Heat Treatment Flashcards
Explain Equilibrium Diagrams?
Thermal equilibrium (also known as Phase or Constitutional) diagrams are of great importance in metallurgy for with their aid it is possible to determine exactly the structure of a particular alloy at any given temperature, provided the alloy has been allowed to reach a state of equilibrium.
Explain how you would Harden Carbon Steel?
In order to harden carbon steel, it must be heated to a temperature of 20-30 0C above its upper critical temperature and kept at this temperature long enough to ensure that the whole mass is at this temperature and the structure is austenitic. Maximum hardness can now be obtained by quenching the steel in water or brine.
Explain Spheroidizing?
A form of annealing consisting of prolonged heating of iron base alloys at a temperature in the neighbourhood of, but generally slightly below the critical range, usually followed by a relatively slow cooling
What are the different types of Quenching Media?
Water,
Salt Water,
Oil,
Polymer Quench,
Cryogenic Quench
Explain Tempering
The hardness and brittleness of quenched steel leaves the possibility of internal stresses. By reheating after hardening, the hardness and brittleness can be reduced and internal stresses relieved.
Explain Annealing?
The process consists of heating the metal to a certain temperature and then allowing it to cool very slowly out of contact with the air to prevent oxidation of the surface.
The annealing temperature should be about 50C above the upper critical point and therefore varies with the carbon content of the steel.
What are the effects from over-annealing?
Over-annealing has the bad effect of promoting grain growth, resulting in reduction in resistance to shock and fatigue.
Explain Full Annealing?
Fully-annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming.
The steel must then be cooled slowly, in the realm of 38°C per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside.
Explain stress Relieving?
The reduction of residual stresses in metal by heating to about 625°C and held at that temperature for sufficient time to allow the internal stresses to be released. After soaking, the metal is allowed to cool slowly in the furnace to about 150°C and completed in still air.
Explain Normalising?
The reduction of residual stresses in metal by heating to about 625°C and held at that temperature for sufficient time to allow the internal stresses to be released. After soaking, the metal is allowed to cool slowly in the furnace to about 150°C and completed in still air. As it isn’t held at such a high temperature, there isn’t a chance for and grain/ crystal growth to take place.
Explain case-hardening and the different processes?
For steel components that have to withstand heavy surface wear. It produces a surface with the required hardness. Several processes are,
Case-Carburising,
Cyaniding,
Nitriding,
Shorterizing
