Carburizing
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- |
Cases to 0.020" |
|
High wear
resistance, moderate to heavy loading- |
0.020" to
0.040" |
|
High wear
resistance, heavy loading, crushing loads or high magnitude alternating
bending stresses- |
0.040" to
0.060" |
|
High wear
resistance, shock resistance, high crushing loads- |
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.
California
Eng & Tool heat tretaing Treat’s 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.
Carburizing
Time:
4 to 10 hours
Carburizing
Temperature:
1750 oF
(above the upper critical temperature-Austenite area)
Quenching:
All of the
carburizing processes (pack, gas, liquid) require quenching from the
carburizing temperature or a lower temperature or reheating and quenching.
Parts are then tempered to the desired hardness.
California
Eng & Tool offers complete heat treating services. Utilizing
state-of-the-art metallurgical processes, we can assure the quality of material
for your gearing. Our capabilities include stress relieving, through hardening
, and surface hardening. We have in-house metallurgical expertise to fully
monitor the chemistry and properties of the materials. Our gas carburizing and
hardening equipment is among the most advanced in the world, with capabilities
of handling 16 foot long, 75 inch diameter parts weighing 60,000 lbs.
