Forster pilots

Pistolero

Well-Known Member
Heat red hot, drop in motor oil, for 4140. That gives the nice black oxide coating.

This oil quench makes it max hard, still pretty tough, because of the chrome and molybdenum
in the alloy, better than if it was a straight 1040, which has the same 0.40% carbon,
but not the added alloying elements. For shock use like a punch, polish back to bare steel and
warm VERY GENTLY, it is super easy to overheat and ruin the hardness, until the polished part JUST
turns from yellow to blue from the heat. Then remove heat and blow on it a bit, if a small part
like punch tip. Put your heat mostly on the small part. If you heat a thick section, by the time the
heat soaks to the thin tip (like a punch) the fat part may be well above the desired temper
temperature, and it can then soak into the tip, overshooting the temp and over softening
the tip. Care at even, gentle heat application when tempering is critical.

4140 is a really good alloy, good machineability, strong, tough in the unheat treated
form. For harder and stronger, you quench and temper. If you want it even a bit harder,
than above, stop the warming at just the point where it turns yellow. This is harder to do,
need to wave a propane torch on, then watch, then on, then watch. Literally 1-2 seconds
of heat, then stop. Easy to overdo and soften too much. This second part is called tempering,
and these colors appear at very specific temperatures, and are called "tempering colors".
Tempering trades a bit of the hardness for less brittleness and more toughness and shock
resistance. If you go past purple on the tempering colors, you are starting to get close
to the un-heat treated state, but will still be a bit stronger, yet very tough.

The basics of heat treating and tempering are pretty much the same for all steel
alloys, but the results you will get will not be the same. Much below 0.30% carbon
you won't get much hardening from quenching. Like 1020 steel (plain steel, no alloying
elements, 0.20% carbon) will not harden noticeably from quenching. 1030, and 4130
will benefit, as will any of the alloys with more carbon. 1090 gets quite hard, needs to
be carefully tempered as the as-quenched behavior will be pretty brittle. Thousands of
alloys, each with a particular set of properties in mind. As a readily available alloy for
the home machinist, 4140 is a great choice. Not expensive, easy to machine, easy to heat
treat and if tempered a bit, very tough for even impact applications. I made a block to
back up some steel rivets in an AK assy. Heated red hot, quenched in oil. Pounding steel
rivets against it with an air riveting tool didn't even mar the surface finish.

Bill
 
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