Give me a lesson on steels

J

John_Wayne777

Joined
Jul 21, 2003
Messages
5
Ok. All these steel numbers getting thrown around are starting to confuse the hell out of me. Would someone be kind enough to clue me in on the differences between the numbers and rockwell ratings and stuff?

Here is what I know (or think I do) so far:

1. The harder the steel, the harder it is to get it to a sharp edge, but the longer it will keep an edge. Higher Rockwell numbers mean harder steel. (right?)

2. Stainless steel knives resist corrosion well, but carbon steel knives are stronger and hold an edge much better than most stainless steels.

3. The more carbon in a blade, the more strength the blade has, meaning it will stand up to prying and the like better, and will also be more resistant to having the blade chip or break.

4. Proper heat treatment really makes a difference in strength of the blade.

Other than that, I am clueless, like which heat treatment methods are best, what grinds are best, what steel numbers mean what...

Educate me...
 
Go to the top of this page and click on TUTORIALS / FAQ's, then scan down to --

Be sure to check out the Sharpening, Steel and other Knife FAQ's in the Knowledge Base section of BladeForums.com
 
There is a steel FAQ that covers a lot of ground.

[link]http://www.bladeforums.com/features/faqsteel.shtml[/link]
 
#3 - You are always looking for a balance in properties. Too little carbon and the blade will be soft, too much will make it brittle. There are two problems with steel 1- picking the right steel ,2 - heat treating it. Read the FAQs a few times so you get the basics then come back and ask questions.
 
I'm not an expert on knife steels by any means, but I can tell you that confusing strength and toughness is common.

Strength is the amount of tensile force a material can take before breaking. But prying and other activities involve shear (bending) loads, which can snap a brittle material even though it is very strong.

Toughness is how well it withstands impact. In steels, usually the harder it is, the stronger it is. But the snag is that a really strong, hard material is more likely to snap when subjected to an impact loading. A tougher, more ductile steel may bend under the exact same conditions. The art of metallurgy is to acheive the best possible compromise among different characteristics for a particular application.
 
The short version:

VG-10, 154CM, ATS-34, AUS-8A, D2, 1095, 0170-6C...Good steel.

Anything marked "Made in Pakistan"...Bad steel.
 
Originally posted by John_Wayne777
Ok. All these steel numbers getting thrown around are starting to confuse the hell out of me. Would someone be kind enough to clue me in on the differences between the numbers and rockwell ratings and stuff?

Here is what I know (or think I do) so far:

1. The harder the steel, the harder it is to get it to a sharp edge, but the longer it will keep an edge. Higher Rockwell numbers mean harder steel. (right?)

2. Stainless steel knives resist corrosion well, but carbon steel knives are stronger and hold an edge much better than most stainless steels.

3. The more carbon in a blade, the more strength the blade has, meaning it will stand up to prying and the like better, and will also be more resistant to having the blade chip or break.

4. Proper heat treatment really makes a difference in strength of the blade.

Other than that, I am clueless, like which heat treatment methods are best, what grinds are best, what steel numbers mean what...

Educate me...
Click to expand...


1 – The higher the Rockwell the harder the material in terms or resistance to compression. Harder steels are not always harder to sharpen.


2 – Stainless steels and other steels with high alloy contents, generally are “stronger” in terms of tensile strength than low alloy steels with similar carbon contents.


3 - Steels intended for high toughness generally have about 0.5% carbon, increasing the carbon content beyond this tends to reduce resistance to chipping and breaking, while increasing hardenability and wear resistance.

For prying you generally want a tough steel with a spring temper.


4 – If by “strength”, you mean tensile strength than heat treating to a higher hardness will always increase tensile strength.



- Frank
 
1. Is generally true.

2. Non-stainless are generally tougher (resist breaking better). High-end stainless compete well with tool steels (complex non-stainless) for edge holding and are often better than 1095 (a high-end simple carbon steel).

3. You have confused. There are two types of strength that are commonly measured and influence performance. Yield strength is how much pressure it takes to bend or stretch a material until it excedes its elastic limit and doesn't come back to its original shape. Adding carbon changes this a little by itself, but you gain a lot more yield strength by being able to heat treat higher carbon steel to be harder. The big confusion is that harder steel is more inclined to fail by breaking instead of bending. There is a second kind of strength called tensile strength. This is how much pressure it takes to break a material. It tends to go up with yield strength, but not proportionally as much as yield strength. The region of stress between bending and breaking is smaller with hardened steels. They are less tough and are MORE subject to chipping and breaking (but they don't bend easily). If you use one for prying you are more likely to break your point.

4. Heat treatment makes a lot of difference. A file made of W2 alloy will break if you drop it on a cement floor. Heat treated down to 58 RC it makes a tough knife blade.
 
Hmm, the way I read the FAQ, and from my understanding of materials, what is described as being "stength" in the FAQ is more correctly stated as "yield strength", or the amount of force per area (lb/in^2 or Newtons/m^2) after which a material begins to permanently deform (yield). In the classic stress/strain curve, this is the top-most height of the initial straight part of the curve (that is up to the point where elastic deformation is happening). Elastic meaning it will "bounce back" like an elastic band. This site has a good diagram and some explanation: http://www.uoregon.edu/~struct/courseware/461/461_lectures/461_lecture24/461_lecture24.html . Another common measure of another kind of strength is "tensile strength" which relates to the highest point of the entire stress-strain curve.

