Properties of Metals| Metal Types and Heat Treatment of Steel
Properties of Metals
Ferrous metals are metal alloys containing iron. The name ferrous comes from the Latin word for iron Ferro. Most are magnetic.
Steel Production Process
Today, iron is the backbone of the world we have built. It is strong, plentiful, and cheap. Iron comes from iron ore, or such as hematite, which is similar to rust(combination of iron and oxygen).
The oxygen is removed by heating the or with carbon from coke. The carbon removes the oxygen leaving iron, but some carbon is left, which affects the iron properties and carbon content has to be adjusted carefully.
Oxygen is then blown through the molten iron. The oxygen burns up carbon, allowing the steelmakers to adjust the amount of carbon. Iron and carbon are known as steel.
The carbon content affects the properties resulting in different types including; pure iron, low carbon steel, medium carbon steel, high carbon steel, and cast iron.
The low carbon steel is soft and easily scratched. The high carbon is much harder, and high carbon steel can cut low carbon steel.
Properties of Metals
Pure iron is soft as butter and can be bent cold into any shape you desire. Once it is bent, it works hardens a little. When we bend pure iron, we call it wrought iron. It’s suitable for decoration, but it’s not very strong.
Mild steel is the most common steel; it has from 0.1 to 2.3% carbon. It is not very strong, but it is safe to use as it will bend without breaking steel.
Cans are made of mild steel with a very thin coating of tin as they can bend and distort rather than crack and break open.
Cars, too, are made of mild steel; if a car is going to crash, the metal bends and absorbs the crash’s energy.
Mild steel is easy to weld without damaging the properties of the steel. It isn’t easy to weld high carbon steels without changing the carbon content around the weld.
Medium Carbon Steel
Guitar strings are made of medium carbon. Steel medium carbon steels are in the 0.3 – 05 %-point carbon range and stronger and harder to bend. They’re used for cables and wires, train tracks.
High Carbon Steels
High carbon steels range from 0.5 to 1.5% carbon. They are almost impossible to bend without breaking, and it gives the rigidity required in a needle.
If you need something to stay sharp, you’ll need high carbon steel. However, if it is too hard, it will chip easily.
With 2.5 to 4% carbon, it is cast iron, and it is very hard, and Very few things will scratch it. We used cast iron for disc brakes because it’s hard and doesn’t wear easily, and when it’s hot, it doesn’t soften or distort as easily as mild steel does, but cast iron is brittle. You can’t afford to drop it. It must be handled like glass.
Water-cooled engine blocks are made of cast iron, and one of the reasons is that it doesn’t rust. It has silicon in it as well as high carbon.
Cast Iron is used a lot outdoors because it doesn’t need to be painted. Nowadays, a lot of decorative casts are being replaced by cast aluminum.
Besides carbon metals such as chromium, nickel, molybdenum, tungsten, and vanadium, it can be added to modify the steel properties.
These metal mixtures are called alloys, and the proportions can be varied to produce an alloy with the particular properties that you want.
The main types of steel alloys are stainless steel, chrome-molybdenum steel, and tungsten steel.
Stainless steels are the most used of these. It contained from 12 to 18% chromium and up to 10% nickel.
The 1810 label on this saucepan means 18% chromium, 10% nickel.
The most significant advantage of stainless steel is that, unlike plain steel, it doesn’t rust. The knife is made of stainless steel.
On the surface, chromium atoms react with the oxygen to form a very thin oxide layer, which protects it from any further corrosion.
Stainless steel is used a lot for food processing because it doesn’t rust and contaminate the food. Stainless steel does have a weakness.
It is corroded by caustic soda, sodium hydroxide, and stainless steel used in hospitals has to have extra chromium to resist the salt in body fluids.
And if it is in contact a lot with salt water, it will suffer chloride cracking unless it has molybdenum included in the alloy.
Molybdenum is the Molly in chrome Molly, the common name for low alloy steels containing chromium and molybdenum.
Chrome- Molly has excellent tensile strength. This means items can be made thinner and lighter.
Chrome molly is now also finding use in products like gas cylinders. The walls could be made thinner and still safely hold gas under enormous compression. Lighter cylinders are cheaper to transport.
Adding molybdenum to the rail tracks in this iron ore mine has dramatically increased their working life. Before this, carbon steel rails were wearing out quickly. The cost is not just the steel but the labor to replace it on the loss of production while the track is out of action.
Tools used to cut steel have to be extremely hard to keep their cutting edge sharp, but the heat generated by high-speed machining can soften them. Thus, tungsten is added.
Tungsten has the highest melting point of any pure metal, which is 3380 degrees Celsius.
High speed tool steel, maybe 18% tungsten, 4% chromium, 1% vanadium and 0.75% carbon.
Heat Treatment of Steel
If we bend a piece of steel back and forth, it undergoes Work Hardening. The working rearranges the material’s inner structure; it becomes more and more brittle and eventually snaps.
Heat Treatment can modify the properties of steel and minimize the effects of work hardening.
A steel needle is tough and springing. When heated, then left to cool slowly, it is called annealing.
After annealing, the needle bends, and it stays bent. It’s lost its springing nous.
When heated, then left to cool rapidly, it is called quenching. It becomes hard and brittle.
When the needle is heat moderately, it just begins to change color and cool slowly. This process is called tempering.
Tempering removes brittleness while retaining the hardness required in the material.
Golf clubs are tempered to reduce brittleness but retained the other qualities of the steel.
The crystalline structure produced by quenching is called Martensitic after a metallurgist called Martins.
Annealing slow cooling produces a different structure that’s called Austenitic after another metallurgist named Austin.
Physical Properties of Carbon Steels.
When choosing the carbon content, you are choosing hardness, strength, and toughness.
Tensile strength is the strength in tension that is a pulling force. To test tensile strength, you put a sample into a 10 centimeter and pull until it breaks.
A sample of mild still can be broken about 8 kg Newtons. A cast-iron break at about 5 Kg Newtons: so, although cast iron is harder, it is weaker in tension.
The Toughness or Impact Strength.
Toughness is a measure of how much work it takes to break steel. If it is strong and bends a lot before breaking, it takes a lot of work to break it.
When a particular machine is used to compare the impact, strength, or toughness of various metals, here are the results.
Mild Steel: Mild steel distorted a little and absorbed the energy of the weighted pendulum. It is very tough.
Cast Iron: Cast iron snaps easily, and there is even energy left over. Therefore, although it is hard, it is not tough. It can’t absorb energy because it doesn’t stretch before it breaks.
Brass: Brass is similar to mild steel.
Malleability is the ability of steel to be distorted by compression or squeezing. How easy it is to be rolled or hammered out into a thin sheet.
The malleability of steel varies with its carbon content. It is very malleable when it is red hot. The steel sheet is being pressed into shape, it needs to be malleable, and it needs to be ductile.
Ductility is the ability to stretch.
It’s melting and softening point of metals varies a lot. A drill point may get hot, so it must have a high melting and softening point.
When steel is left exposed to water, it rusts. This is a chemical change. The metal reacts with water to form a metal oxide. This is a different substance with entirely different properties.
Hot Dip Galvanizing
Steel is often protected from rusting by dipping it in molten zinc. This is known as Hot-Dip Galvanizing zinc oxide protects from further corrosion.
Zinc Alum Coating
Sheet steel is being coated with molten zinc and aluminum alloy, zinc Alum Coating.
Chromium plating works the same way a layer of chromium oxide prevents corrosion.