Thermal fatigue

Originally Posted by Ed Fowler

If you do not understand your steel, if you are afraid of it, fear that you could hurt it by working it, I believe that you should limit yourself to the art knife and not take a chance of sending some client to the field with a knife that could fail when it is needed most.

As a teacher of the craft bladesmithing,... I would never offer even a beginning student any advice or criticism, that I myself could not take and/or practice.

Me, the perpetual beginner...

It's all art!

[Ed Fowler]

It is all about loyalty to the client. If you don't know the limits of your knife and let him find out on his own, you have served him unjustly.

Originally Posted by Ed Fowler

It is all about loyalty to the client. If you don't know the limits of your knife and let him find out on his own, you have served him unjustly.

My point exactly!

Honesty is the best policy.

We all need to learn how to practice what we preach.

Break test every blade you make and what do you have?

Knives are for using not abusing.

[Ed Fowler]

Ignorance and illusions of 'thermal fatigue' are no substitute for honesty.

[toddhill]

I don't know who has read the links Tai posted but they talk about stress to steel from thermal cycling, particularly quenching and tempering. I can see how within reason, the thermal cycles from forging aren't too bad, even if making damascus. They're fairly gentle to the steel--relatively slow heat up and slow cool down. But obviously if you get the steel that hot too many times (when making damascus), you're going to hurt it, at the very least by losing all the carbon in it. However, it seems like the shock of quenching could hurt the steel. The articles talk about that. If you think about it you're putting that steel through quite a shock when you quench it. This shock could cause microfractures especially (according to the articles) if there are certain elements present in the steel, like phosphorous, which is quite common. This might be where multiple quenches could hurt more than they help. I think this is a valid question. If you look at the links, they come from reliable sources. I'm a little leary of the guy who "has all the answers." Nobody ever seems to and questions are good for everyone. Who has read the links and what are your thoughts about them?

Originally Posted by Ed Fowler

It is all about loyalty to the client. If you don't know the limits of your knife and let him find out on his own, you have served him unjustly.

Answer me this,... If a customer asked you, "Exactly how many automatic dish washer cycles can one of your knives go through before the handle fails and the blade rusts through?",... what would you tell them?

...I'd just say, "They will all last a lot longer if you don't abuse them".

[Ed Fowler]

todhill:
The posted information is a good introduction, if you want to read a lot on fatigue failures, you can go to Amazon or Abebooks and pick up a copy of
"Prevention of Fatigue of Metals" Written by the Battelle Memorial Institute back in 1941, it contains very well documneted information that still applies. If you read carefully you will also find a lot of insight into the art of metals.

Originally Posted by John Frankl

Cool thread. I can see where the idea comes from originally, but I think proper knowledge and technique can minimalize if not totally eliminate the dangers.

I think of pattern welded steel as an example. The amount of heating and forging it takes to produce a 300 layer blade from a billet that started out as 10 layers is going to be way more than on a knife of homogenous steel. Yet with proper thermal treatments during and at the end of all this forging/forge welding, I see no problem producing an excellent blade.

John

Forge welding is different than general forging. It can close fractures and heal the steel, although other types of problems may arise.

What I'm saying is whether you are making a 300 layer billet, a 10 layer billet, forging a socket handle, an integral or just beveling a flat bar,... do it as efficiently as possible.

Don't waste a lot of heats, but try to get the most out of each one. If you do this it should be fine.

Todd, depending on the, quality, purity, condition and other possible variables of the steel going into the quench, and the conditions of the quench itself,... "one quench" may be all that is necessary to initiate or propagate embrittlement, fatigue, fractures or failure.

However,... one quench is "absolutely necessary" to harden the steel.

This may be getting a bit off topic, but while we are on the general subject of steel failure, I feel I should at least mention "internal defects" from the manufacturing process. We have talked about impurities, but what I'm bringing up now are things like large inclusions and gas pockets, which are really not that uncommon in steel. If they are on the surface of the steel we may be able to pick up on them by close inspection. However, it seems likely that a few blades are going to make it through the entire bladesmithing process with internal defects going undetected. In the event of a serious internal defect, or any other internal anomaly from the manufacturing process, a bladesmith may do everything prudent and correct, but the blade may still fail.

My advise remains the same,... don't take any unnecessary risks, don't abuse any steel or any blade, and use common sense.

Be honest and do the best you can!

This will not solve the problems, but it is the best we can do.

[Thingmaker]

Originally Posted by toddhill

I don't know who has read the links Tai posted

I did.

Originally Posted by toddhill

but they talk about stress to steel from thermal cycling, particularly quenching and tempering.

No, they don't. They talk about temper embrittlement, tempered martensite embrittlement, and mechanical fatigue.

More than three quarter century of metallurgists have demonstrated "smaller grain = better tool."


If thermal cycling is so very bad, and we should avoid extra heats because of it, does that mean we should only use new steel?

[Ed Fowler]

Just the facts:
The greater the ratio of reduction by forging the greater our ability to align any inclusions, etc with the desired plain of stress. Also the greater our opportunity to observe those defectrs and get rid of that part of the steel. Done right we will not expose the steel to some mystical error or deffect. It takes a lot of learning, but all very well worth it when you come to understand your steel of choice.

thing maker, I believe you will do well!

Without knowing it, we may have stumbled on to a whole new field of study,... "Philosophical Metalology"!

[Ed Fowler]

Yes, but we got to be careful, some newbies might take this kind of talk seriously.

Just think, if Michael Angelo had worried about this kind of stuff, Venus would still be a block of stone.

[Andrew Garrett]

All of a sudden. I'm thinking about engine parts. Cars, jets, etc. These parts are steel and are thermally cycled every time they are used. Then there is the mechanical stresses involved with those same parts.

What of leaf springs? Shouldn't they always be forged to 'repair the stressed steel before shaping a knife from it? A stock-removal knife from a leaf spring would be a knife bearing all they mechanical stress that the original spring had wouldn't it?

Hmmm...

I'm just grasping at staws here. Forgive me. Still a newbie.