First knife learning curve

D

David Peters

Joined
Jan 24, 2021
Messages
4
I'm attempting my first blade ever, a 9" kukri out of 5160, and and I have more questions than I can find answers to. I built a forge out of a 20# Propane tank lined with 2 inches of ceramic wool. It has a single burner in the center at about a 30 degree angle. I drilled a hole for a thermocouple and can keep temps inside fairly steady by finding the sweet spot between the choke, pressure regulator, and the valve. I blocked the opening with fire bricks, leaving a slot for me to pass the blade through. When the tip hits the back of the forge, there is about an inch of the tang hanging out, which is enough for me to grab with a pair of vice grips.

The problem I'm having is the first half of the blade heats to nonmagnetic, but the back end and the ricasso is a dull red and stays magnetic. I'm moving the blade lengthwise as much as I can while still keeping it in the forge and rotating it often. I figured if I could hold the temperature close to 1525, I could just keep it in there until it was heated evenly, but after 20 minutes I was still magnetic at the ricasso end of the blade. I don't know if I should crank the heat higher and keep cooking the front half until the back end catches up, or if I need a deeper forge, or maybe cut an opening in the back end to let heat escape from that end also and be able to pass the tip through that opening to get the ricasso end deeper into the forge.

I'm also having a hard time finding info on how long should the quench be taking. I need to get the temperature below a certain point, but how low should it be? Should I leave it in the oil until it is cool enough to touch? If the oil is at 130 degrees, does it balance out in temperature after the hot steel is submerged, as long as I have a sufficient amount of oil? I also read that after quenching I should grind off the scale before tempering in order to be able to see the colors on the steel, but most of what I read focuses on time and temperature, not on color.

So, such are my struggles, which I haven't quite been able to find the answers to online.

Thanks!
 
15 seconds in oil should be plenty, rock it in a forward/backward, up/down motion (not side to side). The blade will be hot to touch when you take it out, so you still need gloves. After quench you still have couple of minutes where the blade is still soft and you can take out the warps with your hand or a vice.

Use your kitchen oven for tempering and you will not need to see the colors. Colors are indicative but they can be tricky, you want a golden straw color. There are color charts online.
 
Your forge has a "hot spot" in the center where the burner is aimed. It is also too short for the large blade you are doing. It would be a good idea to cut a 2"X2" hole in the back. You can block it with firebrick when not needed. Also, open up the front bricks so you can push the tang all the way to the center with the tongs. You probably need to turn the gas up a bit and lower the air, too. Forge HT is best done in a reducing flame. You should always let the forge run a good 15 minutes to get "soaked" before doing the HT.
Don't try and set it at an exact temperature with a TC unless the forge is built with PID control and of suitable size and construction to fully soak the heat into the refractory.

You have to "pump" the blade in and out of the hot spot of the forge. Push it in until the tang is at the hot spot and then pull out until the tip is at the hot spot. Make these push/pulls even and smooth. Adjust the amount of time at the tang and tip so the whole blade gets evenly heated. Turn the blade over ever few strokes to assure more even heating.
When the blade reaches non-magnetic, continue until the blade is about one shade brighter red. This will be about 75-100° degrees hotter, which is about right for the quench.

Too late on this forge, but the burner would have been better about 20% back from the front and angled back at 30°.
 
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