the section thickness, and the hardness required. Once the cooling rate is decided the proper quenchant is selected which gives the required cooling rate. The cooling rates of quenchants, in turn depend upon the type of quenchant (brine, water, oils, polymer solutions etc), whether they are new or old, clean or contaminated, level of agitation etc.
Is there a way of calculating the required cooling rate for a given component?
Yes, ProQuench software developed by ProVaC Manufacturing Technologies Pvt. Ltd. can calculate surface heat flux and also predicts cooling rate from surface to core. This gives complete insight to the bearing heat treatment. In addition the software is coupled with metallurgical transformation which predicts microstructure and hardness from surface to core. It comes with a probe which uses same bearing steel as specimen and test conducted in the actual quench tank. This is the breakthrough technology every bearing manufacturer can depend upon to decrease the quench failures in the bearing heat treatment.
Some of the key features are:
- In-situ measurement of quenching capacity of different quenching mediums
- Check the ‘health’ of quenchants with continued use
- Select suitable quenchant for specific material to achieve required properties
- Inspection of quenchants in as received condition
- Check the effect of agitation/flow rate/quenching position in the tank
- Check the effect of contamination in quenchants (water in oil; polymer in oil etc.)
- Estimate the heat transfer coefficient during quenching
- Reduce rework, rejection rate and improve quality
- Eliminate destructive testing by conducting cost effective virtual experiments
- As the software runs with standard mathematical models, accuracy of the results are high
If you have additional questions on bearing materials, our team of experts is ready to assist. Pls feel free contact our engineer via www.bywb-bearing.com, we are happy to help!
Metal components in bearings come in contact with other metal or containments, so the hardening process during manufacturing is a vital to ensuring less wear and a longer service life. Hardening also provides the optimal physical characteristics for the bearing’s use.
There are three main types of hardening commonly used on bearing applications: case, induction and through. Each involves:
- Heating to a specified temperature under tight controls
- Holding the temperature for the appropriate amount of time
- Cooling according to the methods which can range from a slow cool in a furnace to a fast cool through quenching.
Case hardening – or surface hardening – is a lot like how it sounds, hard on the exterior and soft in the interior. The process results in the surface metal hardening while the material below the exterior remains soft. This structure allows the core to maintain its toughness. Case hardening is specified for applications where a shock load is likely to occur, such as in construction equipment.
Process: Bearing case hardening uses one of two methods carbonizing or induction hardening.
Carbonizing hardens the bearing in an atmosphere of ammonia gas and dissociated ammonia, while the bearing is heated to 920 degrees Celsius. The length of time the bearing spends in this charged atmosphere determines the depth of the hardness. Using this method, bearings can be case hardened after being machined, quenched and tempered with only a little distortion.
Induction hardening uses an electric wire coil to achieve the same results as carbonizing, however the process can be localized to a specific area, such as a ball bearing’s raceway. This form of non-contact heat treatment uses electromagnetic induction. The depth of the hardness is determined by the frequency of the voltage, material composition and heating time frame.
Through hardening – also known as quench and temper – is specified for applications where uniform hardness, strength and wear resistance is required. When a bearing uses this method, it becomes hard from the surface through to the core. This form of hardening is more cost efficient than case hardening, but may result in brittle steel if improperly tempered.
Process: Through hardening creates fine carbide throughout the steel in a process that uses rapid quenching after heating. The bearing is heated, then quenched in either a salt or oil bath, and tempered to increase ductility.
The goal of hardening is to increase the strength of the metal. A secondary process, such as tempering, is used to soften the material, increase toughness, reduce brittleness or increase ductility.