Case Hardening Explained with Examples

Want to improve the abrasion resistance of your workpiece without affecting its ductility and toughness? Case hardening is the way to go.

But what exactly is case hardening? And how do you perform case hardening of a workpiece?

Case hardening is a heat treatment process that increases the surface hardness of the component while maintaining ductility at the core. The process involves the diffusion of carbon, nitrogen, or both to increase surface hardness and wear resistance. It is prominently used for automobile spares, aerospace components, cutting tools, etc.

This article provides a detailed guide on case hardening by going through its process, types, benefits, and applications.

In the end, the article also discusses the methods of descaling the workpiece after case hardening and differentiates between case hardening and surface hardening.

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What is Case Hardening?

Case hardening is a heat treatment that produces a hard and wear-resistant surface ‘case’ without affecting the ductility of the core of the workpiece.

Unlike other heat treatment processes like annealing, tempering, and normalizing, case hardening only alters the properties of the surface of the workpiece.

Therefore, an annealed steel will have improved ductility throughout its cross-section, whereas a case-hardened workpiece will be hard and brittle on the outer surface while maintaining its ductility and toughness in the core.

It involves heating above 1334 °F in the presence of carbon or nitrogen.

This carbon or nitrogen is then diffused on the surface of the workpiece, resulting in the formation of a hard surface casing.

However, the process is maintained in such a way that it does not affect the core of the workpiece, allowing it to maintain its ductility and toughness.

A tough core and hard surface are ideally required for components used in heavy-duty machinery, such as shafts, gears, and cogs.

Types of Case Hardening

Type of Case Hardening	Characteristics
Carburizing	Suitable for low-carbon steels
Nitriding	Suitable for low alloy steel grades
Carbonitriding	Produces better results than carburizing and nitriding
Cyaniding	Quick process, takes less than an hour

Case hardening can be achieved by different methods and each method requires proper procedure and good process control to get the desired results.

Carburizing

Carbon from the surrounding diffuses in the steel surface when heated above a definite temperature.

The component has to be plunged and socked in a carbon-rich source for a predetermined duration.

Low-carbon steels are commonly case hardened, as they do not have enough carbon to harden them using heating and quenching treatment.

Depending on the type of medium used, there are different types of carburizing, such as pack carburizing, gas carburizing, liquid carburizing, and vacuum carburizing.

Process of Carburizing a Workpiece

The first step is surface preparation, which involves cleaning the surface using any cleaning agent to remove any surface contaminants.

The second step involves heating the workpiece in carbonaceous material. It is achieved by packing the workpiece with 80% granular carbon and 20 % barium carbonate in a heat-resistant steel box and placing it in a muffle furnace.

The carbon powder mixture should be filled at least 1 inch above the component.

You can also use a cyanide salt bath with barium chloride (BaCl2) in place of the carbon and carbonate mixture.

Ensure the steel box has a tight-fitting lid and seal gaps with fire clay or refractory mortar to prevent leakage during heating.

When case hardening, ensure that the workpiece is not touching the steel box or other workpiece in the same box.

The third step is to hold the steel container in the furnace at 1700 °F temperature until the desired case depth is achieved. As a rule of thumb, case depth increases by 0.1 mm every hour.

In the fourth step of case hardening, the workpiece is gradually and intermittently immersed in water or oil for quenching.

It should be noted that oil quenching will produce lower case hardness than water quenching.

The final step in case hardening is tempering, which involves heating the workpiece below a lower critical temperature and allowing it to cool in still air.

The tempering temperature is selected based on the required target mechanical properties.

Nitriding

Nitriding is an alternate method of case hardening where Nitrogen is diffused instead of carbon.

This process is generally suitable for low alloy steel grades like AISI 708A37 and AISI 817A40. 

