My uncle in Toledo always said to quench a blade in old motor oil for a better temper.

I finally tried it on a simple hunting knife last month and it came out with a weird, uneven hardness. Turns out the additives in modern oil can mess with the carbon steel. Has anyone else had this happen with recycled quenching fluids?

3 comments

3 Comments

jamesm382mo ago

My grandpa gave me the same advice with used transmission fluid. Ruined a perfectly good filet knife, left it soft in spots and brittle near the spine. Modern oils just don't act the same as the straight weight stuff from back in the day. What did you end up doing with your blade?

the_stella2mo agoMost Upvoted

Ever wonder if the real problem is the metal itself and not just the oil? Some of those old knives were made from simple carbon steel that reacted well to weird quenches, but modern filet blades often use stainless mixes that need very specific heat treatment. That fluid might have cooled parts too fast or too slow depending on the exact alloy. Maybe your grandpa's trick worked on his old hardware store knife but not on something with chromium in it.

the_jordan2mo ago

@jamesm38 mine's now a glorified butter knife, but at least it can't hurt me anymore