One Quenching crack
(1) Shape effect. It is mainly caused by design factors, such as small fillet R, improper hole location, and poor section transition.
(2) Overheating (overburn). It is mainly caused by inaccurate temperature control or temperature run out, too high process setting temperature, and uneven furnace temperature. Preventive measures include: maintenance, calibration of temperature control system, correction of process temperature, and adding sizing iron between the workpiece and furnace floor.
(3) Decarbonization. It is mainly caused by overheating (or overburning), unprotected heating in the air furnace, small machining allowance, residual decarburization layer in forging or preparation heat treatment, etc. Preventive measures are: controlled atmosphere heating, salt bath heating, vacuum furnace and box furnace are protected by packing or anti oxidation coating is used; The machining allowance is increased by 2-3mm.
(4) Improper cooling. It is mainly caused by improper selection of coolant or undercooling, and the cooling characteristics of quenching medium or tempering treatment should be mastered.
(5) Raw materials are poorly organized. If the carbide segregation is serious, the forging quality is poor, and the preparation heat treatment method is improper, the preventive measures are to adopt correct forging process and reasonable preparation heat treatment system.
Two Insufficient hardness
(1) The quenching temperature is too low. It is mainly caused by improper process setting temperature, temperature control system error, improper furnace loading or entering the cooling tank method, etc. The process temperature should be corrected, the temperature control system should be checked and repaired, and the workpiece should be placed at reasonable intervals and evenly during furnace loading. The workpiece should be scattered into the tank, and it is prohibited to stack or bundle into the tank for cooling.
(2) The quenching temperature is too high. This is caused by improper process setting temperature or temperature control system error. The process temperature should be corrected, and the temperature control system should be overhauled and checked.
(3) Over tempering. This is caused by too high tempering temperature setting, fault error of temperature control system or furnace entry when the furnace temperature is too high. The process temperature should be corrected, the temperature control system should be checked and repaired, and the furnace temperature should not be higher than the setting temperature.
(4) Improper cooling. The reason is that the precooling time is too long, the cooling medium is not properly selected, the temperature of the quenching medium is gradually higher and the cooling performance is reduced, the stirring is poor or the temperature of the tank is too high. Measures: The furnace shall be discharged and put into the tank quickly; Master the cooling characteristics of quenching medium; The oil temperature is 60-80 ℃, and the water temperature is below 30 ℃. When the quenching amount is large and the cooling medium heats up, the cooling quenching medium should be added or other cooling tanks should be used for cooling; Strengthen the mixing of coolant; Take it out at Ms+50 ℃.
(5) Decarbonization. This is caused by the residual decarburization layer of raw materials or quenching heating. The preventive measures are controlled atmosphere heating, salt bath heating, vacuum furnace and box furnace using box protection or using anti oxidation paint; The machining allowance is increased by 2-3mm.
Deformation out of tolerance
In mechanical manufacturing, quenching deformation of heat treatment is absolute, while non deformation is relative. In other words, it is just a matter of deformation size. This is mainly due to the surface relief effect of martensitic transformation during heat treatment.
The prevention of heat treatment deformation is a very difficult work, which has to be solved by experience in many cases. This is because not only the steel type and die shape have an impact on the heat treatment deformation, but also the improper carbide distribution state and forging and heat treatment methods can cause or aggravate it. In many conditions of heat treatment, as long as a certain condition changes, the deformation degree of steel parts will change greatly.
Four Decarburization
Decarburization is the phenomenon and reaction that all or part of the carbon in the surface layer is lost due to the surrounding atmosphere during heating or insulation of steel parts. Decarburization of steel parts will not only cause insufficient hardness, quenching cracks, heat treatment deformation and chemical heat treatment defects, but also have a great impact on fatigue strength, wear resistance and die performance.
Five Cracks caused by EDM
In mold manufacturing, EDM (electric pulse and wire cutting) is a more and more popular processing method. However, with the wide application of EDM, the defects caused by it also increase correspondingly.
As EDM is a processing method to melt the mold surface by virtue of the high temperature generated by the discharge, a white EDM metamorphic layer is formed on the machined surface, and a tensile stress of about 800MPa is generated. In this way, defects such as deformation or cracks often appear in the EDM process of the mold.
Six Insufficient toughness
The reason for the insufficient toughness may be that the quenching temperature is too high and the holding time is too long, which causes the grain coarsening, or the tempering is not avoided in the tempering brittle zone.
Seven Grinding crack
When there is a large amount of retained austenite in the workpiece, tempering transformation will occur under the action of grinding heat, resulting in structural stress and workpiece cracking. The preventive measures are: after quenching, conduct deep cooling treatment or repeat tempering for many times (generally 2-3 times for mold tempering, even for low alloy tool steel used for cold working) to minimize the amount of retained austenite.