Relevance of lateral strength/stiffness and alloy/carbide content of steel?

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shqxk

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This is quit a complicate, specific subject which I can't simply google around for accurate answer. Since English is not my first language then please apologize me if I picked the wrong technical term.

The question is.

In different type of steel, for example 1075, 1095 A2, D2, CPM-3V and CPM-M4

Given all of these steel proper heat treatment with the protocol specific for knife blade application (like what Peters Heat treat does) to the same hardness, lets say 60HRC, with exactly similar shape/geometry.

Will all of these steel took the same force to temporary deformed? or flex to minor degree that in range of elastic region?

Would the higher carbon/carbide able to take more stress before reaching plastic deformation?
 
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If you consider the Young's modulus... all the steels could be considered basically similar.
What changes with the heat treatment it's the position of the yeld point in the stress/strain diagram. Below the yeld point every steels behave the same elastic way, but the makeup of the steel dictates how far you could shift that point, and in that regard you have to fight with the brittleness, which is more pronounced in presence of carbides filling the grain boundaries massively than in steels with fine texture and minimal volume of finely dispersed carbides. The bigger the carbide the more you could consider it a stress riser.
Edge stability wants high hardness and if a small volume fraction of carbides is desired for abrasion resistance, they should be very fine, omogeneous and well dispersed in the whole matrix.
 
I agree. I think it would take an extremely sensitive instrument to measure the difference between the same size bar of 2 different alloys.
 
Yep - that's what I always tell everybody.


stezann said:
If you consider the Young's modulus... all the steels could be considered basically similar.
What changes with the heat treatment it's the position of the yeld point in the stress/strain diagram. Below the yeld point every steels behave the same elastic way, but the makeup of the steel dictates how far you could shift that point, and in that regard you have to fight with the brittleness, which is more pronounced in presence of carbides filling the grain boundaries massively than in steels with fine texture and minimal volume of finely dispersed carbides. The bigger the carbide the more you could consider it a stress riser.
Edge stability wants high hardness and if a small volume fraction of carbides is desired for abrasion resistance, they should be very fine, omogeneous and well dispersed in the whole matrix.
Click to expand...
 
none would compare to INFI ;)


...thought i'd get it in there before someone tried to make the argument, lol
 
shqxk said:
Will all of these steel took the same force to temporary deformed? or flex to minor degree that in range of elastic region?
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Yes, the young modulus varies very little among all steels. There're some high alloys containing 30%+ alloying elements that may be stretching the definition of "steel", but for most steels you can assume the same modulus.

shqxk said:
Would the higher carbon/carbide able to take more stress before reaching plastic deformation?
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Yes. Remember that hardness is a proxy measurement for yield strength. As such, if all steels are treated to the same hardness they have the same yield strength.
 
60 Hrc steel is all the same - until you apply a force to it that deforms, corrodes or abrades it. What happens when it deforms is why we have words like "tough" or "brittle". When it is abraded we talk about "wear resistance". But until you scratch it, or put salt water on it or bend it too far, it could be 440C or 1050.
 
gammarad said:
Yes, the young modulus varies very little among all steels. There're some high alloys containing 30%+ alloying elements that may be stretching the definition of "steel", but for most steels you can assume the same modulus.


Yes. Remember that hardness is a proxy measurement for yield strength. As such, if all steels are treated to the same hardness they have the same yield strength.
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Yeah, I also think that young modulus can be vary even the same HRC hardness. I'm not really sure but from what I ever see steel like D2 or M2 is noticeable harder to bend than 5160 at the same HRC hardness. Some might say its about the hardenability or depth of hardened sector. But 5160 is quite deep hardening steel at 0.15" thickness, I might be wrong though.

HRC hardness test is about the compressive strength, Young modulus might be about tensile strength.
 
Actually, given the same thickness, regardless of the hardness, 5160 and d2 flex exactly the same under the same force before each sample's yield point.
The false impression is related to the fact that if any permanent deformation due to exceeding the yield point is done to the steel, than you will encounter progressively less resistance to bending.
Compressive strenght and tensile strenght are expressions of the same property of the material, described by the Young modulus in the stress/strain diagram
 
stezann said:
Actually, given the same thickness, regardless of the hardness, 5160 and d2 flex exactly the same under the same force before each sample's yield point.
The false impression is related to the fact that if any permanent deformation due to exceeding the yield point is done to the steel, than you will encounter progressively less resistance to bending.
Compressive strenght and tensile strenght are expressions of the same property of the material, described by the Young modulus in the stress/strain diagram
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Thanks you very much for answering stezann.

But I sincerely don't understand how the two steel with different in alloy will has the same lateral strength? Even at the same hardness, the matrix structure, grain, carbide size/type should be totally different isn't it? From my point of view, proper amount of well distributed carbide should act as a sand in the concrete which will strengthen the structure.

