Basic principle and properties of grinding media surface heat treatment
1. Induction Heating Surface quenching definition: is the use of a certain method to produce a certain frequency of induction current on the surface of the workpiece, the surface of the parts quickly heated, and then quickly quenching cooling heat treatment operation. PRINCIPLE: As shown in the figure, electromagnetic induction produces an induced current of the same frequency, I. E. Eddy Current. The distribution of Eddy current on the section of the workpiece is not uniform, the center is almost equal to zero, but the surface current density is very high, it is called “skin effect”, the higher the frequency, the thinner the surface layer of current density. Depending on this current and the resistance of the workpiece itself, the surface of the workpiece is rapidly heated to the quenching temperature, while the core temperature remains close to room temperature. Classification: Induction heating can be divided into three categories:
(1) high-frequency heating frequency is (200 ~ 300) kHz, hardening depth is (0.5 ~ 2.5) mm.
(2) the frequency of medium frequency heating is (2500 ~ 8000) HZ, the depth of hardened layer is (2 ~ 10) mm.
(3) heating current frequency is 50HZ, no frequency equipment is needed, the city can use alternating current, hard layer depth is (10 ~ 20) mm, the city can use alternating current. Advantages and disadvantages: induction heating surface hardening heating fast, high productivity, heating temperature and hardening depth easy to control, grinding media surface oxidation and decarburization, grinding media deformation small, can make all quenching process mechanization and automation.
2. Properties of induction surface hardened.
1) Surface hardness: the hardness of workpiece surface hardened by high and medium frequency induction heating is usually 2 ~ 3 units (HRC) higher than that of common quenching.
2) Wear resistance: The wear resistance of workpiece after high-frequency quenching is higher than that of ordinary quenching. This is mainly due to the combination of fine martensite grains in the hardened layer, high carbide dispersion, high hardness, and high surface compressive stress.
3) Fatigue strength: High and medium frequency surface quenching can improve the fatigue strength greatly, and decrease the notch sensitivity. For the same material workpieces, the fatigue strength increases with the depth of hardening layer in a certain range, but the surface layer is compressive stress when the depth of hardening layer is too deep, so the fatigue strength decreases when the depth of hardening layer increases And increase the Brittleness of the workpiece. General hardening depth (10 ~ 20% D). Is more appropriate, where D for the effective diameter of the workpiece.
3. Chemical Heat treatment carburizing: carburizing is the process of allowing carbon atoms to penetrate the surface layer of steel. Also is to make the low carbon steel workpiece with high carbon steel surface layer, and then quenching and low-temperature tempering, so that the surface layer of the workpiece with high hardness and wear resistance, while the central part of the workpiece still maintains the low carbon steel toughness and plasticity.