So far I've used canola oil heated to 110F for quenching 5160, with excellent results. Someone suggested I quench in used diesel oil. His theory is this: The black soot in the diesel is "free" carbon that is not chemically bonded with the oil molecule, and therefore a quenched blade will absorb some of this free carbon yeilding a slightly higher surface hardness, as compared to a blade quenched in clean oil. Sounds plasuible. I've done a small test piece. My super accurate Rockwell-testing file reads "Yup, that's pretty darn near full hardness, buddy". Does anyone else quench in used motor oil, and any opinions on the "case hardening" effect supposedly due to free carbon in the quench?
I would be real cautious about using diesel fuel and starting a big fire. I have been using automatic transmission fluid for years and have most likely hardened over 300 blades in it. It is not as volatile a diesel fuel and is cheap when you get it at the right place. Just go to a place that fixes transmissions and they will give you the used oil for free. I preheat it to about 110 – 120 deg. F and have never had a blade crack in it, and the Rockwell is always where it is supposed to be for the particular steel I am heat treating. As far as the diesel fuel adding carbon to the blade because of the black soot on the outside of the blade that is a fallacy. The only way to add carbon to steel is through supplying it in the gaseous form. This fellow has written a very detailed book on heat treating and metalllury for blacksmiths and knife makers. If you Google it there is a PDF file that answers any question you may have on the heat treating process. Metallurgy of Steel for Bladesmiths & Others who Heat Treat and Forge Steel John D. Verhoeven Emeritus Professor Iowa State University Since charcoal is a solid form of carbon, one might expect that the C atoms get transportedfrom the charcoal to the iron surface at the points where the charcoal makes contact with that surface. But such is not the case. If one does not have a gas in the porous charcoal no carbon transport into the iron occurs. If one uses pure carbon powder instead of charcoal no carburization occurs. Charcoal, being the remains of burned wood, contains hydrocarbons that vaporize and produce a gas containing carbon monoxide, CO. The transport of the C atoms to the iron surface is carried by gas molecules of carbon monoxide.
Sorry for the confusion, I meant the used engine oil from the crank case of a diesel powered vehicle. I also use tranny fluid and canola, both at 115F, with satisfaction. Sometimes I just like to experiment. I'll be downloading this book shortly, thanks for the tip!
I have also read that even when in a gas state, carbon penetration will only be roughly .01 of an inch per hour that the steel is exposed to it at proper temp. This means even at an hours soak time in a carbon rich atmosphere, any carbon transferred to the steel will be removed anyway during the post HT grind and sand. Also any used oil would concern me as a quench fluid due to hydrocarbons and other possible toxic gasses that could be released suring the quench. Oil is also more volatile than ATF as far as i know, so there could be risk of flare ups. I would think that if canola seems to be working for you i would stick with that or use a commercial quenchant to be safe. It would be a shame to see a knifemaker have his career cut short due to health problems, or burning down your shop.
I just use old engine old from past oil changes and find it works well when left for a bit longer. Then heat between 115f to 200f and requench in veg oil.
I tried used motor oil once...only once. Smoke is like running a 2-stroke in the garage. Yuk. When friends did a turkey fry they donated me 5 gallons of used canola. Still smells like turkey. Oil is cheap. Lungs are expensive. Dan
Thats awesome and yes lungs are expensive that's why I forge and heat treat, quench all outside and have a mask on.
