If you were lucky enough to be at the Tolles Materials Science day on Saturday 5/6, you got to hear Dr. Tom Glasgow and Dr. Glenn Daehn give a presentation on supersaturation, Steel, and Aluminum.
One of the takeaways from this was the usefulness of precipitates in strengthening materials. To make a material stronger, you want to make it harder for dislocations to move through the metal. In the bobby pin experiment, you show that a water quench will create a strong and brittle steel. This is because the steel forms martensite, which is a particular structure of Fe & C that dislocations have a hard time moving through.
Here is a video of martensite forming in steel during a quench:
Tom also mentioned that we form martensite in order to temper it (martensite is brittle, after all, and we don’t want our metals to be brittle). When we heat it up, we allow the steel to change back to its equilibrium and ductile body-centereed-cubic structure with very small hard carbides throughout. This is actually how aluminum is strengthened as well. Watch Mark Midownik talk about that here: