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40Cr quenching and tempering of workpiece quenching and tempering, various parameters of the process card are stipulated, we experience in the actual operation is:
(A) 40Cr parts should be used after oil quenching, 40Cr steel hardenability is better, in the oil cooling can be hardened, and the workpiece deformation, cracking tendency is small. However, in the case of tight oil supply, small enterprises can quench the water in workpieces with uncomplicated shapes and find no cracks. The operator must strictly control the temperature of the incoming and outgoing waters based on experience.
(2) The hardness of 40Cr parts after quenching and tempering is still high, and the second tempering temperature will increase by 20~50°C. Otherwise, the hardness will be reduced.
(C) 40Cr workpiece after high temperature tempering, the shape of the complex in the oil cooling, simple cooling in water, the purpose is to avoid the second type of temper brittleness. After quenching the workpiece after cooling, if necessary, apply stress relief treatment.
Affecting the quality of tempered parts, the level of operators is an important factor. At the same time, there are many reasons such as equipment, materials, and pre-conditioning. We believe:
(1) The workpiece is transferred from the furnace to the cooling tank at a slow speed. The temperature of the workpiece entering the water has dropped below the critical point of Ar3, resulting in partial decomposition. The workpiece is incompletely quenched and does not meet the hardness requirements. Therefore, the small parts of the cooling fluid should pay attention to the speed, large workpieces to be cold to grasp the time.
(2) The amount of the workpiece loading furnace should be reasonable. It is advisable to use 1 to 2 layers. The overlapping of the workpieces causes uneven heating, resulting in uneven hardness.
(3) The arrangement of the workpiece into the water shall be maintained at a certain distance, and the denseness of the workpiece will cause the vapor film in the vicinity of the workpiece to be broken, resulting in a low surface hardness of the workpiece.
(D) quenching furnace, can not be quenched in one go, should see the furnace temperature drop degree, the furnace closed to reheat, so that after the hardness of the workpiece before and after quenching consistent.
(e) Pay attention to the temperature of the coolant. If the temperature of 10% brine is higher than 60°C, it cannot be used. The coolant must not contain impurities such as oil, mud, etc., otherwise, there will be insufficient or uneven hardness.
(6) Hardness will not be uniform without roughening and tempering of raw materials. If good quality of quenching and tempering is to be achieved, rough blanks should be roughed and bar stock must be forged.
(7) Strictly control the quality of the product. After quenching, the hardness is low by 1 to 3 units. The tempering temperature can be adjusted to meet the hardness requirement. However, after quenching the workpiece hardness is too low, and some even HRC25~35 only, must be re-quenched, must not only apply medium or low temperature tempering in order to achieve the drawing requirements, otherwise, lost the meaning of tempering, and may have serious s consequence.
Cr increases the hardenability of the steel, improves the strength and tempering stability of the steel, and has excellent mechanical properties. For large or important quenched and tempered workpieces, Cr steel should be used. However, Cr steel has the second temper brittleness.