For Cotdt and Others Experimenting With S5

[nullack]

Gday Cotdt and others,

Please find below some info to help make decisions about your heat treating process when using s5.

Also while Ive previously talked about how S5 was specifically designed to have maximum hardness, toughness and strength as a tool steel, there is another mechanical property that can be objectively tested that's useful for blades. While engineers figured out how to measure things like yield strength some time ago in classical solid mechanics, it took many deaths and machine failures before the true natural of dynamic mechanics was properly understood.

Specifically for example, the stress of a part in ultimate tensile strength is not the be all and all end. When the dynamic condition of smaller stresses go "on and off" it can break the part well before the UTS is reached. Its even worse when the stresses reverse, to whats called reverse cyclic fatigue. In fact some materials are so hopeless with fatigue that the stress of feathers onto it will eventually crack it (it will take ages though heh).

Whats good about materials like S5 is the silicon content provides excellent fatigue properties. Other materials use this too, such as 9260 spring steel which isnt as good for tool steel applications as S5, but is nonetheless designed for spring applications where fatigue is a big issue.

Anyway best of luck with your experiments. cheers

 

[cotdt]

Thanks for the useful tips and the charts. Is there a correlation between hardness and fatigue wear? I know that especially with stainless steel, under long term chopping the edge has a tendency to simply break apart from fatigue. I'm aiming for 61 rc for the S5, let's see what this steel can do!

 

[nullack]

Im not trying to be vague, but the real answer to that question is yes and no. Let me explain:

1. No - you cant compare hardness and wear resistance between different materials because different carbides effect wear.

2. Yes - you can compare wear resistance within the same materials at different hardness levels.

As I like to provide objective mechanical test data to show the facts, heres some test data in abrasive wear to clearly show what Im saying mate:

Fatigue is also effected by things beyond the official fatigue rating of the material. For example, the surface engineering of the blade - if it is highly polished it will resist fatigue better. If the blade is radiused in shape where possible it reduces stress raisers.

 

[cotdt]

Sorry I wasn't being clear. I was comparing fatigue wear to hardness, like continuous chopping or bending back and forth, which introduces fatigue and the steel breaking along the fatigued area. I was wondering what was the optimal hardness for fatique resistance.

 

[nullack]

Oh I see now sorry. Its known that triple tempering in particular is helpful for improving fatigue life as it helps deal with residual stresses during the pretty chaotic stress of quenching.

As too if different hardness levels achieved by different temps during triple tempering makes a measurable difference to fatigue strength, I simply dont know. Its a dam good question though

 

[nullack]

I'm aiming for 61 rc for the S5

By going from a limit of HRC 60 max to HRC 61 you go from 138 ft lbs to 49 ft lbs on the charpy c notch toughness test according to crucible when their s5 is oil quenched from 1650F. Personally I see that as a major loss of toughness for a minimal wear resistance gain but you'll make your own choices.

Youll find you can exceed these values if you put effort into refining the grain size with triple normalising or quenching. The ASM says the finest grain size of S5 at full hardness will be 9 but I know for a fact this can be exceeded with finer sizes through heat treat cycling like the triple normalising I recommended in the other post.

 

[cotdt]

By going from a limit of HRC 60 max to HRC 61 you go from 138 ft lbs to 49 ft lbs on the charpy c notch toughness test according to crucible when their s5 is oil quenched from 1650F. Personally I see that as a major loss of toughness for a minimal wear resistance gain but you'll make your own choices.

Youll find you can exceed these values if you put effort into refining the grain size with triple normalising or quenching. The ASM says the finest grain size of S5 at full hardness will be 9 but I know for a fact this can be exceeded with finer sizes through heat treat cycling like the triple normalising I recommended in the other post.

49 ft-lb is a pretty huge number for this test, though I agree 60 rc is optimal for this steel. I'm trying to maximize resistance to edge deformation (rolling/chipping), which requires both hardness and toughness. Wear resistance is not important to me. I'm interested in raw cutting ability at really thin angles. That 1 point in hardness allows me to use significantly more acute edge in my experience. I just need to find a way to quench it w/o warping.

 

[nullack]

That'll be an interesting test mate Its totally different to what I use the material for but I'm dam interested in your results cant wait.

 

[mete]

Take impact values only as guides not exact figures.
I don't understand what you mean by 'fatigue wear'. Fatigue and wear are usually two different things.

 

[nullack]

I was comparing fatigue wear to hardness, like continuous chopping or bending back and forth, which introduces fatigue and the steel breaking along the fatigued area.

Ive been puzzling over this question since. What we know as fact is that the tensile, yield, compressive and shear strengths rise as the hardness of the material rises. We know that some materials are hopeless with fatigue and actually have no fatigue rating. Others do have a fatigue rating. For those that do have known fatigue ratings, since the fatigue rating of the material is related to the stregths of the material Im going to offer an educated guess that you get more fatigue strength from tool steels tempered to be harder. Id like to see a study on this specifically before Im willing to upgrade it from educated guess to fact.

