Accuracy of rockwell C hardness testing
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Posted by: calvindral

12/20/2005, 14:10:48

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How accurate can one expect hardness readings to be on the rockwell C scale. We have specified a part hardness of 40-45 HRC. Our current supplier is stating that statically they can not meet that largely due to the error in measurement. They are saying that we should allow for a 3 point range due just to error.






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Re: Accuracy of rockwell C hardness testing
Re: Accuracy of rockwell C hardness testing -- calvindral Post Reply Top of thread Engineering Forum
Posted by: randykimball
Barney
12/20/2005, 20:39:18

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That depends on a few things:
Quality of the tester
Calibration accuracy of the tester
Expertise of the operator
Accuracy tolerance of your calibration coupon
.. and several other things..

Your tester should have a certification sheet to explain the tolerance you can expect.

Your tester should be calibrated by an instrument certification service on a periodic basics to certify it back to the National Bureau of Standards in which your heat treater and vendor are also being certified back to. That service will at the time of calibration provide you with the answer to your question in the certification paper work.

When I require a heat treat specification, I too, usually state an RC value. I am then showing faith in my vendor to have certified testing equipment and to test according to their best ability. Realize that you, or I in my case, may request a copy of their certification provided by a certified calibration service, any time we wish. You may too, request a certification sheet to come with the product showing at what RC value they tested it to be, but you will pay for it.

This proceedure should do for most cases as a couple of points difference in RC hardness doesn't usually make a huge difference in performance in most products. .... However, this suggested method will NOT do in any aircraft products. In aircraft products you will have to have a currently certified Rockwell tester and provide your inspectors name on the dotted line that the product is to the value required, on your own certified tester, ... or a certification report provided by an aircraft certified heat treat vendor certifying the testing to a specific product batch by a travel number with the material heat lot numbers, and more attached. ... depending on the spec. required.

... wink ... I certify this to be more or less, sort of, accurate...

I hope I have helped




The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

Modified by randykimball at Tue, Dec 20, 2005, 20:51:34

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Re: Accuracy of rockwell C hardness testing
Re: Re: Accuracy of rockwell C hardness testing -- randykimball Post Reply Top of thread Engineering Forum
Posted by: yates

12/28/2005, 08:05:27

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Your supplier is not entirely wrong. There are many details where you can lose accuracy in Rockwell C testing, and if you're not careful you can effectiveley end up with three points of uncertainty.

-Follow randykimball's advice and make sure your machine is certified.
-Make sure the hardness test block you are using to check the machine has a hardness within about 5 Rockwell C points of the min or max hardness you are measuring.
-Make sure you do not space too tightly together the indents on the test block (three indent diameters min. and no less than 3D from test block edge)
-When taking the hardness reading on the part, make three indents and average the three readings, apply here the same indent spacing requirement as for the block.
-Get your load application times right, in accordance with ASTM
E10;
-Make sure the part is not being deformed during the test. If that is the case, use a lower load.
-Thoroughly wipe the anvil surface before testing. Any dust on this will allow part displacement to be incorrecly interpreted as indent depth.

The above is standard practise within the aerospace industry, and should reduce total uncertainty to approximately 1.6 HRc points.







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Re: Accuracy of rockwell C hardness testing
Re: Re: Accuracy of rockwell C hardness testing -- yates Post Reply Top of thread Engineering Forum
Posted by: yates

12/29/2005, 09:15:53

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Sorry, my mistake. That should have been ASTM E18, not E10






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Re: Accuracy of rockwell C hardness testing
Re: Re: Accuracy of rockwell C hardness testing -- yates Post Reply Top of thread Engineering Forum
Posted by: randykimball
Barney
12/29/2005, 22:40:56

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Thank you for your input.
I can tell you are an inspector somewhere or have been one.

I'm sure you realize you can edit your posts by simply using the edit "tab" to the left of your post and typing your password, and sometimes your user name?

...but... wink... I much more enjoy it when an inspector admitts their mistakes. ...

you made my day,

thank you,... have a Happy New Year.

I rarely sign this way, but... this time I choose to sign..
randykimball, moderator




The worst suggestion of your lifetime may be the catalyst to the grandest idea of the century, never let suggestions go unsaid nor fail to listen to them.

Modified by randykimball at Thu, Dec 29, 2005, 22:42:50

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