Toughness is more related to "work of fracture" or the amount of energy (foot-pounds) needed to permanently deform or break a material. This would be correlative to the area underneath the curve up to the point of failure. The area under the curve up to the point of the beginning of inelastic behavior, is called "resilience". Of course, this elastic resilience is not to be confused with resistance to abrasion which is a much more microscopic phenomenon.

So where does hardness come in? Well, Rockwell hardness is measured by deforming a sample with a specific blow, and then measuring the resulting indentation. Since there is a resulting indentation, the material in question is undergoing plastic deformaion. So Rockwell hardness must be a measure of a combination of tensile strength and toughness. My brain isn't big enough to figure out how it would be related to yield strength.

Of course, further confusing the matter is term "stiffness" which relates to the slope of the stress-strain curve, and thus is a measure of the "elastic hardness" if you will.

All of the above relates only to the material properties of materials taken as a whole. As has been mentioned many times in these fora, when it comes to the act of cutting, much of the material properties derive from microscopic phenomena (carbides, micro-serrations etc.) that measuring strength, hardness and stiffness is just the beginning. Heat treating affects properties on both the micro and macro scale, requiring the balancing of more variables than I can think of at one time.

Some examples:

Glass has high stiffness, low toughness and practically equal yield and tensile stengths. Glass has low toughness and low resilience.

Lead has low stiffness, medium toughness and a yield strength much lower than yield strength. It has high toughness but much lower resilience.

Compared with the general range of steels, cast steel has high stiffness, highish yield strength but not much higher tensile strength.

Of course, not being a Mat. Sci. major, I could be totally wrong about all of the above so take it all with large grain of salt.
 
Einkidu, Toughness - usually refers to impact strength -measured by tests such as Charpy Impact test. Hardness tests - originally developed to give approximate measure of tensile strength. When used for other things such as wear resistance it is not a good test.Best example of this is Talonite - low hardness, very high wear resistance. " Cast steel" , this tells you the forming method only, it tells you nothing of what type of steel is used. Brittle - glass when it breaks. Ductile - when a car crashes the metal bends and stretches greatly before it tears.
 
The final mysteries of steel are "stress concentration" and "crack propagation".

You almost never cause steel to fail by compressing it too much. In any normal application you bend or stretch it until it splits somewhere. Although it may happen so fast that you don't see it, the split usually starts as a small crack at one point that causes a tear to run across the material and splits it. This is called crack propagation. If you stress something smooth and symmetrical like a polished cylindrical rod the tension is smoothly distributed across the material and it will hold together until you reach a fairly consistant stress level. This is how you measure tensile strength. However if there are nicks or microscopic cracks in the material it is weakened much more than you would expect. You may have only decreased the cross-sectional area of the rod by .001%, but in some cases you may have lowered the breaking point by 50%.

It depends (among other things) on the hardness of your material. If your material is soft enough to stretch plastically (by plastically I mean such that it does not return to its original shape) sufficiently easily it will stretch near your crack and even out the stresses. If the material is hard the stresses concentrate at the edge of your crack. The stresses concentrate to such a high degree that the material breaks at the edge of the crack and it starts to grow. By this mechanism hard materials are vulnerable to stress concentration and crack propagation. The classic example is glass. If you want to break glass in a straight line you score the surface with a glass cutter and apply bending force with the scoring on the outside of the bend. An extremely hard steel like a file can break the same way. A very high hardness knife blade is also vulnerable. Invisible flaws in the blade may dramatically compromise the blade strength.
 
It is fascinating how "generally accepted" truths are only true over a small range. With steels, you really cannot extrapolate.

Once the carbon content gets over 3 or 5 % (can't remember which, I would have to dig out my old metallurgy books) it becomes cast iron. The "extra" carbon then forms long microscopic ribbons, which give cast iron it's tremendous compressive strength by acting as buffers or shock absorbers. These same "ribbons" make cast iron tremendously weak in tension by providing many additional fracture lines.

Fascinating stuff!
 
mete: thanks for the info. I meant to type cast-iron (mega high carbon levels) not steel, but my fingers were steel happy. I wasn't trying to say that hardness measured wear resistance. Sorry for the confusion and thanks for the correction.

Jeff, good explanations. "Stress concentrations" returns "Griffith" from my rusty memory banks (Griffith crack?). I believe he was the first one to come up with the concept of stress concentrations at crack tips. The smaller the tip diameter, the higher the stress. What happens in ductile material is that the material "flows" around the tip of the crack to increase the tip diameter, thus reducing the stress concentration.

In any structure placed in shear or tension, you want your crack tips to have large diameters to avoid the problem of stress concentrations. That's why frame-locks have bigger holes drilled in the base of the lock-bar crack: increase the diameter, lower the stress concentration, prevent crack propogation. This is a more macro-scopic design issue though.

For knife blades, you are still left balancing hardness, toughness and strength. In the end, there can't be a free ride because the strength of the molecular bonds and the ductility of teh molecular structures are limited.
 
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