RC hardness explanations

Atlas Knife Company

Atlas Knife Company

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Feb 16, 2010
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I'm having trouble wrapping my mind around the hardness scale. I get it that 30 is soft and easily worked, and files are around 80 and will shatter.

Here's where I would like to learn. What is the difference between 59 and 61? Currently my folder blade is untempered, supposedly around 61? 61 will stay sharped but be more likely to chip? 59 won't stay as sharp, but would be less likely to chip. 1% more likely to chip? 10%? how big is the difference?
 
since nobody else has tried I'll give it a shot. The Rc numbers are determined by how far a diamond cone will penetrate the material. First off files are 63-65 Rc steel can go no higher than about 72 Rc various carbides are in the 80s Rc also I have heard, and I think this is true that the relationship between Rc numbers and wear resistance is actually exponential so 1 or 2 points can make a real difference.
 
Zaph1, first off, for the most part steel should be tempered as soon after hardenening as practicable. To partially answer your question, each steel has a "sweet spot", where there is the best balance of edge holding strength, and sharpness. Look up the steel that you are using to see what the manufacture recommends, or to see what knifemakers using that steel recommend. Again, each steel is different, and each application is different. Without knowing what steel you are using no one will be able to provide you with an educated response.

~Alden
 
this is a generic tempering chart but all steels will be different sometimes drastically so

tempero1.jpg
 
You have to look at hardness and toughness at the same time, from the above chart it shows at 65 RC (direct from quench) you have the hardest but the toughness is at its minimum. The impact toughness is done by swinging a weight at a test piece and see how much force is needed to break it. Tempering you gain toughness very quickly but give up some hardness. You have to pick your steel based on the properties you want and then do the HT w/ temper to get the combination of HC and toughness you need.

On folders I think toughness is more important than hardness since the bladed is thin and small you need to make sure it will not break, typically folders are not used alot so retaining an edge us not as important as with a skinner. I am sure there are situations where this may not apply and so you pick your steel, hardness and toughness to match the use of the knife.
 
just have ot add that folders might be the 2nd most used knife out there right behind kitchen knives
its not like most use hunters every day or carry a bowie :)
but there is much truth in pic the steel for you app. and then make sure you HT to a range that gives you the best of the steel for that app.
 
OK, that makes a little more sense. When I first started I read that annealed was around 30, then you harden to 80 and temper it back to around 60. I'm guessing that isn't correct. I thought there was a much bigger numerical difference between annealed and hardened and tempered.

Can anyone tell me what hardness s30v would come from the factory at? Also, with a target of 59-61, three points is a big variation. If I cut a lot of tape and paper, I would want a lasting edge so stick around 61, but if I use it to whittle with I should go for 59 to prevent breakage?

(the chart shows up now) Sounds like I need to re-anneal and HT my blade.
 
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There are different rockwell hardness scales and some have a range that goes up into the 80's. Rockwell A, B, and C can overlap and become very confusing if what you read misquoted the hardnesses. A google search should bring up a chart so you can compare all the different hardness tests. Some are more common in the USA, while others are used in England and other countries.
 
In the U.S. the most common hardness test used is Rockwell. There are Rockwell A, B, and C scales. A is for very hard items where superficial hardness if most often measured, B is for rather soft metals and uses a ball shaped penetrator, C is the one we most often use in knifemaking and covers the ranges from around 20 to 70. The scale is simply a series of numbers representing the depth of penetration between the minor load and the major load. The minor load is applied to give a good solid seat in substantial material thus eliminating many false readings before the major load is applied.

Rockwell test measure penetrative hardness which only translates rather indirectly into true abrasion resistance. What it is really measuring is the materials overall strength, and with enough strength abrasion resistance can be enhanced, however, things like scratch tests and files more directly measure abrasion resistance, but materials with very hard particulates suspended in a softer medium can have a very high abrasion resistance and still have a very low Rockwell, or strength.

