questions about a break

M

mitch8

Joined
Oct 11, 2010
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180
so i decided to crank out a throwing knife out of a leaf spring while i had the forge running for another blade. i forged it into some sort of a kunai but with the smother curves characteristic of an old kunai not the modern ones. anyway i let in austenize before quenching it in water and putting it in the temper. it was at 450f for about 45 mins.
i got antsy to throw it so i took it out and started using it once it cooled. it broke on the 3rd throw after but it broke at the widest part of the leaf shaped blade. when i looked at the blade in the blade it was brown around the edges of the break. is this a different micro structure or was there a crack before tempering and this is just temper color? hope this isn't to confusing, just really curious about the color change.

thanks again, mitch
 
Mitch,
I am not a pro knife thrower but from being a knife maker and having a retail cutlery store the vast majority of throwing knives are not hardened or down around about 50 RC. I do not know what caused the coloring around the break?
It sounds like there was a crack in the steel before hand?

Laurence
 
Couple of things I see.

Use known and unstressed steel, quench in appropriate oil, temper hotter. JMHO

It could be a crack from 40 years ago, or it couldn't handle that fast of quench. Even water hardening steels are safer to quench in a fast oil.

Best luck, "neighbor"
 
It is not rare for auto leaf springs to have cracks and micro cracks. That is why most savy makers use new 5160 or similar spring steels. Water quenching alloy steels is not a good idea either.
 
There are several issues:
1) A leaf spring is not a water quench steel. You probably put all sorts of cracks in the steel with that quench.
2) A leaf spring that was used on a vehicle is just about the worst source of steel one can use. It is almost sure to have many stress areas and micro-cracks. Forging would have acerbated these.
3) Until the steel if fully tempered, it will still have brittleness. Your haste made waste.
4) The temper for a thrower is in the low Rc50s at most. Many are in the mid to upper Rc40s. A temper of about 700F is required in a steel like 5160 to be Rc50-52.
5) You didn't give any temps or method of how you austenitized the blade, but overheating or undersoaking of a spring steel and then water quenching it would be one more bad thing that happened to the blade.
 
It sounds like the crack was present before you tempered it. The exposed inside of the crack oxidized during the tempering.

Ditto on everything everyone else said.
 
Thanks guys, this is good to know. I figured there would be some problems with leaf springs which is why I don't use them on important things like real knives, just throwers, forge tools, etc.
I thought that the crack might have come just before or during temper but the microstructure change is a more interesting explanation although probably not true.
 
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