The modulation heat treatment process of 40Cr steel in gear manufacturing

Cr can increase hardenability, strength and tempering stability of steel, and has excellent mechanical properties. Cr steel shall be used for large section size or important quenched and tempered workpieces. But Cr steel has the second kind of temper embrittlement.

The quenching and tempering of 40Cr workpieces are specified in various parameter process cards. Our experience in actual operation is as follows:

(1) After quenching, 40Cr steel should be oil-cooled. The hardenability of 40Cr steel is better, and it can be hardened when it is cooled in oil, and the deformation and cracking tendency of the workpiece are small. However, in the case of tight oil supply, small enterprises can quench the workpieces with uncomplicated shape in water without any cracks, but the operator should strictly control the temperature of water in and water out by experience.

(2) After quenching and tempering, the hardness of 40Cr workpieces is still high. The second tempering temperature should be increased by 20 ~ 50 ℃, otherwise, it is difficult to reduce the hardness.

(3) After high temperature tempering of 40Cr workpiece, the complex shape is cooled in oil, and the simple shape is cooled in water, so as to avoid the influence of the second kind of tempering brittleness. If necessary, stress relief treatment shall be applied to the workpiece after tempering and quick cooling.

The quality of quenched and tempered workpieces is affected by the operator’s level. At the same time, there are many reasons such as equipment, materials and pre-processing of quenched and tempered workpieces

(1) The speed of workpiece transferring from heating furnace to cooling tank is slow, the temperature of workpiece entering water has dropped below the critical point of Ar3, resulting in partial decomposition, and the workpiece has incomplete quenching structure, which can not meet the hardness requirements. Therefore, the cooling liquid of small parts should pay attention to speed, and the precooling of large workpieces should master time.

(2) The charging capacity of the workpiece should be reasonable, with 1-2 layers as appropriate. The overlapping of the workpieces causes uneven heating and uneven hardness.

(3) A certain distance should be kept for the arrangement of the workpiece into the water. Too close to the workpiece, the vapor film rupture is blocked, resulting in the low hardness of the near surface of the workpiece.

(4) Open furnace quenching can not be completed in one breath. Depending on the furnace temperature drop, the furnace shall be closed in the middle to raise the temperature again, so that the hardness of the front and rear workpieces after quenching is the same.

(5) Pay attention to the temperature of the coolant. If the temperature of 10% brine is higher than 60 ℃, it cannot be used. The coolant shall be free of oil, mud and other impurities, otherwise, the hardness will be insufficient or uneven.

(6) If we want to get good quality, the rough should be rough turned and the bar should be forged.

(7) Strictly control the quality, the hardness is 1 ~ 3 units lower after quenching, and the tempering temperature can be adjusted to meet the hardness requirements. But after quenching, the hardness of the workpiece is too low, and some of them are only HRC25 ~ 35. It must be re quenched. It is not allowed to use only medium temperature or low temperature tempering to meet the requirements of the drawing. Otherwise, the significance of quenching and tempering is lost and serious consequences may occur.