what exactly does rockwell hardness tell us?

[scott.livesey]

As the subject says, what does Rc tell us? I understand that the test shows how far a diamond shaped piece of material can penetrate material under a calibrated load. the harder the material, the smaller the hole. What kind of progression is there from Rc50 to Rc60? geometric, exponential, linear?
I did my homework and found that tensile strength of tool steel increases about 20% from Rc55 to Rc60. Am I on the right track thinking:
higher the Rc, higher the tensile strength, material is less likely to bend or deform, it takes more force to bend or deform the material.
is a piece of 2mm Rc 60 steel as strong as a 3mm piece of Rc55 steel?
any and all comments welcome.
scott

 

[Kevin R. Cashen]

... Am I on the right track thinking:
higher the Rc, higher the tensile strength, material is less likely to bend or deform, it takes more force to bend or deform the material...

Yes, It has been my understanding from reading that the Rockwell style test was developed as an alternative aid in assessing tensile strength. Hardness itself is not so much an actual property as it is a concept for expressing strength, the opposite of which would be ductility. In tensile loading, higher resistance to deformation = higher strength, low resistance to deformation =ductility. In many materials, steel included, tensile and compressive strengths correspond well with each other, exceptions can be things like cast iron or concrete where the compressive and tensile strengths are considerably different. The Rockwell "C", as well as the "A" scale, are more depth measurements, the distance the penetrator moves between the minor load and the major load. I guess in Rockwell the "B" scale would be the same but I tend to see similarities to the Brinell scale which measures the diameter of the dimple created rather than its depth.

 

[tomwatson]

There is more to reading the results of a hardness test than just the "R" factor. The R test can tell you if oven temps are on track. One should follow R tests through hardening, tempering, etc, to see if the complete heat treating is accomplished. I know everyone thinks tensile strength is a good factor to have in a knife, but this only tells me if is a strong steel when the ends are pulled opposite each other. How many times in using a knife have you ever had it try to pull itself apart. A better test is the Charpy Impact. A knife will probably come under other loads like side impact, bending, compression, and wear resistance. Hardness alone is not a good indicator of wear resistance. One R test you do need to do is a hardness test down the edge. Do one a 1/4" from the edge, and every 1/2" from tang to point. If you get too many readings that vary more than 1 point, then you most likely ran the temp too hot and too long at hardening temp. Soft spots in the edge are the result of this.