Due to the massive amount of yield and tensile strength that HRC60 S5 offers as shown in the strengths table above, it could be reasonable to assume that the fatigue rating for light duty cutting tasks will be a fatigue rating of inifite cycles. If the fatigue becomes a reverse cyclic high stress fatigue scenario, the performance of S5 with its silicon fatigue resisting makeup will survive far longer without breaking from fatigue failure compared to lesser materials.

 

[hardheart]

Take impact values only as guides not exact figures.
I don't understand what you mean by 'fatigue wear'. Fatigue and wear are usually two different things.

I think he means microchipping, perhaps.

 

[nullack]

Can I offer some definitions off my head, others with more time are welcome to look up text books on:

Fatigue Strength : a measure of the number of cycles a part can endure before failure. Infinite imples the loads will never result in fatigue failure not that the part will last forever.

Fatigue Rated : this might be aussie engineer slang but typically implies that the design is fit for purpose over service life from cyclic stresses

Surface fatigue : the surface engineering has gotten messed up by cyclic fatigue loading but the fatigue strength has not yet been reached so there is no total part failure yet

Abrasive Wear : wear when one hard surface meets a softer surface

Adhesive Wear : wear when two smooth surfaces are pressed into each other

 

[nullack]

Before I goto sleep I forgot to mention in my original post the reasons why I included some data at varying amounts of C all under the s5 specification. For those of you who are researching this in detail you will see that Crucible do C @ 0.6% while Carpenter does C @ 0.55% and LaTrobe go for C @ 0.6%.

 

[cotdt]

Take impact values only as guides not exact figures.
I don't understand what you mean by 'fatigue wear'. Fatigue and wear are usually two different things.

Should read "fatigue strength". Sorry for the confusion.

 

[mete]

S-5 is very close to 9260 .9260 is a common automotive suspension spring material.Springs must have good fatigue strength ! The only blades that fatigue would matter are those used for fencing - and as it happens in my old fencing days they were made of 9260 !! That particular steel was so dirty that it usually failed at the large inclusions or at nicks in the blade from other blades.

 

[nullack]

Hey my friend I think we need to dig deeper into your statement. We know some materials have no fatigue resistance so thats obviously an issue for any blade. If we assume were only going to use materials that have some sort of fatigue rating, can we assume that it will only be in fencing where that matters?

Im not so sure. What happens if your trying to chop your way through say a knotted hard wood log to make a shelter. I can image how shear stress could indeed reach a point of mechanical significance where lesser fatigue strength materials would rapidly break under that cyclic loading. You could take a strain gauge and measure the stresses to exactly know the forces were talking about.

 

[nullack]

Cotdt I now have the full answer to your question RE hardness and fatigue. The ASM say ductility is usually only important in low cycle fatigue situations. More info below.

Also, it is noted by them that residual stress and grain size is an issue. Finer grains not only have more toughness, but also greater fatigue strength. So grain refinement through heat treating is important, coupled with the side benefit of stress relieving.

As too the surface engineering were a highly polished surface is better for fatigue.

They also provide detail of vacuum melted steel being more clean and better than electric furnace melted steel - the vacuum melted extra clean steel in the comparison has higher fatigue strength and higher hardness.

 

[cotdt]

Ah now we have a reason for acheiving super-fine grains! As for the steel, powder metallurgy could also produce very clean steel, but I don't think they have PM-S5, though they do have 1V.

Very interesting that hardness improves fatigue limit though, very interesting indeed.

 

[nullack]

It would need to a be a custom order for a super high cleanliness S5 AFAIK of which suppliers do S5. That means a minimum of a 1/4 tonne buy for most custom supplies.

CPM1V is quite different to S5.

EDIT: Suppliers methods are:

Carpenter use electric arc furnace melting for s5
LaTrobe dont say in their product info
Crucible dont say in their product info

 

[nullack]

Id like to offer some anecdotal observations on knife tribology. This isnt properly understood but I have some experience thats interesting in my book.

Traditional measures for measuring wear like adhesive and abrasive wear cant be directly related to all knife applications.

For example when your cutting with a larger blade and good amount of force I found so called high abrasion resistant materials to actually fail from a lack of toughness - all those hard vanadium carbides and other hard carbides just pull out, the edge microchips itself and basically it wont retain an edge. Tougher materials dont microchip like this and actually hold the edge better than materials with high abrasive wear resistance.

This is why when you look up crucibles s5 datasheet they say s5 has "good edge retention" even though its abrasive wear character is low to medium.

Then theres the converse situation where those high vandium carbide materials with excellent abrasive wear outlast high tougness materials because the tribology situation is one of abrasive wear - there is no mechanical impact of significance and its wear from light duty cutting of soft materials.

One "universal knife" approach Ive been thinking of is using a laminated knife structure consisting of fine grain heat treated S5, inside of which is say carpenters micro melt maxamet alloy. This material can do HRC70, is immensely strong and wear resistant. It was developed to replace sintered cerments where more toughness is needed than what those cerments can provide.