Microhardness testing questions 4

[coreman73]

I have been asked to measure the hardness of a 0.005" thick piece of C1050 grade steel. Since surface hardness clearly isn't possible, I mounted a sample and will do microhardness. The specification for this thin steel requires testing by Vickers using load 10kg and is supposed to show results in the range of 560 +/- 30 HV. The problem I'm experiencing is that the microhardness tester I have access to only goes up to a 1kg load. I've checked with some outside labs and they also can only test at 1kg.

My questions:

1. Could I use a lesser load such as 1kg or 500g to carry out the microhardness testing and still expect to receive similar results as if I used 10kg?

2. If not why and how could I get accurate hardness readings if using a 10kg load is not possible?

3. Could I simply get results first by Knoop and then convert to Vickers? It seems this may be the best option.

Thanks in advance for the advice. It is always most appreciated.

 

[redpicker]

I doubt you will be able to get a 10kg vickers test performed on a 0.005" thick piece. The indention will not be a valid test becasue the material is too thin. The best you can do is a test with a lighter load and report the load you used. The results should be comparable, but not exact.

You are correct, probably the best option would be a Knoop test, report the KHN and report what the corresponding 10kg vickers result would be when converted per ASTM E140.

rp

 

[coreman73]

Thanks rp. I will just go with the Knoop test and report what the corresponding 10kg Vickers result would be.

 

[EdStainless]

VHN (and NHN) should be more or less independent of load. That is why we use them. Testing at 1kg should give you a good hardness number.
I'll bet that they were doing 10kg VHN on a flat sample.

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Plymouth Tube

 

[CoryPad]

At 530 HV and using 10 kgf, the indent diagonal would be 0.187 mm, which is greater than your part thickness.

ASTM E 384 requires the indent center-to-edge distance to be a minimum of 2.5 * diagonal. Using 530 HV, you could use 200 gf and meet the requirement.

It is possible that the hardness of your material is not constant from 200 gf to 10 kgf. The same is true for a low-force Knoop test.

 

[unclesyd]

Is there any compelling reason to require an absolute hardness value instead of a comparison?

With thin materials such as yours the thermal history can cause variation due to grain size. We used small diameter 1095 wire and the grain size was across the scale where as the calibration block was very fine grain.

Things to ponder.

One of our uses for 1095 wire allowed us the use bending test to insure that in operation was within the elastic limit of the material.

On wires used for shear pins and other small pats we always used a first temper that produced a hardness higher than we needed. After testing we would draw the material down into the desired range.

You might be able to use a small anvil and stack two pieces to check the hardness.

Can your instrument do scratch hardness?

 

[coreman73]

Definitely lots to consider. I only have a standard test block for 500gf with both Vickers and Knoop. I will probably just test with each and see how my numbers compare.

Unclesyd,
I don't believe it's necessary for an absolute value so I will just provide the comparison.

Thanks to everyone for the input.