Case hardening is a process used to harden the surface of metal while keeping the inside soft and tough. The process creates a hard wear-resistant outer layer ideal for gears, bearings, and other high wear parts. There are several methods of case hardening, each with their own advantages and best uses. The most common types of case hardening are carburizing, nitriding, flame hardening, and induction hardening.
What is Case Hardening?
Case hardening is a heat treatment process that hardens the surface of a metal part while the core remains soft. The soft core allows the part to be machinable, resist impact, and withstand high loads. Meanwhile, the hard outer case provides wear resistance, fatigue strength, and high surface hardness.
Case hardening is ideal for parts like gears and shafts that require a wear-resistant exterior and shock-resistant interior. Through selectively hardening the surface, parts can be made at lower cost than using a high carbon steel alloy throughout.
Why Case Harden a Part?
There are several key benefits to case hardening a metal part:
- Improved wear resistance – The hardened case can withstand friction, abrasion, and erosion.
- Higher fatigue strength – Compressive stresses formed in the case increase fatigue life.
- Better corrosion resistance – Hardened cases are less prone to corrosion damage.
- Higher surface hardness – The case has a higher hardness value for scratch and indentation resistance.
- Tougher core – The soft core can absorb more energy before deforming or fracturing.
- Lower cost – Only the case needs to be hardened, not the entire part material.
Case hardening allows cheaper low carbon steels to be used in high wear applications where through-hardened alloy steels would normally be required. The process also enables complex parts to be case hardened after final machining, avoiding the difficulties of machining a fully hardened part.
Types of Case Hardening
The most common types of case hardening used today are:
Carburizing
Carburizing diffuses carbon into the surface of a low carbon steel part at high temperature. The added carbon forms hard martensite upon quenching. Carburizing creates a case depth up to 0.1 inch thick with a high carbon content of up to 1%.
Nitriding
Nitriding diffuses nitrogen into the surface of a steel part through heating it in ammonia or a nitrogen-rich atmosphere. The nitrogen forms hard nitride compounds. Nitriding typically produces a 0.001-0.010 inch case depth with surface hardness up to Rc 65.
Flame Hardening
Flame hardening rapidly heats the metal surface with an oxy-fuel torch followed by quenching. This forms martensite and creates a thin hardened case less than 0.025 inches deep.
Induction Hardening
Induction heating is used to rapidly heat and quench the surface area on parts like gears and bearings. This can produce a localized case hardened area while keeping the core soft.
When to Use Each Method
| Method | Case Depth | Hardness | Best Applications |
|---|---|---|---|
| Carburizing | Up to 0.1″ | Rc 60-63 | Gears, automobile parts, bearings |
| Nitriding | 0.001″- 0.01″ | Rc 65 max | Gears, cams, crankshafts |
| Flame Hardening | Under 0.025″ | Rc 50-65 | Shafts, rolls, valves |
| Induction Hardening | Up to 0.25″ | Rc 45-60 | Gears, shafts, cams |
Each case hardening process has advantages unique to its mechanism of hardening. Selecting the right method depends on the required case depth, surface hardness, economics, and other factors.
The Carburizing Case Hardening Process
Carburizing is one of the most common case hardening processes. It involves diffusing carbon into the surface of low carbon steel parts to form a high carbon martensitic case. Carburizing provides deep case depths up to 0.1″ for excellent wear resistance.
How Carburizing Works
In carburizing, the steel parts are placed into a carbon-rich furnace atmosphere at a temperature of 1650-1700°F. The carbon penetrates the surface and diffuses inwards. As carbon content increases, the steel’s hardenability is raised. When cooled rapidly by quenching in oil or water, the surface transforms into very hard martensite while the core remains soft pearlite and ferrite. A secondary tempering operation can relieve stresses and reduce brittleness in the martensite case.
Types of Carburizing
- Gas carburizing – Parts are heated in gas furnace atmosphere enriched with a hydrocarbon gas.
- Pack carburizing – Parts are embedded in a carbon-rich powder pack inside a furnace.
- Liquid carburizing – Parts are submerged in a molten salt bath containing energized carbon.
- Vacuum carburizing – Uses vacuum furnace to eliminate oxidation and control carburizing.
Gas and vacuum carburizing offer the best control over case depth and carbon diffusion with precise temperature regulation in the furnace. Pack carburizing can be simpler and less expensive but doesn’t allow the same level of process control.
Applications of Carburizing
Carburizing is widely used to case harden:
- Gears – For good bending and contact fatigue strength.
- Bearings and bushings – To resist wear from rolling contact.
- Pump parts – For resistance to abrasive wear and corrosion.
- Automotive components – Like gears, shafts, and drive sprockets.
Any component requiring a thick wear resistant exterior and shock resistance inside are ideal applications for carburizing case hardening.
Nitriding Process for Case Hardening
Nitriding forms a thin, hard case by diffusing nitrogen into the surface of a steel part. The nitrogen reacts with iron to form hard nitride compounds. Compared to carburizing, nitriding offers a thinner case depth up to 0.01″ but with greater hardness up to Rc 65.
How Nitriding Works
In nitriding, parts are heated to 950-1250°F in an atmosphere of ammonia gas or a plasma containing nitrogen. The nitrogen diffuses into the surface and reacts with the steel to form hard iron nitrides. Common nitrides include Fe3N, Fe4N, and Fe2-3N. No quenching is required since the nitrides form directly from the nitriding process. The case can be left soft with a fine, feathery morphology or hardened through low temperature tempering.
