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

As other people have said, it really depends on the metal. And not just the metal, but also on the alloying elements added to the main one. The most common thing you would quench is steel. What happens there is there’s carbon present which is dissolved in the steel when it’s really hot. Then (depending on the alloy), if there’s enough carbon you can cool it quickly to harden, or quench it.

The thing is kinda the problem with the terms you’re using, what you’re seeing, and then like technical definitions. First is strength. There’s a few different definitions here, one of the more common ones is yield strength, or when the material starts to deform. Another is ultimate strength, or how much it takes for it to actually break. Materials that are ductile tend to start deforming and then can bend for a while before they finally break. Brittle materials take more to start deforming, but then break soon or immediately after this point.

So getting back to steel, when you quench it, you change it from ductile to hard and brittle. Applying heat can help make it less brittle, and enough over time will change it back to ductile. However, this is just steel. Other materials respond differently, for example aluminum is what you call hot short where if you hear it up and try to forge it with a hammer it is brittle and will crack.

Tldr there’s a lot of variables at play that makes it hard to explain simply without specifying more than “metal”. If there’s more specifics I might be able to help more