Bruce,
Thanks for the details. As you can probably deduce from my description,
I was indeed describing the Rockwell "C" scale testing I am familiar
with for our high strength steel. Anything less than 120,000 psi is
"pot metal" in the Rocket Biz. You are much more on the ball about the
cast parts. We also have a test for soft plastics, similar with a much
larger ball, and a depth measure for a preset load under load, as the
dent goes away when the pressure is removed. I think is was a "shore
hardness" test.
Thanks for the reminder that this isn't a rocket we drive, although we
like to think so. :-) , but stuff we relegate to "ground support
equipment and tractors." ;-)
I take it, other than the testing details, you agree with the processing
of HiPo cranks selection.
Steve
bmelusky@netscape.net wrote:
>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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--
Steve Laifman
Editor
http://www.TigersUnited.com
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