Rockwell/Microhardness Reporting

[mrfailure]

A few years ago I asked how everyone reported their hardness results. It seemed like every respondent reported in a different way.

With digital Rockwell testers now the norm, I want to ask: do you report to the 0.1 point or do you round to the nearest integer value. Do you think the wording of ASTM E18 requires you to report Rockwell as integer values? If you round, how do you handle conversions between Rockwell and other scales (Vickers/Knoop/Brinell)?

I'll put out my opinion: I think hardness should be reported to 3 significant digits as this is accurate for properly calibrated modern testers. Rounding to integers degrades the data. Also, using 3 digits generally allows straightforward conversion between Rockwell and Vickers/Knoop/Brinell, which would report to integer values unless the material is really soft.

 

[EdStainless]

Considering that precision and bias (or gauge R&R) leads you to conclude that even using integers is likely a overstatement of the resolution of the test. Reporting Rockwell with a decimal point is ridiculous.

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P.E. Metallurgy, Plymouth Tube

 

[mrfailure]

Not if you consider you are measuring on other scales to a precision of 3 digits, and you often have to move back and forth when you convert. I think it is a matter of equivalency.

 

[EdStainless]

The conversions are approximate, so unless you have built your own higher accuracy ones all that I see is an artificial image of accuracy.

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P.E. Metallurgy, Plymouth Tube

 

[mrfailure]

Nothing artificial about the accuracy of the measurement. Routine rounding of Rockwell to integer values is exactly the same as rounding Vickers/Knoop/Brinell to the nearest 10 to get 2 significant digits, which I am sure would not be acceptable. In addition, some hardness requirements I deal with call out that 0.1 digit for specifications (this happens most frequently for fasteners). Much easier to have a consistent rule that can be applied where Rockwells are reported as measured than having special exceptions to a rounding rule.

 

[EdStainless]

The measurement that being made in HR versus Vickers is very different.
I can actually measure the indention sizes in HV and while single digit values may be a stretch there is high resolution.
HR is measuring the depth of the indentation, so that <0.020" deep intention is translated and hence the more limited resolution.

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P.E. Metallurgy, Plymouth Tube

 

[MagBen]

We report one decimal (0.1) only when converting, we round to integer even the spec. calls out one decimal, regardless of Rockwell, or superficial, or Brinell, or Vickers. Anyway, technologically, it makes no much difference. Hardness is a dirty, cheap test. In most cases, it should be used for reference, not a guidance for rejection (I know this is not the case in reality).

 

MagBen,
(1) Tech specs can be stupid. I don't know of any practically used scale or method where reporting to decimal places is scientifically justified. Just read ASTM E92 and E384 in depth to learn about the chain of factors that accumulate uncertainty. Finally, there is the error in E140 conversions (really a set of best-fit correlations).
(2) Hardness is a dirty, cheap test. For such a simple test it is notoriously difficult to do consistently well. But hardness represents a complex aggregate of physical and metallurgical properties. Many organizations do not pay enough attention to training in this area, IMO because the tests are just so easy to do.
(3) Don't even mention field hardness testing around me. It is done wrong more than 80% of the time.


"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[dbooker630]

On the older Wilson Rockwells with the dial scale it is difficult to justify rounding tighter than 0.5 HRC points. I'm in the middle of a discussion with two of our plants where Brinell microscope readings are the issue. Not the test machine itself but how the impressions are measured and reported!

 

dbooker630,
Parallax error.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[mrfailure]

You never should report readings from old dial machines beyond a whole number - I think that is in ASTM E18. I wonder if some resistance to reporting the decimal value is from procedures developed for those dial machines that everyone is used to.

 

mrfailure,
There's a newer thing I call the Digital Delusion. Instruments are based on the same fundamental principles but now have digital displays which are never questioned (because hey, it's Tech). Case in point is PMI guns - most of the numbers displayed are meaningless random numbers but they get mindlessly scribbled onto reports. It's borderline fraudulent.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

I can actually measure the indention sizes in HV and while single digit values may be a stretch there is high resolution.

True Ed, which is why I prefer it.
But "there's many a slip 'twixt the cup and the lip", and optical measurement is a significant source of error that grows larger with harder materials.
George v.d. Voort has much to say on this subject. His text from around 1985 (published by Wiley I think) is my go-to reference for metallography and related activities.
Being a welding engineer a failure analyst, my first rule for hardness testing is choosing the most appropriate method for the material. You wouldn't use Brinell on a weld cross section and you wouldn't use Vickers in a foundry.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[mrfailure]

Why do people think that the depth measurement made with indentation for Rockwell is not as accurate as using area measurements for Vickers/Brinell/Knoop? Because they cannot see it or measure it themselves? Modern technology does allow for better measurement.

 

[EdStainless]

How precise and accurate do you think your machine is measuring a 0.006" deep indention?


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P.E. Metallurgy, Plymouth Tube

 

Rockwell hardness, based on depth measurement, is a completely legitimate approach. As with anything, the sources of error must be understood and mitigated/eliminated.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[MagBen]

I think Rockwell is generally the most accurate method than others, but NOT due to the measurement of depth vs length/area.

 

MagBen,
Horses for courses ... I don't think it is possible to declare one method as 'best'.
The critical step is determining an appropriate method.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[Maui]

The calibration block that you use to verify your Rockwell hardness testing machine is operating within range for HRC testing is only good to plus or minus 1 point Rockwell C. This is usually stamped directly on the block itself. Why on earth would you believe that the test results for a given part can be relied upon to be more accurate than the hardness readings from the calibration block?

Maui

 

[mrfailure]

Maui: You have that same degree of scatter in cal blocks for all hardness testing. There is a similar tolerance band for tensile testing. Why is Rockwell testing so special in this regard that everyone wants to first degrade the actual reading automatically? The fact that a decimal value is measured seems to be hard for many to accept, but if you have to round Rockwell to the nearest whole number then you should also be rounding Knoop/Vickers/Brinell to the nearest 10 using the same logic. Crazy, right?