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
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for any given metal, it’s down to the actual temps you’re talking about. there are certain changes that happen to the crystaline structure of metal at certain specific temperatures. It’s SIMILAR to phase changes like 100*C to boil water, etc. Not exactly the same, but similar. So whether a cold/hot cycle strengthens or weakens a metal depends a great deal on how hot it got (what was happening to the crystals at the peak), how long it took to get there (what happened to the crystals on the way up), how long it stayed at the highest temp, and how long it took to cool down.
The point of ‘qeunching’ is because at certain temps the crystals form in a way you want, but it’s way back to room temp they would be undone by time spent at lower temperatures. So maybe 1000 degrees does stuff you want, but 800 degrees will undo the stuff you want. so you try to spend however much time at 1000, and then quickly get back down to some lower temp as fast as possible, skipping the 800 degree zone that undoes all your work.
Alright, here’s a simple way to think about it: When you heat and quench metal, you’re making it harder by changing its internal structure. But this hardening also makes it more brittle, like turning a rubber ball into a glass marble. Repeated heating and quenching can make the metal’s structure more uneven, which can lead to it breaking more easily when you bend or stress it. It’s like if you keep freezing and thawing a sponge, it starts to crack and crumble. For better results, try to avoid overheating and quenching too many times and follow proper heat treatment guidelines!
Heat cycling makes structures with different materials in them more likely to break, because those different materials expand differently. It’s like the thing is rubbing against itself constantly, twisting and stretching as it does.
As for heat treating metals, this one is fascinating. The metal forms a crystal lattice – a certain way that the atoms are organized. Now, different lattices are stable at different temperatures, and with different alloys. Having the proper carbon (and usually to a lesser extent other elements) content of steel is crucial to heat treatment.
The size and type of crystals formed in the metal has huge consequences for its behavior. Heat treatment allows these crystals to grow to the right size and type, and then be cooled (at a specific rate) to cement them into the structure you want.
One thing that people don’t seem to be mentioning…..
There are multiple types of metal. Many react differently to heat and work. Some like to be worked hot, some cold, some are fine being worked for a long time without hardening
When metal cools down slowly it forms long unbroken grains, which can bend easily. The slow cooling allows the crystals in the metal to grow in a consistent, orderly pattern that is soft and flexible. This is great for something like a spring, but would lead to a knife that dulls quickly.
When a metal cools quickly it forms tons of tiny interlocking grains that don’t bend easily. The sudden shock of cooling causes the metal to start crystallizing everywhere at once, and these crystals meet up in harsh boundaries and lock together. This makes the metal hard and it will hold an edge well, but it will snap before it bends much. When you try to bend it, these harsh boundaries between grains will split apart.
So depending on what you’re making you will want different grain sizes. So heating and cooling it uncontrollably can mess that up. You can make a spring too brittle by heating it up then cooling it down quickly, or you can soften hardened steel by heating it up and letting it cool down slowly.
Btw, if you want to learn more about this and related stuff, in a book focused much more on the practical side of “why” not just showing you some heavy engineering math and being like “because math says so”,. Then this book is goddamn amazing – [New Science of Strong Materials: Or Why You Don’t Fall through the Floor: 58 : Gordon, James Edward, Ball, Philip: Amazon.com.au: Books](https://www.amazon.com.au/dp/0691180989?ref_=mr_referred_us_au_nz)
Literally a “Curious laymans guide to why basically every material does the things it does and how we worked that out”
Were you heating up aluminum?
Think of it like clay. Instead of heat, we add water to make it soft.
Soft, wet clay is malleable. If you punch it, it bends. This is equivalent to hot steel.
If you let clay sit and dry out and you hit it, it’ll just crumple. It’s not much harder, but it’ll sort of chip and turn to dust. This is like annealed steel. Easily machinable.
Quenching is like fire hardening clay. Now if you punch it you’ll break your hand because it is way harder, but if you manage to break it, it will shatter. That’s because it’s very brittle, but very hard ceramic.
Like clay, steel can be softened and hardened, except with heat instead of water.
Also, like clay, If you let steel harden, and you repeat the cycle a lot, the expansion/contraction makes it brittle.
There is a LOT about steel that is unique to it. AFAIK you can’t just add water back to fire hardened clay, but with steel you can do the cycle many times over if you do it right.
If you temper steel, which is what you do in knife making, you’re keeping a lot of the toughness of the fire hardened clay, with some of the flexibility of the wet clay.
If you could do that with clay, it’d be like punching a clay bowl, breaking your hand, and also leaving faint dents where your knuckles hit it. It had some give to it so that it absorbed the blow enough to not shatter, but still broke your hand.
Master’s degree in material science:
Not every metal is “heat treatable”.
Steel is funny because the carbon acts like a road bump to the iron matrix it resides in – a road bump in the sense that iron remains ductile until it encounters defects (caused by carbon).
If you increase carbon enough you get something very brittle (cast iron).
If you add just the right amount of carbon and heat treat it you can control how much iron-carbon solute comes out of solution (this is all solid state, but it helps to think of something like sugar and water in solution).
If you heat a steel and quickly quench it, you freeze a lot of iron-carbon solute – this is not stable. This is very brittle, but also very hard.
If you now heat treat the steel (raise to a certain temperature and leave it) the unstable iron-carbon solute tries to reach stability – BUT – you can just cool it down to “freeze” that state. The perfect combination of strength, toughness (ability to not crack), and hardness.
Heat treatment is nothing but slowly introducing equilibrium back to a steel but stopping it before it gets all the way there. If you leave steel quenched but not heat treated, it’s very unstable, which is why it’s like glass.
Making it harder also makes it easier to break
Tldr; hard is tough until it breaks, ductile deforms before it breaks, strong is a balance between the two that meets the needs of the product.
There’s a lot of tried and tested science behind it but when you’re heat treating something you’re more or less looking for a balance that meets the needs of the product. Hard enough not to deform but not too hard where it snaps.
Also under certain conditions heat cycling can cause stress and repeating that just adds stress, if you create a crack even a micro fracture upon striking something it will relieve that stress by growing the crack until the material snaps.
Also while working with hot metal like forging blades the metal crystalizes in particular ways and if you don’t address it properly before quenching you could end up with a crystalized structure resembling sand with a much weaker bond between them than the velvet you want, this leads to a very fragile blade.
Lastly to remove stress of quenching from material you can temper it which is to ‘warm’ the material up to a desired temperature for a certain amount of time. This relieves some of the stress and makes the material a little more ductile leaving you with something that should be hard but not too hard ie strong.
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