Properties and Grain Structure

the safety barriers around this Junction are coated with a thin layer of zinc if we look closely at the surface of the zinc we see it's built up of many different patches you can find this effect on a number of zinc coated objects the patches you see are crystals or grains of zinc here's a piece of a different metal it's been machined along one edge you can't see any grains in this surface however if we turn it over it's quite a different story this piece of metal has been broken in half along the broken edge you can see some of the grains from which this material is made all metals are made up of grains although you'll rarely see them like this in fact with most metals including this piece of aluminium the grains aren't visible not even on the surface here's a way of making them visible first we must give the aluminium a mirror-like finish and there we are next we treat the carefully prepared surface with a very powerful acid gloves are essential for this operation as soon as the acids have time to react the metal must be washed it's then given a second treatment with another special chemical we call this process edging the particular chemicals used depend on the metal we're trying to etch one final wash and there are the grains grains of pure aluminium they're all similar both in size and in structure they appear as different shades only because of the way they reflect the light by etching we can reveal the grain structure of any metal here's a different sample of aluminium in this case the grains are much larger and they vary in size and here's a piece of copper in this sample the grains seem to get smaller as you get nearer the middle zinc a sample whose grains are all shapes and sizes but how do grains form to find out we've gone back to the stage when a metal is molten in this furnace the metal is aluminium when the time is right the furnace is tapped the molten metal runs out along channels into molds to form huge slabs of aluminium in turn the solid slabs will be rolled into sheets from time to time a small sample of the molten aluminium is taken for analysis the metal is poor and left to solidify now we can see what happens as the aluminium solidifies in a special film at a number of points in the liquid metal tiny crystals begin to form and grow each crystal grows outwards in all directions until it meets the surfaces of its neighboring crystals in engineering terms each fully grown crystal is called the grain in this piece of solid aluminium there's a very large number of grains now once the aluminium has been cast into slabs its rolled into sheets to get the metal down to this thickness it's been rolled many times it's now relatively cold so as the metal is squashed between the rollers it's being cold worked let's find out what effect the cold working has on the grain structure of the aluminium here we're itching a piece of the aluminium before cold working at this stage the grains are all approximately the same size and the same shape remember they appear different shades only because of the way they reflect the light now will cold roll a similar piece of the same aluminium in one pass this machine will reduce the thickness by only a very small amount so will reduce the gap between the rollers and put the metal through again right let's see what that's done to the grain structure the cold rolled piece of metal is the one at the top can you see the difference we seem to have changed the shape of the grains they've become elongated we can get a better idea of what happened in a diagram first we'll look at the grain structure of the metal before it's deformed here the grains are normal but as the metal is squashed between the rollers you can see how the grains become elongated and distorted in the direction of rolling the change in grain structure that results from cold working is accompanied by a change in the mechanical properties of the metal it's hardness and tensile strength increases while the ductility decreases after cold working a metal it's usually heated to a sufficiently high temperature let's see what effect heating has on the distorted grain structure in the case of this particular metal nothing happens until the temperature reaches about 350 degrees centigrade now at the grain boundaries new grains begin to form these grow rapidly until a new undistorted grain structure completely replaces the old distorted one we call this process recrystallization let's see what effect this has had on the mechanical properties of the aluminium hardness for example here we're measuring the resistance to indentation of a piece of cold work aluminium how does this compare with the size of the dent produced in a piece of recrystallized aluminium it's much deeper so recrystallization has restored softness and what about the tensile strength first the cold worked aluminium that needed a force of about seven units to pull it apart now for a recrystallized piece the force is going to be much less this time the tensile strength has decreased if we put the broken bits back together again we find the recrystallized piece stretched the most so we've also restored the ductility however the resulting properties depend on the temperature at which recrystallization is carried out if the temperature becomes too high some of the grains will grow at the expense