We will most commonly achieve a maximum of around 65 or 66 HRC in our shops with common bladesmith heat treating practices. Drawing the blade back to at least 62 is advisable for most U.S. users due to our quirky ideas of what knives should do. A good skinner could be from 60-62 HRC, while a large chopper could be from 57 to 61 HRC depending on the alloy used. A 60 HRC machete made from O-1 may not be a good idea but one made from L6 just might work. There are many who think that less than 57 will work for a knife, but the loss of strength necessitates awkward geometries that would not be necessary if the maker would gain a better understanding of the strengths in their alloy of choice.

In Europe Rockwell is not as common and one must be careful to note where the numbers are coming from. If your charts are from the U.K. the numbers will mostly likely be in Vickers, which uses a elongated pyramidal penterator to create a dimple that is then microscopically measured to generate the hardness level. There you may see something like 653 HV or DPH. Or if the materials are commonly not of extremely high hardness you may see 578 BHN, or Brinell hardness number based upon a very common test using measurement of the diameter of a dimple created by a ball shaped penetrator. Brinell is commonly used in the U.S. as well for things like bullet lead and other non ferrous.

To complicate matters even more it is very important to remember that toughness is most often used (particularly on the chart posted above) to refer to impact resistance, which really has nothing to do with the materials ability to stretch or bend in gradual flexing. I just scared my wife with a demonstration of the concept the other night when she brought home several packages of giant chewy Sweet Tarts. These things are great for showing the radical differences between gradual loading and sudden loading and I think I am going to take a package to all my lectures on the topic in the future. You can bite the things and stretch them like putty, fold them over in a chewy mass, but if you quickly slap the package in the table they shatter like glass. This is the distinct difference between true toughness (impact resistance) and bending blades in a vice. In Charpy or Izod “V” notch tests all but the softest of samples break as if they are brittle even though they would just bend in a slow loading situation. Sudden loading works beyond the capacity of the materials slip systems to deform without failure and this is why being able to slowly bend a bowie blade (or flew is edge on a rod) is no guarantee against the edge chipping out in chopping and why I shifted my focus to impact strength long ago (sword making brings the topic into focus even clearer).

The chart posted above is definitely a generic one or one for a very simple carbon steel due to the lines being almost straight. Any amount of alloying will turn them to curves with dips or plateaus between the sweet spots if optimum hardness vs. toughness.
 
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Zaph, it appears that 58-59 Rockwell C is a common range for s30v in folders. The manufacturer calls for 58-61. Tempered into this range the knife will probably neither chip or dull readily at either end of that range. But I believe that it should have been tempered soon after quenching, and should be done now, even though it is in the target range.

~Alden
 
It's been 11 days since HT. Should I temper now or anneal and re-HT?

To answer my own question from earlier, annealed s30v is around 255bhn, or roughly 25 Rc. Thanks, Kevin, now I what they are talking about when they list bhn on the spec sheet from Crucible.

As quenched, it should currently be 62 Rc. Tempering would take it to 57.5 Rc. I would like to keep it on the upper end, around 60-61 Rc.
 
zaph1 said:
...To answer my own question from earlier, annealed s30v is around 255bhn, or roughly 25 Rc. Thanks, Kevin, now I what they are talking about when they list bhn on the spec sheet from Crucible...
Click to expand...

Yes, steels that are more thoroughly annealed will be measured in Rockwell "B" or even Brinell due to the fact that the pointed "C" penetrator plunges unreliably deep into materials in the 20's and below, and the 150kg load is a bit much. I have had to switch to "B" scale when testing the spines of some old Japanese swords which achieved unbelievable softness in steel, and I often switch the the "B" scale for spheroidized steel to be sure.
 
I'll add a thought to what Kevin already said.
The fact that tempering reduces hardness to add toughness and resilience (impact resistance) often gives the impression that there is a negative causal nexus between the two: you add hardness and reduce toughness, so you reduce hardness and get toughness back.
The two properties are related, but one does not necessarily disrupt the other.
Other factors come into play, like the structure of the metal itself.
For example, a blade with grossly overgrown cristalline structure will remain brittle no matter how much hardness you take out of it, while a very fine-grained powder steel can maintain a very high toughness even at higher hardness levels.


