Rockwell C Hardness Scale 3

[Maui]

Can anyone provide a reference that states the actual upper limit for the Rockwell C hardness scale? I would expect it to be the hardness of diamond, since a diamond indenter is used in this test. To my surprise, I am unable to locate a reference that contains this information. My search has included ASTM standards, a Google search, and contacts at industrial research facilities. They all came up empty.


Maui

 

[redpicker]

I think the problem is that there is no upper limit based on technical justification. That is, the lower limit, 20 HRC, is based on tolerances of the conical section of the braile, but there really isn't an upper limit.

I know they like to use the A scale (100 kilogram load) when testing carbides and other very hard materials. I was told (by someone, I can't remember who) this was because the lighter load was less likely to break the diamond.

rp

 

[EdStainless]

ASTM E18 Standard Method for Rockwell Hardness.
HRC, conical diamond indenter, 150kg load
65+ is the maximum reading for this range.
per E140 68HRC is roughly equal to 940HV in a steel.

You can work higher with microhardness.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[NickE]

Hmmmm... Desk Edition of the Metals Handbook says:

well it doesnt come right out and say it but I can SWAG these statements together for you.

The rockwell test is a measurement of the depth difference betweeen the minor and major loads.

And:

"One Rockwell number represents and indentation of 0.002mm. Therefore a reading of 60 HRc indicates indentation from minor to major load of
(100-60) * 0.002mm = 0.080mm."

So from that equation I would figure that the real absolute on the HRc is 100...

Hmmmm....

So using that equation and shifting around gives...

100 - d/0.002mm = HRc

Nick
I love materials science!

 

[SMF1964]

At what point does the backing material supporting the diamond tip on the indenter come into play if you are running a hardness test on something "really hard"?

You also have cracking of the sample. I've taken Rockwell C measurements of a 52100 bearing steel that was end quenched (from 1850°F) in water. The material cracked all around the indentation and radially outward. If someone could tell me how, I can post a picture.

I know for ceramics you can actually get an estimate of the toughness of the material by measuring the cracks that run off the corners of the microhardness indentation.

 

[Carburize]

There is a statement - without any supporting reference - in Mechanical Properties of Metals by J.G.Tweeddale that the Rockwell C scale becomes inaccurate above 70.

 

[Carburize]

Interestingly 70 is the highest number on the C scale quoted in ASTM E 140.

 

[NickE]

And according to the above equation 70HRc relates to a penetration differential of ~.060mm wich is a big difference...

Is it possible that the resolution of the mechanical machine was the limit of the test? I have used tons of wilson testers... Few need any sort of power whatosever... Possibly the dial indicator becomes not sensitive enough at HRC>70?

The hardness of the anvil (platen) shouldnt be an issue since the sample thickness should be thick enough to completely contain all deformation...

 

[EdStainless]

Yes, each scale is limited by the resolution of the indicator. Hard materials will have very small indentions.
Most of us use traditional hardness readings, and we are aware of the limitations, but you need to be careful.
Indention hardness methods assume all sorts of things about sample uniformity, stiffness (of sample and system) and microstructural variations.
The smaller the indention the more sensitive the test is to outside influences.
In any case, remember that hardenss is a proxy for other properties. It is a means, not and end.

= = = = = = = = = = = = = = = = = = = =
Corrosion, every where, all the time.
Manage it or it will manage you.

http://www.trent-tube.com/contact/Tech_Assist.cfm

 

[SMF1964]

Got it.

:<imgsrc="

http://flypicture.com/file/?id=rd70m6vb">

 

[CoryPad]

Here is some information from NIST Special Publication 960-5 Rockwell Hardness Measurement of Metallic Materials.
[green]
Each Rockwell scale is an arbitrarily defined range of numbers from 0 to 100 covering a specific range of material hardness. Although, theoretically, the entire scale can be used for hardness testing, there are practical limitations on the range of testing for many of the Rockwell scales. At the low hardness end of the scales, these limits result from the indenter penetrating too deeply into the material, possibly causing contact with the indenter cap for ball indenters.

In the case of diamond indenters, the sensitivity of the test diminishes as the diamond indenter penetrates further down the conical portion of the diamond. At the high hardness end of the scales, these limits result from the likelihood of fracturing or significantly reducing the life of a diamond indenter. In the case of ball indenters, the sensitivity of the test diminishes, and there is increased possibility of flattening a steel indenter ball. The ISO standard [ISO 6508] suggests the limits given in Table 2 for some Rockwell scales.[/green]

Table 2 shows the 70 HRC max mentioned by a previous poster.

This document is a very good one. It can be obtained at:

http://www.msel.nist.gov/practiceguides/SP960_5.pdf

Regards,

Cory

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

 

[Maui]

It looks like you were right about the upper limit Nick. Thanks for the link Cory.

Maui

 

[limct]

In Gilbert Ford Kinney's Engineering Properties and Application of Plastics, page 202. There's a comparison of several hardness scales.

HRC range is from 20 to 80, where 80 is about in line with hardness of (8)Topaz in Mohs hardness. It also corresponds to approx. 1000 HBN.

Diamond which is in (10), the highest position in Mohr scale is close to 7000 HBN.

Best regards,
ct

 

[Kalroy]

I know our Wilson (Instron) chart only lists HRC20 through HRC80.

Dunno if that helps.

Kal