Self organising steel balls explain metal heat treatment

there's this really cool desk toy called Atomics that you can't buy anymore so a couple of years ago I made my own it's about three thousand ball bearings sandwiched between two layers of acrylic they're free to move in two dimensions and they naturally fall into this regular lattice structure except that there are imperfections in the lattice and actually that's why I built this thing in the first place because I wanted to make a video about lattice imperfections in things like diamond or metals and this Atomics toy is a really good model for that if you want to watch that video the link is in the card and in the description but someone commented on that video saying that this Atomics toy might also be useful for demonstrating what's going on when you heat treat metals like annealing or tempering and that's what this video is about if I allow the balls to fall quickly into position they form these small regions of regularity you get the same kind of arrangement in the atomic lattice of metals these small regions of crystalline metal that are neighboring each other and they can't form one single crystal because they're oriented differently these regions are called grains in a metal and it turns out that the size of grains in a metal really has a profound effect on the physical properties of that metal so let's see what happens to the grain size when we heat treat a metal this is a model of an atomic lattice so we need to think about what heating means for the model well on the atomic scale heat is just kinetic energy it's how much are the atoms and molecules jiggling around so if we want to increase the temperature in our model we need to jiggle the balls around I'm going to jiggle these balls with a vibration generator which is basically a speaker with a prong and because I'm filming at 50 frames per second I'm gonna try vibrating at 50 Hertz and a hundred Hertz by picking a multiple of the frame rate I don't need to worry about the stroboscopic effect you can see how the other kinetic energy gives the lattice an opportunity to rearrange itself but extra movement means that the atoms can slip and slide into new positions and the grain size increases so heat treating a metal lattice in this way increases the grain size but what effect does that have on the physical properties of the metal well in metallurgy you have all these different terms for the physical properties of a metal hardness toughness strength they all sound really similar but they all have different technical definitions that I don't want to get into in this video so we're just going to gloss over all that and say how hard or easy is it to bend a metal permanently out of shape before and after heat treating in this way well my assumption would be that if you have larger grains larger crystal structures then it will be stronger because a regular crystal arrangement is really strong but it turns out that larger grains make it easier to bend a metal out of shape to see why we need to look at what happens at the molecular level when you permanently deform a metal it's called plastic deformation when you permanently change the position of atoms and you might theorize that when you ponies deform metal maybe the grains are sliding over each other or they're breaking up into smaller grains or joining together it's actually none of that it turns out the most important thing when it comes to plastic deformation is imperfections in the crystal specifically imperfections called dislocations dislocations come in a few different flavors but generally speaking they look a little bit like this and it turns out that dislocations are easy to move you can see why if you look at the bonds around a dislocation they're already quite strained so you don't need much energy to break one of those bonds so when you apply forces to a metal in different directions these dislocations will move around in different directions like this the important thing is it takes less energy to move dislocations through a grain than it does to move grains against each other that's why dislocations are sometimes called the carrier of plasticity in you might have thousands of dislocations within a single grain in a metal so when you're bending it you're causing those dislocations to glide through the grain but crucially when a dislocation gets to the edge of a grain it can't go any further it can't hop to the next grain that's the end of its journey so if your metal has small grains then the dislocations within those grains won't have far to travel before their journey ends before they reach a grain boundary and on a macroscopic level that means that your metal isn't very ductile you can't plastically deform it very much and if you keep applying force then it's just going to fail in other ways whereas if you grow the size of your grains like we did here then you're giving those dislocations further to travel you're increasing the capacity of your metal to plastic Li deform before breaking you're increasing the ductility of your metal that's about the limit of what I can demonstrate with this model and obviously it's an oversimplification of the whole field of metallurgy because there are different ways to heat treat a metal with different results what I've done is probably closest to annealing but there's also tempering and quenching and there's loads of different variables like what temperature do you raise the metal to is it above or below the recrystallization temperature how quickly or slowly do you let it cool down and you get all these different properties it's really really interesting and it's not something I can demonstrate with this model but I found a couple of really good videos if you want to dive a bit deeper I'll leave those in the card and in the description [Music] [Applause] [Music] let me tell you what I've been doing with the sponsor this month and pleased to say it's go share again online community of learners with thousands of courses in 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mold for that's school Dutch slash Steve mold for someone told me that I should say slash instead of forward slash because I guess the Ford bit is implied it's never back slash so in case anyone was wondering why didn't you say four words - I made this longer really by explaining every knife anyway I hope you enjoyed this video if you did don't forget to hit subscribe and I'll see you next time [Music]

Channel: Steve Mould

Views: 3,028,145

Keywords: Understand, metallurgy, materials, lattice

Id: xuL2yT-B2TM

Length: 8min 45sec (525 seconds)

Published: Thu Aug 22 2019

Reddit Comments

[u/Guysmiley777]

It's not a perfect analogy but I really enjoyed this guy's demonstration of the difference in structure in the model he built and how it mirrors how heat treating affects grain structure and size in steel.