Vickers Indentation Loads For Use In Hardness Conversion Per ASTM E140

[mrfailure]

ASTM E140 specifies different minimum indentation loads for Vickers hardness in order for the measurement to be considered valid to use for conversion to other hardness scales. These minimums are:

Non-Austenitic Steel

No limit​

Nickel Alloys

1 kg​

Cartridge Brass (70-30)

No Limit​

Copper

100 g​

White Iron

50 kg​

Wrought Aluminum Alloys

15 kg​

Of these, only copper shows different hardness conversions using different loads.

Since Vickers testing is supposed to stay proportional no matter what load is applied, I wonder about these limits. The aluminum 15 kg limit is absolutely nutso and would never be used it the real world (not to mention the white iron 50 kg minimum load); users mostly just ignore that limit and use the conversion when they are perform microhardness testing. At the same, testing has shown the proportionality deviates at low loads, certainly by 25 g. I think a case could be made that for all conversions in the spec applying a minimum load of 100 g would be reasonable. Thoughts/comments?

 

[EdStainless]

Have you take samples of different materials and run Vickers at different loads?
No, I wouldn't use 50kg for white iron, I would use 100kg.
Our work was mostly Autenitic SS and we used our own conversion tables made in house.
I have also never figured out why some of these don't have upper limits.

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P.E. Metallurgy, consulting work welcomed

 

[MagBen]

there are basically two types of Vickers hardness, macro-load (>1kg) and micro-load (<=1kg). Macro-Vickers gives only bulk hardness data, due to a large indent, it is more representative, more accurate on average. itis also easily related to strength. while micro-Vickers can test different phases, it is more sensitive. when load<100g, it is said dislocation interaction with indenter. smaller load, smaller the diagonal size, and the higher the error. when load>1kg, it is said no load issue.

I think these are (perhaps some of) the basis used to set the rules in ASTM E140 (also ASTM A370). All correlations between hardness test
scales are empirical in nature and involve a degree of uncertainty in the conversion.

 

[mrfailure]

Ben, what you say is totally correct. I suspect each material conversion was developed piecemeal. The net result is the conversions probably are used regardless of the applied load, especially for aluminum which has an unrealistic limit in the real world. I would want to reframe the question to practical lab testing: Can the conversions shown for each material be used for each material be used in E384 microhardness testing at 100 g or greater loads? The standard already accounts for variability as the conversion is considered an estimate.

Ed, I also know of an in-house allowable conversion for austenitic SS that is considered proprietary, so I use it but can never say where I got it. Given real world needs, I would love for ASTM to test and publish Vickers/Knoop conversions for this material.

 

[EdStainless]

If ASTM ever does an extensive revision of this spec they may be required (by their current rules) to do a round-robin and develop precision and accuracy statements. We can dream.
Our SS charts were considered so proprietary that we were forbidden from having copies of them. I wish.
I never tried to work out the exact relationship but the sensitivity to load is related to the uniformity of microstructure, dislocation/strain hardening characteristics, and phase of the moon.
To be honest when we cast white irons we tested using Brinelle (10mm) with a load of 1500kg (or more).
The big carbides make it a real pain.

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P.E. Metallurgy, consulting work welcomed