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Brian Vanspeybroeck

Flexability and stiffness

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Here is a question I thought I knew the answer to but read stuff (posted by some very schmart people..) on various other forums on heat treat that has confused and bewildered me. :mellow:

 

OK...the question. If I take two dimensionally identical bars of the same particular steel chemistry (let's say 5160!) and I fully harden both of them and then temper one bar at say 400f for 2 cycles of 1 hour each and then temper the other bar at say 600f for two cycles of 1 hour each, will not the bar that is harder (tempered at a lower temperature) give me a "stiffer" spring? Both bars should be able to be deflected to some degree and return to shape without permanent deformation (unless they are flexed beyond their ability to do that....) and that to me is the definition of "spring". But I have read somewhere that the amount of resistance to being flexed (the stiffness of the spring) is simply and soley a function of material thickness. That the heat treat will not affect the spring qualities and that the harder bar will not yield a spring that takes more force/energy to deflect. :blink:

 

This is contrary to a whole lot of experimentation on my part...1/16" blades of 5160 tempered at 425f are "stiffer" than an identical blade of the same bar stock tempered at 525f for the same number of cycles. Not a lot....but a significantly longer cheater or more weight/energy on the end of the blade is required to make the harder blade flex the same distance.

 

Or am I living in another dimension metallurgically? :wacko:

 

Brian

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I agree with you, everything I've seen in the last 20 years tells me higher hardness=higher stiffness.

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I think it is safe to jump into this one with both feet. Although stock thickness relates to resistance to deflection, the heat treating of the steel is the way to get the desired results.

 

Just my .02 worth,

Chuck

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We metallurgists always live in a different dimension !! :rolleyes: The flex or spring of metal is [defined as] within or below the permanent deformation of the steel. The difference between your two steels is that a 400F temper will be stronger and therefore have a higher limit before permanent deformation than the 600 F steel. If we do a bend test ,the 600F will bend [permanent deformation] at a smaller angle than the 400F steel . If we want flexibility [as in a fileting knife] we are concerned about thickness not strength. Your confusion is common partly due to definitions [flex vs bend] and what you think is 'intuitive' !! :wacko:

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I take two identical blades, one tempered at 400F and one at 600F and I clamp them both in identical vises with the blades parallel to the ground at the same point of the tang. I then hang 100lbs off the tip of each blade and measure the amount of deflection in terms of degrees of angle that each blade is flexing because of the weight on the end.

 

In my world the blade tempered at 600F has a greater angle of deflection...it will droop at a steeper angle because it is less rigid than the identical blade tempered at 400F. Yes? The blade with the deeper temper (tempered at 600F...) is a less rigid structure and can be forced into flexion with less force.

 

Whether or not either blade is permanently bent is not the issue with me. I simply can't wrap my head around the statement that both blades will have an equal amount of rigidity and that only changing the dimensional thickness can make the blades more rigid.

 

Brian

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NO ! Your flexing test only involves Modulus of Elasticity not strength !! The strength question only comes up when you are going to exceed the elastic limit. 400F temper has higher elastic limit than the 600F temper. :excl:...I can draw a graph if that would help ??

Edited by mete

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I'm going to have to say, thickeness and taper will effect stiffness and flexibility much more than temper between 400f and 600f. I think with the same force applied to your two blades they will flex the same amount.

Edited by Don Hanson

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Graph = picture?! I'm good with pictures...please post any that will help. B)

 

Am I the only one who doesn't get it? I feel a little....stupid. :mellow:

 

Brian

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Hmm, is what Brian is calling ‘stiffness’ the same as ‘modulus of elasticity’? If so, is that affected by temper temp? If not, what else could cause a blade to seem as if that were the case?

I don't quite get it yet, either... :huh:

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Daryl is right of course but sometimes we engineers are criticized for being to 'picky' in using words .Well until we all speak the same language there will be confusion. Picture will follow but I do it the old way -draw it on paper and photograph it !! :P

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Here's a quicky photo .As we apply a force the blade flexes [springs, none permanent ] .If we exceed the elastic limit the blade takes a permanent set [bends] .The straight line D-A slope defines the modulus of elasticity .This is dependent on the material and is different for aluminum, titanium etc. If we have , for example ,a steel in three different conditions , we will have the same M of E but the bend will take place at different forces. If the steel is un- heat treated it might have an elastic limit [ yield point] at C. Heat treated and tempered at 400F an elastic limit at A, and tempered at 600 F an elastic limit at B. The curved sections are of course the permanent bend . ...The language problem of course is that we engineers call it 'flex' when it's non-permanent and 'bend' when it's permanent and exceeds the elastic limit.....I hope that clarifies things a bit . :wacko:

DSC00679.jpg

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Thanks Mete. Clarifying terminology helps. B) Your graph illustrates the principles that I find to be somewhat intuitive and pretty much mimic my empirical studies.

