RC Hardness Testing Q

[dePaul]

"In Rockwell hardness testing, a conical diamond is
first pressed with a force F0, and then with a force
F0+F1 into the material being tested. After unloading to
F0, the increase (e) of the depth of the impression
caused by F1 is determined. The depth of penetration
(e) is converted into a hardness number
(HRC) which is read directly from a scale on the
tester dial or read-out".

I was reading this info regarding the Uddeholm Elmax powder steel. At the end of the article there was this appendix.
Can somebody plz clarify this for me? Does this statement mean that the needle is pressed into the material twice on the same spot? If so is the case, then why?? What's the purpose of this procedure?

Thanks,
~Paul~

 

[BigFunWMU]

"In Rockwell hardness testing, a conical diamond is
first pressed with a force F0, and then with a force
F0+F1 into the material being tested. After unloading to
F0, the increase (e) of the depth of the impression
caused by F1 is determined. The depth of penetration
(e) is converted into a hardness number
(HRC) which is read directly from a scale on the
tester dial or read-out".

From what it says it sounds like they press the needle to the material with a small force, then increase the force, then remove the needle. And the HRC sounds like a ratio between the change in force compared to the change in depth.

 

[Bear Creek]

I own an old 70's-era Puma pocket knife that was tested/checked for "hardness" by Puma. It has a small needle-sized indent on one side of the blade. I don't know if it was a real "test" or merely a marketing feature.

 

[dialex]

I'm not sure, but I think that the F0 is the force needed to move the cone to the surface of the metal. Once the diamond cone is on the knife, an increased force (F1) is added and the diamond "sinks" into the material. Then, they reverse to F0 (aka lift the cone) and measure how deep the "pit" is.
I'm not very thrilled about those tests. AFAIK, it is possible that a blade can have different HRCs depending on the zone you test (closer to the tang, edge aso.)
BTW, some makers (eg. Spyderco) mark their seconds with a point stamped on the blade. Those could also be mistakenly considered HRC test marks.

 

[dePaul]

Hm, BigFunWMU might just be closest with his explanation.

Alex, the method you describe sounds like a hybrid of the Rockwell and the Vicker's test. In Vicker's hardness testing a pyramid shaped diamond with a square base and a peak angle of 136° is pressed under a load against the material and the hardness is hereby determined.

After unloading, the diagonals d1 and d2 (both sides of the diamond) of the impression are measured and the hardness number (HV) is read off a table. When the test results are reported, Vickers hardness is indicated with the letters HV and a suffix indicating the mass that exerted the load and (when required) the loading period.

~Paul~