The heat treatment of the mold includes preheat treatment, terminal heat treatment and surface hardening treatment.
Usually heat treatment defects refer to various defects that occur during the terminal heat treatment of the mold or in subsequent processes and during use, such as quenching cracking, poor deformation, insufficient hardness, cracking in electromachining, grinding cracks, and early failure of the mold, etc.
The following is a more detailed analysis.
The causes of quenching crack and preventive measures are as follows:
- Shape effect.
It is mainly caused by design factors, such as too small rounded corners R, improper positioning of holes, and poor cross-section transition;
It is mainly caused by inaccurate temperature control or losing temperature, too high process temperature setting, and uneven furnace temperature.
Preventive measures include maintenance, proofreading of temperature control system, correction of process temperature, adding iron between workpiece and furnace floor, etc.
Decarburization is mainly caused by overheating (or overburning), unprotected heating of the air furnace, small machining margin, and residual decarburized layer in forging or preheat treatment.
Preventive measures are: controlled atmosphere heating, salt bath heating, vacuum furnace and box furnace using box protection or using anti-oxidation coating and increasing machining allowance by 2-3mm;
- Improper cooling.
It is mainly caused by improper selection of coolant or over cooling. You should master the cooling characteristics of the quenching medium or tempering treatment;
- The mold steel has poor structure
Such as severe segregation of carbides, poor forging quality, improper preheat treatment and so on.
The preventive measures are: adopt the correct forging process and reasonable preheat treatment system.
The reasons and precautions for insufficient hardness are as follows:
- The furnace or the way into the cooling tank is improper.
Correct the process temperature then overhaul and check the temperature control system.
When installing the furnace, the workpieces should be spaced reasonably, placed evenly, dispersed in the trough, and stacked or bundled into the trough for cooling is prohibited.
- The quenching temperature is too high
It is caused by improper process setting temperature or errors of temperature control system.
The process temperature should be corrected as well as the temperature control system should be overhauled and checked.
It is caused by the high tempering temperature setting, the failure error of temperature control system, or entering the furnace with high temperature.
The process temperature should be corrected as well as the temperature control system should be overhauled and checked. Enter not higher than the set furnace temperature.
- Improper cooling
The reason is that the pre-cooling time is too long, the cooling medium is not selected properly, the temperature of the quenching medium is gradually increased while the cooling performance is reduced, the stirring is poor or the outlet temperature of the tank is too high.
Solutions: get out of the oven and enter the tank quickly, master the cooling characteristics of the quenching medium, when the quenching amount is large to heat up the cooling medium, the cooling quenching medium should be added or cooled by other cooling tanks, strengthen the stirring of the coolant and take out at Ms + 50 °C.
It is caused by the residual decarburization layer of the raw materials or during the process of quenching and heating.
Preventive measures are:
Control the atmosphere and salt bath heating. Vacuum furnace and box furnace adopt box protection or use anti-oxidation coating. Increase the machining allowance by 2-3mm.
In mechanical manufacturing, the quenching deformation of heat treatment is absolute, while the no-deformation is relative.
In other words, it’s just a matter of size.
This is mainly due to the surface relief effect of martensite transformation during heat treatment.
Preventing heat treatment deformation (dimension and shape change) is a very difficult task and in many cases has to be solved by experience.
This is because not only the type of steel and the shape of the mold have an influence on the heat treatment deformation, but also the improper distribution of carbides and the method of forging and heat treatment will also cause or exacerbate.
And among the many conditions of heat treatment, as long as a certain condition changes, the degree of deformation of the steel piece will change greatly.
Although for quite a long time, the problem of heat treatment deformation was mainly solved by experience and heuristics.
However, it is extremely meaningful to correctly understand the relationship between mold steel forging, module orientation, mold shape, heat treatment method and heat treatment deformation, to grasp the heat treatment deformation law from the accumulated actual data, and to establish archives of heat treatment deformation.
Decarburization a kind of phenomenon and reaction that 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.
The 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, electrical discharge machining (electric pulse and wire cutting) is a more and more common processing method, but with the widespread application of electrical discharge machining, the defects caused by it have also increased accordingly.
Since electric discharge machining is a machining method that melts the surface of a mold by means of the high temperature generated by electric discharge, a white electric discharge machining deteriorative layer is formed on the machining surface and a 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, when adopting EDM molds, it is necessary to fully understand the impact of EDM on the mold steel and take corresponding preventive measures in advance:
1) Prevent overheating and decarburization during heat treatment, and fully temper to reduce or eliminate residual stress;
2) In order to fully eliminate the internal stress generated during quenching, high temperature tempering is required. Therefore, steels that can withstand high temperature tempering (such as DC53, ASP-23, high-speed steel, etc.) should be used for processing under stable discharge conditions;
3) After electrical discharge processing, stabilize the relaxation treatment;
4) Set reasonable process holes and slots;
5) Fully eliminate the re-solidified layer for use in a sound state;
6) Using the principle of vector translation, the internal stress of the cutting outpost has to be cut through drainage and dispersed.
The cause of insufficient toughness may be caused by excessively high quenching temperature and too long holding time causing grain coarsening. Or because tempering is not avoided in the brittle zone.
When there is a large amount of retained austenite in the workpiece, the tempering transformation occurs under the action of grinding heat, resulting in structural stress and the cracking of the workpiece.
The preventive measures are: do the cryogenic treatment after quenching, or repeat tempering (mould tempering is generally 2-3 times, even for low-alloy tool steels in cold working), to minimize the amount of retained austenite.