Regarding flexibility

fitzo

fitzo

Gold Member
Joined
Aug 14, 2001
Messages
6,648
Mete, I hope you will lend us your knowledge here.

On a thread about a filet knife, differential HT came up. It got me to thinking a separate thread regarding flexibility may be informative.

There is of course a difference between the ability to bend without breaking (ductility??) and the ability to bend substantially and return to true. I don't know what term is used other than springiness.

Now, the idea of differential quenching is to promote a martensite edge and a pearlite spine.

My question would then be what is the best crystalline structure for springiness, and how "springy" can pearlite become? What's happens to pearlite (if anything) during the tempering process? (Pardon my lack of terminology. I'm sure I will learn it soon! ;)) Is it thus best to "martensite" the whole blade and temper or what?
 
I have talked about 'flexibility' here and on sword forum before. There is a general misconception that flexibility has something to do with heat treatment .It only has to do with thickness [assuming that the blade doesn't take a permanent bend in the process]. If you've ever taken apart a mechanical clock you've seen the spring is extremely flexible - able to be wound in a very tight spiral. That's because it's very thin. So flexible blade =thin blade....The ability to flex and return to straight [springiness] without permanent deformation requires a high yield strength That you get from tempered martensite. Pearlite has a much lower yield strength and will permanently deform long before martensite. Therefore differential hardening with it's pearlite spine doesn't help you.....Ductile - Take a look at a car that has been in a crash - the metal undergoes considerable bending and stretching before it breaks .That is ductile metal [low yield strength much higher tensile strength] .That is the opposite of brittle where there is no bending or stretching before breaking ,like glass [ where the yield and tensile strenths are very close].
 
Mete, What makes A-2 so flexable? A number of years ago I made a sushe knife out of 1/8" A-2, pure stock removal. It was like trying to grind a snake. It could have been 3/32. I've done some thin blades in the past but they were nothing compaired to the A-2. It would have been easier to grind if I had heat treated it.....
 
mete said:
There is a general misconception that flexibility has something to do with heat treatment
Click to expand...

Thank you for your response, Robert.

Wouldn't you agree, though, that flexibility does have "something" to do with heat treatment in that you want tempered martensite?

Doesn't hardness have something to do with flexibility?

I'm sorry to belabor this but the audience changes here, some folks may not have been exposed to the info, and there's always the possibility that sufficient repetition will make it stick! :)
 
Perhaps we can clarify terms a bit .When I go to a major knife supplier like Dexter-Russell ,I can get a boning or fileting knife either flexible or stiff.Flexible here means springy or elastic, that is , when bent it will return to it's original shape.The difference in the two knives is only thickness. But it takes strength to resist permanent bending. The same knives of a lower strength steel [pearlite] will permanently bend sooner than the martensite one. Want to pass the MS bend test easier ? make it thinner.
 
Would a flexable knife have to be a bit softer? the reason I ask is that we make thousands of gun springs in 6150 steel and if they are a little hard they brake but where they are flexable they are also very easy to cut with a file.
 
Pete Allan said:
Would a flexable knife have to be a bit softer? the reason I ask is that we make thousands of gun springs in 6150 steel and if they are a little hard they brake but where they are flexable they are also very easy to cut with a file.
Click to expand...

I have bent 1/16 A2 hardened to RC60 past 90° without permanent effects.

Ductile, or plastic, deformation is non-recoverable strain, the effects do not go away after the stress is removed. This occurs above what is called the yield point of the material. Below that point, elastic strain occurs, that is recoverable, that is the material goes back to where it was once the stress is removed. Steel has a large elastic strain field, just drop a steel sphere, like a bearing on a solid surface and watch it bounce. In material that is rigid, once the yield strength is exceeded fracture occurs.

Just to keep our terms straight, stress is only the force applied. It does not build up in material. The effects of stress that accumulate are called strain.
 
Mete, would you give us the right temperature to temper at on say 5160 , 1080 & 01 assuming that we are going to make the blade thin. Thanks. Gib
 
For a full hardened [ not differentially] filet knife I would aim for 55-57 Rc. That would be somewhere around 450-475F. Remember that those very thin filet knives will overheat very easily when grinding !
 
Thanks Mete that is something I have always wondered about. Now what about a spring would you use the same temperature? Gib
 
Gib, I've been searching the internet for info about 5160. The HT on ajh-knives apparently was taken from info about it's use for automobile springs since it's tempered at a very high temperature .For that use BTW it's chosen because of toughnes and good fatigue strength.The same characteristics are found in 9255 which apparently no one uses for knives .Knife use , of course is different. So for spring use it depends on the application .
 
You must log in or register to reply here.
Back
Top