There are many applications, such as gears and rolling contact bearings, where the components are subjected to shock loads and wear and tear due to a combination of sliding and rolling contact with mating surfaces in relative motion. (For more on this topic, read Understanding Carburization: The Positive and Negative Impacts on Metals.) Such applications require special materials that are processed through controlled heat treatment. Case hardening is one such heat treatment process.
What is Case Hardening of Steel?
Solid metals contain molecules and atoms packed tightly into crystalline structures. (This structure is examined in the article The Crystalline Structure of Metals.) When carbon diffuses into the crystalline structure from carbon-bearing matter on the surfaces of low carbon steel or alloy steel, the strength and the hardness of the outer layer increases dramatically. The depth of the outer case of carbon diffusion can be finely controlled, thus determining the mechanical properties of the case.
Hardening of ferrous materials is accomplished via two main methods: through hardening and case hardening.
During through hardening, the component part is heated in such a way that both the surface and the inner core become uniformly harder. Parts that require through hardening are heated in a high carbon environment to ensure uniform hardening of the entire mass of the parts. Through hardening can, however, make the entire metal mass brittle.
The case hardening method aims at overcoming this problem of brittle mass formation.
Key benefits of case hardening of steel are discussed in the following sections.
1. Case Hardening Improves the Durability and Usage Economy of the Steel Parts
One of the key benefits of case hardening is the enhanced durability it provides to steel parts. The mechanical strength and surface hardness developed by case hardening, as well as the retention of the soft core, significantly enhance the wear resistance and fatigue life of components. Retention of a softer core improves the ability to absorb energy released by impact loads and thus contribute to longevity and economic advantage.
2. Case Hardening Enables Use of Steel with Superior Machinability for Heavy-Duty Applications
Typically, the alloys used for heavy-duty applications have low machinability because they are required to be harder as well as stronger. In this context, the case hardening process enables the use of low carbon steel with precision machinability for weapons and firearms applications and other similar heavy-duty uses that require mechanical strength, fine finishes and accurate geometry. Subsequent case hardening after machining provides superior wear resistance and hardness on the surface of the precision machined parts.
3. Case Hardened Steel Enhances Performance of Taper Roller Bearings
Case hardening leaves a softer core that is advantageous for accommodating a misalignment condition common in taper roller bearings. A strong surface layer with a soft inner core ensures fracture resistance under an extreme operating environment that includes misalignment, vibration and severe shock loads. Bearing rings and rolling elements are case carburized to ensure their suitability for shock loads and a combination of axial loads with thrust loads.
4. Case Hardening Leads to Noise Reduction and Energy Savings for Wind Turbine Gears and Other Heavy Equipment
Case hardened steel is well-known for fracture resistance under impact load conditions. Case hardened steels are preferred for engineered components such as camshafts, link components, driving pinions and universal joints. These components are essential for ships, trucks, earth moving machinery and race cars. Because case hardened steel components can withstand cyclic stresses, they are used to produce turbine gears for hydroelectric and steam power houses. Due to their fatigue strength and superior wear resistance, case hardened steel components are also used in metal rolling mills, heavy-duty cranes, mining machinery and other mechanical engineering applications.
For wind turbine gears, the case hardening process has undergone further technical developments to meet the challenges of extreme loads caused by sudden increases in wind speed and the need to apply hard stops to wind turbines.
Although proper material selection and case hardening improvements aim to significantly reduce the noise produced by gear box operation, the noise level inherently increases due to abrasive wear of the gear teeth. Therefore, alloy steels with deep-hardening ability are chosen for increased abrasion resistance and higher fatigue life. Increasing the steel's molybdenum content improves its hardenability. Reducing abrasive wear, noise and vibration lowers the waste heat produced by the gears, thus contributing to energy savings during turbine operation. Case hardening improves the performance of cam shafts that are subjected to severe impact loads in engineering applications.
5. Case Hardening Enables Formation of Residual Compressive Stresses in Gear Teeth
According to one study, case hardening of gears enables the formation of residual compressive stresses both inside the case depth and on the surface, which avoids crack formation and neutralizes bending stress. Advanced induction hardening methods have enabled accurate case hardening of heavy-duty spiral bevel gears, pinions, hypoid gears and helical gears of complex geometry.
Other advantages of advanced induction hardening include long life, energy efficiency, accurate hardening and minimization of part deformation due to superior control over the heating process.
6. Bearings Made of Case Hardened Steel have Superior Fracture Resistance
Bearings made of case hardened steel such as tapered roller bearings and cylindrical roller bearings have high fracture resistance and toughness, as compared to through hardened steel bearings. Case hardening prevents the catastrophic failure of bearings because the fatigue resistance is improved.
Case hardened cylindrical and tapered roller bearings are used in earth moving equipment, excavators, drilling equipment, suction pumps, presses and washing machines. The rollers used in rolling element bearings are essentially case hardened in order to enhance their ability to absorb shock loads. The inner and outer races of large tapered roller bearings are also case hardened because they must accept radial and thrust loads. Cups and cones are case hardened as well.
7. Case Hardening (Nitriding) with Duplex Hardening Improves Corrosion and Abrasion Resistance
Case nitriding with martensitic hardening (duplex hardening) of bearing steels leads to superior corrosion resistance and abrasion resistance. These bearings can operate under starved lubricant conditions and are used in high speed applications such as aircraft engines.
8. Weldability of Steel is Improved by Case Hardening
Case hardening improves the weldability of steel, which is important for certain engineering applications.
9. Case Hardening of Gears by Nitriding Minimizes Distortion
Case hardening of steel gears by nitriding produces low cost, wear-resistant load bearing surfaces with minimal distortion. Nitrided surfaces do not lose their hardness at temperatures in the vicinity of 150°C (302°F), as is the case with carburized steels. (Also read: Nitriding for Corrosion and Wear Fatigue Resistance.)
10. Case Hardening Produces Improved Fatigue Resistance for Gear Teeth
The load bearing teeth of heavy duty power transmission gears are almost by necessity always case hardened. In a typical gear manufacturing scenario, the heat treatment cost accounts for approximately 30% of the total cost, which is second only to production process cost at 55%. A properly chosen case hardening process contributes to energy savings, cost control, reliability, maintainability and serviceability. To accomplish superior bending and fatigue resistance at the root of the teeth, the surface hardness and residual compressive stresses resulting from case hardening are critical factors.
Case hardened steel is preferred in applications where the components are subjected to shock loads, vibration and misaligned conditions. Unlike through hardened steel, case hardened low carbon steels and alloy steels become tough, strong and hard without being brittle. Case hardening also results in a wear-resistant surface that provides durability and reliability.