Metal finishing Q&As since 1989

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Why are there multiple hardness scales?

2006

Dear Sir,

Please explain for the difference between HRC,HRA,HRD,HRB hardness nos.Please give the details for why we need to use different methods like HRC,HRA etc.

Thanks,

Kaviarasan
CQS PRECISION INDUSTRIES INC,Automobile stamping parts. - Hochi Minh,Vietnam

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You may not need all of those scales, Kavi, or you may need all of those and more, like Brinnell hardness and Knoop hardness, etc. A particular scale rates the hardness of a material using a particular penetrating tool under particular conditions. But some penetrating tools and methods may be inappropriate for measuring a given item.

As an example you might whack a truck bumper with a sledgehammer and measure the indentation on a facetious "Sledgehammer Scale" to see which of two bumpers is stronger, but you can't use a sledgehammer to determine which of two eggshells is stronger. You might check eggshell strength by tapping an egg with a small teaspoon, but you can't measure the strength of a truck bumper that way. Each of the scales that you mentioned has very particular specifications and applications.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2006

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First of two simultaneous responses -- 2006

What Ted said is correct. To help a tiny bit more,HRA means Rockwell hardness scale A, or to stay in the same order, hardness, Rockwell scale A. B,C and D correspond to different indenters. They are not directly correlated, even if you see charts that might indicate a cross reference. PS, Rockwell is one of the original companies that made hardness testers.
You do need to do some additional homework, as the tester is supposed to be checked with a standard block every day for the indenter that you are going to use.
You should also get training from a qualified peson, because it will lie to you if used slightly incorrectly.

James Watts
- Navarre, Florida

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Second of two simultaneous responses --

It was to answer just such questions as these, and more, that Samuel Low of the US National Institute of Standards and Technology wrote special publication 960-5, on Rockwell Hardness Testing of Metallic Materials. Check it out at www.metallurgy.nist.gov/reports/ recommendedpractice/SP960_5.pdf. It's great.

Lee Gearhart
metallurgist - E. Aurora, New York
2006