Heat Treatment Defects In Molds: Causes & Preventive Measures

The heat treatment of the mold includes preliminary heat treatment, final heat treatment and surface strengthening treatment.

Generally, heat treatment defects refer to the various defects that occur in the final heat treatment process or in the subsequent processes and use of the mold, such as quenching cracks, deformation overproof, insufficient hardness, electrical machining cracks, grinding cracks and early failure of molds etc.

Heat Treatment Defects In Molds

A more detailed analysis is as followed.

I. Quenching  cracking

The causes and preventive measures of quenching cracking are as follows:

Shape effect

It is mainly caused by design factors, for example, if the fillet R is too small, the hole position is not set properly, and the cross-section transition is not good;

Overheating (overburning)

It is mainly caused by factors such as inaccurate temperature control or running temperature, excessively high process setting temperature and uneven furnace temperature etc.

Preventive measures include overhauling, proofreading the temperature control system, correcting the process temperature and adding a shim between the workpiece and the furnace floor etc.;

Decarbonization

It is mainly caused by factors such as overheating (or overburning), unprotected heating of air furnaces, small machining margin, and residual decarburization layer from forging or preliminary heat treatment etc.

Preventive measures are: controllable atmosphere heating, salt bath heating, boxing protection for vacuum furnaces and box furnaces or use of anti-oxidation coatings; machining allowance is increased by 2-3mm;

Improper cooling

It is mainly caused by the improper selection of coolant or excessive cooling and should master the quenching medium cooling characteristics or tempering treatment;

Poor organization of mold steel

Such as severe segregation of carbides, poor forging quality and improper preparation heat treatment methods etc.

The preventive measures are: To use the correct forging process and a reasonable pre-heat treatment system.

II.  Insufficient hardness

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

1) It is caused by furnace or incorrect method into the cooling tank. which should correct the process temperature and calibrate the temperature control system.

When installing the furnace, the workpieces should be spaced reasonably, placed evenly, and dispersed into the tank.

It is forbidden to pile up or bundle 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 calibrated.

3) Over tempering, which is caused by the high tempering temperature setting, the temperature control system fault error or the high furnace temperature.

The process temperature should be corrected, and the temperature control system should be calibrated, and the furnace temperature should not be higher than the set furnace temperature.

4) Improper cooling: The reason is long the pre-cooling time, improper selection of cooling medium, gradually higher quenching medium temperature, decreased cooling performance, poor mixing or relative high tank temperature etc.

Preventive measures are: to be fast out of the furnace or into the tank etc.; to master quenching medium cooling characteristics;

In the case of oil temperature 60-80 ℃ and water temperature 30 ℃ below, when the quenching volume and the cooling medium temperature rise, it should add cooling quenching medium or change to other cooling tanks for cooling;

Intensify stirring of coolant; remove at Ms+50°C.

5) Decarburization, which is caused by residual decarburization of the raw material or when quenching and heating.

Preventive measures are: controlled atmosphere heating, salt bath heating, vacuum furnaces, chamber furnaces with case protection or oxidation-proof coatings; the machining allowance is increased by 2-3mm.

III. Deformation superbad

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 steel type and the die shape have an impact on the heat treatment deformation, improper carbide distribution and forging and heat treatment methods can also cause or aggravate it.

Moreover, in many heat treatment conditions, as long as a certain condition changes, the content of deformation of the steel will change greatly.

For a long time, experience and trial methods are mainly used to solve the problem of heat treatment deformation, but the relationship between die steel forging, module orientation, die shape, heat treatment method and heat treatment deformation needs to be correctly mastered.

It is very meaningful work to mastered the heat treatment deformation law from the accumulated actual data and establishes the file information about the heat treatment deformation.

IV. Decarbonization

Decarburization is the phenomenon and reaction in which all or part of the carbon in the surface layer of a steel part is lost due to the action of the surrounding atmosphere when the part is heated or kept warm.

Decarburization of steel parts will not only cause insufficient hardness, quenching cracking, heat treatment deformation and chemical heat treatment defects, but also have a great impact on fatigue strength, wear-resistance and die performance.

V. Cracks caused by electrical discharge machining

In mold manufacturing, the use of electrical discharge machining (electric pulse and wire cutting) is a commonly used method.

But with the widespread application of electrical discharge machining, the number of defects caused by the corresponding increase.

Since electrical discharge machining is a processing method that melts the mold surface by means of the high temperature generated by electrical discharge, a white electrical discharge machining deterioration layer is formed on the processed surface and tensile stress of about 800 MPa is generated.

In this way, defects such as deformation or cracks often occur during the electrical processing of the mold.

Therefore, the use of electrical discharge machining molds must fully master the impact of electrical discharge machining on the mold steel, and take corresponding preventive measures in advance:

1) Prevent overheating and decarburization during heat treatment, and perform sufficient tempering to reduce or eliminate residual stress;

2) In order to fully eliminate the internal stress generated during quenching, high temperature tempering is required.

Therefore, steel grades that can withstand high temperature tempering (such as DC53, ASP-23 and high-speed steel etc.) should be used for processing under stable discharge conditions;

3) After EDM, stabilization and relaxation treatment are required;

4) Set reasonable process holes and grooves;

5) Fully eliminate the resolidified layer so that it can be used in health status;

(6) Using the vector translation principle, the cutting outpost has been concentrated part of the internal stress, to cut through the drainage and scattered release.

VI. Lack of toughness

The lack of toughness may be caused by the high quenching temperature, long holding time caused by grain coarsening, or because there is no tempering brittle zone to avoid tempering.

VII. Grinding cracks

When there is a large number of residual austenite in the workpiece, tempering transformation occurs, resulting in tissue stress, causing cracking of the workpiece under the action of grinding heat.

Preventive measures are: after quenching for sub-zero treatment or repeated repeatedly tempering (die tempering is generally 2-3 times, even cold working with low alloy tool steel is also the same), the amount of residual austenite is minimizing to the largest extent.

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