Carburizing

Introduction
Plain carbon steels are generally defined as alloys of iron and carbon that
contain up to 2.0 wt.% carbon. Pure iron at temperatures below approximately
910°C has a Body-Centered Cubic structure (BCC). When heated above this
temperature, the iron structure changes to a Face-Centered Cubic structure
(FCC). When cooled, the change is reversed and a BCC structure is again
formed. Up to 2.0 wt.% carbon can dissolve in FCC iron, forming what is
known as a solid solution, whereas in BCC iron no more than 0.02 wt.% carbon
can dissolve in this way (Smith and Hashemi, 2009). The solid solutions of
carbon atoms in FCC iron and BCC iron are called austenite (γ) and α-ferrite,
respectively. As a piece of FCC steel cools slowly and changes to its BCC form,
any dissolved carbon present in excess of 0.02 wt.% will be precipitated,
whereas if it is cooled rapidly this precipitation is prevented. When carbon is
precipitated from austenite it does not assume the form of elemental carbon, but
rather the compound iron carbide, Fe3C, known as cementite. Slow cooling
(equilibrium condition) of steel, in turn, forms alternate layers of Fe3C and α-
ferrite, known as pearlite.

Carburizing, also referred to as Case Hardening, is a heat treatment process that

produces a surface which is resistant to wear, while maintaining toughness and
strength of the core. This treatment is applied to low carbon steel parts after
machining, as well as high alloy steel bearings, gears, and other components.

Carburizing increases strength and wear resistance by diffusing carbon into the
surface of the steel creating a case while retaining a substantially lesser hardness
in the core. This treatment is applied to low carbon steels after machining.

Strong and very hard-surface parts of intricate and complex shapes can be made
of relatively lower cost materials that are readily machined or formed prior to
heat treatment.

Most carburizing is done by heating components in either a pit furnace, or

sealed atmosphere furnace, and introducing carburizing gases at temperature.
Gas carburizing allows for accurate control of both the process temperature and
carburizing atmosphere (carbon potential). Carburizing is a time/temperature
process; the carburizing atmosphere is introduced into the furnace for the
required time to ensure the correct depth of case. The carbon potential of the gas
can be lowered to permit diffusion, avoiding excess carbon in the surface layer.

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After carburizing, the work is either slow cooled for later quench hardening, or
quenched directly into oil. Quench selection is made to achieve the optimum
properties with acceptable levels of dimensional change. Hot oil quenching may
be used for minimal distortion, but may be limited in application by the strength
requirements for the product. Alternatively, bearing races may be press
quenched to maintain their dimensional tolerances, minimizing the need for
excessive post heat treatment grinding. In some cases, product is tempered, then
cryogenically processed to convert retained austenite to martensite, and then
retempered.

Metlab has the ability to carburize and harden gears and other components that
are small enough to be held in one's hand, up to 14' in diameter and 16' tall,
weighing as much as 50,000 pounds. Shallow cases only 0.002 - 0.005", and
deep cases, up to 0.350" have been specified and readily achieved.

The press quench located in the facility allows for the dimensional control,
therefore precise hardening of gears and bearings up to 16" in diameter.

Carburization is a case hardening process that uses carbon absorption to develop

a hard exterior shell, or “case,” on a steel workpiece. Case hardening is often
used interchangeably with carburization, but this is inaccurate. Case hardening
is the overarching term for any process that hardens a steel part’s surface, while
carburization is a specific type of case hardening.

The process transforms low-carbon steel, also called mild steel, into high-
carbon steel by soaking it in an extremely hot, carbon-dense environment. This
treatment enables loose carbon atoms to attach to the workpiece’s surface,
resulting in a rigid surface with a soft inner core. Carburizing is sometimes used
to fix undesired decarburization that occurred in prior manufacturing processes.

The Carburizing Process

Generally, the carburizing process is relatively simple:

1. A steel workpiece is heated in the presence of a carbon-dense medium in any

form — gas, liquid or solid.
2. The carbon from the medium diffuses into the workpiece surface, forming a
hard case.
3. The steel is removed from the furnace and quenched, hardening the carbon
in the case.

