Technology

Common defects in mold heat treatment

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The vacuum heat treatment of the mold includes preliminary heat treatment, final heat treatment and surface strengthening treatment. Generally, heat treatment defects refer to various defects that occur in the final heat treatment process or in the subsequent processes and use of the mold, such as quenching cracks, out-of-tolerance deformation, insufficient hardness, electrical machining cracks, grinding cracks, and early mold failure Wait. Let me learn more about these defect prevention measures with the editor below!

Quenching

The causes of quenching and preventive measures are as follows:

  • 1. The shape effect is mainly caused by design factors, such as too small fillet R, improper placement of holes, and poor cross-sectional transition.
  • 2. Overheating (overburning) is mainly caused by inaccurate temperature control or running temperature, irregular and unreasonable vacuum heat treatment process, especially insufficient tempering. The setting temperature is too high, the furnace temperature is uneven and other factors. Preventive measures include maintenance, proofreading the temperature control system, correcting the process temperature, and adding a shim between the workpiece and the furnace floor.
  • 3. Decarburization is mainly caused by factors such as overheating (or overburning), unprotected heating in air furnaces, small mechanical addition, and residual decarburization layers from forging or preliminary heat treatment. The preventive measures are controlled atmosphere heating and salt bath heating. Vacuum furnaces and box furnaces are protected by packing or anti-oxidation coatings; machining allowance is increased by 2 to 3 mm.
  • 4. Improper cooling is mainly caused by improper selection of coolant or overcooling. The cooling characteristics of quenching medium or tempering treatment should be mastered.
  • 5. Poor organization of raw materials, such as severe segregation of carbides, poor forging quality, improper preparation heat treatment methods, etc. The preventive measures are to use correct forging processes and reasonable preparation heat treatment systems.

Insufficient hardness

The reasons and preventive measures for insufficient hardness are as follows:

  • 1. The quenching temperature is too low, which is mainly caused by improper process setting temperature, temperature control system error, improper method of installing the furnace or entering the cooling tank, etc. The process temperature should be corrected, the temperature control system should be overhauled, and the workpiece spacing during furnace installation Place them reasonably and evenly, disperse them into the tank, and prohibit stacking or bundles into the tank for cooling.
  • 2. The quenching temperature is too high, which 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 checked and repaired.
  • 3. Over tempering, which is caused by the tempering temperature setting is too high, the temperature control system fault error or 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 is not higher than the set furnace temperature. Into.
  • 4. Improper cooling, the reason is that the pre-cooling time is too long, the cooling medium is selected improperly, the temperature of the quenching medium is getting higher and the cooling performance is reduced, the stirring is not good or the temperature of the tank is too high, etc., measures: the furnace, the tank, etc. should be fast; master the quenching medium Cooling characteristics; oil temperature 60~80℃, water temperature below 30℃, when the quenching volume is large and the cooling medium is heated up, cooling quenching medium should be added or other cooling tanks should be used for cooling; strengthen the stirring of the coolant; at Ms+50℃ When taken out.
  • 5. Decarburization, which is caused by the residual decarburization layer of raw materials or during quenching and heating. The preventive measures are controlled atmosphere heating, salt bath heating, vacuum furnaces, box furnaces with boxing protection or use of anti-oxidation coatings; Increase the amount by 2~3mm.

Out of tolerance

In mechanical manufacturing, the 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 martensite transformation during heat treatment. Preventing heat treatment deformation (dimension change and shape change) is a very difficult task, and in many cases it has to be solved by experience. This is because not only the steel grade and the shape of the die have an effect on the heat treatment deformation, but also the improper carbide distribution and forging and heat treatment methods can also cause or aggravate it. In addition, among the heat treatment conditions, as long as a certain condition changes, the deformation of the steel The degree will vary greatly. Although for a long time, experience and heuristic methods are mainly used to solve the heat treatment deformation problem, but the relationship between raw material forging, module orientation, mold shape, heat treatment method and heat treatment deformation is correctly grasped, and the heat treatment deformation law can be grasped from the accumulated actual data , It is a very meaningful work to establish file information about heat treatment deformation.

Decarburization

Decarburization is due to the phenomenon and reaction that part of the carbon in the surface layer is completely or partially lost due to the effect of the surrounding atmosphere when the steel is heated or insulated. Decarburization of steel parts will not only cause insufficient hardness, quench cracking, heat treatment deformation and chemical heat treatment defects, but also have a great impact on fatigue strength, wear resistance and mold performance.

Cracks caused by electrical discharge machining

In mold manufacturing, the use of electrical discharge plastic machining (electric pulse and wire cutting) is an increasingly common processing method, but with the wide application of electrical discharge machining, the defects caused by it increase accordingly. Since electrical discharge machining is a processing method that melts the mold surface by means of the high temperature generated by the electrical discharge, a white electrical discharge machining deterioration layer is formed on the processed surface, and a tensile stress of about 800 MPa is generated. In this way, the electrical machining process of the mold Defects such as deformation or cracks often appear in the battery. Therefore, the use of electrical discharge machining molds must fully understand the impact of electrical discharge machining on mold materials and take corresponding preventive measures in advance. To prevent overheating and decarburization during heat treatment, and fully temper to reduce or eliminate residual stress; in order to fully eliminate the internal stress generated during quenching, high temperature tempering is required, so steel grades that can withstand high temperature tempering (such as Crl2 type, ASP-23, high-speed steel, etc.), processing under stable discharge conditions; after discharge processing, stabilize and relax treatment; set reasonable process holes and grooves; fully eliminate the resolidified layer to be in a healthy state Use under; use the principle of vector translation to disperse and release the internal stress of the concentrated part of the cutting outpost.

Insufficient resilience

The reason for the insufficient toughness may be that the quenching temperature is too high, and the holding time is too long to cause the grain coarsening, or the tempering brittle zone is not avoided.

Grinding cracks

When there is a large amount of retained austenite in the workpiece, tempering transformation occurs under the action of the grinding heat, resulting in structural stress and causing the workpiece to crack. The preventive measures are: cryogenic treatment or repeated tempering after quenching (tempering of the mold is generally 2 to 3 times, even for low-alloy tool steel for cold working) to minimize the amount of retained austenite.

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