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Analysis of heat treatment technology of cast iron for machi
Pubdate date:2019-10-17Views:0
Unlike other processing techniques, heat treatment of machine tool castings is an important process in mechanical manufacturing. Heat treatment generally does not change the shape of the workpiece but changes the microstructure of the workpiece or changes the chemical composition of the cast iron platform and machine surface. Improve the performance of the workpiece. This paper focuses on the heat treatment process of cast iron in machine tools.

    First, multi-temperature quenching of ductile iron; ductile iron can obtain high strength after austempering, and at the same time has good plasticity and toughness. The selection of the multi-temperature quenching heating temperature mainly considers that the original structure is completely A-formed, and no F remains, and A-grain growth is also avoided. The heating temperature is generally 30 to 50 ° C above Afc1, and the isothermal treatment temperature is 0 to 350 ° C to ensure the lower bainite structure with comprehensive mechanical properties. After the austempering of rare earth magnesium-aluminum ductile iron, σb=1200~1400MPa, αk=3~3.6J/cm2, HRC=47~51. However, attention should be paid to adding a tempering process after austempering.

    Second, stress relief annealing; due to the uneven wall thickness of the casting, effect force and tissue stress will occur during heating, cooling and phase transformation. In addition, large parts are subject to residual stresses after machining, and all these internal stresses must be eliminated. The stress-free annealing usually has a heating temperature of 500 to 550 ° C for 2 to 8 hours, and then furnace cooling (gray iron) or air cooling (ball iron). This process can eliminate 90~95% of the internal stress of the casting, but the cast iron structure does not change. If the temperature exceeds 550 ° C or the holding time is too long, it will cause graphitization, which will reduce the strength and hardness of the casting.

    Third, the high-temperature graphitization annealing of the white mouth of the casting is eliminated; when the casting is cooled, the white layer is often produced at the surface layer and the thin section. The white mouth tissue is hard and brittle, has poor processing properties, and is easily peeled off. Therefore, annealing (or normalizing) must be used to eliminate white-mouth tissue. The annealing process is: heating to 550-950 ° C for 2 ~ 5h, then cooling to 500-550 ° C and then air cooling. During high temperature insulation, the high cementite and eutectic cementite are decomposed into graphite and A, and the secondary cementite and eutectoid cementite are also decomposed during the subsequent cold protection process, and the graphitization process takes place. Due to the decomposition of the cementite, the hardness is lowered, thereby improving the machinability.

    Fourth, the normalization of ductile iron; the purpose of the normalization of ductile iron is to obtain the pearlite matrix structure, and to refine the grains and uniformly structure to improve the mechanical properties of the casting. Sometimes normalizing is also the preparation of the surface quenching of the ductile iron, the normalizing of the high temperature normalizing and the low temperature normalizing. The high temperature normalizing temperature generally does not exceed 950~980 °C, and the low temperature normalizing is generally heated to a common folding temperature range of 820~860 °C. After normalizing, four people are usually required to eliminate the internal stress generated during normalizing.

    Fifth, quenching and tempering of ductile iron; in order to improve the mechanical properties of ductile iron, the casting is generally heated to 30~50 °C above Afc1 (Afc1 represents the final temperature formed by heating), and is quenched into oil after heat preservation to obtain Markov Body organization. In order to properly reduce the residual stress after quenching, tempering should be carried out after quenching. The low-temperature tempering structure is tempered Markov and residual bainite plus spheroidal graphite. This type of tissue has good abrasion resistance and is used for parts requiring high wear resistance and high strength. The medium temperature tempering temperature is 350-500 °C. After tempering, the structure is tempered troostite plus spheroidal graphite, which is suitable for thick parts requiring good wear resistance, stability and elasticity. The high temperature tempering temperature is 500-60D °C. After tempering, the structure is tempered Soxhlet for adding spherical graphite. It has good comprehensive properties of toughness and strength, so it is widely used in production.

    Sixth, chemical heat treatment; for castings requiring surface wear resistance or oxidation resistance and corrosion resistance, a chemical heat treatment process similar to steel, such as gas soft chlorination, chlorination, boronizing, sulfurizing, etc., may be employed.

    This process is related to the use efficiency and efficiency of machine tools, so we must pay more attention to it.
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