Types of Nitriding
- Gas nitriding – Parts are nitrided in a furnace with ammonia gas enriched atmosphere.
- Plasma nitriding – Nitrogen plasma provides faster diffusion and lower processing temperature in a vacuum furnace.
- Salt bath nitriding – Parts are submerged in a molten cyanide salt bath heated to 930-1100°F.
Modern plasma nitriding methods provide the best case hardness and control over case depth. Salt bath nitriding is a cheaper option but with less control and salt residue left on the parts.
Applications of Nitriding
Nitriding is commonly used to case harden:
- Gears – For wear resistance with compressive residual stresses.
- Camshafts and crankshafts – To resist wear and scuffing damage.
- Valves – To withstand hot gases and combustion chamber conditions.
- Extrusion tooling – To prevent galling and sticking.
The thin, hard nitrided case works well on highly loaded parts seeing cyclic stresses and operating against mating surfaces. The process has become a popular replacement to carburizing due to its low distortion and low environmental impact.
Flame Hardening Process
Flame hardening rapidly heats the surface of a metal part with a high temperature oxy-fuel torch followed by quenching. This produces a thin martensitic case ideal when only minor wear resistance is needed.
How Flame Hardening Works
In flame hardening, an oxy-acetylene or oxy-propane flame quickly heats the metal surface to its austenitizing temperature of 1450-1600°F. The torchtraverse speed controls the depth of heating. Spray quenching immediately after heating rapidly cools the surface to form martensite while the core remains unaffected. The result is a thin hardened case just 0.02-0.06 inches deep depending on the steel being treated.
Types of Flame Hardening
- Progressive hardening – The torch oscillates back and forth across a surface to produce even hardening.
- Spot hardening – The torch focuses on one section at a time to produce a localized hardened area.
- Spin hardening – Used for symmetrical parts rotated while the torch hardens the entire surface evenly.
Modern computer controlled systems allow for precise control over the torch motion and timing to optimize the hardening pattern. Robotics can also automate parts handling for higher volume production.
Applications of Flame Hardening
Flame hardening is ideal for applications including:
- Gears – Localized hardening of gear teeth.
- Cams and camshafts – Hardened working surfaces.
- Shafts – Selective surface hardening of bearing surfaces.
- Valves and valve seats – Hardened areas exposed to wear.
The low case depth produced makes flame hardening suitable for less demanding applications requiring mild wear resistance combined with a tough core.
Induction Hardening for Case Hardening
Induction hardening uses electromagnetic induction to rapidly heat just the surface area of parts requiring local hardening. No furnaces are involved – the heating and quenching take place at a work station containing induction coils and quenching equipment.
How Induction Hardening Works
In induction hardening, an alternating current running through a copper coil creates a high frequency alternating magnetic field. When placed in the coils, the part acts as a short circuit generating eddy currents in the surface. This instantly heats a localized area of the part surface. Quenching then rapidly cools the heated zone to form martensite while the core remains unaffected. The process can be repeated to harden multiple areas on a complex part.
Types of Induction Hardening
- Spin hardening – Rotating part is heated by an encircling coil.
- Scan hardening – Coil scans back and forth across the surface.
- Single-shot hardening – Coil positioned to heat specific area.
Modern power supplies allow for precise control over the heating patterns and immediate on-off switching of the magnetic field. This enables very localized hardening without heating entire parts.
Applications of Induction Hardening
Induction hardening is widely used to selectively harden areas on parts including:
- Gears – Hardened teeth while retaining a tough core.
- Shafts – Journal surfaces and mounting points hardened.
- Axles and driveshafts – Hardened spline teeth and bearing surfaces.
- Cutting tools – Localized hardening of tool tips and edges.
The process is ideal when different areas on a complex machined component require different hardness levels or performance. It prevents distortion compared to hardening an entire part.
How to Choose the Right Case Hardening Method
Selecting the optimal case hardening process depends on factors including:
- Required case depth – Carburizing and nitriding allow the deepest hardening.
- Surface hardness level needed – Nitriding offers the hardest case.
- Type of steel being treated – Alloy and tool steels require different processing.
- Geometry of part – Complex shapes benefit from local hardening methods.
- Production volume – Automated processes like gas carburizing suit high volumes.
- Cost considerations – Pack carburizing and salt baths are most economical.
Discussing application requirements with heat treating suppliers helps determine the best case hardening method. The table below summarizes the key characteristics of each process:
| Method | Case Depth | Hardness | Steels Treated | Equipment Cost |
|---|---|---|---|---|
| Carburizing | Deep | Medium | Low carbon steels | High |
| Nitriding | Shallow | Very high | Low and medium carbon steels | High |
| Flame Hardening | Very shallow | Medium-high | Medium to high carbon steels | Low |
| Induction Hardening | Shallow-medium | Medium-high | Medium to high carbon steels | High |
Conclusion
Case hardening allows inexpensive low carbon steels to be selectively hardened at the surface for wear resistance while retaining a tough, ductile core. The main processes include carburizing, nitriding, flame hardening and induction hardening. Carburizing provides the deepest case depth up to 0.1″ while nitriding can achieve surface hardness up to Rc 65. Flame and induction hardening offer localized hardening for complex parts. Carefully selecting the case hardening method based on the application produces the optimal combination of surface wear characteristics and internal properties.