Any alloy suitable for nitriding will typically have alloying elements within the following range:

Alloying Element	Percentage Composition
Carbon	0.1 - 0.5 %
Aluminum	0.75 - 1.25 %
Chromium	1 - 1.5 %
Molybdenum	0.2 %

Process of Nitriding a Workpiece

The first step is to clean the component's surface to remove any contamination, dust, or oil.

After cleaning, preheat the component to around 50°F higher than the nitriding temperature.

To perform nitriding, place the component in a heated furnace to a temperature between 1022 °F and 1122 °F.

Anhydrous ammonia is introduced into the furnace, causing the nitrogen to diffuse into the workpiece surface.

Depending on the desired case depth and component size, the process will be completed in 21 to 100 hrs.

Once the heating is completed, remove the component from the furnace and allow it to cool in the air.

Tempering can be performed to enhance the nitriding layer's properties and improve the component's overall performance.

Moreover, depending on the medium used, there are different types of nitriding, such as gas nitriding, ion nitriding, salt bath nitrocarburizing, etc.

Carbonitriding

Carbonitriding is the process of diffusing carbon and nitrogen to increase the surface hardness of the workpiece.

In this process, a gaseous mixture of ammonia, methane, and argon is introduced into the furnace atmosphere while heating the workpiece to around 472 °F  to 1578 °F.

Compared to carburizing and nitriding, carbonitriding takes a longer duration to complete and produces parts with better surface hardness.

Generally, the thickness of the case produced by carbonitriding is maintained between 0.07 mm to 0.75 mm.

Increasing the thickness beyond that reduces the efficiency of the product by significantly increasing the time and cost.

Cyaniding

Cyaniding is the process of heating the workpiece to around 1600–1750 °F in a bath of sodium cyanide.

Unlike carburizing and nitriding, it is a quick process and can be performed in under an hour (around 30 minutes).

This process is suitable for applications where a case thickness of around 0.01" to 0.03" is required on small workpieces like nuts, bolts, etc.

However, sodium cyanide is a poisonous material and should not be used without proper safety gear.

Ferritic Nitrocarburizing

Ferritic nitrocarburizing (FNC) is a case-hardening process primarily used for ferrous metals.

Based on the medium used for case hardening, ferritic nitrocarburizing can be divided into 4 categories: Salt bath, gaseous, fluidized bed, and ion or plasma ferritic nitrocarburizing.

The salt bath method is also known as Tufftride, Melonite, and Tenifer, while the gaseous FNC process is known as Nitrotec.

Unlike other case hardening techniques, this process involves heating the workpiece within its ferritic phase, without transitioning into the austenitic phase.

Generally, FNC involves heating the workpiece around 525℃ (977℉) to 625℃ (1,157℉), which allows for better dimensional stability of the workpiece throughout the process.

This process involves diffusing nitrogen and carbon in the ferritic state of the workpiece which results in improved abrasion resistance, fatigue-bearing capabilities, and corrosion resistance.

Flame Hardening or Surface Hardening

Unlike other case hardening processes, flame hardening does not involve the diffusion of external elements in the workpiece.

Instead, it involves heating high-carbon workpieces to diffuse the already present carbon molecules towards the surface, leading to case-hardening of the workpiece.

The workpiece is heated either by an oxy-gas flame or in an induction furnace, followed by quenching it in water to induce thermal shock.

This results in the transformation of the workpiece material along the surface into the martensite phase, forming a hardened case.

However, this process is only suitable for workpieces having a carbon content of around 0.3–0.6 % by weight.

Benefits of Case Hardening

Case Hardening Improves Surface Hardness

Process	Carburizing	Nitriding	Carbonitriding
Hardness range	62-64 HRC	62-65 HRC	55-62 HRC

Case hardening increases the surface hardness of the component, making it more resistant to wear, abrasion, and erosion.

A lower wear rate of the component increases its life span in service.

Gears used in automotive applications are case-hardened to prevent wear while maintaining their core toughness to avoid failure under load.

Cutting tools, such as drill bits and milling cutters, have a hardened surface, improving tool life.

