During the heat treatment process, the reworked parts must be normalized, and the structural parts that require mechanical performance indicators must be quenched and tempered after normalizing to meet the mechanical performance requirements. Medium and high alloy steels and large forgings must be tempered at high temperature after normalizing to eliminate the internal stress generated during normalizing.
Some alloy steels undergo partial martensitic transformation during forging to form hard structures. In order to eliminate this kind of bad tissue, when normalizing is adopted, the normalizing temperature is about 20°C higher than the normal normalizing temperature, and it is heated and kept for normalizing.
1 .Multi-process processing
NSK bearing parts have high requirements, and there must be many production processes. Generally, bearing production requires 20 to 40 processes, and more than 70 processes.
The working surface of the bearing parts is a rotary forming surface, which is suitable for processing by forming method. For example, the forging, rolling and turning grinding of the raceway of the ring are all processed by forming tools or anti-type plates.
3 .Precision Machining
Most of the surfaces of NSK bearing parts have to be ground, and the grinding size and geometric accuracy are in μM. In particular, the precision of the raceways and rolling elements of the rings is higher, and ultra-finishing or grinding is required.
1) Annealing cooling method: When steel is annealed, it is generally cooled with the furnace to below 600-550 °C and then air-cooled after being released from the furnace.
2) Normalizing cooling method: when steel is normalizing, it is generally cooled in air.
3) Quenching cooling method: When steel is quenched, the cooling rate of steel in the most unstable range of supercooled austenite (650-550°C) should be greater than the critical cooling rate, so as to ensure that the workpiece does not transform into pearlite structure; while in The cooling rate near the Ms point should be as low as possible, so as to reduce the internal stress of quenching and reduce the deformation and cracking of the workpiece. Therefore, when quenching, in addition to selecting a suitable quenching cooling medium, the quenching method should also be improved. For workpieces with simple shapes, a simple single-liquid quenching method is often used. For example, water or brine is used as the cooling medium for carbon steel, and oil is often used as the cooling medium for alloy steel.
4) Tempering cooling method: when carbon steel is tempered, it is generally cooled in air.
The normalizing process is a heat treatment process in which the steel is heated to 30-50°C above Ac3 (or Acm), held for an appropriate time, and then cooled in still air. Normalizing that heats the steel to 100-150°C above Ac3 is called high-temperature normalizing.
For medium and low carbon steel castings and forgings, the main purpose of normalizing is to refine the structure. Compared with annealing, after normalizing, the pearlite sheets are finer and the ferrite grains are also smaller, so the strength and hardness are higher.
Because the hardness of low carbon steel is too low after annealing, the knife will stick to the knife during cutting, and the cutting performance is poor. The hardness can be increased by normalizing, which can improve the cutting performance. Some medium carbon structural steel parts can be normalized instead of quenched and tempered, simplifying heat treatment craft.