Tempering knives made from rail car springs.

[Southpaw92]

I recently forged a hunting knife from a section of rail car spring. After forging the blank I annealed it and then proceeded to refine the profile and cut in the bevel with a file and sandpaper. When this was done I heated the blade and quenched it in room temp. used veggie oil. After cleaning the knife off I put a steel plate over my forge and tempered the blade on it. I drew the temper to a peacock blue. At one point the color started to run really fast on the edge and point so I place just the first 1/4" of the point and edge into my water quench to slow the colors. When the tempering was complete I placed the knife in a container of wood ash. When it had cooled I placed the knife in the vise and tried flexing the blade. It ended up snapping about 1/2" back from the point.

I'm wondering if this is a result of me placing the point and edge of the blade in my quench during the tempering process, or if I'm running the colors to fast on the knife, (The knife blade was 4" long, an 1 1/4" wide, and 3/16" thick and it took about 12 minutes to temper) or a combo of the two. Should I try tempering my blades of said steel in the oven instead?

Any input would be greatly appreciated.

[MattBower]

Tempering takes a while -- normally at least an hour or two at the target temperature. If you're going to temper by letting the colors run, you really need to do it multiple times to try to achieve the necessary time at temp. Just letting the colors run once isn't really sufficient. So under-tempering could definitely have been part of your problem.

I suggest you temper in an oven to get the edge the hardness you want, then draw the spine back further with a torch, while keeping the edge and tip in some cooling medium -- wet sand, something like that -- to prevent oopsies. (That's assuming that selective tempering is what you want to do, which appears to be the case.)

Quenching from a proper tempering temp for a knife blade will not cause the problem you had.

However, how was the grain size in the steel at the site of the break? Can you take pictures and post them? I've run into a lot of guys who overheat their steel during heat treating. That causes large grain, which is brittle. That could also be the problem (or part of it).

Added later: I should mention that you don't say how far the blade flexed before it snapped, or how much force it took. The fact that it's possible to break a blade does not necessarily mean it's defective. You can break anything if you try hard enough.

[thingmaker3]

I'm wondering if this is a result of me placing the point and edge of the blade in my quench during the tempering process, or if I'm running the colors to fast on the knife

Neither. It possibly is a result of either too thick of a blade for the temper, or too little temper for the blade thickness. Or too large a grain structure, as Matt has already suggested.

If you have enough of that railroad spring left over, try this: http://blacksmith.org/forums/attachment.php?attachmentid=1529&d=1290409793

[Don A]

Don't neglect to normalize your blade before you harden it.

After all of the filing, grinding, drilling, etc., heat the blade to critical and then let it air cool to black.

Repeat, but perhaps a shade cooler (slightly below critical) than the first cycle.

Repeat again, but perhaps a shade cooler than the second cycle.

Then you are ready to harden.

Normalization relieve stress in the steel. Understand that this method is not at all scientific, but is far better than nothing. I have found that it works for me.

Would this have prevented your blade from breaking? Maybe not. But it is good practice.

In agreement with what Matt said, I normally run two seperate one hour cycles at a fixed temperature in the oven and then a third temper with the stove eye, forge, or torch (watching the colors). Again, not scientific, but I have had good results with simple steels.

Don

[Southpaw92]

When I inspected the break I did notice the grain in the core of the knife reflected light differently than the grain around the edge of the break. It didn't flex a full inch. I would post pictures but I already fixed the break. I'm going to try the oven tempering method. Is 450 F. the right temp?
Thanks for the input.

[Don A]

Is 450 F. the right temp?

If you know what type of steel it is, do a little search/research for the optimum temp according to the hardness you hope to achieve.

If you're flying blind (like I do most of the time), I would start at around 425F. Run it at an hour and then check the hardness. If it's still too hard, bump up the temp on the next cycle.

Again, not scientific, but it works for me... most of the time.

[MattBower]

When I inspected the break I did notice the grain in the core of the knife reflected light differently than the grain around the edge of the break. It didn't flex a full inch. I would post pictures but I already fixed the break. I'm going to try the oven tempering method. Is 450 F. the right temp?
Thanks for the input.

With junkyard steel, you have to experiment. There are too many variables for me or anyone else to tell you the "right" tempering temperature. Among many other things, it depends on the geometry of your knife. A 3" paring knife can be left much harder than a 12" camp knife. For most spring steels, in larger blades, 400 F isn't a bad place to start experimenting -- assuming you got the blade full hard in the first place.

Steel grain should look silky, only slightly rough, like fine grit sandpaper. If you can see individual grains with the naked eye, it's too big. If you want to know what really good grain looks like, snap the tip off a high quality file (Nicholson, Simonds, etc.) and keep that handy for comparison. (Just lock it in the vise and give it a solid rap with a 3 pound hammer, right where the file meets the vise.) The normalizing cycles that Don suggested will also help reduce your grain size, which may be large due to the effects of high heat during forging.

How did you fix the break?

[Frank Turley]

When chasing color, I learned a tip from the old book, "20th Century Toolsmith and Steelworker." The author suggests tying a small length of cotton rag on the end of a stick and keeping the rag dampened with water. If you see the color run to fast at any place, stop it down with the wet rag, and let the other color catch up to it. This may help on a knife because of the knife's conformation. I have used the method on hatchet blades where the cutting edge may be fuller in the center (convex). The colors tend to run to the corners first rather than the blade center.

http://www.turleyforge.com Granddaddy of Blacksmith Schools

[MLMartin]

Little wet rag on a stick sounds handy. I have used a blowgun off my air compressor a hand full of times to slow temper lines, takes some getting use to but can be vary handy. I also use the blowgun to selectively cool parts of hot steel if the part is small enough.

[Golden_eagle]

Yeah, I know- necropost -but I had to chime in

most leaf springs are 5160, which tempers to 57-58 @475,I like a harder edge- so I temper my blades in a 2 inch deep pan of preheated clean super fine sand mixed with ash @425-450 in the oven (wait for the wife to leave.) for about 30 minutes per 1/4th inch of thickness, letting it all cool to room temp in the oven, then I clay or submerge (clay for thin blades, water for thick.) the edge, and draw the spine to either a deep purple or light blue, depending on how flexeble the blade needs to be, hold temp for about 2 minutes per 1/8 inch thickness, then I let it cool to room, finish, then put on your ceremonial robes and sacrafice a chicken to the elder god of steel tempering, temparuthlu. yeah, its complex, but I haven't been able to dull a knife tempered in this fashion yet.