Causes and treatment of heat treatment defects of die steel
Table of Contents
Shape effect. It is mainly caused by design factors, such as too small fillet R, improper hole location and poor section transition.
Overheating (overburning). It is mainly caused by inaccurate temperature control or running temperature, too high process setting temperature and unequal furnace temperature. Preventive measures include maintenance, calibration of temperature control system, correction of process temperature, addition of sizing block between workpiece and furnace bottom plate, etc.
Decarburization. It is mainly caused by overheating (or overburning), unprotected heating of air furnace, small machining allowance, residual decarburization layer after forging or preliminary heat treatment. The preventive measures are: controllable atmosphere heating, salt bath heating, packing protection or anti-oxidation coating for vacuum furnace and box furnace; The machining allowance shall be increased by 2-3mm.
Improper cooling. It is mainly caused by improper selection of coolant or undercooling. We should master the cooling characteristics of quenching medium or tempering treatment.
Poor organization of raw materials. For example, serious carbide segregation, poor forging quality and improper preliminary heat treatment methods, the preventive measures are to adopt correct forging process and reasonable preliminary heat treatment system.
Quenching temperature is too low. It is mainly caused by improper process temperature setting, temperature control system error, improper method of charging or entering the cooling tank, etc. the process temperature should be corrected, the temperature control system should be overhauled and verified. During charging, the workpieces should be placed evenly at reasonable intervals and scattered into the tank. It is forbidden to stack or bundle into the tank for cooling.
Quenching temperature is too high. This is caused by improper process temperature setting or temperature control system error. The process temperature shall be corrected and the temperature control system shall be overhauled and checked.
Over tempering. This is caused by too high tempering temperature setting, fault error of temperature control system or entering the furnace when the furnace temperature is too high. The process temperature shall be corrected, the temperature control system shall be overhauled and checked, and it shall not be higher than the set furnace temperature.
Improper cooling. The reason is that the precooling time is too long, the cooling medium is improperly selected, the temperature of the quenching medium increases gradually, the cooling performance decreases, the mixing is poor or the outlet temperature is too high. Measures: discharge the furnace and enter 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 rises, the cooling quenching medium shall be added or cooled by other cooling tanks; Strengthen the mixing of coolant; Remove at MS + 50 ℃.
Decarburization. This is caused by residual decarburization layer of raw materials or quenching and heating. The preventive measures are controllable atmosphere heating, salt bath heating, packing protection or anti-oxidation coating for vacuum furnace and box furnace; The machining allowance shall be increased by 2-3mm.
Deformation out of tolerance
In mechanical manufacturing, quenching deformation of heat treatment is absolute, while no deformation is relative. In other words, it’s just a matter of deformation size. This is mainly due to the surface convexity effect of martensitic transformation during heat treatment.
Preventing 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 grade and die shape have an impact on the heat treatment deformation, but also the improper carbide distribution and forging and heat treatment methods will cause or aggravate it. Moreover, in many conditions of heat treatment, as long as a certain condition changes, the deformation degree of steel parts will change greatly.
Decarburization is the phenomenon and reaction that the carbon in the surface layer is completely or partially lost due to the action of the surrounding atmosphere when the steel parts are heated or insulated. Decarburization of steel parts will not only cause insufficient hardness, quenching crack, heat treatment deformation and chemical heat treatment defects, but also have a great impact on fatigue strength, wear resistance and die properties.
Cracks caused by EDM
In mold manufacturing, electrical discharge machining (electric pulse and wire cutting) is a more and more common machining method, but with the wide application of electrical discharge machining, the defects caused by it are also increasing.
Because EDM is a processing method that melts the die surface with the help of the high temperature generated by 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 crack often appear in the EDM process of the die.
The reason for insufficient toughness may be that the quenching temperature is too high and the holding time is too long, resulting in grain coarsening, or because the tempering brittle zone is not avoided.
When there is a large amount of retained austenite in the workpiece, tempering transformation occurs under the action of grinding heat, resulting in structural stress and workpiece cracking. The preventive measures are: deep cooling treatment or repeated tempering after quenching (generally 2-3 times for die tempering, even for low alloy tool steel for cold working), so as to minimize the amount of retained austenite.
Source: China Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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