In our second piece on bearing quality parameters, we will focus on technology & procedures, more precisely on the heat treatments for bearings. The quality of heat and surface treatments directly affects the bearing’s performance, durability, and resistance to wear and fatigue. These treatments enhance the material properties, improve surface hardness, increase wear resistance, and extend the bearing’s operational life. 

Heat treatment involves subjecting the bearing components, such as rings, balls, and rollers, to controlled heating and cooling processes to alter their microstructure and mechanical properties. This optimizes material properties, enhances strength, and improves resistance to wear and corrosion. 

Main hardening/heat treatment methods used for steel bearings

Through-hardening

Through-hardening is the most used heat treatment method for bearings. It is also known as quenching and tempering and is specified for applications where uniform hardness, strength and wear resistance are required. When using this method, the bearing 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.
Through-hardening creates fine carbide throughout the steel in a process that uses quenching, a rapid cooling process after heating. The bearing is heated and then quenched in either a salt or oil bath. Tempering follows quenching and involves reheating the parts to a lower temperature to reduce internal stresses and enhance toughness.
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. 

Case-hardening

Case hardening is a surface treatment method that increases the hardness and wear resistance of the bearing’s outer layer while maintaining a soft and ductile core. It involves introducing carbon or nitrogen into the surface layer through processes like carburizing or nitriding. This treatment ensures the bearing can withstand higher loads and resist surface wear. It is usually specified for applications where a shock load is likely to occur (construction for example). There are two methods of carbonising or induction hardening: 

• Carburising is a case hardening process in which carbon diffuses into the surface layer of a steel part at a temperature high enough to change the steel grain structure. This change enables the steel to absorb carbon.  The bearing is hardened in an atmosphere of ammonia gas and dissociated ammonia, while the bearing is heated to 920 degrees Celsius.
  The length of time the bearing component 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. Here the quality of the end product depends on the quality of the heat treatment process. It is important for the manufacturer to ensure the right temperatures are applied and that the atmosphere is pure.
   
• Induction hardening uses an electric wire coil to achieve the same results as carburising, 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. 

The right heat treatment procedures used in the production of bearings can significantly influence their quality. Here’s how it can impact bearing performance.

• Enhancing Strength and Durability of Materials – by carefully managing the heating and cooling procedures, the microstructure of the material is optimized, leading to increased hardness, toughness, and resistance to wear, fatigue, and impact.
• Achieving Dimensional Stability and Precise Tolerances – with careful control of the expansion and contraction of the material, heat treatment minimizes internal stresses and distortions, ensuring that bearings retain their precise dimensions and fit seamlessly within machinery assemblies, especially in environments with high temperatures.
• Enhancing Resistance to Wear, Corrosion, and Fatigue – advanced heat treatment techniques and surface modification processes create a protective barrier that enhances the bearing’s ability to withstand abrasive wear, fretting corrosion, and chemical reactions. This fortification extends the service life of bearings, reduces maintenance requirements, and increases productivity by minimizing downtime
• Optimizing Fatigue Life and Reliability – heat treatment plays a crucial role in optimizing the fatigue life and reliability of bearings. By strengthening the material structure, heat treatment reduces the occurrence of internal defects like inclusions and voids.
   

Also, heat treatment offers the ability to customize bearings to meet diverse application requirements. Manufacturers can tailor the heat treatment processes to achieve specific material properties, hardness levels, and surface characteristics. This flexibility enables bearings to meet industry-specific demands, including load capacity, speed, and environmental conditions. Customized heat treatment ensures that bearings are optimized for their intended applications, delivering superior performance and extended lifespan. 

Precise control of the treatment parameters, adherence to industry standards, and appropriate selection of treatment methods ensure consistent quality and reliability of bearings. Manufacturers with robust heat treatment facilities and strict quality control processes can produce bearings with superior material properties, optimized surface hardness, and extended service life, meeting the demanding requirements of various applications.

In case you need any assistance, Codex Team is ready to help you with expertise & further information.