ARE YOU GETTING WHAT YOU PAY FOR?
If you buy sprockets or gears with
hardened teeth you should know that not all hardening is created equal. In many
cases, parts sold as having hardened teeth are sub-standard in terms of what we
consider to be good heat treating.
Fundamentally, heat treating is a
process which changes the microstructure of steel in order to achieve an even
distribution of carbide crystals throughout the base structure. Carbide starts
out in "large globs" which are diffused into smaller crystal groupings when the
part is heated to a high temperature. Quenching in fluid immediately after
heating causes a very rapid reduction of temperature which "freezes" the carbide
crystals in a well distributed condition. After quenching, heat treated parts
should be tempered (reheated to a lower temperature and slow cooled) to increase
ductility (reduce brittleness) by lowering the post-quench hardness a few
points. The resulting structure of small, evenly distributed grains gives heat
treated steel its hardness. There are three processes which are used to do the
majority of sprocket and gear hardening - flame, induction and carburizing. All
of these are good processes when done properly.
The first, flame
hardening requires the least investment in equipment and training. In fact, it
can be done, in an unsophisticated manner, with tools found in most any work
shop. A part is simply heated with any gas torch until it is red hot, then it is
quenched, usually in water. This process is difficult to do well and is probably
the easiest to do poorly. It is also one of the most common methods for
hardening sprockets and gears.
What we commonly see with parts flame
hardened by non-professionals is inconsistency. Often we find very hard areas
and very soft areas within a single part. Sometimes only the very tips of the
teeth are actually hardened properly, or there is insufficient depth of
hardness. Coarse pitch gears sometimes have major variations of hardness across
the face of a single tooth. Multiple strand sprockets are often hard on one end
and get softer as you progress across the strands. We even see cases where all
that was achieved by flame "hardening" was a discoloration of the part without
any hardening result.
The old "buyer beware" motto seems to be
appropriate in many cases. The only way to know that you are getting what you
pay for is to buy hardened sprockets and gears from sources which have the
ability to do the job right, or have the parts you buy tested with the proper
specialized testing devices, either in your own plant or a third party testing
lab.
Such testing usually comes in the form of checking the hardness of a
given part at various points in the area to have been hardened and comparing
them with targets set prior to heat treating. Hardness can be rated on several
different scales. The Brinnell and Rockwell scales are very common in the U.S.
As a point of reference, 1035 hot rolled and annealed steel usually measures
about 5 - 15 on the Rockwell C Scale. Gears are typically hardened anywhere from
Rockwell C 30 to 65, depending on their type, size, material, application and
hardening method. We harden sprockets to R/C 40 - 50. In addition to testing
parts for their actual hardness, parts must also be checked for cracking using a
spray penetrant or magnetic particle method.
The other two heat treating
processes, induction and carburizing, are more complex methods requiring large
investments in equipment and personnel. As a result they are usually seen only
in professional heat treating environments such as you would find in a
commercial heat treater's shop or a manufacturer's dedicated shop such as our
own induction facility.
In this environment the results of heat treating
processes can be controlled by qualified people using the right equipment to
achieve the desired results.
Induction hardening is a good general purpose heat treating method which
requires the use of medium carbon steel (.030 - .050 C). Parts are heated by
placing the target hardening area (usually the teeth in this context) in close
proximity to a copper coil which surrounds the entire part, or a pointed tip the
shape of one tooth space in the case of larger parts. An electrical current is
run through the coil or tip and the magnetic resistance between the work piece
and the coil or tip causes the work to heat up at a very rapid pace. When the
work reaches its hardening temperature it is then quenched. Part distortion
rates are similar to or slightly better than that of flame hardening.
The
carburizing method of heat treating results in the highest hardnesses of the
three methods mentioned here, typically R/C 57 - 63, but the depth to which
parts can be hardened is less than with either flame or induction hardening
(.030 - .060 inches as opposed to .100 - .200). This method utilizes low carbon
steel (.010 - .030 C). The part is heated in a carbon rich atmosphere, usually a
furnace filled with a gas high in carbon content. As the part heats up it
absorbs carbon into the exposed surface areas. After enough time has passed for
the part to have absorbed an appropriate amount of carbon, and to have reached
its hardening temperature, it is quenched. Unlike flame or induction hardening,
parts are hardened all over rather than strictly on certain target areas such as
the teeth of a sprocket or gear. Because the entire part is heated to a high
temperature the distortion rates seen with carburizing are higher than with
other methods. This can cause problems for gearing which requires highly
accurate tooth forms, thus resulting in the need for post-heat treat finishing
such as grinding of teeth or other critical surfaces such as bores or bearing
journals.
We would like to help you in any way we can in supplying your
heat treated sprocket and gear needs, or determining the quality of products you
are currently purchasing. If Classic Gears and Machining can be of service to you please don't
hesitate to contact us.
