感应加热钢丝回火温度与保温时间的选择

依据以下两点来选择感应加热调质处理时的回火温度。

（1）回火处理后弹簧钢丝的力学性能根据钢丝的回火温度与 力学性能的关系曲线来选定所需要的回火温度，这是最常用的方法。SAE 9254冷拉弹簧钢丝感应加热调质处理回火温 度与钢丝的抗拉强度、弹性极限和塑性的关系曲线。调质处理后钢丝再经375〜400°C感应加热炉低温回火、保温40min 后钢丝的力学性能。通常低温回火是用于弹簧制成品提高弹性极限 的热处理。

（2）根据弹簧钢丝的回火组织选择回火温度 共析和亚共析弹 簧丝的回火组织可分为三类（见表8-21）。其中珠光体为高温回火 产物，强度低，不适合做弹簧材料；索氏体为中温回火产物，具有 较高的强度和韧性，可以用作部分弹簧材料；屈氏体为低温回火产 物，具有最细小的组织结构、最高的强度和韧性，更为重要的是具 有最佳的抗应力松弛性能，即低的松弛率。因此，在回火处理时应 尽量获得回火屈氏体组织。要想得到回火屈氏体，应当具备的条件 是：首先要得到由细小奥氏体晶粒淬火而形成的淬火组织；其次要 选择较低的回火温度，这样方能获得屈氏体占比例较多的回火组 织。感应加热弹簧钢丝的回火组织中应当由屈氏体和索氏体组成， 前者比例越多越好。

表8-21共析钢感应加热回火温度与回火组织的特点

①回火保温时间的确定确定感应加热回火保温时间的原则 是保证钢丝透热和完成组织转变。实际上完成组织转变所需要的时 间很短，主要是使表面与心部均温花费的时间。如果设计表面加热 功率密度不是很大，对于直径在6mm以下的钢丝可以不要保温时间。对于使用表面功率密度较小、直径大于6mm以上的钢丝，回火保温时间可取5〜10s。

②回火处理后钢丝的冷却方式感应加热回火后冷却对钢丝的性能影响不大。对于有回火脆性倾向的钢丝应釆取喷水冷却。不过为了缩短生产线长度，弹簧钢丝均可釆用喷水冷却，但是最好使钢丝保持50℃左右余温，以便使表面水分自行挥发。

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 375—400°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

①Determination 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 .

②The 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.