|Tanto heat treatment, round one
||[Jul. 16th, 2003|12:12 am]
Yesterday, monday evening, I fired up the heat-treat oven and attempted to heat-treat the tantos. If you remember, I had clayed up the tantos beforehand, using furnace cement and baking it in my oven to set it. With five tantos , and a fairly good-sized heat treat oven, I thought heat treatment would be no problem.
I was a trifle optimistic, I think.
Now, according to the heat treat data, the opitmal temperature for the heat treatment of a 10xx steel such as I'm using should be between 1485 and 1550 deg. F. (yes, I double checked to make sure I'm using the right scale.) So, I heated the oven until it was above that range, let it soak for a good half hour, then cooled it down to 1550 and put in two tantos. I set a timer for 15 minutes and let the temperature fall down to around 1480 during that time.
Some notes about the oven: this is an ancient beast, basically a metal box lined with heavy duty furnace cement, with a blower-driven two-burner heating system. There's a raised platform above the burners (opposed design, one burner on the left and one on the right, one in front one in back) on which the work rests. The thermocouple is at the top of the box, enclosed in an iron pipe. The thermocouple element is a type K, moden design, hooked up to a digital readout. The oven originally had a much larger type K thermocouple, hooked up to an analog gage, but I prefer the digital readout as it's far more accurate and easy to calibrate. The regulator for the forge has two outputs -- one for a minimum output that results in the oven losing about one degree every two seconds, good for annealing and other slow cooldowns, and an auxiliary valve, that lets you really dump in the gas, which will heat up the box at the rate of about 300 deg/minute. This valve is a bit tricky to operate, as it's definitely non-linear in operation. You also have to fiddle with the air intake on the blower to adjust it to the gas flow you select.
Anyhow, after letting the tantos soak in what should have been the right temperature for about 20 minutes, I quenched one in a fast oil. It should have come out at about RC 65, but a quick file test showed that the metal hadn't hardened. I increased the temperature a bit, to 1600, and gave the 2nd tanto an additional half hour, but it didn't harden fully either.
Having failed on two, I had three more to go. I put them in the furnace, and changed the fast quenching oil to water. After a half-hour soak at 1600, I pulled one out and quenched it. Still too soft, though the very tip got hard. I increased the temperature to 1800, and pulled the fourth blade. More of the blade was hardened, but there were still spots on the blade, particularly near the tang, that were not hardened enough. I increased the temperature to 2000, and let the last blade sit for another half hour at this temperature. It was quenched, and fully hardened.
Now, obviously something is wrong here. On the one hand, this is the first time I've tried heat treating a japanese style blade in what I hope is a truly controlled environment -- it may be that with the extra insulation and thermal mass of the clay coating it just takes much longer, requires a higher temperature, or possibly my thermocouple setup is mis-reading the temperature somehow. Maybe with all that extra-thick, high density cement I need to pre-heat a lot longer (the outside of the oven wasn't even boiling hot after over 2 hours of operation!) in order to get the true temperature of the interior to match that of the relatively exposed, lightweight thermocouple. Still, if I have to heat each blade to the 2000 deg. mark and hold for a while, that works, though technically I'm probably getting a lot of grain growth and decarb. I'll clean off the four failed blades, re-clay, and heattreat them again.
This time maybe they'll all work.