From your logic every material at the same thickness and HRC would bend exactly the same under similar load?
 
It is a little tricky. Basically, the steel is still all iron atoms bonded together, mostly. Alloying does change Young's Modulus (Modulus of Elasticity) slightly, but for steels it is very close and for knife purposes are essentially the same. Up until the iron atoms (and others) start slipping past each other during permanent bending/stretching, the bonds act as springs. Since it's all mostly iron bonded to iron, it's essentially the same spring. The difference happens when you consider how much it takes to make them slip past each other. This is where the yield point is and it can vary over a huge amount just in steels alone.

Concrete is a decent analogy for steel, simply because most people are familiar, at least a little, with what is inside it and that it's not a uniform material once you get past the surface. Beyond a certain point, the concrete analogy doesn't work, and I think we are at that point in this discussion.

It's helpful to use the word bending when you mean something stays bent when you let it go. Flexing is when you let it go and it goes back to where it started. We are only talking about flexing when we discuss steels reacting the same to a given load, and only up to the yield point. HRc is only an approximation of strength, and it is an approximation of compressive strength. However, since steels are generally the same (or very close) in tension and compression, it can be used to estimate tensile strength. However, it is just an estimate. Actual strength tests and hardness tests will show different strengths for the same steel at the same hardness.

For structural applications, there are different strength grades of steel beams. The difference in use between the stronger and the weaker ones are the amount they are allowed to flex when doing design. Higher strength beams are permitted to deflect more, because they haven't reached yielding yet. A 36,000 psi material may be permitted to deflect 1/4", while a 50,000 psi material may be allowed to deflect 1/2". The same load will deflect the different beams the same, but the higher strength one can handle more load because it will deflect more before "failing" (yielding). It's a bit more complicated than that for beams as beams don't just start stretching then fall, but you get the idea.
 
The stiffness of all steel is basically the same, with a modulus of elasticity of a little under 30 million PSI. Some steels with a large carbide and alloy volume can get a little over 30 million PSI. The range is only a few percent and for all intents and purposes is the same.

There is a difference in compressive yield strength, so you can't really say that all steels of the same hardness will have the same yield strength, but there is a strong correlation.
 
Nathan the Machinist said:
There is a difference in compressive yield strength, so you can't really say that all steels of the same hardness will have the same yield strength, but there is a strong correlation.
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True. That's why I said it was a "proxy" measurement. As the penetrator digs in, it first has to overcome the yield strength of the material, but then it forces it into plastic deformation. Two materials with the same yield strength, but with different slopes on the plastic region will measure differently.

However my point is that on a first approximation, assuming that hardness equals yield strength will get you close.

I see a lot of post confusing elasticity modulus with strength (yield mostly). They are completely different and independent characteristics:
- Rubber band: low modulus, low strength
- Kevlar: low modulus, high strength
- Calcite: high modulus, low strength
- carbon fiber: high modulus, high strength
 
me2 said:
It is a little tricky. Basically, the steel is still all iron atoms bonded together, mostly. Alloying does change Young's Modulus (Modulus of Elasticity) slightly, but for steels it is very close and for knife purposes are essentially the same. Up until the iron atoms (and others) start slipping past each other during permanent bending/stretching, the bonds act as springs. Since it's all mostly iron bonded to iron, it's essentially the same spring. The difference happens when you consider how much it takes to make them slip past each other. This is where the yield point is and it can vary over a huge amount just in steels alone.

Concrete is a decent analogy for steel, simply because most people are familiar, at least a little, with what is inside it and that it's not a uniform material once you get past the surface. Beyond a certain point, the concrete analogy doesn't work, and I think we are at that point in this discussion.

It's helpful to use the word bending when you mean something stays bent when you let it go. Flexing is when you let it go and it goes back to where it started. We are only talking about flexing when we discuss steels reacting the same to a given load, and only up to the yield point. HRc is only an approximation of strength, and it is an approximation of compressive strength. However, since steels are generally the same (or very close) in tension and compression, it can be used to estimate tensile strength. However, it is just an estimate. Actual strength tests and hardness tests will show different strengths for the same steel at the same hardness.

For structural applications, there are different strength grades of steel beams. The difference in use between the stronger and the weaker ones are the amount they are allowed to flex when doing design. Higher strength beams are permitted to deflect more, because they haven't reached yielding yet. A 36,000 psi material may be permitted to deflect 1/4", while a 50,000 psi material may be allowed to deflect 1/2". The same load will deflect the different beams the same, but the higher strength one can handle more load because it will deflect more before "failing" (yielding). It's a bit more complicated than that for beams as beams don't just start stretching then fall, but you get the idea.
Click to expand...

Thanks you. That was definitely great explanation.
 
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