how heating metal and quenching makes it stronger, but heat cycling over time makes it more likely to break

Question

I’m just an amateur guy who messed with metal on occasion

And straight up not following the logic

I know heating and quenching makes it harder, which is good for knives and such, but also makes it more brittle I guess? And likely to crack?

The descriptions on this subject are literally “over explaining the scientific molecular composition of metal” or “so anyway make hot then make un hot, dat good”

But I was trying to bend some metal today, heated it up a few times and got it near its shape, then cooled it by quenching so no one would grab it and burn their hands on it while I stepped away, came back and heated again and it just broke lol

In: Other

Answer 1

A few precise terms to unpack here.

First, lots of different metals behave in different ways. But the most common type of steel behaves this way, so I’ll explain that.

Second, “strong” and “less likely to break” are closer to opposites than synonyms. Almost anything you do to make metal stronger will also make it more brittle. A pencil is “stronger” than an eraser, but also easier to break.

In common steel, heating it up causes it to change phase (>!from ferrite to austenite, if you want to google more!<). If you cool it slowly, it returns to the original phase. If you cool it quickly by quenching, the phase basically freezes halfway between the 2 phases (>! We call this martensite!<).

This 3rd phase is very hard, but very stressed out. It usually has residual stress from the rapid cooling because the phases change volume (fun fact, this is why katanas are curved. The edge is quenched more rapidly than the rest, causing it to expand slightly).

Typically, you want to follow the quench with a lower heat–enough to relax the atoms enough to get rid of the residual stress, but not enough to change phases again).

In your specific case, I’d guess your initial quench was too fast and caused cracking. Rehearing expanded the metal and pulled the crack apart.

TLDR: tell your kid that they need an A to get a good job and they will be stressed, but motivated to study. Tell them if they don’t get an A then they’ll be homeless and unloved, and they will be so stressed they snap. And maybe your neighbor’s kid is made of aluminum instead of steel and needs a totally different motivation.