of their neighbors this can give rise to properties which are highly undesirable for most engineering applications this is molten steel when it cools it will solidify and grains will be formed let's find out what the grain structure of a piece of plain carbon steel is like we've given this piece of steel and mirror-like finish now we're itching it the steel contains point four percent carbon so far so good but with the unaided eye we can't see any grains we'll have to take a small sample of the steel and view its surface through a microscope here it is magnified nearly 250 times let's take a closer look at this in a diagram in the case of Steel there are two different types of grain we'll look at each in turn the light grains like this one are made up of iron Engineers call them ferrite these give steel the property of ductility the other grains like this one are made up in layers the white layers are iron the black layers are a chemical compound of iron and carbon called iron carbide pearlite is the name given to this type of grain they give steel the properties of hardness and strength this particular piece of steel is made up of roughly equal numbers of the two types of grain so far we've only looked at 0.4% carbon steel however steel can be produced with other carbon contents what effect does this have on the grain structure we'll start with 0.4 percent carbon and let's add some more can you see what's happening the number of pearlite grains is increasing now there are no ferrite grains at all the steel now contains 0.8 percent carbon under the microscope a similar piece of steel looks like this you can probably guess what will happen if we now reduce the amount of carbon in the steel the number of perlite grains decreases leaving a lot of ferrite grains now there's only about 0.1% carbon left this is what a similar piece of carbon steel looks like under the microscope now we can change the mechanical properties of plain carbon steel by a carefully controlled sequence of heating and cooling by heat treatment let's find out what effect heat treatment has on the grain structure of the steel the rings were treating contain 0.8 percent carbon so all the grains are of the same type pearlite nothing happens until the temperature reaches about 720 degrees centigrade now the grain boundaries new grains begin to grow these new grains are quite different to the original ones and they grow until they completely take over the old structure here we're normalizing so the components are taken out of the furnace and left of cool in air let's see what happens to the grain structure as the temperature reaches about 720 degrees centigrade the old type of grains begin to reappear these grow until they meet their neighbors this structure appears to be very similar to the one we started with but if we compare the two we find we've reduced the size of the grains and made them more uniform we've also changed the properties of the steel here's a similar piece of untreated steel let's see how tough it is remember toughness is its resistance to shock loading or impact about sixty units the broken surface reveals a very coarse grain structure now we'll test another piece of the same steel that's been heated to a high temperature and left of cool in air about a hundred units it's much tougher this time the broken surface reveals a much finer grain structure in another form of heat treatment plain carbon steel is heated to a high temperature and is then cooled rapidly or quenched in water this treatment increases the hardness of the steel let's find out what it does to the grain structure we're going to heat up a piece of 0.8 percent carbon steel to seven hundred and fifty degrees centigrade remember with this particular amount of carbon we have only one type of grain nothing happens until the temperature reaches about 720 degrees now exactly the same thing happens here as it did in normalizing new grains of a completely different structure grow out at the old grain boundaries right now for the quench as the temperature falls in each new grain a needle-like structure forms this structure is very hard indeed it's also brittle we can relieve the brittleness by tempering in this case we're going to temper at about 500 degrees centigrade tempering modifies the structure of the needles inside each needle small flakes of carbon begin to appear now the steel is much less brittle but it's still harder than it was before heating and quenching so far we've only looked at the heat treatment of 0.8 percent carbon steel what happens if we try to harden steel containing 0.1 percent carbon here the grains are mainly iron again nothing happens until the temperature reaches about 720 degrees centigrade and once again the new grains begin to form until they completely take over the old grain structure in this case we have to take the temperature up much higher to nearly 900 degrees centigrade if we quench it now there's insufficient carbon for the hard needle-like structure to form we finished up exactly as we started you

Channel: moodlemech

Views: 824,536

Rating: 4.9564018 out of 5

Keywords: Engineering, Mechanical, Metallurgy

Id: uG35D_euM-0

Length: 18min 29sec (1109 seconds)

Published: Wed Sep 03 2014

Reddit Comments

[u/Erikwar]

Great material, got any more

[u/whymydookielookkooky]

https://youtu.be/U1iCZpFMYd0