Micro hardness testing

[myates]

I am trying to find a solution to somehow test the hardness of some very small parts. These parts a basically a 4" piece of 17-4 wire with a diameter of 0.010". There does not need to be any certified testing equipment needed. All that is needed is a way to quickly analyze the hardness of each part after an H900 heat treating process. A non-destructive test would be prefered, but a small sample to do some destructive tests on could be allowed.

Is there anyway some other measurement could be used to correlate to the hardness of the material without doing an indentation? One idea that has been thrown around is using some kind of torque or cantilever test to basically correlate stiffness to hardness. We have tried simply scratching the surface with a file, but it produced a lack of consistent visual feedback.

This a territory I have not ventured in yet so any suggestions would be greatly appreciated.

 

[Maui]

The method you need to use depends on the accuracy that you require from the test. The simplest method that I can think of is to use a calibrated file set to get an idea of what the hardness is. These files are usually calibrated so that they differ from the next file in the set by 5 points Rockwell C. A typical set can be found here:

http://www.mscdirect.com/product/06534309

If you shop around you can usually find them for less than $100 a set. Take each file, and see if it will actually file the wire sample. If the file is softer than the wire, then it won't mark the wire. Go to the next harder file. Eventually you will find one that will mark the wire. At that point you will know that the wire's hardness must fall between the hardness of the last two files you used. And the hardness for martensitic steels correlates with the ultimate tensile strength:

http://www.mwsco.com/kb/articles/19990630e.htm

This would be my first thought. If you require more accurate results, a more sophisticated technique would be required.

Maui

http://www.EngineeringMetallurgy.com

 

[myates]

Thanks, that file set should work great. It should give us a good go/no-go gauge to quickly test these parts before production.

 

[TVP]

Keep in mind that 17-4PH can have quite a bit of variation in hardness for a given tensile strength when heat treated to H900, etc. And the hardness typically will be in the 40-45 HRC range after H900, ~ 35 HRC in the annealed A condition.

 

[myates]

The 17-4 part just needs to be harder than the 304 stainless hypo tube it comes in contact with. These parts are too expensive for us to just be scrapping a bunch because somewhere down the line someone didn't heat treat the 17-4 properly and we don't find out until we wasted the time on assembly of the product. Knowing the 17-4 is in the 40-45 HRC range will put us at ease until another issue arises.

 

[Ron]

Put it in tension like a guitar string. Determine the required frequency for the hardness you need then check from there.

 

[EdStainless]

The easiest destructive test would be to measure the force that it takes to bend a piece.
If you built a little fixture to bend a piece around a pin. Once you are in the plastic deformation range the load will only be a function of the wire size and strength.
Maybe move the arm of the fixture with a peak recording torque device.

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