Bearings used in heavy machinery are case-hardened to reduce wear and increase the machine's life.

Case Hardening Improves Fatigue life

Case hardening increases the component's fatigue life, making it suitable for higher variable loading applications.

Crankshafts in internal combustion engines are case hardened to reduce the chances of failure due to repeated lading.

To reduce the chances of landing gear failure during take-off and landing, the landing gear components of aircraft are case hardened.

Case Hardening Improves Corrosion resistance

The diffusion of carbon and nitrogen on the surface enhances corrosion resistance, making the workpiece suitable for harsh working conditions.

Valve components used in the chemical processing industry are case-hardened to reduce degradation in a corrosive environment.

Case hardening the muzzles of firearms reduces the chances of failure when used in a wet and humid environment.

Fasteners used in marine applications are case-hardened to reduce the chances of degradation due to highly corrosive marine environments.

Case Hardening Retains Toughness

Unlike other heat treatment processes that alter the properties of the workpiece as a whole, case hardening only affects the surface of the workpiece.

As a result, case-hardened workpieces are hard and brittle at the surface while being ductile and tough at the core, allowing the workpiece to resist wear while maintaining its durability.

Tools such as hammers and chisels undergo case hardening to improve the abrasion resistance of the surface while maintaining its ductility to withstand high-impact loads.

Bucket teeth, bulldozer blades, and other mining equipment components are an example where a tough core and a hard surface are a must.

Case Hardening Enhances Dimensional Stability

 Case hardening reduces the risk of failure from dimensional instability and warping.

Shafts in rotary machines are case-hardened to reduce the chances of dimensional instability due to surface wear.

Lead screws and feed screws are often case-hardened to maintain their precise dimensional tolerances and reduce the risk of failure.

Application of Case Hardening

Components such as gears, cams, shafts, and crankshafts are case-hardened to increase fatigue strength and reduce surface wear.

Landing gear components such as axles and struts are case-hardened to improve resistance to wear.

Bolts, screws, and other fasteners are case-hardened to improve shear and tensile loading resistance.

Therefore, for every application that demands excellent abrasion resistance without affecting the toughness of the workpiece, case hardening is the way to go.

Types of Metals Suitable for Case Hardening

Low-carbon steels with carbon percent lower than 0.25 % are suitable for carburizing and are called carburizing steels.

AISI 1018, AISI 8620, AISI 5015, AISI 4023, and AISI 1117 are a few examples of carburizing steel.

AISI 8620 steel is a low alloy, Ni-Cr- Mo steel generally used for manufacturing shafts and gears.

AISI 4320 steel is a low alloy steel with excellent hardenability, most commonly used in structural components and gears.

AISI 4140 steel is a low alloy steel best suited for landing gear applications.

AISI 9310 steel is a Ni-Cr-Mo low alloy steel but suited for high-strength mechanical components.

AISI 140 and AISI 4340 steels are very commonly nitrided before using them for applications.

AISI H13 is a high-carbon steel grade commonly used in high-temperature applications and is nitrided before being used.

AISI M2 is a high-speed steel best suited as a cutting tool for machining and is also nitrided to increase hardness and tool life.

Along with steel, cast irons, aluminum alloys, and nickel-based alloys are case-hardened for different applications.

Descaling of Case-hardened Steel

Case hardening involves heating the workpiece and then quenching it. This process often results in the formation of scales on the surface of the workpiece.

These scales are more prominently formed when heating low-carbon steels, whereas high-carbon steel or alloy steels show good resistance to scale formation at lower temperatures.

There are different methods of removing the scales from the surface of the workpiece, such as sandblasting, laser removal, acid pickling, ultrasonic cleaning, etc.

Sandblasting involves using compressed air with abrasive particles to remove the scales from the workpiece.

On the other hand, laser rust removal involves using a high-energy laser to vaporize the unwanted material (scales), leaving behind a smooth surface free from scales and other impurities.

