Steve Laifman <SLaifman@SoCal.rr.com> wrote:
>
>few small dimples on it from the tests. Much like a fine point, spring
>loaded center punch. Of course it was a much more sophisticated piece of
>equipment, but the depth of penetration with a preset impact is what the
>criteria is for hardness, which equates to the crank tensile strength
>from heat treat.
>
>Steve
>
A Brinell test uses a fixed load (in this case 3000 kg) and a 10 mm ball. The
surface is ground primarily to give a clean edge to the impression, but it is
also desireable to remove any decarb (metal that had the surface carbon
depleted) that would skew the results. The impression is measured by a small
10X scope with a reticle. The size of the impression determines the hardness.
This is the preferred method for high nodular cast iron (or ductile iron) such
as used in the Hipo crank. Nodular iron is made by pouring the material over
magnesium to create spherical graphite instead of long flakes typical of gray
iron. For those that are curious here are some microstructures:
http://www.metallography.com/technotes/iron/nodular.htm
As you can see from the photomicrographs, the material is not homogeneous, so a
large ball is desireable to get a representative reading. Ductile iron is
superior in strength and can take higher loads over gray iron. It is rated by
the number of nodules, and the precentage of ferrite (weak) versus pearlite
structure(desireable for strength).
A Rockwell type test would be a small prick punch impression. Harder material
would require a diamond indentor and softer material would generally use a 1/16
inch ball penetrator, although I have seen special penetrators up to 3/4 for
some coatings. A small indent is generally not suitable for cast iron, given
its varied structure. Rocwells measure depth of penetration and neither test
uses impact, as impact can cause work hardening and change the results. For
both tests the rate of load application and duration are important parameters
to assure repeatability
Probably more than most want to know, but there it is.
Melusky
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