| Carburizing is one
of the most widely used surface hardening processes. The
process involves diffusing carbon into a low carbon steel
alloy to form a high carbon steel surface.
Most steels specified for carburizing
contain less than 0.25% carbon, with sufficient alloys
to improve case and core hardenability. Depending on the
application, any of numerous grades may be used:
- Type
4620 Steel -
Lower cost, chrome/nickel/molybdenum steel where
only nominal hardenability and core response is required.
- Type
8620 Steel -
Most commonly specified grade. Excellent carburizing
response, with good hardenability for most section
sizes.
- Type
4320 Steel -
Higher hardenability for improved core response in
heavier sections.
- Type
4820 Steel -
Increased nickel content for improved core toughness;
slower response results in longer process times.
- Type
9310 Steel -
Maximum nickel content for maximum core toughness;
slower response results in longer process times.
Though plain carbon steels may
be carburized, the lack of alloying elements reduces the
carburizing response of the case. These steels respond
better to the Carbonitriding process.
The carburizing process is typically
specified by a case depth and a surface hardness. Typically,
the surface is specified at Rockwell C 58 to 62 (or equivalent),
with other hardnesses occasionally specified for special
applications. Carburized case depth is typically specified
to an "effective" hardness, or the depth where
a specified hardness is obtained, generally Rockwell C
50. The effective hardness is determined by preparing a
metallographic sample from either the product or a representative
test bar of the material, and then testing the microhardness
at various depths.
Case depths from as light as
0.003" to as deep as 0.250" may be specified,
depending on the service requirements of the product.
| Application |
Depth
of Case |
High
wear resistance, low to moderate loading-
Small and delicate machine parts subject to wear |
Cases
to 0.020" |
High
wear resistance, moderate to heavy loading-
Light industrial gearing |
0.020" to
0.040" |
High
wear resistance, heavy loading, crushing loads or high
magnitude alternating bending stresses-
Heavy duty industrial gearing |
0.040" to
0.060" |
High
wear resistance, shock resistance, high crushing loads-
Bearing surfaces, mill gearing, rollers |
0.060" to
0.250" |
The response of a particular
steel to carburizing is dependent on the diffusion of carbon
into the steel; the depth of penetration is controlled
by temperature and time. The most typical carburizing temperature
is 1750F, although lower temperatures may be used to reduce
distortion or improve control of the case depth tolerance.
The disadvantage of reducing temperature is that the time
necessary to achieve the specified case is increased. In
all cases, the carbon diffused into the steel is provided
by a carbon rich gaseous environment.
After carburizing, the work
is either slow cooled for later quench hardening, or quenched
directly into various liquid quenches. Quench selection
is made to achieve the optimum properties with acceptable
levels of dimensional change. Hot oil quenching is preferred
for minimal distortion, but may be limited in application
by the strength requirements for the product.
When maximum properties are
required, cold oil quenching may be specified with some
sacrifice in distortion.
When both maximizing quench
response and dimensional control is required, die quenching
is an option. The part is quenched in cold oil in a special
fixture under a clamping load to hold critical dimensions.
In some cases, it may be desirable
to have the benefits of a carburized surface in only certain
areas of the part. For these applications, a carburizing
stop-off can be applied to the areas to be protected. The
stopped-off, or localized, areas will remain carbon free
and relatively soft. The paint is removed through normal
processing in the heat treatment operation.
Milwaukee Gear's
heat tretaing affiliate, Treat
All Metals, specializes in quality heat treating, and our
carburizing capabilities are no exception.
Our process equipment is certified
to ensure accurate control of the carburizing variables,
utilizing the best control equipment available. We have
capabilities to harden in a variety of quenches, including
our rotary die quench facility for close dimensional control.
Our auxiliary equipment includes
capacity for deep freezing to control retained austenite,
straightening, blasting, and magnetic particle inspection.
The Quality Control Metallurgical Lab is well equipped
to test your product to your most exacting requirements.
When necessary, all process steps and inspections can be
certified or substantiated with documentation.
Finally, our experience in successfully
applying the gas carburizing process to a wide variety
of products will allow us to help recommend the best process
for your application.
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