Rockwell Testings -

R

rlinger

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Mar 29, 2002
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I just tested some un-hardened stock and thought it just interesting:

O1 (precision ground 9/64 thick) 6 HRc

ATS (prec. ground 5/32) 16 HRc

D2 (prec. ground 1/8) 25 HRc

CPM S30V (3/32, actually 0.122 inch) 25 HRc


I don't remember O1 working that much easier but its probably because I've been using the other stuff on a steady basis and have just gotten used to it.

RL
 
In addition, I don't think hardness is a direct indication of machinablity. As an example: D2 at 6 HRc may well not be as machinable as another steel of the same hardness.

D2 is a non-stainless tool steel.
 
The original definition of stainless steel was- steels that contain at least 12% chromium - therefore D2 would qualify, though it is usually listed as a tool steel. Technically the Rc scale ends at 20, other scales are used below that. The tool steels such as O1 or D2 are sold in the spherodized anneal condition otherwise they would be extremely difficult to machine. Hardness tests give a measure of the strength of the matrix not machinability and not wear resistance. Tool steels are sometimes made in free machining grades to make machining easier , such as D2 ,but I wouldn't want a knife made of that. Metallurgy rules.
 
I need this "technically Rc ends at 20" explained further please. Perhaps I misunderstand that sentence. The way I read it is literal and that is the top end of that scale. Can you give a breif history of why it is recognised above that value. Of course though, I may have misunderstood. I am not questioning you. I am interested in knowing.

On another matter but relative to your post, isn't 440C - a widely recognised stainless - also known as a tool steel??

Thanks, RL
 
The high chromium, high carbon tool steels are the D series the most common of which is D2. 440 is not in that list. I believe that the 440 is just refered to as a cutlery steel. There are a number of hardness tests for metals ( still others for plastic and rubber). In the Rockwell tests the C scale goes from 20-70 below that we use the B scale. Rc 20 =Rb 98. It is important to know that the hardness tests were developed to give an approximate measure of tensile strength. I often see it misapplied to wear resistance or toughness. The best example is talonite, though fairly low in hardness it is extremely wear resistant, because the hardness measures the strength of the matrix not the wear resistance of the carbides. Strength, wear resistance, machinability, grindablity, toughness are all destinct properties and cannot be simplified by just measuring hardness.
 
So now I think I understand more properly your first post. I was thinking opposite of what you ment to explain. The value 20 is the low end. I was thinking you ment it to be the high end of the C scale.

Should I test the same O1 again that measured 6 on my C scale do I want to measure it with a ball indentor and read it, instead, on the B scale?? - OR, do you mean, rather, that those items testing less than 20 but greater than 0 Rc should be converted and expressed in B values??

Roger
 
I said "technically" because sometimes Rc values below 20 are used especially when comparing steels to those above 20. Conversions to another scale are approximate as are the below 20 Rc. Not to loose sleep over. Also Crucible lists 440 under stainless tool steel though its primery use is cutlery. I cannot find CAT in any steel listing.
 
In order to be stainless, there has to be a certain percentage (I believe 10%) of Cr left over after carbide formation to make a layer of CrO on the surface of the steel. This layer protects the steel from corrosion and makes it "stainless".
Thus the necessary amount of Cr varies with the amount over Carbon in the steel. The high amount of C in D2 keeps it out of the stainless realm.
 
Steve,

That little paragraph you wrote helps to explain why some stainless over heat treated losses corrosion resistance.

RL
 
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