Why Doesn't Heat Treating Affect Steel Flex?

Larrin

Larrin

Knifemaker / Craftsman / Service Provider
Joined
Jan 17, 2004
Messages
5,013
In my latest metallurgy-related article I wrote about steel flexing and bending, where I answer such questions as:

1) Why doesn't a softer steel flex more than a harder one?
2) At what point does steel permanently bend rather than flex?
3) Why is it that a fillet knife can flex to such extreme angles?
4) What is happening in the ABS 90 degree bend?
5) What exactly is the "brass rod test" even testing?

All this and more here: http://knifesteelnerds.com/2018/03/13/why-doesnt-heat-treating-affect-steel-flex/
 
Thanks Larrin. I hop over there to do some reading whenever I have time. It's a great resource and a good read.
 
Thanks, added to the metallurgy sticky.

I have explained/argued Young's modulus with some folks here and in person for years and it is hard to get people to believe it. I think hearing it from a professional may help the doubters.
I really get a laugh from some of the old timers who state that their HT method makes the blade more flexible. As a certain friend of ours says, "You are entitled to your own beliefs, but you are not entitled to your own science."
 
Last edited:
I have a couple of questions. I was big into making bows at one time in my life. And when making a laminated bow you try to pair a material that is strong in tension with one strong in compression. I understand that the thickness matters for stiffness far more than material but material does affect stiffness. A common thing with a wooden bow is that moisture changes affect the draw weight with the bow gaining weight when it's dry even though wood shrinks in cross section.

My question is what would affect stiffness (I'm assuming that stiffness and flex are what we are talking about ). If heat treatment doesn't affect it then does alloy ? And if you call permit set a failure is that fail a tension or compression fail? My understanding is that a broken blade would always be a tension fail. My hypothetical question would be that I make a leaf spring type object like a bow and use a very hard alloy on one side and second alloy on the other side that is high in tension strength would that change the stiffness? Or is the mechanism that some materials essentially pre load themselves and that steel would have a change if one side of a object was preloaded?

I mean no disrespect but am trying to wrap my head around what is going on. Or did I not understand the point being made?
 
Last edited:
skillgannon said:
I have a couple of questions. I was big into making bows at one time in my life. And when making a laminated bow you try to pair a material that is strong in tension with one strong in compression. I understand that the thickness matters for stiffness far more than material but material does affect stiffness. A common thing with a wooden bow is that moisture changes affect the draw weight with the bow gaining weight when it's dry even though wood shrinks in cross section.

My question is what would affect stiffness (I'm assuming that stiffness and flex are what we are talking about ). If heat treatment doesn't affect it then does alloy ? And if you call permit set a failure is that fail a tension or compression fail? My understanding is that a broken blade would always be a tension fail. My hypothetical question would be that I make a leaf spring type object like a bow and use a very hard alloy on one side and second alloy on the other side that is high in tension strength would that change the stiffness? Or is the mechanism that some materials essentially pre load themselves and that steel would have a change if one side of a object was preloaded?

I mean no disrespect but am trying to wrap my head around what is going on. Or did I not understand the point being made?
Click to expand...

I am not an expert in this field like or there but I will take a poke at a few of these things. Comparing wood to steel is not accurate. Steel for the most part is one uniform material where wood has fibers, voids, moisture exc. So wood can’t be use as a comparison in this aplacation.

All kinds of things can affect stiffness but in steel. That’s not what we are talking about because the comparison is between two identical alloys. The only difference between the two would be the heat treat, everything else is the same.

A failed blade is a complicated thing but it’s not just tension. When you flex a blade the inside curve is under compression and the outside is under tension.
 
Last edited by a moderator:
I will quote from the article: "The strength of those bonds does not change with heat treatment, and changes by only small amounts when other elements are added (such as with chromium additions for a stainless steel)." It's typically less than 10% difference even when talking about a low carbon steel vs a stainless steel with large amounts of chromium and nickel added. The difference is more academic than practical.

Yield stress is pretty close between tension and compression with steel, so deformation is equally likely with either. Failure in terms of fracture is more likely to occur in tension.
 
Larrin said:
I will quote from the article: "The strength of those bonds does not change with heat treatment, and changes by only small amounts when other elements are added (such as with chromium additions for a stainless steel)." It's typically less than 10% difference even when talking about a low carbon steel vs a stainless steel with large amounts of chromium and nickel added. The difference is more academic than practical.

Yield stress is pretty close between tension and compression with steel, so deformation is equally likely with either. Failure in terms of fracture is more likely to occur in tension.
Click to expand...

Thanks. That's basically what I was asking. I can definitely understand that it is that academic difference. Just to clarify though if using a laminated structure and preloading of a single or both sides into tension or compression it should be possible to increase stiffness for the same cross section?

I understand that regarding knives this is a totally academic question but just trying to make sure that the world is still flato_O
 
skillgannon said:
Just to clarify though if using a laminated structure and preloading of a single or both sides into tension or compression it should be possible to increase stiffness for the same cross section?
Click to expand...
I don't think so. Shot peening is a relatively common practice for creating a compressive stress in the surface of different types of parts and I've never read that that impacts the modulus in any way.

Edit: I found a nanoindentation study that claims a difference in Young's modulus with shot peening: http://nanovea.com/App-Notes/shotpeen-nanoindentation.pdf
We must be outside of my area of knowledge.
 
Last edited:
Larrin said:
I don't think so. Shot peening is a relatively common practice for creating a compressive stress in the surface of different types of parts and I've never read that that impacts the modulus in any way.

Edit: I found a nanoindentation study that claims a difference in Young's modulus with shot peening: http://nanovea.com/App-Notes/shotpeen-nanoindentation.pdf
We must be outside of my area of knowledge.
Click to expand...

My confusion was coming from my look at bow making and not realizing that not just wood but other materials used probably have much greater variations in strength including directional strength. One interesting thing that was found was that if materials were pulled into tension (tension side compressed , compression side pulled into tension. ) prior to bonding it seems to move the tension and compression zones further into the cross section. It allows for greater stiffness without a increase cross sectional thickness. Probably not something that can be used in knife making but a fun fact
 
I think the results from the nanoindention study are due to the fact that we are not looking at the same geometry as a flat bar or steel. Thus, Young's modulus isn't comparing apples to apples. With millions of microscopic craters and raised rims, there is no way to calculate the actual dimensions and use the modulus to calculate the deflection with accuracy.
 
Last edited:
Noooo, Larrin, what did you do!! Now all the bead blasted tacticool blades will be regarded as they have the same modulus of carbide's!! :)
 
stezann said:
Noooo, Larrin, what did you do!! Now all the bead blasted tacticool blades will be regarded as they have the same modulus of carbide's!! :)
Click to expand...
Tactical knives will continue on without my help.
 
You must log in or register to reply here.
Back
Top