Case hardening refers to a group of processes that produce a hard wear-resistant surface layer on metal parts. The core of the metal part remains soft and tough. This allows the part to resist wear and fatigue while staying resilient. Case hardening is commonly used on gears, bearings, cams, and other machine parts that require a hard surface. But the inner core must remain soft and impact resistant.
Why Use Case Hardening
There are several reasons why manufacturers use case hardening processes:
- Improve wear resistance – The hard surface prevents wear, galling, and erosion.
- Increase fatigue strength – Harder surfaces better resist cracking from cyclic stresses.
- Lower manufacturing costs – Cheaper low-carbon steel can be selectively hardened.
- Maintain core toughness – The soft core handles impact loads and prevents brittle failure.
- Enhance product performance – Optimal combination of surface hardness and core toughness.
Common Case Hardening Processes
The most popular case hardening processes include:
Carburizing
Carburizing involves diffusing carbon into the surface of a low-carbon steel part. This is typically done by heating the steel to 1600-1700°F in a carbon-rich environment. The heat allows carbon to dissolve into the steel up to 0.8-1% carbon content. Once cooled, this forms a hard martensitic case layer on the surface. Carburizing provides the deepest case hardening depth, up to 0.120 inches thick.
Nitriding
Nitriding thermally diffuses nitrogen into the steel surface. The nitrides formed greatly increase surface hardness while maintaining core ductility. Nitriding is done between 950-1100°F. The case depth is typically 0.001-0.012 inches deep. The process does not require quenching media.
Flame or Induction Hardening
With flame or induction hardening, the surface of the steel is rapidly heated with a flame, laser, or induction coil. This heats a thin layer, which is then quenched to harden. The fast heating and quenching creates a martensitic case without significantly hardening the core. Hardened case depths range from 0.05-0.25 inches thick.
Carbonitriding
Carbonitriding combines carburizing and nitriding. First, the steel is carburized at a high temperature to introduce carbon. Then it is nitrided at a lower temperature to diffuse additional nitrogen. This dual treatment creates a very hard case. The case depth ranges from 0.02-0.12 inches.
How Does Case Hardening Work?
The key to case hardening is controlling how carbon or nitrogen diffuses into the steel’s surface. This is done by carefully regulating the temperature, time, and chemical environment. There are several stages to the case hardening process:
1. Pre-Treatment
Before case hardening, the steel parts must be cleaned to remove oils, oxides, and other contaminants. This ensures proper diffusion. Parts may also be preheated to improve the hardened case depth and properties.
2. Case Hardening
During this stage, carbon, nitrogen, or both diffuse into the surface. Parts are heated in environments rich in the hardening element. For carburizing, the steel is heated in contact with carbon-rich gases, liquids, or packed solids. Nitriding uses ammonia or nitrogen gases. Hardening times range from 1-20 hours depending on desired case depth.
3. Quenching
For carbonitriding and carburizing, the steel must be rapidly cooled or quenched after treatment. This produces a martensitic transformation that hardens the case. Quenching is done in oil, water, or air jets. Nitriding does not require quenching since the nitrides themselves greatly strengthen the steel.
4. Tempering
Quenched parts may be tempered by reheating to 400-650°F and air cooling. This reduces brittleness and relieves stresses in the case. Tempering is often done to tailor the final hardness and toughness.
Carburizing Process
Carburizing is one of the most common case hardening processes. Here are the typical steps:
- Clean and preheat parts to 1400-1700°F.
- Heat parts to carburizing temperature, typically 1600-1700°F.
- Expose parts to carbon-rich environment for 1-20 hours depending on desired case depth.
- Quench parts in oil or water to harden the case.
- Temper if needed to reduce brittleness.
There are three main methods of carburizing – gas, liquid, and pack.
Gas Carburizing
In gas carburizing, parts are heated in furnace atmospheres with hydrocarbon gases like methane, propane, or natural gas. The gases thermally decompose and deposit carbon into the steel surface. Gas carburizing offers excellent control and uniformity.
Liquid Carburizing
Liquid carburizing submerges parts in molten salts like sodium cyanide. As parts are heated, the molten salts release carbon that diffuses into the steel. Liquid carburizing provides deep case depths up to 0.2 inches.
Pack Carburizing
Pack carburizing uses solid carbon compounds packed around the parts. As the material decomposes at high heat, carbon absorbs into the steel surfaces. Packs are often made from charcoal, coke, or carbon black mixtures. This method is simple but can cause uneven hardening.
