Heat Treat Question for a Kunai

[squigly1965]

ok so i'm making a Kunai for my nephews girlfriend. I am using O1 and wondering what i should be shooting for temperwise. It's made from 1/8" stock. it's 9" overall. the blade edge is 4 3/4". I asked if they were planing on throwing it. They said it wasn't planned on, but since i asked they want to be able to throw it now. I'm a little worried about my normal temper being too hard and the tip possibly breaking off
heres a pic



i was thinking of shooting for 55rc

thank you for your time and help in advance

Chris

[jimmontg]

Throwing knives are typically in the 45-50 RC range, 55 is too hard maybe for O1. They use 4140, 1055 or 5160 for throwing weapons, medium carbon steels typically. Never made a thrower from O1 though so you may want to experiment some as it is a high carbon steel and will tend to break more easily.
That point looks pretty thin and I would think maybe a differential HT may be helpful, but that takes some practice to get right. Obviously you're going to have to make some experimental blanks. Or just temper it back down to 50 if isn't a cutter and you should be OK Chris.

[squigly1965]

thanks for the info. been looking around and from what i've been reading i'm afraid to even heat treat it. if i do, it will be probably 50ish

[squigly1965]

i may grind the tip back so its not so skinny

[jimmontg]

You can do a down and dirty differential HT. Look at the temper specs for RC 50 and do your temper in that range. Then if you have an electric stove you can turn up the burner just under where it glows and lay the polished tip of the knife on the burner until it turns blue, turn off and let cool and the tip should be around 40-45. It may be a bit soft, but it won't break. The stove top burner will be a bit more controlled than a torch.

[Crex]

Nice job on yours, but like JM mentioned O1 would not be my choice either. 5160 would probably be the only "real" blade steel I'd consider for that type of knife/dart.
Hey maybe if you did some fancy file work and a dragon etching on the blade they'd think it too nice to throw....

My grandson went through a "kunai" period. I had to study up just to figure out what he was talking about. He had the design spot on (I'd just never heard of them). When I found out what they were for, I decided rebar was a better material for them. Not going to waste good blade steel on a tosser. They have held up quite well under heavy usage from a 12 year old (hit everything but the target).

[squigly1965]

Well I shortened the blade and gave it more of a convex point to it. Think that will help a lot. Still going to keep the temper low.

[jimmontg]

Yeah throwers have sharp bevels not thin ones like cutters, just look at an axe.

[jimmontg]

That definitely looks better for throwing, you did a good even grind Chris. By the way O1 is too soft in the annealed condition for throwing. That ring will get dented and bent. The book calls for a 800 to 1000 degree temper, 50 to 44 RC. Chris that means you'll have to use your HT oven for the temper.
Here are some concise and clear instructions for O1.
http://cintool.com/catalog/Oil_Hardening/O1.pdf

O1 is never used for shock uses like throwing knives. Like Carl said he would use 5160 or rebar which runs at lower carbon content. O1 is just too expensive for throwing knives, but I understand you didn't intend to make a throwing knife initially.

Problem with 5160 is most suppliers sell it 1/4" thick, which is fine if you want to forge it. New Jersey Steel Baron list 1045 for sale, but is out of stock. Now also you can check with a local machine shop and see if they will sell you some 1045-55 flat bar or 4140 if you want to make throwers of high quality steels. Problem with some of the online metal companies is they charge way too much for shipping and they carry these steels as do many machine shops.

[squigly1965]

my girlfriend works for a machine shop. I'll ask her if she can get anything like that. They've mostly been using stainless for processing systems though. i've asked before about drops of tool steels. and that was the answer i got. but i will try again.
I'm not really looking to get into the throwing knife market. but using other steels would expand my horizons.

thank you for the info

Chris

[Crex]

That's pretty much why I grabbed rebar. Yeah, it's a crap shoot at best as far as consistent quality goes, but seems to have just enough "integrity" to work for tossers. No expense other than the forging fuel and time (which is quite minimal).
I did get lucky and picked up some old school rebar from a bridge footing being demolished that was dated 1912. Little odd looking but turned out to be reasonably predictable as far a consistent quality goes. Made a lot of tossers out of it and it hardened up quite similar to 5160....go figure. I'm thinking 1040-1050 range from quench-break-spark testing. A "super quench" actually made it quite brittle, but a little grainy. Never could find the magic spot for good grain refinement (may not be one).

[jimmontg]

The L6 substitute 8670 or 8670M would make a good throwing knife as well now that I think about it Chris. It would make a great thrower tempered down to around 50. It is basically an improved 5160 with nickel in it for toughness. Admiral steel carries it as does Alpha knife supply if you want a smaller piece. L6 is available, but in thick sizes like 5160, but it's not as cheap as 8670. Heck Cincinatti Tool Steel where those HT instructions come from sells L6 in 3/8 and 1/2 plate sheared to the size you want if you want to forge an axe or sword.LOL Their shipping was a flat $20 last year whether you bought 10 lbs or 100.

Some of the stainless steels make good throwing knives as well, but that would take some research, but I know that 410-416 will harden up to around Rc40-44. Just take up to nonmagnetic around 1425 and air cool. 17-4PH is a really tough stainless and you do not need to HT it as the PH stands for Pre-Hard. Exactly how hard it is I do not know, but it was some pretty tough stuff to work with. It wouldn't hold an edge well, but would make a pretty good throwing knife.