 

I myself am less confused/concerned about "Bending" (permanent deformation) than I am about "Flexing" and the force it takes to get an equal amount of flex in two blades that are identical. My original confusion stems from the statement that two identical blades, of the same material, will flex equally and identically if "X" amount of force is applied to them in a lateral direction and that the relative state of the material (the heat treat) of the blades cannot affect this degree of flex. It is counter intuitive and does *not* conform to my empirical studies that one identical blade tempered at 400F will flex with exactly the same amount of lateral deflection as a blade (identical) that was tempered to a softer state.

 

It is counter intuitive and against my empirical studies that if I wish to make one of these blades more rigid, that is, it takes more force to cause an equal amount of flex, that the only way to accomplish this is to increase the thickness of the blade. That two identical blades, one of tempered Martensite at rc50 and one of tempered Martensite at rc57 will flex identically with "X" amount of lateral force applied simply does not fly with me...it is counter intuitive and does not jive with any empirical/destruction/use testing I have conducted over the last 15 years.

 

I'm willing to believe I have missed something and that my data and my methods are flawed and that what I believe to be true is actually not.....the reason is unimportant to me. Faulty testing/bias/ignorance is less important to me than understanding if modern metallurgy supports the concept that the only way to increase resistance to flexing in a steel blade is to make the blade thicker. The only way to make the blade more susceptible to flexing is to make it thinner.

 

I'm interested in all points of view. But particularly the engineering/metallurgical statments that can either prove this concept to me or refute it.

 

Thanks for the clarification! B)

 

Brian

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I'm going to have to say, thickeness and taper will effect stiffness and flexibility much more than temper between 400f and 600f. I think with the same force applied to your two blades they will flex the same amount.

 

I agree that cross-section will affect the amount of flex moreso, yes, but I also think the level of hardness will affect it as well. The 600F section will Flex a little furthur than the 400F section with the same force applied.

Perhaps not enough to see easily in small sections, but in the context of sword blades, it's a very easily expierienced and obvious difference. A sword blade with higher level of hardness will definately seem "stiffer".

 

Edited to clarify, I said "bend" where I should have said "flex".

Edited by RHGraham

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Lightbulb on! I get it now. We are really talking about modulus of elasticity as it relates to flex and deformation.

 

Thanks,

Chuck

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"Modulus of Elasticity. The ratio of the unit stress to the unit deformation (strain) of a structural material is a constant as long as the unit stress is below the elastic limit. 4th edition of practical metallurgy.

 

I ran this test in collage with 1/4"x1.5x36" bars of 01 tool steel (precision ground to a high tolerance). I drilled and bolted the ends to a common bar and mounted that bar in a vise. Then I suspended 10 pound weights on the ends. They both dropped the same amount. Both were annealed. I Heat-treated one and tempered it back 400f for an hour. They both still dropped the same amount. It is not intuitive, but it is real. It is a test you can do yourself. most steels have very similar Modulus of Elasticity so they yield basically the same results even between different alloys. It is important to remember that the the Modulus is a Ratio. No matter how the materials strength is altered from heat treatment the Ratio for the Modulus remains the same within the elastic limit. The book said to use mild steel and heat-treated tool steel. I used identical steel bars because they were dimensionally identical. I could not make the mild steel bar the same dimension without going to great lengths, but the results should have been the same even using mild steel/01.

So if you are working within the steels limits i.e. applying stress, but not so much as to create strain the deflection should be the same for a given load between hard and soft steel. I would never really have wrapped my brain around it until I did a dedicated test.

Patrick

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Well, I'll be danged, Patrick. <_<

 

I do appreciate your explainin' it in a way guy like me can understand it. It's still disturbing. Great! There goes my recovery time off of the day job. I had planned on just spending the next few weeks convalescing and finishing a sword mount but I have some tests to run on some 5160 bar stock it seems. :lol:

 

Got some brain wrappin' to attempt. B)

 

Brian

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Are we seriously saying that an dimentionaly identical piece of o1 full hard and one anealed will deflect the same underthe same load (asuming that the elastic limit is not exceded) .

 

asuming that flexing is an act of compressing one side of a bar and putting the other in tension then surely a harder material will require more force to compress it ? and therefore more force to give the same flex .

 

I sit with baited breath .

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Sorry Owen, but the rules still apply in England -within the elastic limit deflection is the same regardless of heat treatment .Try Patrick's experiment !

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Sorry Owen, but the rules still apply in England -within the elastic limit deflection is the same regardless of heat treatment .Try Patrick's experiment !

 

Thanks I shall.

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Wow! Thanks Patrick and Mete. Way cool explaination. Lets make sure I have it:

The higher temp temper steel will permantly deform with less deflection than the lower temp temper.

 

Do I have it ?

 

Thanks,

Chuck

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