Nitriding Process
Here are the main steps in a nitriding case hardening process:
- Thoroughly clean and heat parts to process temperature.
- Expose parts to nitrogen-rich environment at 950-1100°F for 1-20+ hours.
- Cool parts slowly to diffuse nitrogen and form iron nitrides.
The main methods for nitriding include gas, salt bath, and plasma nitriding.
Gas Nitriding
In gas nitriding, parts are heated in a nitrogen-rich gas atmosphere, usually ammonia which decomposes to nitrogen and hydrogen. The nitrogen reacts at the surface to form nitrides. Gas nitriding can be done as a batch process in large furnaces.
Salt Bath Nitriding
Salt bath nitriding submerges parts in molten nitrate/nitrite salt mixtures around 1000°F. The salts donate nitrogen which hardens the steel case. Salt baths allow precise control of case depth and uniform results.
Plasma Nitriding
Plasma nitriding generates a plasma discharge around the heated parts in a vacuum chamber filled with nitrogen and hydrogen gases. The ionized plasma promotes faster and deeper nitrogen diffusion for thinner cases. Plasma nitriding is free of polluting salts.
Induction Hardening
Induction hardening uses electromagnetic induction to rapidly heat just the surface layer of the part. The local heating quickly austenitizes a thin case zone. Immediate quenching then hardens this case while the core remains unaffected. Here are the basic steps:
- Preheat part to improve heating efficiency.
- Position part inside an induction heating coil.
- Generate a high-frequency alternating current through the coil.
- This induces eddy currents on the part surface, heating a thin layer.
- After a few seconds, quench the hot surface layer to harden.
- Temper if needed to adjust hardness.
Induction hardening provides excellent control over the hardened depth and pattern. It is fast and does not affect the core properties. The case depth ranges from less than 1 mm up to 6 mm deep depending on power intensity and time.
How Deep Does Case Hardening Go?
The depth of the hardened case depends on the case hardening process used. Here are typical case depth ranges:
| Process | Case Depth |
|---|---|
| Carburizing | 0.02 – 0.12 inches |
| Nitriding | 0.001 – 0.012 inches |
| Induction Hardening | 0.05 – 0.25 inches |
| Flame/Laser Hardening | 0.01 – 0.1 inches |
Carburizing provides the thickest case depth, up to 0.12 inches deep from diffusion over several hours. Nitriding produces the shallowest case. Induction and flame hardening lie in between. However, case depth also depends on process parameters like time and temperature.
Case Hardening Applications
Here are some examples of components commonly case hardened:
- Gears – Carburizing provides wear resistance for gear teeth while maintaining tough cores.
- Bearings – Hard bearing surfaces prevent galling but resilient cores handle cyclic stresses.
- Cams and tappets – Hardened cam surfaces improve wear in valve train components.
- Drive shafts – Case hardened shafts resist torsional fatigue cracks.
- Ball screws – Hardened ball tracks prevent wear while threads stay tough against fractures.
- Pistons – Nitrided ring grooves prevent scuffing and wear from piston rings.
The main goal is optimizing the balance of hardness and toughness for each part. Case hardening does this by targeting only the critical surface layer that needs wear resistance or fatigue strength.
Benefits of Case Hardening
Here are some of the main benefits of case hardening processes:
- Improved wear resistance and prevention of scuffing, galling, and erosion.
- Better fatigue and impact strength from hardened surfaces.
- Maintains core toughness compared to through hardening.
- Allows use of low carbon steels which are cheaper than alloy steels.
- Provides design flexibility compared to expensive exotic alloys.
- Environmentally cleaner than some coating processes.
- Often less energy and processing steps than through hardening.
Limitations of Case Hardening
Some limitations include:
- Only provides shallow case depths, usually less than 0.1-0.2 inches.
- Not suitable for very complex geometry parts.
- Same equipment often cannot process a wide range of part sizes.
- Some methods produce undesirable surface oxidation.
- Quenching can cause parts distortion if not properly designed.
- Carburizing and nitriding take multiple hours to complete.
Conclusion
Case hardening is an essential manufacturing process when parts require optimized hardness and toughness. By selectively hardening only the surface layer, components get excellent wear resistance where needed without sacrificing impact strength. Carburizing, nitriding, and induction hardening are the most popular methods to produce a hard, wear-resistant case over a tough ductile core.
Understanding these main case hardening processes allows designers to properly apply them. Parts can be engineered to gain the ideal balance of surface hardness and core resilience needed for their working conditions. The capabilities of modern case hardening continue to expand, allowing ever more precise control over hardened depth and properties.
