Properties and Grain Structure

The safety barriers around this crossing are coated with a thin layer of zinc. If we look closely at the surface of zinc, we see that it is made up of many different patches. You can find this effect on various zinc-coated objects. The patches you see are crystals or

grain

s of zinc here is a piece of a different metal, it has been machined along one edge, no

grain

s can be seen on this surface, however if we turn it over it is a story Completely different, this piece of metal has been split in half along the broken edge you can see some of the grains this material is made of.

All metals are made of grains, although you will rarely see them like this. In fact, with most metals, including this piece of aluminum, the grains are not visible even on the surface. Here is a way to make them visible. First we must give the aluminum a mirror-like finish and that's it, then we treat the surface. carefully prepared with a very powerful acid. Gloves are essential for this operation as soon as the acids have time to act. reacts, the metal must be washed, then it is given a second treatment with another special chemical, we call this process edging, the particular chemicals used depend on the metal we are trying to etch, a final wash and there are grains of pure aluminum that are . all similar in both size and

structure

appear in different shades just because of the way they reflect light when etching we can reveal the grain

structure

of any metal here is a different sample of aluminum in this case the grains are much larger and vary in size and here is a piece of copper in this sample the grains seem to get smaller as you get closer to the middle zinc a sample whose grains come in all shapes and sizes, but how are the grains formed to discover that we have returned to the scenario when a metal is melted in this furnace the metal is aluminum when the right time comes the furnace is hit the molten metal runs along channels into molds to form huge slabs of aluminum in turn the solid slabs will be rolled into sheets of from time to time a A small sample of the molten aluminum is taken to analyze the metal is poor and allowed to solidify.

Now we can see what happens as the aluminum solidifies into a special film at various points in the liquid metal. Small crystals begin to form and each crystal grows. It grows outward in all directions until it meets the surfaces of its neighboring crystals In engineering terms, each fully developed crystal is called a grain in this piece of solid aluminum, there are a large number of grains now, once the aluminum is has been cast into slabs, it is rolled up. sheets to reduce the metal to this thickness, it has been rolled many times and is now relatively cold, so as the metal is crushed between the rollers, it is cold worked, let's find out what effect cold working has on the aluminum grain structure.

Here we are. Re-chip a piece of aluminum before cold working it. At this stage, the grains are all approximately the same size and shape. Remember that they appear in different shades just because of the way they reflect light. You will now cold roll a similar piece of the same aluminum. In a single pass, this machine will reduce the thickness only by a very small amount, thus reducing the gap between the rollers and passing the metal again. Let's see what that does to the grain structure. The cold rolled metal piece is the one on the bottom. Above, can you see the difference?

It seems that we have changed the shape of the grains, they have become longer. We can get a better idea of ​​what happened in a diagram. We will first look at the grain structure of the metal before it deforms. Here. The grains are normal, but as the metal is crushed between the rollers, you can see the grains elongate and distort in the rolling direction. The change in grain structure that results from cold working is accompanied by a change in the mechanical

properties

of the metal. Hardness and tensile strength increase while ductility decreases after cold working a metal, 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 approximately 350 degrees Celsius. Now at the grain boundaries new grains begin to form and 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 aluminum hardness, for example here. When measuring the indentation resistance of a piece of cold-worked aluminum, how does this compare to the size of the dent produced in a piece of recrystallized aluminum?

It is much deeper, so recrystallization has restored the softness and what about the first tensile strength of cold worked aluminum we need a force of about seven units to separate it. Now for a recrystallized part, the strength will be much lower this time, the tensile strength has decreased. If we put the broken pieces back together, we find that the recrystallized piece is more stretched, so we have also restored the ductility, however, the resulting properties depend on the temperature at which the recrystallization takes place. If the temperature rises too high, some of the grains will grow at the expense of their neighbors, which can lead to properties that are highly undesirable for most. engineering applications this is molten steel when it cools it will solidify and grains will form let's find out what is the grain structure of a piece of simple carbon steel as we have given this piece of steel a mirror type finish now that we are If it itches , steel contains four percent carbon so far so good, but with the naked eye we can't see any grains.

We will have to take a small sample of the steel and view its surface through a microscope. Here it is enlarged almost 250 times, let's take a closer look at this in a diagram. In the case of steel, there are two different types of grains that we will see each of them. Light grains like this are made of iron. Engineers call them ferrite. These give the steel the property of ductility the other grains like this are formed 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 give the steel the properties of hardness and strength.

This particular piece of steel is composed of approximately equal numbers of the two types of grain. Until now we have only analyzed steel with 0.4% carbon, however, the steel can be produced with other carbon contents, what effect does this have on grain structure? We'll start with 0.4 percent carbon and add a little more. Can you see what's happening? The amount of pearlite grains is increasing now there are no ferrite grains. The steel now contains 0.8 percent carbon. Under the microscope, a similar piece of steel is seen. You can probably guess what will happen if we now reduce the amount of carbon in the steel, the amount of pearlite grains decreases, leaving many ferrite grains, now there is only about 0.1% carbon left, this is what a piece looks like similar to carbon steel.

Under the microscope we can now change the mechanical properties of plain carbon steel through a carefully controlled sequence of heating and cooling through heat treatment. Let's find out what effect heat treatment has on the grain structure of steel. The rings they were trying contain 0.8 percent carbon, so all the grains are the same type pearlite nothing happens until the temperature reaches around 720 degrees Celsius now the grain boundaries start growing new grains these New grains are quite different from the original ones and grow until they completely take on the previous structure here We are normalizing, so the components are taken out of the oven and allowed to cool in the air.

Let's see what happens to the grain structure as the temperature reaches approximately 720 degrees Celsius, the old types of grains begin to reappear, they 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 that we have reduced the size of the grains and made them more uniform. We have also changed the properties of the steel. Here is a similar piece. untreated steel let's see how strong it is remember that hardness is its resistance to shock or impact loading about sixty units the broken surface reveals a very coarse grain structure now we will test another piece of the same steel that has been heated to a temperature high and has been left. of cold in air about a hundred units is much stronger 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 then quickly cooled or tempered in water this treatment increases the hardness of the steel let's find out what it does to the grain structure we are going to heat a piece of steel with 0.8 percent carbon to seven hundred and fifty degrees Celsius remember that with this particular amount of carbon we only have one type grain nothing happens until the temperature reaches about 720 degrees now exactly the same thing happens here as it happened in normalization.

New grains of a completely different structure grow in the old grain boundaries right now to die down as the temperature drops in each new grain a needle- how the structure is formed this structure is very hard in fact it is also fragile we can alleviate the fragility tempering in this case we are going to temper at about 500 degrees Celsius tempering modifies the structure of the needles inside each needle small carbon flakes begin to appear Now the steel is much less brittle but it is still harder than before heating and tempering it . So far we have only looked at the heat treatment of 0.8 percent carbon steel.

What happens if we try to harden steel that contains 0.1 percent carbon? Here are the grains. mainly iron again, nothing happens until the temperature reaches about 720 degrees Celsius and once again new grains begin to form until they completely take over the old grain structure. In this case we have to raise the temperature much more to almost 900 degrees Celsius if Turn it off now, there is not enough carbon for the hard needle-like structure to form. We end exactly as we began.