Blade Hardness: Where do you stand?

It's called a "Knovelhamickife", Tai....... at least get the name right! Ha!
 
james terrio said:
Toughness can be measured with a Charpy notch test, and that information is available for many alloys in the manufacturer's data sheets, at different hardnesses. Unfortunately some folks are hesitant to rely on the manu's specs, and few if any makers have a Charpy tester of their own.
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So you are going to perform a destructive test on a knife to determine its toughness?
 
james terrio said:
Toughness can be measured with a Charpy notch test, and that information is available for many alloys in the manufacturer's data sheets, at different hardnesses. Unfortunately some folks are hesitant to rely on the manu's specs, and few if any makers have a Charpy tester of their own.
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What would an acceptable number be for a Charpy test?
 
Kostoglotov said:
So you are going to perform a destructive test on a knife to determine its toughness?
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I do destructive testing of finished blades fairly regularly... a lot of makers do. It's helpful and fun to beat one knife out of a batch to death from time to time. Things like stress-risers you didn't think were important but are, edges that are too thin and of course the HT protocol itself will quickly come to light.

However, that's not 100% necessary in this instance: one could simply grind a small sample of the same steel to the same cross-section as one's blades, HT it along with the blades and test that. EDIT: or trust the manu's test results at a given hardness.
 
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Carl_First_Timer said:
What would an acceptable number be for a Charpy test?
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That depends on what/how you want to cut. I don't think Nathan is terribly concerned about high toughness values because his knives are designed for precise cutting like skinning and field-dressing. Rick builds his heavy-duty knives specifically for high toughness, because he fully intends to chop, dig and pry with them. (forgive me if I'm speaking out of turn, gentlemen)

I don't have all my datasheets in front of me, but this fellow has some thoughts on the matter and reports this:
154CM/ATS 34, D-2 and S90V, at RC 60 are in the 16 to 20 ft lbs.range.
A2 at RC 60 is about 40 ft-lbs
CPM 3V at RC58 is about 85 ft-lbs and drops to about 50 ft-lbs at RC 60
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Of course it's not just hardness or toughness that comes into play; 3V excels across the board due in part to the way it's made (CPM process) and also to relatively huge amounts of vanadium. Those vanadium carbides are very hard (70Rc or something crazy like that, IIRC) but not brittle, and evenly distributed... so even at a lower hardness and with very high toughness, it has outstanding edge-holding even with a thin edge. Then again it's a real pain to sharpen if/when it does begin to dull. So aiming for a certain Rc or Charpy number as being "good" or "bad" is only part of the picture.

Who the heck opened this can of worms, anyway?!? :D
 
grizzled gizzard said:
This sounds interesting, could you elaborate a bit? What steel? I would really like to understand the paper cutting part.

I like my knives hard and keen as well, and this sounds like a good test. Please tell me how you do whatever you do with the paper. I like to impress my friends and influence strangers. Especially girls. :D
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The knife was (I removed the handle for a new one and haven't finished yet) a clip point utility with a convex edge and no primary grinding, like a tiny machete. Hardness is between 64 and 66. The steel is M2 or a similar High Speed Steel. The blade was ground/formed from a full hard mechanical hacksaw blade 12" x 1" x 0.058".

The paper cutting test used a piece of my kids construction paper folded in half length wise 2 times and set on the corner of my kitchen table. I've also done the rolled newspaper test, and folded a single sheet from a 3" x 5" pocket notebook and cut it while resting on the table, though these last 2 didnt stay in place after the cut. The 3 x 5 test was done after cutting a laundry basket full of cardboard. The knife also flexed 90 degrees a few times with no trouble. I have one saw blade left to use with this steel, then I'm switching to A2, just for availability issues.
 
sort of in the middle... hard enough to cut and not roll
- that changes for different alloys
myself i like the carbonsteels aswell.. some tool steels (basically any of the steels i'd use for a sword )
-
-what are we cuttin with this, anyway?
i wouldn't lose any sleep over an rockwell point here or there.. aslong as the knife fits your idea of what a knife should be ..
 
I HT and temper according to what is best for the given steel I'm working with. For example A2 is at it's peak performance level at 60/61 rc. I use data provided by Crucible Metalurgy. Just as important as properly HTed & tempered steel is grind/edge geometry. Knowing the proper edge type for the given task is very important.
Scott
 
I tend to be somewhat conservative on hardness and I leave my edges a bit thicker than most except on kitchen knives. For my customers I want to provide a product that will take years of hard use rather than somehing that is geared towards short term performance. An overly thin and hard edge that chips can be lots of trouble for a guy using a stone or crock stick. It isn't the sexiest approach but in my opinion it is practical.
 
james terrio said:
I do destructive testing of finished blades fairly regularly... a lot of makers do. It's helpful and fun to beat one knife out of a batch to death from time to time. Things like stress-risers you didn't think were important but are, edges that are too thin and of course the HT protocol itself will quickly come to light.

However, that's not 100% necessary in this instance: one could simply grind a small sample of the same steel to the same cross-section as one's blades, HT it along with the blades and test that. EDIT: or trust the manu's test results at a given hardness.
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While I agree that there is a lot of advantages to practical destructive testing, IMO there’s a difference between “beating a knife to death” and performing a scientific test.

If you’re going to use a charpy impact test on a knife cross section, do you have a baseline to correlate it with the manufacturers standard test coupon data?
 
blackcatt said:
I tend to be somewhat conservative on hardness and I leave my edges a bit thicker than most except on kitchen knives. For my customers I want to provide a product that will take years of hard use rather than somehing that is geared towards short term performance. An overly thin and hard edge that chips can be lots of trouble for a guy using a stone or crock stick. It isn't the sexiest approach but in my opinion it is practical.
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I like to take my edges as thin as possiblE and heat treat as hard as I can without chipping. I tell my customers that they will have no problem resharpening my knives if they invest in DMT brand diamond stones, and strongly advise against any carborundum or similar furnace made stones, haven't had any complaints in the 20 years I have been selling my knives about them being hard to sharpen or chipping, the only problems folks have had with them so far is cutting themselves from careless handling

-page
 
hi james.

for my experience the impact test is kind of unreliable. i remember doing unnotched impact test to Vanadise 4 extra, 3 test was made, 120, 60, 60 j/cm2 was the result. and also did 3 time U-notched impact test to S5 tool steel at hrc60(quenched at 890 °C, tempered at 205°C）, and 27, 9, 24 was the result. impact test sometimes giving funny numbers though i do not know the excaltly reason.
 
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