Hardening a sear

[Puzzledprofessor]

I know this has been asked and answered a million times but I can't find the answer,
so I will ask. How do I harden a trigger sear?
I think I know what I am doing but I need my hand held.
Heat, quench in motor oil, and temper in an oven, right? 
Okay, so how hot do I get the part and at what temp do I temper it and for how long?
Sorry for the sophomoric question, but I don't want to jack this up.
Thanks in advance for the help.

[Mark Elliott]

Heat quickly and evenly to a salmon color (this is actually too hot, but small parts cool quickly in the air) and quench in light oil; heat treat oil if possible.   If not, use 10-20W motor oil.  Check with a file to make sure it is hard.   If not repeat in a lighter quench medium.   I temper my tumblers and sears to 500F.   So put it in a oven with an accurate pyrometer lead near the part.   Bring to temp and hold for 30 mintues.  Then let the part cool normally.   

[Puzzledprofessor]

Thank you for the detailed answer. I will give it a go. If I don't have oil that light would transmission fluid work?s

[smallpatch]

Tranny fluid is what I use.  Works great.

[Puzzledprofessor]

I thought I had read that somewhere. Thank you.

[Bob Roller]

I've used 0-1 and hydraulic jack oil for 50 or more years and so far so good.

Bob Roller

[rich pierce]

I guess we are assuming we know what steel the sear us made of.  Be sure to check it's hard after you quench it and before you temper it.

[smart dog]

Hi,
I quench in Canola oil which smells a lot better than any transy or motor oil.  It does go rancid after a while and needs to be replaced but not very often.  It is also much easier to dispose off.

dave

[LRB]

  Canola oil, in most cases, works well enough for the 10xx steels, but it cools 01 a little faster than is necessary or desired, and presents at least a small risk of cracking the part. The thinner the part, the higher the risk. The slower cooling speed of ATF is better for 01, but avoid breathing the fumes. Common motor oil is better used for it's intended purpose. It is a very poor choice of a quench oil for any steel.

[FDR]

I use a magnet. Just heat until the part becomes non-magnetic and quench in your choice of light oil. Transmission fluid, canola, olive oil etc. will all work.  Check the part with a new file to make sure it is hard.  Draw at 450 to 500 F in an oven. I usually us 450 as home ovens are usually inaccurate.

Most originals were case hardened but that is another process.

[David Rase]

I have been using a gallon can of tranny fluid forever, and yes, it smells.  I will have to trade it out for canola oil.  Time to go green!
David   

[LRB]

I use a magnet. Just heat until the part becomes non-magnetic and quench in your choice of light oil. Transmission fluid, canola, olive oil etc. will all work.  Check the part with a new file to make sure it is hard.  Draw at 450 to 500 F in an oven. I usually us 450 as home ovens are usually inaccurate.

Most originals were case hardened but that is another process.

Non magnetic is not really a good quench temp, and all oils are not the same in cooling speed. File checking for hard is no guarantee of full, or even good hardness. Only an indication of maybe. Non magnetic is 1414°. With most steels you should be shooting for 1475° minimum, with some steels liking a tad bit more. 1475° is a good shade of red above non magnetic. Usually red-orange to most eyes. ATF oil is slower than warmed canola, which is the fastest cooling oil in the cooking, or lubrication oil types.  10xx steels require fast cooling. 01 is best with a slower oil.
   David, it depends on what steel you are heat treating. 01 is better with ATF. 10xx types with canola. 01 has many more added alloys than 10xx types which are basically iron, carbon, and manganese.

[Mark Elliott]

I should note that I use a heat treat oven (goes to 2100F) with a digital controller, and I place the part on an upside down crucible in the middle of the oven.   I will also check the temperature with a separate thermocouple next to the part.