There are many reasons affecting the heat treatment deformation and crack of stamping dies, mainly related to the original organization, the chemical composition of steel, the heat treatment process, the structural shape and size of the parts, and many other factors.
Today, we talk about the 8 ways to improve heat treatment deformation and crack of stamping dies.
Generally, cracking is often preventable, but heat treatment deformation is always difficult to avoid.
In engineering practice, due to the differences in cross-sectional dimensions, the structure, and shape of the stamping die parts, the heat treatment (heating – holding – cooling) process due to the different rates of heating and cooling, and thus under the combined effect of thermal stress, tissue stress, and phase change volume changes and other factors, will lead to volume expansion or contraction of the parts, so that the die size and shape of a certain deviation and deformation, and even cause Cracking.
Firstly, preparatory heat treatment
For stamping die forgings made of eutectic steel, it is recommended first to normalize and then spheroid and anneal the forgings, the purpose of which is to eliminate the reticulated secondary carburized bodies in the forgings, refine the grains, eliminate the internal stress and prepare the organization for the subsequent (or final) heat treatment.
For stamping concave die parts, before their quenching, should be low-temperature tempering heat treatment (i.e. stabilization treatment).
For some of the more complex shapes, high precision requirements of the concave die parts, after rough machining and finishing before, should first use the tempering treatment, the purpose is to reduce the quenching deformation of the parts, try to avoid cracking tendencies, and for the final heat treatment process to prepare the organization.
Secondly, optimize the quenching and tempering treatment process
To improve heat treatment deformation and crack of stamping dies way 1. Tempering treatment control
Mold parts from the coolant after removal, should not stay in the air for a longer period of time and should be promptly put into the tempering furnace, tempering heat treatment.
Tempering treatment should avoid the occurrence of low-temperature tempering brittleness and high-temperature tempering brittleness. For some high precision requirements of the mold parts, it is recommended that after quenching for multiple tempering treatments, the purpose is to eliminate internal stress and reduce the deformation of features to avoid cracking tendencies.
To improve heat treatment deformation and crack of stamping dies way 2. Quenching parts protection
Quenching and tempering treatment, is a vital link affecting the heat treatment of stamping die parts deformation or cracking.
For quenching important mold parts (such as convex die, concave die, etc.), the parts prone to deformation or cracking, must take some effective protective measures, and strive to make the shape of the heat treatment parts and cross-sectional symmetry, so that the internal stress balance.
Commonly used protection methods are the filling method, bundle method, and blocking method.
To improve heat treatment deformation and crack of stamping dies way 3. Heating temperature determination
If the quenching heating temperature is too high, it will make the austenite grain coarse and will appear to be oxidation, and decarburization phenomenon, so that the part’s deformation and cracking tendency to increase.
In the specified heating temperature range, if the quenching heating temperature is low, it will cause shrinkage of the inner hole of the part, and the hole size becomes smaller, it is recommended to use the upper limit of the heating temperature specification;
For alloy steel parts, if the heating temperature is high, it will cause the expansion of the bore of the part, and the size of the bore becomes more extensive, therefore, it is recommended to use the lower limit of the heating temperature specification.
To improve heat treatment deformation and crack of stamping dies way 4. Improvement of the heating method
Some small stamping convex and concave die, or slender cylindrical parts (such as small punches, etc.), can be preheated to a certain temperature, such as 520-580 ℃, and then put into the medium temperature salt bath furnace.
When heated to a quenching temperature, then the direct use of an electric furnace or reflection furnace heating quenching, the parts of the deformation is significantly reduced and can control its cracking tendency, especially for those high alloy steel mold parts, the correct heating method is recommended: first preheat (temperature to 530-560 ℃), and then heated up to quenching temperature.
In the heating process, the high-temperature section should be as short as possible, the purpose is to reduce the quenching deformation of the parts and avoid the production of small cracks.
To improve heat treatment deformation and crack of stamping dies way 5. The choice of coolant
For alloy steel parts, the best way to reduce quenching deformation is to use potassium nitrate and sodium nitrite heat bath isothermal quenching method or graded quenching method.
This method is particularly suitable for dealing with more complex shapes, and dimensional accuracy requirements of high stamping die. For some porous die parts (such as porous concave die, etc.), the use of isothermal quenching time should not be too long, otherwise, it will cause the hole diameter or hole distance of the parts to become larger;
If the use of cooling in oil shrinkage, or nitrate in the cooling expansion characteristics, the reasonable use of dual-media quenching, can greatly reduce the deformation of the parts.
To improve heat treatment deformation and crack of stamping dies way 6. Optimization of cooling methods
When the parts from the heating furnace, into the coolant before, should be placed in the air, appropriate pre-cooling for a period of time, followed by quenching into the coolant, which is to reduce the quenching deformation of the parts, as well as prevent the elements from cracking tendency of one of the standard effective methods.
Mold parts into the coolant should be properly rotated, and the direction of rotation to change, so that the details are more conducive to maintaining a uniform cooling rate, which can significantly reduce the deformation of parts, and prevent cracking tendency.
To improve heat treatment deformation and crack of stamping dies way 7. Choice of coolant
For alloy steel, the best way to reduce quenching deformation is to use potassium nitrate and sodium nitrite heat bath isothermal quenching or graded quenching, this method is particularly suitable for dealing with complex shapes, and precise size requirements of the stamping die.
For some porous die parts (such as porous concave die), isothermal quenching time should not be too long, otherwise, it will cause the hole diameter or hole distance becomes large.
If the use of oil cooling shrinkage, as well as the characteristics of cooling expansion in the nitrate, the reasonable application of dual-media quenching, can reduce part deformation.
To improve heat treatment deformation and cracking of stamping dies way 8. Quenching treatment before the wire cutting
Some need to use wire-cutting processing of stamping die parts, before the wire-cutting process, should be used in the graded quenching and tempering (or high-temperature tempering) heat treatment process, the purpose is to improve the hardenability of the parts and make its internal stress distribution more uniform, and in a smaller internal stress state.
In general, the smaller the internal stress, the smaller the tendency of deformation and cracking of the parts after wire cutting.
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