Heat Treatment Process Used for Castings30 June 2020
It is highly essential to use heat treatment process to change the physical and chemical properties of cast alloy depending on how it will be used. As it undergoes a heat treatment process, most of its mechanical properties be it ferrous or non-ferrous in nature will be observed through their microstructure. The heat treatment process will alter the crystalline structure of the cast alloy so that the mechanical properties will behave in a manner appropriate for its application. The heating and cooling will be controlled so that the vital properties namely hardness, strength, toughness, ductility and elasticity will change. However, one alteration no matter how minimal can affect the other, most often in a non-beneficial manner. To further understand this scenario, below are the heat treatment process used for castings.
During the solidification of the cast alloy, the outcome usually results into a build-up of hardness and stress and less ductility. The annealing process is done to alter these conditions. This process is done to reduce hardness, improve ductility and relieve the stress. Also, the annealing process makes the ferrous and non-ferrous castings more machine-able. These process vary in many ways so the results would be dependent on its application. The only similarity between them is the actual process of heating to itd desired temperature and afterwards its controlled cooling.
Precipitation Strengthening Process
Precipitation strengthening process occurs when there is a controlled release of constituents to eventually form precipitate clusters. These precipitate clusters are proved to significantly increase the strength of the casting. The precipitation strengthening process usually improves the yield strength of castings such as aluminium, nickel and titanium. It also works with certain steels and stainless steels.
The tempering process is done by reducing the hardness of the casting alloy which will in turn increase its ductility. As a result, the casting will be less brittle. As the casting undergoes tempering, it will be quenched to its maximum hardness. Afterwards, it will be reduced to the desired level. The process mainly consist of heating the casting to a temperature below its point. Then the austenite will form and the cooling process will be initiated. It normally works with ferrous alloys.
Carburising or Case Hardening Process
Carburising or case hardening process occurs when the casting is heated above its transformation temperature in a carbon-rich environment. Afterwards, it will be quenched and will result into a surface layer intended to increase surface hardness and wear resistance without sacrificing its loading performance brought by its softer core. It is known as a thermo-chemical diffusion that involves a carbon element in a ferrous casting.
The normalising process is done to ensure a uniform and fine-grained structure so that its mechanical behaviour is predictable without the tendency for underperforming machining qualities. It is done by heating the casting to its hardening temperature then afterwards soaking it before cooling. The cooling is executed in an environment with a protective gas atmosphere that prevents oxidation and decarburisation. The normalising process is usually done because steel alloys have a non-homogenous microstructure with large grains and unwanted structural components – all by-products of casting.
The quenching process varies for ferrous and non-ferrous casting alloys. For ferrous alloys, the goal is a martensite transformation for a harder metal. While for non-ferrous alloys, the end result should be a softer than normal material. This is done to ensure corrosion-resistant castings.
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