Heat Treatment/toughness/charpy Values

Cobalt · Feb 14, 2004

Ok, in nearly 5 years of this forum, all of these subjects have been discussed time and time again. However, whether your an Engineer/Knifemaker/Knife user-tester there is obviously a lot of factors here, so I await some comments based on experience with knife using-testing, making and Engineering.

As a mechanical/materials Engineer, I studied much concerning properties of metals and other materials and have teste knives that have proven whats in books and have tested knives that has disproven what's in books, so here goes my commentary and question:

Lets take two steels, D-2 and O-1. the Charpy value of D-2 at 60Rc is 23. The charpy value of O-1 at 57 Rc is 32, so these two steels are very similar in Charpy value. The way Charpy works is a test sample of the steel notched and impacted to fracture. however, the steel is heat treated to whatever Rc is desired. The Heat Treat used, I am presumming is from the steel's MSDS. So who determines if the heat treat given to the steel prior to testing was the best possible treatment and can't it be improved upon. I guess what I am saying and wondering about is whether the numbers given really mean much in a business where heat treatment can be so complicated and much better than what the standard is on the test samples. for example, test samples are not cryo'd. Test samples, are not differentially tempered, nor are they subjected to a Top Secret heat treatment that improves the steels performance. It seems to me that many of todays steels perform much better than their specs dictate and some perform much worse. So do the numbers given and spoken of so often really mean anything or is everyone just using them as a baseline.

So in comparing lets say carbon steels or even the two steels above, D-2 and O-1, charpy values would not be enough. The best possible heat treat for each and made in an identical knife would be the only real way to determine which steel performs best in toughness and wear resistance.

Hopefully someone is awake and willing to read this boring thread.

mete · Feb 15, 2004

The first problem is that Charpy values ,while ok for comparative tests,may not be translatable to actual use . That has always been a problem for knives and other things. It is always important in producing an item to get real world tests.There are many variables ; what type of use and what type of knife, the steel chosen and the many variables in heat treating. The best steel , heat treated poorly makes a poor knife. There is at this time significant experience with the various steels so that we can choose the best steels for a application and the best heat treatment.

me2 · Feb 15, 2004

I think that many times, the heat treat instructions given by the manufacturer are a starting point, especially regarding knife making. The instructions are generic, and are meant to cover industrial uses ranging from extrusion dies to film slitting blades to wood chipper blades etc. The end user can experiment and fine tune their heat treatment for their use. Very few steels were designed with knife making in mind, so heat treatments given by the mill may not be optimal for cutlery. Also, there is quite a range of compositions from different steel makers for the same grade of steel. D2 varies quite a bit, as do other steels such as 304 and O1. The makers' instructions may vary from one to another. This is something else to consider, and allows even more fine tuning. One manufacturer may make a steel that works better for a certain application. Finally, to further cloud the issue, the mill's sometimes change processes in an attempt to improve and dont tell the customers. I've seen this happen with 304, and the life of the parts was cut by 1/4. Makers like Dozier and Fowler dont offer a lot of variety in their steels, but they know how to ring every last ounce of performance out of the chosen grades. I would be willing to bet that makers like these dont change sources very often and without a lot of experimentation.

Cobalt · Feb 15, 2004

Originally posted by mete
The best steel , heat treated poorly makes a poor knife.

Originally posted by me2
I think that many times, the heat treat instructions given by the manufacturer are a starting point, especially regarding knife making. The instructions are generic, and are meant to cover industrial uses ranging from extrusion dies to film slitting blades to wood chipper blades etc. The end user can experiment and fine tune their heat treatment for their use. Very few steels were designed with knife making in mind, so heat treatments given by the mill may not be optimal for cutlery.

I guess this is what I am talking about. The fact that the impact tests rely on very basic heat treatment, not some of the very specialized HT's that Knife Makers are using. These specialized HT's do help to increase toughness considerably.

So again, in the real world do these Charpy impact values really ean much.?!?!?!

mete · Feb 15, 2004

As a general guide they are useful.But understand the Charpy numbers and take a look at things like tempering temp vs impact strength.Some steels have impact strength low at a 300F temper, high at 400F low at 500F and again get to the 400F values at 800F.This type of info shows just how critical proper tempering temps can be. Hardening temperatures can also greatly effect toughness . Overheating is especially bad for toughness and of course it increases retained austenite.

Cobalt · Feb 15, 2004

Originally posted by mete
As a general guide they are useful.But understand the Charpy numbers and take a look at things like tempering temp vs impact strength.Some steels have impact strength low at a 300F temper, high at 400F low at 500F and again get to the 400F values at 800F.This type of info shows just how critical proper tempering temps can be. Hardening temperatures can also greatly effect toughness . Overheating is especially bad for toughness and of course it increases retained austenite.

I have plenty of bucks from my ME days showing Charpy values and Rc and HT diagrams. However, never have I seen a Charpy value where the HT process was explained in detail, including tempering, cryo, forging, etc. where the effects of such would definitelly make a difference in the Charpy numbers.

Take a look at A-2 and 3V:

3V, toughness of 50 at an Rc of 60
A2, toughness of 41 at an Rc of 60

Although A-2 is an excellent steel, 3V is head and shoulders above it in every test I have ever seen, when the issue of toughness is taken into account, yet the difference in the two on paper is not that great, no more than 440C (23 toughness) is to ATS-34 (14 toughness). How many have actually noticed a marked increase n toughness in 440C over ats-34? According to paper 440C is nearly 80% tougher than ats-34.

