Heat treatment of bearing steel ball surface

- Oct 31, 2019-

1. Induction heating surface quenching

Definition: It is a heat treatment operation method that uses a certain method to generate a certain frequency of induced current on the surface of the workpiece, rapidly heats the surface of the part, and then rapidly quenches and cools.

Principle: As shown in the figure, electromagnetic induction produces an induced current of the same frequency, that is, eddy current. The distribution of eddy currents on the cross section of the workpiece is not uniform, the core is almost equal to zero, and the surface current density is extremely large, called the “skin effect”. The higher the frequency, the thinner the surface layer with the highest current density. Relying 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 is still close to room temperature, and then immediately sprayed with water to cool the surface of the workpiece.


Classification: Induction heating can be divided into three categories:

(1) High frequency heating

The commonly used frequency is (200-300) KHZ, and the depth of the hardened layer is (0.5-2.5) mm.

(2) Intermediate frequency heating

The commonly used frequency is (2500 ~ 8000) HZ, and the depth of the hardened layer is (2 ~ 10) mm.

(3) Power frequency heating

The current frequency is 50HZ, no frequency equipment is needed, the city can use AC power, the hard layer depth is (10~20)mm or more, and the city uses AC power.


Advantages and Disadvantages: Induction heating surface quenching heating speed, high productivity, heating temperature and depth of hardened layer are easy to control, steel ball surface oxidation and decarburization are less, steel ball deformation is small, can make all quenching process mechanized and automated.


2. Inductive surface quenching performance

(1). Surface hardness: The surface hardness of workpieces subjected to high- and medium-frequency induction heating surface quenching is often 2 to 3 units (HRC) higher than ordinary quenching.


(2). Abrasion resistance: The wear resistance of the workpiece after induction hardening is higher than that of ordinary quenching. This is mainly due to the fineness of the hardened martensite grains, high carbide dispersion, high hardness, and high compressive stress on the surface.


(3). Fatigue strength: High and medium frequency surface quenching greatly improves the fatigue strength and reduces the notch sensitivity. For the workpiece of the same material, the depth of the hardened layer is within a certain range, and the fatigue strength increases as the depth of the hardened layer increases. However, when the depth of the hardened layer is too deep, the surface layer is compressive stress, so the depth of the hardened layer increases the fatigue strength and reduces the workpiece. Increased brittleness. Generally, the hardened layer has a depth δ=(10-20)%D. More suitable, where D is the effective diameter of the workpiece.


3. Chemical heat treatment

Carburizing: Carburizing refers to the process of infiltrating carbon atoms into the surface layer of steel. It also makes the low carbon steel workpiece have the surface layer of high carbon steel, and after quenching and low temperature tempering, the surface layer of the workpiece has high hardness and wear resistance, while the central part of the workpiece still maintains the toughness of low carbon steel. Plasticity.