Heat Treating Leaf Spring by Air

[showlandjr]

Hi Folks!

I am just getting back into blacksmithing again after a long hiatus and have decided to try my hand at knife forging. This is something that has interested me in the past though I otherwise stuck to more practical work such as hooks and the like.

Here's my question: What do people think the steel is and is there something going on in the heat treating that could be done better?

I have a pile of leaf springs which I got from my local mechanic shop. They are off some larger trucks, probable f350 or so. So I decided they might make some fine knives so I forged and ground them and then proceeded to attempt to heat treat them based on information I gleaned from the internet.

Here's what my results were. All quenches were done after I brought the steel up to nonmag in a gas forge and then put back in forge for a few more seconds and rotated regularly to make sure the heat was even.

Water quench: Soft and cause a bend no matter what. I didn't have much hope for this quach but since it was a unknown steel I figured I should try all the method to see what was the best.

Gear Oil Quench: Harder but not file skating hard, the oil was a very dirty so maybe not a useful testing medium.

Vegetable Oil: Prewarmed to the point of too hot to touch for long. then I quenched, this was a similar result to the gear oil.

Air Hardening: This seems to have made them the hardest by far. After bring them up to nonmag I just let them cool and after they were handleable I tried filling them and found the file to not bite at all.

I have at this point 3 quenched in oil and 2 air. I am tempering them all to 450 in the oven and will attempt some stress testing on them after to see what the qualities of the steel are.

Any thoughts would be greatly appreciated, Thanks!

 

[Kevin McGovern]

Water-hardest then oil. Air won't do it. Did you send off the decarb before testing hardness?

 

[showlandjr]

No the Air was the hardest the Water was the softest. and I filed at a corner to test so I knew I was through the scale very quickly.

 

[DevinT]

Those results are the opposite from what is normal.

Hoss

 

[milkbaby]

It seems like 90% of the "why didn't my steel harden" issues on this forum are not getting past the soft decarb layer. Decarb is not the same as scale, it's the layer under scale that the carbon has migrated out from and therefore is soft. So even if you've filed through the scale, you may only be filing the soft decarburized layer.

 

[kuraki]

I think there's issue with the temperatures. Even if the steel was air hardening, it would get hard when quenched in a medium faster than air.

If you water quench A2, it may crack, but it will get at least as hard as if you quenched in still air.

This would be the first leaf spring I've ever heard of that was deep hardening enough to skate a file after an air quench, additionally, if it was air quenchable I don't know that going slightly past magnetic for a few seconds would even be hot enough, since every air hardening steel I know of needs 1725 or better.

Last, was this reheating the same piece of steel over and over?

 

[showlandjr]

Here are my results after sharpening two knifes, one air hardened and one oil. both were tempered in an oven at 450.

Oil is bottom and right. Air is top and left. As I'm sure all of you can see the oil hardening created a much more even development of grain structure. The Oil test also held up much better to being smacked with a crescent wrench, though that somewhat more difficult to see in the pictures.

I also did a edge retention test and while the air hardened knife held up okay the oil was better.

The test consisted of hanging a piece of bailing twine and cutting it 5 time in the air to which both knifes did great immediately after sharpening. I then smacked both knifes on the end grain of a price of oak for about 2 minutes. I then repeated the string cutting test and while the oil knife still cut just fine the air knife cut twice but failed to cut with three of the slashes. it also send the sting flying a lot more, even when it did cut.

So my conclusion is that this is a oil hardening steel and after more research I image that I can perform much better with some heat cycling and some better soak times at hardening temp. I hope that I haven't wasted anybodies time here with the fact that I have more or less answered my own questions but I hope that others can gain some useful information from what I am doing. Just for kicks If people are interested I can continue to to post my future testing of knife making from this same spring. Too bad I don't know that make and model of the truck, maybe I can find that out.

 

[Rick Marchand]

I think you are on the right track. I'll offer up some opinion for your consideration.

Here is a decent (low-tech) recipe for an unknown spring steel.

- Forge Hot... do not let it get to dull red.
After forging...
- Heat to bright orange (1650-1700F), let cool to magnetic.
- Heat to bright red (1500-1550F), let cool to magnetic
- Heat to red(just past non-magnetic, 1450-1500F), quench in oil
- Heat to dull red(still magnetic) 3 times, let cool to black in between.
You have redneck-spheroidized your blade. It will be easy to grind, file and drill and the internal structure should be even, refined and well distributed. When you are ready to harden...
- SLOWLY heat to red(past non-magnetic, orange should just be creeping into the edge. Keep the blade moving in and out of the forge)
- Quench in warmed oil. You should be able to dip your finger in the oil but not be able to hold it for long.(think, moderately hot tapwater)
- Temper at 400F, for an hour, twice.

You will have decarb... the file test only works after you grind a bit off. "calibrate" your file by first filing the unhardened portion of the tang. Some files can be hard enough to scratch into fully hardened steel. And remember, the more you file test, the duller the file becomes. That's why the "calibration" is important.

 

[Stacy E. Apelt - Bladesmith]

Since everything is you did was pretty much a guess - I guess something is wrong.
Your test results make no sense. They are backwards for any carbon steel.

You need to austenitized 5160 ( and similar steels) at 1525 F and hold evenly for 5 minutes. An oven is the best way to do this.
It needs to quench in a medium speed oil which is at 120-130F. Temper twice at 425-450 for one hour. canola oil will work. Parks AAA is what I would use.

Water should crack 5160 if quenched from 1525 F. Air cooling should make it pearlitic and softer.

I would grind an edge on the test pieces and see what they are like inside.

BTW, the grain is set in the austenitizing, not the quench. It doesn't matter what quenchant you use, the grain is the same if the steel was heated to austenitization the same. Overheating grows grain.