Modification of Materials

Syllabus Dot Point

modification of materials

– work hardening

– heat treatment

– alloying materials

Work hardening is the strengthening of a metal by plastic deformation.

This strengthening occurs because of dislocation movements and dislocation generation within the crystal structure of the material.

Task: Bend a metal coat hanger back and forth. It will eventually break. Why?

Heat treatment consists of heating the metal near or above its critical temperature, held for a particular time at that finally cooling the metal in some medium which may be air, water, brine, or molten salts. The heat treatment process includes annealing, case hardening, tempering, normalizing and quenching, nitriding, cyaniding, etc.

Types of Heat Treatment Processes

Following are the different types of heat treatment processes:

Annealing - Depending on the carbon content, the steel is heated to a temperature of about 50° to 55°C above its critical temperature range. It is held at this temperature for a definite period of time depending on the type of furnace and nature of work. The steel is then allowed to cool inside the furnace constantly
Normalizing - The steel is heated to a temperature of about 40° to 50°C above its upper critical temperature. It is held at this temperature for a short duration. The steel is then allowed cool in still air at room temperature, which is known as air quenching.
hardening - The steel is heated above its critical temperature range. It is held at that temperature for a definite period of time. The steel is then rapidly cooled in a medium of quenching.
Tempering - The steel after being quenched in the hardening process is reheated to a temperature slightly above the temperature range at which it is to be used, but below the lower critical temperature. The temperature here varies from 100°C to 700°C.

Precipitation hardening, also called age hardening is a heat treatment technique used to increase the yield strength of malleable materials, including most structural alloys of aluminium, magnesium, nickel, titanium, and some steels and stainless steels.

We will look at this process in more detail in the 'Aeronautical Module Yr12'

Glass can also be heat treated!

Standard glass, sometimes referred to as annealed or float glass, breaks into large, sharp pieces with jagged edges. This presents a serious risk of injury if you were to fall against it. To reduce this risk, we put the standard glass through a toughening process. This process changes the structure of the glass so that if it breaks it shatters into lots of small pieces. These small pieces are about the size of your finger nail, and is better than a few large sharp pieces.

Alloying Materials

What is alloy steel?

Alloy steel is a class of steel that, in addition to carbon, is alloyed with other elements which are used to enhance the material’s various properties. These elements commonly include manganese, nickel, chromium, molybdenum, vanadium, silicon, and boron

Alloying elements are used to alter the mechanical and chemical properties of steel to give them advantages over standard carbon steel. While there are many alloying elements used to achieve various enhanced properties, certain elements are much more common than others.

Chromium - Chromium added to carbon steel in percentages usually greater than 11% creates stainless steel. The iron does not oxidize because the chromium will oxidize first and form a protective layer over the steel. Chromium also helps to improve mechanical properties, even in smaller amounts. It will increase the steel’s strength, hardness, and ability to be heat treated.

Molybdenum - Molybdenum, like chromium, has an effect on the corrosion resistance of steel. Molybdenum can also increase the hardenability, toughness, and tensile strength of steel

Vanadium - Vanadium is used to help control the grain size of the steel, keeping it small.

Nickel -Nickel is used to manufacture austenitic stainless steels because it is an austenite promoter. When amounts of chromium around 18% or greater are used and nickel composition is greater than 8%, austenitic stainless steel is created. This combination is extremely corrosion resistant, and austenitic grades are some of the most widely used stainless steels. Nickel is also used to improve the mechanical properties of steel. It is used to increase toughness and impact strength, even at lower temperatures

What are some other examples of alloys?.

Below is a diagram showing some metal alloys.

Compile a list showing the metals in each alloy for - Cast Iron, Solder, Duralumin, Brass, and Bronze

What is the difference between interstitial and substitutional solutions?

Substitutional solutions form only if the solute atoms are large enough to replace the solvent atoms in the lattice. Moreover, the atomic size of solutes is nearly similar to the size of solvent atoms. Interstitial solid solutions are solid state solutions that form when solute atoms enter into the holes between solvent atoms of lattice.

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