Mtal heat treatment is to put the metal workpiece on a medium and heat to a suitable temperature.Keep at this temperature for a certain time,cool the metal workpiece in different medium with different speeds.Through this method,it can change the structure of metal material surface or interal microsturcture to control the performance.Heat treatment is very meaningful method to improve the quality of materials.The way to use heat treatment may change or affect the organization and nature of the iron,and can obation higher stength,hardness,and to improve the wear resistance of the castings,such as valve body casting,machine tool castings, impeller casting,etc.
Because of the difference of purpose,there are many heat treatment ways,that can be divided into two major categories basicly.The first category is that the organization structure will not change by heat treatment or should not change and the second category is that the basic organization structure changes.The first heat treatment procedure is mainly used to eliminate internal stress and the internal stress due to different cooling conditions in the process of stainless steel casting. The organization, strength and other mechanical properties has no obvious changes due to heat treatment.
For the second type of heat treatment, matrix structure has an obvious change,that can be roughly divided into five categories:
Softening annealing: The purpose is to decompose carbide, reduced its hardness,and improve the processing performance.For ductile iron, Its aim is to gain more ferrite.
Normalizing treatment: The main purpose is to obtain pearlite and sorbite structure and to improve the mechanical properties of the stainless steel casting.
Quenching: Itís main purpose is to obtain higher hardness and wear and tear strengthpearlite and at the same time to obtain the higher surface wear-resistant characteristics.
Surface-hardening treatment: Mainly to obtain the higher surface hardening layer, and get very high surface wear-resisting properties.
Precipitation hardening treatment: Mainly to obtain high strength and elongation and not drastic changed.
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