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    Selection of Tempering Temperature and Holding Time of Induction Heating Steel Wire

    Choose the tempering temperature during induction heating quenching and tempering treatment based on the following two points.

    ( 1 ) The mechanical properties of the spring steel wire after tempering are used to select the required tempering temperature according to the relationship curve between the tempering temperature of the steel wire and the mechanical properties. This is the most commonly used method. The relationship curve of  cold drawn spring steel wire induction heating quenching and tempering treatment tempering temperature and steel wire tensile strength, elastic limit and plasticity. After quenching and tempering, the steel wire is subjected to low-temperature tempering in an induction heating furnace at 375400<C and the mechanical properties of the steel wire after being kept for 40 minutes . Usually low temperature tempering is a heat treatment used for spring products to increase the elastic limit.

    ( 2) Select the tempering temperature according to the tempering structure of the spring wire. The tempering structure of the eutectoid and hypoeutectoid spring wire can be divided into three categories (see Table 8-21) . Among them, pearlite is a high-temperature tempered product with low strength and is not suitable for spring materials; sorbite is a medium-temperature tempered product with high strength and toughness and can be used as part of the spring material; troostite is a low-temperature tempered product , Has the smallest structure, the highest strength and toughness, and more importantly, has the best resistance to stress relaxation, that is, a low relaxation rate. Therefore, the tempered troostite structure should be obtained as much as possible during tempering. In order to obtain tempered troostite, the conditions that should be met are: firstly, a quenched structure formed by quenching of fine austenite grains; secondly, a lower tempering temperature must be selected so that the proportion of troostite can be obtained. More tempering organizations. The tempered structure of the induction heating spring steel wire should be composed of troostite and sorbite, the more the ratio of the former, the better.

    Table 8-21 Characteristics of induction heating tempering temperature and tempering structure of eutectoid steel

    Microstructure

    Tempering temperature

    /<c

    Interlayer spacing

    / nm

    Hardness HBS)

    Pearlite

    680-600

    150 450

    170 25 .

    Sorbite

    600 550

    80-150

    250 320

    Troostite

    550-400

    30-80

    320-400

     

    Determination of the tempering and holding time The principle of determining the induction heating and tempering holding time is to ensure the steel wire to penetrate the heat and complete the structure transformation. In fact, the time required to complete the organizational transformation is very short, mainly the time it takes to equalize the surface and heart temperature. If the design surface heating power density is not large, in the diameter 6mm steel may do the following when incubated between. For smaller surface power density, diameter greater than 6mm or more wires, preferably Tempering Time . 5 10s .

    The cooling method of the steel wire after tempering. Induction heating and cooling after tempering have little effect on the performance of the steel wire. For steel wires with a tendency to temper brittleness, water spray cooling should be adopted. However, in order to shorten the length of the production line, the spring steel wire can be cooled by spraying water, but it is best to keep the steel wire at a residual temperature of about 50 ≧ so that the surface water can evaporate by itself.


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