A phase is an easily seen part of a microstructure (i.e. different colour, grain shape and grain size to other grains). A phase has the same composition throughout
What is the deffinition of an alloy?
An alloy is made up of two or more elements, one of which must be a metal and the alloy
itself must show metallic characteristics.
What are the diffrent types of solid solutions?
Interstitial solid solutions – solute atoms are very small in comparison with the solvent atoms.
Substitutional solid solutions – atoms of the solute material take up positions normally occupied by some of the atoms of the solvent material.
a) Ordered – if the atoms of the solute material occupy similar lattice points within the crystal structure of the solvent material.
b) Disordered – if substitution is random.
What is thermal equilibrim?
Equilibrium means here that the cooling of the alloy is carried out so slowly that at every temperature all possible changes are allowed to go to completion.
Explain cooling curves.
Cooling curves show the temperature and time at which metals cool from a liquid state. These diagrams are used to construct thermal equilibrium diagrams.
Undercooling generally occurs if the cooling rate is not slow enough to allow the necessary nuclei to form.
What are the names of these cooling diagrams?
Left = 'a pure metal.'
Right = 'an alloy.'
Name the four types of thermal equilibrium diagrams.
1. Binary alloys forming acontinuous series of solid solutions. (eg. Copper and Nickel)
2. Binary alloys forming Eutectic mixtures.
a. Mutual liquid solubility but solid insolubility. (eg. Cadmium and Bismuth)
b. Mutual liquid solubility, partial solid solubility and a Eutectic mixture. (eg. Lead and Tin)
4. Layer-type diagram. (eg. Aluminium and Lead)
What is a Eutectic Mixture?
A Eutectic mixture is formed in an alloy system when distinct solid phases separate simultaneously and at constant temperature from a single liquid phase.
What are the three main parts in the Heat Treatment of Steel?
1. The heating of the metal to
the pre-determined heat treating temperature.
2. The soaking of the metal at
that temperature until the structure becomes uniform throughout the section.
3. The cooling of the metal at some pre-determined rate such as will cause the formation of, or will maintain desirable structures within the metal.
How does the hardening of steel work?
Hardening is done by heating steel with sufficient carbon (i.e. above about 0.3%) to austenite zone and cooling quickly. This is what happens during rapid cooling.
What is Annealing?
Annealing is the heating and cooling of a metal to produce the softest possible state.
What is tempering?
Tempering consists of heating the hardened steel to some temperature below its lower critical temperature.
1. to relieve internal stresses
2. to improve the ductility and toughness of the steel
What is surface hardening of steel?
Surface hardening of steel produces a product that has a hard surface while its core or centre portion is relatively soft.
Name the basic procedures of surface hardening of steel?
1. Carburising – to impregnate the surface of low carbon steel with sufficient carbon to raise its composition to the eutectoid level.
2. Nitriding – steel is heated at 500oC for 40 to 90 hours in a gas-tight container in which ammonia is circulated. Its surface will become very hard due to nitride formation while its core will remain unaffected.
3. Selective hardening – Heating the surface into the austenitic range then quenching rapidly. The surface will become martensitic while the core remains soft.