Recommendation? Thermal cycling stressed 5160 springs

H

Hammarfest

Joined
Apr 20, 2022
Messages
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I was given a bunch of old tractor trailer leaf springs, which are massive.
The first blade I made was riddled with cracks, I had just cut off a piece and forged it straight away.
I am thinking that because the stress these springs have undergone, I need to relieve that stress and "reset" that steel by thermal cycling.
I am intending to take the steel, heat to orange, let air cool to black, repeat, doing 3 cycles of that. Then I will forge.

Does this make sense? Is 3 cycles right?
I would love to hear any advice on how to properly prepare spring steel that has been stressed.
Thanks! :)
 
The constant flexing of the spring under great weight has created micro-cracks along the grain boundaries. These don't repair with a stress relief or normalization. Forging makes it worse by propagating the micro-cracks until they look like spiderwebs in the steel. It is best to get some good unused 5160 or other steel to forge a blade from. You said in your bio that you have a good shop with good equipment, so it makes sense to use good steel, too.

I have a stack of leaf springs people have brought me saying, "I know this is good knife steel." I politely take them and set them out in the boneyard. They may get used for some sort of sculpture one day or in some non-knife welding project.
 
Stacy E. Apelt - Bladesmith said:
The constant flexing of the spring under great weight has created micro-cracks along the grain boundaries. These don't repair with a stress relief or normalization. Forging makes it worse by propagating the micro-cracks until they look like spiderwebs in the steel. It is best to get some good unused 5160 or other steel to forge a blade from. You said in your bio that you have a good shop with good equipment, so it makes sense to use good steel, too.

I have a stack of leaf springs people have brought me saying, "I know this is good knife steel." I politely take them and set them out in the boneyard. They may get used for some sort of sculpture one day or in some non-knife welding project.
Click to expand...
That is an excellent explanation, and I'll start looking for a good steel supplier instead of relying on the springs, thank you Stacy!
 
Is it possible for these micro cracks to form if the steel is forged a tad too cool? It's really easy to have the billet at a good forging temp and as it's forged allow it to get a tad cool for the last couple of hammer blows that could form a tiny crack inside the billet so it's not visible at the time
 
Yes, forging below the correct temperature is a bad idea. I know there are certain old timers who talk about cold forging and reducing the heats as they get close to the final shape, but metallurgically it is a bad thing. Look at the forging temps in the ASM guide and you will be surprised at how high they are. For 5160 you start forging at 2200°F and stop at 1600°F. That is yellow-white (2200°F) to light-red range (1600°F). If it is dull red, it is a good 200 degrees too cold to forge.
Most folks forge far too cold.
 
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