What is Near-surface hardness? 1

[jmarkus]

I just received a test report from a lab we ask to do a surface hardness and core hardness test of a part. They reported the surface & core hardness test results using the Vickers Low Load test. They also reported the "near-surface" test results using the Rockwell test. The part drawing specified a Rockwell C hardness in a certain range. I expected surface & core hardness done with a Rockwell test (in addition to the HV0.3) but I am confused by the "near-surface" moniker. Can someone help explain?

Thanks,
Jeff

 

[Carburize]

If the lab is testing a carburized part it is typical to do the tests on a mounted cross section of material using a Vickers microhardness or Knoop hardness tester, the results of which are converted to Rockwell. The case hardness of a carburized part is often defined as the hardness at 0.0025-inch below the surface this avoids problems with errors due to the part deforming if the indentor is too close to the edge and effects of any oxidation - decarburization etc local to the surface.This is usually defined as the "case" hardness but may be what the report is calling "near surface".
The hardness usually defined as the "surface" hardness is exactly as you describe which is a test performed directly on the surface using an approriate load depending on the case depth.

 

[metman]

Carburize,
A star for you. Such an elegant answer. I cannot imagine anyone argueing with this explanation but I have been wrong before.

 

[jmarkus]

I've been told that one should not convert between hardness methods (e.g. Rockwell & Vickers), but instead run the actual test method required. Since the drawing specified a Rockwell Hardness, shouldn't I expect a Rockwell Hardness test be performed?

Thanks,
Jeff

 

[Carburize]

If a laboratory is determining effective case depth using a mounted sample and determining the hardness profile, the standard procedure in documents such as AGMA 923-A00 "Metallurgical Specification for Steel Gearing" reads as follows:
" The effective case depth is measured normal to the finished gear surface to a location where the hardness number is 50HRC ( 542 HK 500 or 515 HV 500 min) by conversion from a microhardness test result."

The microhardness test provides the necessary accuracy for these determinations particularly for effective case depths of thin cases.

 

[unclesyd]

Generally for a Rockwell hardness for a case or surface reading the callout should be for Ra or a Rockwell Superficial hardness. If your callout was for Rc, one would generally switch to Ra or the Superficial test and report as such. If for some reason the tester deemed a cross section necessary he would generally switch to Knoop or Vickers as posted by Carburize.

As you state conversion isn't always the best approach but has to be used at times. Generally with good equipment and competent technicians there isn't any problem.

 

[jmarkus]

There's part of my problem. I am not testing a gear, but a fastener (a proprietary weld stud). This falls under ISO 898-1, which in turn calls out the Rockwell ISO 6508-1 spec which states "There is no general process for accurately converting Rockwell hardness into other scales or hardness into tensile strength." The 6507-1 spec for Vickers says the same thing for Vickers.

The lab wasn't asked to determine case hardening depth (the part should have through hardened, by my limited heat treating understanding), but Rockwell hardness values.

 

[Carburize]

AH! For a through hardened part there should be no problem with a full Rockwell C number at the surface and in the core however, if the part is small diameter the test lab may have ground a small "flat" on the OD to get a better reading so the hardness measures is not "at the surface" but "near surface"

 

[Carburize]

metman - mine was apparently an elegant answer to the wrong question!

 

[CoryPad]

Carburize may be correct that a small grinding operation was performed, hence the term "near surface". It also may be that a sectioned/mounted sample was used for the Rockwell test, and the indentation location was "near surface" because the large force (1471 N, or 150 "kilogram force") required a substantial edge distance to insure an accurate measurement.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[redpicker]

If you just gave the lab a sample and asked for a surface and core hardness, they may have assumed it was a case-hardened part and tested it using their standard methods for case-hardened parts (microhardness at the near-surface and core). If what you need is surface and core Rockwell 'C' hardness, you should ask them to re-test the sample so you can get the information you need.

While this is generally a true statement: "There is no general process for accurately converting Rockwell hardness into other scales or hardness into tensile strength." when the discussion is limited to quenched and tempered low alloy steels, then the conversions given in ASTM E140 are pretty good. In my experience, they usually fall within +/- 1 HRC value which is the generally accepted accuracy of the Rockwell test. It needs to be remembered that any such conversions are only apporximations. To demonstrate compliance to a specfied requirement, the test really should be performed by the test method specified.