Why negative Rockwell Hardness value is considered as "no good"?

[null void]

I have read some pdf files and it says negative Rockwell hardness value is not used because of it would cause confusion.

What I currently know is :

HR(C/B) = N - h/s;

for C scale, N = 130; while for B scale N = 100;
s always = 0.002

and h is the permanent depth of the indentation.

I think these 2 addition question might related to the reason for not using negative value:

i) negative value means the indentation is like the indentation exceed 0.26mm for C scale,
0 > (130 - h/0.002); h bust be at lease 0.26mm, does this 0.26mm means anything significant?

ii) And i also read that 130 in C scale is the indicate infinite hardness, any simple way to explain why it is 130?

 

[mfb]

I don't know about (i), but for (ii), a value of 130 means h=0, no indentation at all.

 

[null void]

Thanks for your reply. Is that 130 is determined through experimenting the indentation on materials? Probably they can't get a that kind of condition to happen i guess they extrapolate the graph to 0 indentation.

Do you know the N value for other scale?

I also find that I never see negative value on the Rockwell scale, is this the reason they don't use negative value in convention?

 

[null void]

Ok I fond the maximum value of other N scale in this scale table:
http://www.buehler-asia.com/brochure/download_poster_common_applications.pdf

 

[mfb]

Thanks for your reply. Is that 130 is determined through experimenting the indentation on materials? Probably they can't get a that kind of condition to happen i guess they extrapolate the graph to 0 indentation.

It is an arbitrary value to put the hardness values in a convenient range.

I also find that I never see negative value on the Rockwell scale, is this the reason they don't use negative value in convention?

Very soft materials could have that, but I guess then you should use a different scale.

 

[Chronos]

Generally, the indenter has limited travel. At .26 travel the C scale value would be zero. Similarly, a B scale indenter would bottom out at .20 travel. The indenter itself is also different. For B scale measurements, a ball bearing is normally used. For C scale measurements a conical diamond is usually the indenter. The indenter configuration varies according to the material and test type. For surface specific hardness, like case hardening, you don't want the indenter to penetrate deeper than the zone of interest, which may only be in the thousandths. If the indenter bottoms out, or you get something silly like negative hardness, you are doing the wrong test.

 

[null void]

Yeah right, the measurement become error when they penetrated too deep, thanks guys for clearing my confusion up