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I've been in this business for awhile, but I still think of rain when I hear the phrase "Precipitation Hardened". I picture drops of some chemical raining down inside the heat treating furnace, magically changing the properties of the metal. I know I'm wrong, so what is precipitation hardening?

To begin to understand it, we first need to define a couple of terms:

Precipitate
A distinct solid mass created within another mass.
Crystal Lattice
The blueprint of how matter is repeated to form a crystal. My favorite explanation is found at the King's College website. I like it because not only is it very clearly written, but it has graphics from the beginning of the world wide web. It reminds me that we're learning about things that don't change.
Crystal Structure
What fills in the lattice. Again, hat's off to King's College and the page's author Professor Barbara Sauls for an easy to understand explanation.
Dislocation
An irregularity within the crystal lattice
Supersaturation:
When there is more of a substance in a solution than can be dissolved. This changes based on the temperature of the solution (in our case the bar of metal). This is commonly explained using rock candy.

Disclaimer!

I'm not a metallurgist or a scientist of any kind, so what I say should be taken with a precipitate of salt. There's a lot of very technical explanations out there if you're interested in that side of things.

So, here goes the simplest explanation I can come up with:

Dislocations create weak points within the structure, in our case the metal. If you reduce dislocations you make the metal stronger. Precipitation hardening does this in a three stage process.

Stage 1. Solution treatment:

First the metal is heated up to the point that things start dissolving and moving around. This is called the "solution" phase of the heat treatment. I used to think that the metal sat in a liquid solution, like in a bathtub, but the metal itself becomes the solution when it is heated.

Stage 2. Quenching:

The metal is brought down to room temperature relatively quickly, or "quenched". This forces a state of supersaturation. Sort of how like carbonation bubbles just waiting to blow out the top of your soda bottle when you open it.

Stage 3. Aging:

Reheating the metal to something in between the solution treatment temperature and the quenching temperature and letting it sit for awhile (the "aging" phase) gets everything moving again. Particles that don't have a place to fit (because supersaturated) start to find particles like themselves to join up with. The lump they make is a "precipitate".

Now the real fun starts. Where things would normally have the potential to break through the weak points in the lattice (the dislocations) these precipitates get in the way--like a bumper in a bumper pool table. A lot of times they bounce around long enough to create a new part of the lattice that doesn't have a dislocation. They become a part of a stronger crystal structure.

And that's what makes the precipitation hardened metal stronger.

 

Further Reading:

A more detailed, but still easy to understand, explanation can be found on Reddit, courtesy of user terevos2. Some really great graphics are included, including a youtube video that demonstrates dislocations within a crystal lattice. I highly recommend reading it for a short, but accurate understanding of the process.