PS: one thing more.
Rockwell scale is not linear. Since it is based upon the penetration of a sphere, any given step is more than proportionally "harder" than the previous one.
For example, the resistance to penetration between 20 and 30 HRc (10 HRc degrees) is 13 tonnes sq/in.
Between 30 and 40 (10 HRc degrees again) is 18.
Between 40 and 50 is 26, and so on.
 
There really need to be two better adjectives than hardness and toughness. Perhaps strength and edgeability? I think edgeability is a better term for use here than toughness. If I mention edgeability, most people would understand that I am referring to the steels ability to retain an edge. Toughness makes me wonder if it was over cooked steak.

<thinking>

Is there a better term than hardness? Or, was hardness the same as edgeability?
 
These are not pub-talk adjectives as they may seem to the layman's eye.
They are proper technical terms which define very specific properties of steel.

Hardness
Toughness
Resiliency
Tensile strength

are all specific properties of steel. There's no ambiguity in what they mean.;)
And the most appropriate characteristic to associate with "edgeability" would be hardness, not toughness. ;)
Obviously, one needs to do his homework and study the books to know what they mean. Yes, it may be boring, especially if one's eager to get his hands on a piece of steel and make a knife, but at least a cursory education in metallurgy pays tenfold in time spent correcting errors, quality of the finished manufacts and satisfaction in knowing what you are actually doing and getting the results you want, not the ones you happen to stumble upon.
Believe me, it's worth it. :)
 
hardness is not that hard to understand and toughness isn't to bad either. could be worse.
 
To confuse you more - hardness measurements were originally developed to give an approximate measure of strength. This is why typical hardness conversion tables give approximate tensile strength !
Strength is needed in blades to give you an edge that won't roll over , for example.
Hardness measurements are not good for wear resistance info because they don't tell you about carbides which are a major wear resistance factor.

Some day I'll confuse you more by talking about dislocations !!
 
zaph1 said:
There really need to be two better adjectives than hardness and toughness. Perhaps strength and edgeability? I think edgeability is a better term for use here than toughness. If I mention edgeability, most people would understand that I am referring to the steels ability to retain an edge. Toughness makes me wonder if it was over cooked steak.

<thinking>

Is there a better term than hardness? Or, was hardness the same as edgeability?
Click to expand...

The confusion comes from years of misunderstanding and misapplication of the terms by knifemakers, if you approach it from the standard terms used in industry it all makes sense. Hardness is a measure of strength, and strength is defined as the ability to resist deformation. 40HRC steel has a low hardness and thus a low strength, or a lower ability to resist deforming to allow the penetration of the Rockwell stylus. 60HRC steel has a high hardness and thus a high strength, or the greater ability to resist the deformation. For years I have heard bladesmiths occasionally make fools of themselves by claiming a soft backed blade was stronger, when it is in fact much less resistant to deformation than a fully hardened blade, and thus has much lower strength. Since, short of chipping out, the secret to edge holding is edge stability through strength it is hardness/strength that would more correspond with your idea of edgeability.

Toughness on the other hand is not necessarily the opposite of strength but is instead almost another form of strength since it is the ability of the material to resist brittle failure ("snap!") under sudden loading, the problem is that it is left open ended on the bottom side leaving the door open for bladesmiths to confuse it with ductility. Eliminate the notch in impact tests and many steels designed to be "tough" will take a lot more pounding in the mid 50's HRC than steels that are dead soft and just fold up under the load.

If there would never be sudden loading (even on the extreme micro scale), high strength is all you would need for ultimate edge stability, however to avoid brittle failures on the micro edge level some degree of toughness is needed. But despite how blasphemous it is to say it, there really is no need for ductility (not tough, just soft) in a knife blade at all, it is just introducing unnecessary weakness.

I have had folks get quite upset by these concepts over the years and have also grown resentful of the definitions and terminology, but that is just a matter of killing the messenger. We can change the words all we want but the laws of physics and the same concepts will still remain for bladesmiths to deal with by any other name.
 
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