I don't know what I am trying to get at, but I guess what I am sayng is that Charpy is only good for identifying steels that may be acceptable for use in hard use knives. That's all. In order to really know how these steels stack up aganst each other, the best HT processing for each steel must be used, or in other words the most performance must be extracted from each steel sample and the sample must be shaped exactly like the others for there to be an equitable comparison.

So take a Fehrman 7 inch blade. Have one in 3V, one in A-2, one in 5160, one in 52100, one in L6, one in A-8, one in S-7, one in S-5 and add to that several other hard use steels in stainless class like, D-2, 440C, S30V and ATS-34 for comparison sake. Then HT these to get the maximum performance out of each specific steel type at an Rc ofr say 58(or whatever seems right) for all of them, and then test the knives together using the exact same tests at the exact same time and temp conditions. What do you get??

TLM · Feb 15, 2004

Fracture toughness is also a function of the thickness of the test piece, thinner ones being better. This can sometimes have a remarkable effect.

TLM

Cobalt · Feb 16, 2004

True, but if all the test samples are the same, then you remove that from the equation.

TLM · Feb 16, 2004

True, but if all the test samples are the same, then you remove that from the equation.

If you are comparing final geometries at final HT then yes.

If you are comparing materials then plane stress/plane strain dependency should be taken into account.

TLM

Cliff Stamp · Feb 16, 2004

The most sensible charpy values I have seen are listed in Brysons book on Tool Steels, for example the v-notch toughness of D2 (960F, 58/60 HRC), A2 (400,60) and L6 (300,60/61) are 8, 17, and 72 ft.lbs respectively.

Even then though you have to realize that this is one aspect, the other is their unnotched values, then there is their ductility and resilence (of which spring steels are very high in both and steels like D2 are very low).

As with any testing information a lot of it comes from the manufacturers and tends to promote their own products (surprise). I wish you would get error bounds on this as I am always left wondering how much variation there is seen, do they even test batches and examine this at all.

I have used 1095 blades for example that were both very brittle and others that were very weak (Ontario), it would not be informative to note the charpy values of them without also noting the spread was massive.

-Cliff

Cobalt · Feb 16, 2004

There is no doubt that numbers from mfg's may be skewed. I also agree that unnotched values would be helpful, but the forse required would be very high and the steel may take a bend before fracturing, which may be the reason why the notch is used. In essense you are introducing a stress riser into the test piece which causes the piece to begin to fail at the highest point of the stress riser. Who knows what would happen if there was no notch.

Most good knifemakers avoid making a blade with stress risers, although some are unavoidable. I like the fact the Busse radiuses the tang to blade transition on his Basic and Swamprat knives, unlike most hidden tang manufacturers who make a square shoulder transition which is in itself a stress riser. Those notches on th TOPs blades spine are another stress riser.

Thomas Linton · Feb 16, 2004

Originally posted by Cobalt
Most good knifemakers avoid making a blade with stress risers, although some are unavoidable. I like the fact the Busse radiuses the tang to blade transition on his Basic and Swamprat knives, unlike most hidden tang manufacturers who make a square shoulder transition which is in itself a stress riser. Those notches on th TOPs blades spine are another stress riser.

Should one infer that the regular INFI Busse knives (E.g. Steel Heart) are not radiused at the blade/tang transition?

TLM · Feb 17, 2004

but the forse required would be very high and the steel may take a bend before fracturing, which may be the reason why the notch is used. In essense you are introducing a stress riser into the test piece which causes the piece to begin to fail at the highest point of the stress riser.

The notch is there for just about the reasons you mentioned.
- To get a clear and repeatable starting point for the fracture.
- To somewhat lessen the required force but mainly trying to diminish the spread.
- When fatigued crack initiation is used to get the crack in the wanted spot.

Some hard low_breaking_strain steels are 'somewhat' sensitive to the radius at the bottom of the notch.

TLM

Cliff Stamp · Feb 17, 2004

Cobalt :

unnotched values would be helpful, but the forse required would be very high and the steel may take a bend before fracturing

You can control the force by the size of the sample, yes the behavior can be very different which is why it is of benefit to know. However you could argue notch sensitivity is of maximum benefit because propogation of cracks around the edge is far more of a problem than cracks of the blade in half from spine to edge.

Thomas :

Should one infer that the regular INFI Busse knives (E.g. Steel Heart) are not radiused at the blade/tang transition?

No, it is just in plain view on them. The tang flows right off the front talon hole, very smooth transition. However on the Swamp Rats and Basics it was hidden and thus a square transition can be concealed from the user. I always thought it was ironic that McClung who always heavily critized high stress geometries in other knives puts a very large one on all his knives, but it is ok there because the user can't see it.

-Cliff

Cobalt · Feb 17, 2004

Originally posted by Cliff Stamp

but it is ok there because the user can't see it.

-Cliff

LOL
That is true of more than MD. We can name many companies that do that exact same thing and provide square transitions in hidden tang areas. At least Swamp Rat and the Basics were radiused, even though you could not see the radius. This tells me that the maker really believes in his own product as he is not turning out crap and covering it up.