Acid pickling uses a pickle liquor or a mixture of acids to remove the oxide scale. During the process, the workpiece is completely immersed in the acid bath and allowed to stay there until the acid decomposes the unwanted layer.

Generally, hydrochloric acid, sulphuric acid, or a mixture is most commonly used to descale carbon steels.

Inhibitors should be added to the acid bath to reduce the ill effects of acid treatment. Hydrogen embrittlement is one such ill effect.

Apart from that, hot alkaline solutions can also be used to descale a workpiece.

Ultrasonic cleaning in a high caustic solution will also remove the oxide scale in cast iron and steel parts.

Using a moderately alkaline solution for ultrasonic cleaning is beneficial for aluminum and zinc.

The scale can be dissolved electrochemically by passing a current through the acidic solution and submerging the component into it.

Case Hardening Vs Surface Hardening

Parameters	Case hardening	Surface hardening
Definition	Surface hardness increases by diffusing carbon or nitrogen.	Surface hardness increases by altering surface microstructure.
Process	Carburising, Nitriding, carbonitriding, cyaniding	Flame hardening, induction hardening, laser hardening, ion hardening
Outcome	Increased surface hardness and improved wear resistance.	Increase in surface hardness, wear resistance, and overall durability of the component.
Hardening depth	In few millimeters	a few tenths of a millimeter
Residual stress	High	low
Microstructure	Contains nitrides, carbides, and martensitic microstructure	Martensite on the surface and a soft central core
Applications	Gears, crankshafts, and camshafts	Cutting tools, bearings, dies, punches, etc.

Unlike case hardening, surface hardening is the process of increasing surface hardness without introducing any chemical elements like carbon, nitrogen, etc.

It uses heat to modify the internal structure of the workpiece to increase its surface hardness, without altering the core properties.

In this process, the workpiece is heated to an extremely high temperature (above 1670 °F), by using an oxy-acetylene torch or an induction furnace.

After the desired temperature is achieved, the workpiece is immediately quenched in water. This sudden temperature shift alters the internal structure and increases the surface hardness of the workpiece.

Generally, surface hardening is only suitable for metals with high carbon content, such as high-carbon steel, whereas case hardening can be performed on almost every grade of steel.

In surface hardening, there is no need to heat the whole workpiece. Localized heating and quenching can be done to improve the surface property of the region. In case of hardening, the whole workpiece is heated and treated. 

Induction surface hardening is a very fast process compared to any carburizing method and is accomplished by induction heating, and the depth can be controlled by alternating current frequency.

High-frequency induction heating produces shallow hardening depth, and low frequency produces deeper depths. Depth control is easier in induction surface hardening than case hardening.

For localized hardening, electron beam and laser hardening are preferred over induction and flame hardening, but the process is costly.

However, case hardening produces a harder surface and is more durable than a surface hardened surface.

Final thoughts

Case hardening is very effective in increasing the surface hardness of the component without changing the core properties.

Although carburizing and nitriding are the most commonly used technique, carbonitriding is preferable if a deeper case thickness is desirable.

On the other hand, cyaniding is preferable for applications where a quick cycle time is desirable, irrespective of the case thickness.

The addition of carbon makes carburizing ideal for case hardening of low-carbon steels, whereas flame hardening is preferable for high-carbon steels.

Frequently Asked Questions (FAQ)

Can case hardening be used for precision components in electronic devices?

Yes, case hardening can be used for precision components in electronic devices. Case hardening does not change the dimensions of components. It increases dimensional stability by reducing the chances of wear and corrosion. The case-hardened surface does not allow any ingress of corrosive substance.

Can you perform case hardening for a DIY project?

Yes, you can perform case hardening for a DIY project, provided that you use the appropriate safety gear and strictly follow the procedure. However, surface hardening or flame hardening is preferable for DIY applications due to its simple process.

How does case hardening improve the life of robotic arms?

Case-hardened components help improve the robotic arm's functionality by maintaining its dimensional stability and enhancing its abrasion resistance.