First forged blade, how to heat treat without cracking?

[SurvivalKnives]

I made a small utility/bush knife from a spring tine off an old harrow on the 25th last month but I didn't heat treat it as I was afraid it would crack. I have normalized the whole knife about 3 times and I thought I might be able to skip the heat treat, but I tested it with a file and the blade is way too soft to hold an edge. How would all of you experienced knife makers go about heat treating the blade? All I know is that it's some sort of spring steel with a higher carbon content than the mild steel I was also working with.
Should I quench in oil or brine? How long should I let it soak at critical?

Here are some pictures:

Is the blade straight and is the thickness uniform enough? I know the bottom loop of the handle isn't quite lined up but it's not very noticeable.

 

[3fifty7]

I can't see your pics on my phone so here are a couple of questions.
What are the dimensions, spine and edge thicknesses?
I have made a dagger out of a harrow spike before, and it's decent spring steel. I soaked for about a minute at critical then quenched in warm canola oil. If you've normalized 3x and have left a little thickness to it you should be fine.

 

[samuraistuart]

Normalizing by definition is a process only done once. With most high carbon steels this is in the 1650F range (1600-1700 depending on the steel). If you forged the spike, normalize it once with an air cool only, then "thermal cycle" the steel 3 times at descending heats around critical temp ~1500F. Like 1550F, 1500F, 1450F, air cooling only. To harden, bring up to ~1475F-1500F and quench in 130F canola oil (provided you do not have a commercial quenchant like P50 or AAA....if you have those...use them instead). Check for hardness, skating a file is very common...but do not be fooled by the decarb layer that will make you think you failed. It is a thin layer of carbon robbed steel that is very soft. Grind thru it, and check for hard steel. If you do not have any hard steel, it may be that the steel is a shallow hardening type (spring steels usually are, and W1/W2) I would try a brine quench.

 

[SurvivalKnives]

I can't see your pics on my phone so here are a couple of questions.
What are the dimensions, spine and edge thicknesses?
I have made a dagger out of a harrow spike before, and it's decent spring steel. I soaked for about a minute at critical then quenched in warm canola oil. If you've normalized 3x and have left a little thickness to it you should be fine.

I fixed the pictures so you should be able to see them now. I couldn't see them on my computer either. The spine starts at about 4 mm thick then tapers down to 3 mm in the middle and 2 mm at the point. The edge is about 2 mm throughout. It's about 11 mm at the widest part of the blade. The overall length of the knife is nearly 16 cm and the blade is 77 mm long from the finger guard to the point.

 

[SurvivalKnives]

Normalizing by definition is a process only done once. With most high carbon steels this is in the 1650F range (1600-1700 depending on the steel). If you forged the spike, normalize it once with an air cool only, then "thermal cycle" the steel 3 times at descending heats around critical temp ~1500F. Like 1550F, 1500F, 1450F, air cooling only. To harden, bring up to ~1475F-1500F and quench in 130F canola oil (provided you do not have a commercial quenchant like P50 or AAA....if you have those...use them instead). Check for hardness, skating a file is very common...but do not be fooled by the decarb layer that will make you think you failed. It is a thin layer of carbon robbed steel that is very soft. Grind thru it, and check for hard steel. If you do not have any hard steel, it may be that the steel is a shallow hardening type (spring steels usually are, and W1/W2) I would try a brine quench.

Can I use olive oil if I warm it up? I don't have canola oil. Also, will the blade crack if it isn't perfectly even? Or does spring steel not crack that easily?

 

[R.C.Reichert]

Can I use olive oil if I warm it up? I don't have canola oil. Also, will the blade crack if it isn't perfectly even? Or does spring steel not crack that easily?

I would warm the olive oil to 130°F prior to quenching and temper it in the kitchen oven for two hours at 400°F immediately after hardening. Only reason the blade would crack is if you tried quenching it in cold water, ground the blade paper thin prior to quenching, or you tried straightening or bending the blade without tempering in it's hardened state. After a couple hours tempering I usually try to run a file down the edge to see if it's still too hard. You want it hard, but not so hard that it can't be sharpened easily. If it still seems too hard to sharpen, give it another hour or two at 400° in the oven. Btw, if you wait too long before tempering, it could potentially crack the blade too because there will be stresses in the steel from hardening, so just temper within a half hour or so of hardening is what I usually do.

 

[SurvivalKnives]

I would warm the olive oil to 130°F prior to quenching and temper it in the kitchen oven for two hours at 400°F immediately after hardening. Only reason the blade would crack is if you tried quenching it in cold water, ground the blade paper thin prior to quenching, or you tried straightening or bending the blade without tempering in it's hardened state. After a couple hours tempering I usually try to run a file down the edge to see if it's still too hard. You want it hard, but not so hard that it can't be sharpened easily. If it still seems too hard to sharpen, give it another hour or two at 400° in the oven. Btw, if you wait too long before tempering, it could potentially crack the blade too because there will be stresses in the steel from hardening, so just temper within a half hour or so of hardening is what I usually do.

Thanks for the information.

 

[SurvivalKnives]

I've successfully hardened and tempered the knife without it warping or cracking. I'll post new pics when I've filed and sharpened it and made the sheath.

 

[mete]

There is a shortage of olive oil especially in Italy and Spain !! With knife makers using it no wonder there's a shortage ! Canola oil makes an excellent quenching oil with lots of it available .Many makers use it. Canola doesn't have some nasty dangerous chemicals in it like motor oil has .

 

[Stacy E. Apelt - Bladesmith]

Another thing that often isn't mentioned in advice to new makers is the volume of oil. You should be quenching a blade in a gallon ( or more) of 130°F oil.