There are many factors affecting the heat treatment deformation and cracking of Dongguan punching die, which are mainly related to the chemical composition of the original structure and steel, the structure shape of the parts, the size of the section and the heat treatment process. Cracking is often preventable, but heat treatment is always difficult to avoid. In practice, the difference in section size, structure shape of stamping die parts, heat treatment (heating - heat preservation - cooling) is different in the process of heating and cooling. The volume expansion or contraction of parts is caused by the combination of thermal stress, tissue stress and change of phase change volume, which makes the size and shape deviate, Deformation or even cracking.
1. Preparatory heat treatment
For the Dongguan stamping die forgings of eutectoid steel, normalizing should be carried out first and then spheroidizing, in order to eliminate the two carburized carburizing body inside the forgings, to refine the grain and eliminate the internal stress, and to prepare for the subsequent (or final) heat treatment. Before quenching, the stamping die should be tempered at low temperature (stabilized). Some concave die with more complex shape and high precision should be tempered before rough machining in order to reduce the quenching deformation, avoid cracking tendency as much as possible, and prepare for the final heat treatment.
Two. Optimize the process of quenching and tempering
Control of 1 tempering treatment
After the Dongguan stamping dies are removed from the coolant, they should not stay in the air for a longer time. They should be placed in the tempering furnace for tempering. Temper temper brittleness and high temperature temper brittleness should be avoided during tempering treatment. For some precision parts of die and mould, multiple tempering is used after quenching to eliminate internal stress, reduce deformation and avoid cracking tendency.
2 protection of quenching parts
Quenching and tempering are important links that affect the heat treatment deformation or cracking of stamping die parts. For important parts of quench die parts (such as punch and die) which are easily deformed or cracked, effective protective measures should be taken to make the shape of the parts and the cross section symmetrical and the internal stress balanced. The commonly used methods of protection are as follows: a. Bale method; b. Filling method; C. Blocking law.
The determination of 3 heating temperature
The high temperature of quenching and heating makes the austenite grain coarse and will cause oxidation and decarburization, and the tendency of deformation and cracking will increase. Within the prescribed heating temperature range, the lower the quenching heating temperature, the smaller the pore size and the smaller the pore size. Therefore, the upper limit planting of the heating temperature should be chosen, and for the alloy steel, the high heating temperature will cause the expansion of the inner hole and the size of the pore size, so the lower limit of the heating temperature should be chosen.
The improvement of 4 heating mode
For some small punching and concave die or slender cylindrical parts (such as small punch), it can be preheated to 520--580 C in advance and then heated to the quenching temperature in the medium temperature salt bath furnace, and the deformation of the quenching part is obviously reduced than that of the direct electric furnace or the reflecting furnace, and the cracking tendency can be controlled. In particular, high alloy steel mold parts, the correct heating method is: first preheating (temperature 530--560 degrees), and then rise to quenching temperature. During the heating process, the high temperature time should be shortened as far as possible, so as to reduce the quenching deformation and avoid the production of small cracks.
Selection of 5 coolant
For alloy steel, the best way to reduce the quenching deformation is the use of potassium nitrate and sodium nitrite hot bath isothermal quenching or graded quenching. This method is especially suitable for dealing with complex and accurate stamping dies. For some porous mold parts, such as porous die, the isothermal quenching time should not be too long, otherwise the aperture or hole spacing will increase. If the cooling shrinkage of oil and the characteristics of cooling expansion in nitrate are used, the deformation of parts can be reduced by the proper application of double medium quenching.
Quenching treatment before 6 wire cutting
For the stamping die parts for wire cutting, the process of heat treatment should be used before the cutting process, such as graded quenching and multiple tempering (or high temperature tempering), in order to improve the hardenability of the parts, and make the distribution of internal stress tend to be uniform, and in a small internal stress state. The smaller the internal stress, the smaller the tendency of deformation and cracking after wire cutting.
Selection of 7 coolant
For alloy steel, the best way to reduce the quenching deformation is the use of potassium nitrate and sodium nitrite hot bath isothermal quenching or graded quenching. This method is especially suitable for dealing with complex and accurate stamping dies. For some porous mold parts, such as porous die, the isothermal quenching time should not be too long, otherwise the aperture or hole spacing will increase. If the cooling shrinkage of oil and the characteristics of cooling expansion in nitrate are used, the deformation of parts can be reduced by the proper application of double medium quenching.
Optimization of 8 cooling mode
It is one of the effective methods to reduce the quenching deformation of the parts and prevent the cracking tendency of the parts when the parts are removed and put into the reheating furnace before they are put into the cooling agent. After the mold parts are put into the coolant, it should be rotated properly and the rotation direction changes. This will help the parts keep the uniform cooling rate, which can obviously reduce the deformation and prevent the cracking tendency.