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

In summary, the Rockwell hardness scale does not use negative value because it would cause confusion.
  • #1
null void
102
1
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?
 
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  • #2
I don't know about (i), but for (ii), a value of 130 means h=0, no indentation at all.
 
  • #3
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?
 
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  • #5
null void said:
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.
 
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  • #6
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.
 
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  • #7
Yeah right, the measurement become error when they penetrated too deep, thanks guys for clearing my confusion up
 

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

A negative Rockwell Hardness value indicates that the material being tested is softer than the indenter ball used in the test. This means that the material is not as hard or resistant to indentation as the standard reference material, making it less desirable for use in certain applications.

2. Can a negative Rockwell Hardness value be accurate?

Yes, a negative Rockwell Hardness value can be accurate if the material being tested is significantly softer than the indenter ball used. However, it is rare to see a negative value and it is generally seen as an indication of poor quality or unsuitability for certain uses.

3. How is a negative Rockwell Hardness value calculated?

A negative Rockwell Hardness value is calculated by measuring the depth of indentation made by the indenter ball on the material's surface, and then comparing it to the depth of indentation on the standard reference material. If the material is softer, the difference will result in a negative value.

4. What factors can lead to a negative Rockwell Hardness value?

The most common factor that can lead to a negative Rockwell Hardness value is an error in the testing process, such as using the wrong type of indenter ball or applying too much force during the test. Other factors can include the composition or structure of the material, as well as the presence of impurities or defects.

5. Are there any materials that are expected to have a negative Rockwell Hardness value?

No, there are no materials that are expected to have a negative Rockwell Hardness value. In general, materials with a higher hardness are considered to be of better quality and more desirable for use in various applications. However, there may be exceptions for certain specialized materials or alloys that have unique properties or purposes.

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