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 What are the Benefits of Carburizing Steel?
The main advantage of steel carburization is that it is a practical and affordable
process for steel products in various industries. Other benefits include:

 Cost-effective production: Carburization is significantly less costly than other

steel hardening techniques, making carburized steel pieces more affordable than
other case-hardened steel parts. As a result, carburized steel is an ideal material
for mass production.
 Increased hardness: Carburized steel gains a highly wear-resistant surface with
improved fatigue strength. These properties allow it to handle more force
without wearing down as quickly as un-carburized steel.
 Improved corrosion resistance: Carburizing mild steel creates a protective,
carbon-dense outer layer on the workpiece, making it more resistant to
corrosion.
 Malleable core: Although the surface of a carburized workpiece hardens during
the treatment, the core remains soft and ductile, enabling manufacturers to
create more intricate pieces. In contrast, steel that has been hardened through
other methods is more rigid, making it difficult to shape.

These advantages make carburized steel an excellent material for various

products, particularly delicate machine parts such as gears, bearings, piston
pins, and sprockets.

What are the Different Types of Carburizing

Many different carburizing techniques are used today, each with various
advantages and disadvantages. The following are the four most common
methods for industrial applications.

Gas Carburization is one of the most popular carburization techniques used

today because it produces a highly uniform result, making it an advantageous
step in mass production.

In this process, you heat mild steel in a furnace filled with a carbon-rich
atmosphere. This atmosphere is typically a mixture containing:

 A hydrocarbon, such as methane, propane, or chemical acetylene

 An inert carrier gas, such as nitrogen or hydrogen

When the furnace reaches the carburizing temperature, which is approximately

950 degrees Celsius, the hydrocarbon gas decomposes into carbon and
hydrogen atoms. The crystalline microstructure of the steel workpiece’s surface

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 shifts, allowing the piece to absorb the carbon atoms from the air. This process
can last anywhere from two to 36 hours, depending on the workpiece and the
desired case depth.

Once the steel has been carburized, you remove it from the furnace and quench
it in oil, gas, water, or a caustic soda. This step rapidly cools the metal,
permanently locking in the carbon.

Vacuum Carburization, or low-pressure carburizing (LPC), is a specific

method for gas carburization that takes place in a low-pressure, oxygen-free
vacuum chamber.

The lack of oxygen in the environment allows you to use temperatures up to

1,000 degrees Celsius without the risk of oxidation, which can cause corrosion.
Higher temperatures increase carbon solubility and diffusion rate, accelerating
the process and minimizing the time it takes to reach the desired case depth.

The atmosphere used for LPC must be highly controllable and reproducible to
ensure consistent results. Advanced nitrogen and methanol technologies,
combined with a precise atmosphere control system, provide a high degree of
control over the process.

Liquid Carburization requires placement of the workpiece in a carbon-dense

salt bath and heat it to the temperature required to change its microstructure to
austenite, which is around 900 degrees Celsius. As it soaks, the workpiece
absorbs carbon from the bath and hardens.

The liquid bath typically contains a mixture of sodium carbonate, sodium

chloride, and silicon carbide. Cyanide salts are another common ingredient due
to their nitrogen content — the nitrogen can help prevent oxidation and further
increase surface toughness.

Although liquid carburization is generally the fastest technique, it is difficult to

use for small batches. Mass production is often the best application for this
process.

Solid Carburization, or pack carburization, involves packing your workpiece

in a container with a solid carburizing medium and heating that container in a
furnace at approximately 900 degrees Celsius.
Common materials used include:

 Container: The container is often carburized steel, aluminum-coated carbon

steel, or heat-resistant iron-nickel-chromium alloys.

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 Carburizing medium: Common carburizing mediums include coke, sodium
carbonate, or a charcoal-barium carbonate that supplements carbon absorption.

As the container heats up, the carbon monoxide from the medium dissociates
and decomposes into carbon and carbon dioxide on the workpiece’s surface.
Solid carburization is the simplest technique out of the four, but it is ultimately
unreliable for producing consistent results — uniform carburization is difficult
to achieve across the entire workpiece.

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Depth

3 At Time = 3h
At Time = 5h
2 At time = 7h

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temperature

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Depth

0
2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5
Time

640

620

600

580

HV
HV 560

540

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Temp