Deformation of mold heat treatment is one of the main defects in the mold treatment process. For some complex and complicated molds, they are often discarded due to heat treatment deformation. Therefore, controlling the deformation of a complex and complicated mold has become a key issue in heat treatment production.
As we all know, the mold in the heat treatment, especially in the quenching process, due to the various parts of the mold section heating and cooling rate caused by inconsistent temperature difference, combined with the unequal time and other organizational changes and other reasons, making the mold section of each part of the volume expansion and contraction Uneven, non-uniform organizational changes, resulting in "tissue stress" and mold temperature difference between inside and outside caused by thermal stress. When the internal stress exceeds the yield limit of the mold, it will cause deformation of the mold.
Therefore, reducing and controlling the deformation of complex and complicated molds is an important research topic for heat treatment workers.
This article attempts to study the deformation and deformation of precision and complicated molds, and discusses the measures to reduce and control the deformation of precision and complicated molds so as to improve the quality and service life of the molds.
First, the impact of mold materials
1 mold selection
A machinery plant from the material selection and heat treatment simple consideration, choose T10A steel manufacturing cross-section size difference between the requirements of less deformation after quenching more complex mold, hardness requirements 56HRC-60HRC. Die hardness after heat treatment in line with technical requirements, but the larger mold deformation, can not be used, resulting in mold scrapped. Later, the plant was made of micro-deformed steel Cr12 steel, mold hardness and deformation after the heat treatment are in line with the requirements.
Therefore, the manufacture of sophisticated precision, smaller deformation requirements of the mold, we should try to use micro-deformation of steel, such as air-quenched steel.
2. Mold material effects
A factory sent a number of more complex Cr12MoV steel mold, mold with ¢ 60mm round hole, mold heat treatment, some mold oval ellipse, resulting in mold scrapped.
In general Cr12MoV steel is micro-deformed steel, should not appear larger deformation. Metallographic analysis of the heavily deformed molds revealed that the mold steel contained a large amount of eutectic carbides and was banded and agglomerated.
(1) mold oval (deformation) causes
This is because the presence of uneven distribution of carbide carbide in the die steel, carbide expansion coefficient of 30% less than the steel matrix organization, when it is heated to prevent the mold hole expansion, cooling and then prevent the mold cavity contraction , So that uneven deformation of the mold bore, the mold of the oval hole.
(2) Preventive measures
① in the manufacture of sophisticated precision molds, we should try to choose a smaller segregation of carbide die steel, do not figure cheap, the use of small steel production of poor quality steel. ② severe segregation of existing carbide die steel to be reasonable forging, to break carbide ingots, reducing the uneven distribution of carbide grade, to eliminate the anisotropy of performance. ③ After forging steel to be quenched and tempered heat treatment, so that the distribution of carbide uniform, small and diffuse sorbite, thereby reducing the complexity of complex mold heat treatment after the deformation. ④ For the larger size or can not forging the mold, the solution can be used double refinement of the carbide refinement, distribution, angular rounded, can reduce the deformation of the mold heat treatment purposes.
Second, the impact of mold structure design
Some mold material selection and steel are very good, often because of the mold design is not reasonable, such as thin edges, sharp corners, grooves, abrupt steps, thickness disparity, resulting in deformation of the mold after heat treatment larger.
1, the reason for the deformation
Due to the thickness of the mold uneven or sharp fillet, so in the quenching caused by the various parts of the mold between the thermal stress and the stress of different organizations, resulting in different parts of the volume expansion, the mold after quenching deformation.
2, preventive measures
Design mold, to meet the actual needs of the production situation, should minimize mold thickness disparity, the structure is not symmetrical, at the junction of the thickness of the mold, as far as possible the use of smooth transition design. According to the deformation of the mold, set aside allowance, after quenching will not die because of mold deformation and scrapped.
Particularly complex shape of the mold, to quench the cooling uniform, can be used to give the structure.
Third, the mold manufacturing process and the impact of residual stress
Often found in the factory, some complex shape, high precision molds, deformation after heat treatment larger, after careful investigation found that the mold in the mechanical processing and the final heat treatment without any pre-heat treatment.
1, deformation reasons
In the machining process of residual stress and quenching stress superimposed, increasing the deformation of the mold after heat treatment.
2, preventive measures
(1) After roughing, a stress relief annealing should be carried out before semi-finishing, that is, (630-680) ℃ × (3-4) h furnace cooling to below 500 ℃ air cooling, 400 ℃ × (2- 3) h to stress treatment.
(2) reduce the quenching temperature, reduce the residual stress after quenching.
(3) The oil quenching 170 ℃ C air-cooled (graded quenching).
(4) The use of isothermal quenching process can reduce quenching residual stress.
The above measures can be used to reduce the residual stress quenching mold, mold deformation smaller.
Fourth, the impact of heat treatment heating process
1, the impact of heating rate
Die deformation after the heat treatment are generally considered to be caused by cooling, which is not correct. Die especially the complex mold, the processing technology is correct or not, the deformation of the mold often has a greater impact on some mold heating process can be clearly seen, the heating speed is faster, often have greater deformation.
(1) the cause of deformation of any metal heating to be expanded, because the steel in the heating, the same mold, all parts of the uneven temperature (ie, uneven heating) will inevitably cause the expansion of various parts of the mold inconsistent So as to form the internal stress due to uneven heating. In the steel below the phase transition temperature, uneven heating mainly produces thermal stress, over the phase transition temperature heating uneven, but also produce unequal time-varying tissue, both to produce tissue stress. Therefore, the faster heating rate, the mold surface and the heart of the greater the difference between the temperature, the greater the stress, mold deformation after heat treatment greater.
(2) Precaution - For complex molds, heat slowly when heated below the phase transition point. In general, mold vacuum heat treatment deformation is much less than heating and quenching of salt bath furnaces. Preheating can be used for low alloy steel mold preheat (550 ℃ -620 ℃); for high alloy die should be used twice preheating (550 ℃ -620 ℃ and 800 ℃ -850 ℃).
2, the impact of heating temperature
Some manufacturers in order to ensure the mold to achieve a higher hardness, that the need to improve the quenching heating temperature. However, production practice shows that this approach is not appropriate, for complex molds, the same is the case with the normal heating temperature quenching, the maximum allowable temperature after the heat treatment deformation than the allowable minimum temperature heating heat treatment deformation Much bigger.
(1) the cause of deformation As we all know, the higher the quenching temperature, the steel grains grow more, due to larger grains can increase the hardenability, then quench cooling the greater the stress. Furthermore, since most of the complicated molds are made of medium-high alloy steel, if the quenching temperature is high, the amount of retained austenite in the structure is increased due to the low Ms point, and the deformation after the mold heat treatment is increased.
(2) Preventive measures In the case of the mold to ensure that the reasonable choice of heating temperature, try to use the lower quenching temperature, to reduce the cooling stress, thereby reducing the complex heat treatment deformation.
Fifth, the impact of residual austenite
Some high alloy die steel, such as Cr12MoV steel mold quenching and tempering at low temperature, the mold length, width, height are shrinkage phenomenon occurs, it is because the mold after quenching excessive austenite caused.
1, deformation reasons
Because alloy steel (such as Cr12MoV steel) after quenching contains a large number of retained austenite, a variety of organizations have different than the volume of steel, austenite than the smallest volume, which is high-alloy steel mold quenching low-temperature tempering volume reduction The main reason. The specific volume of the various tissues of the steel decreases in the following order: martensite - tempered sorbite - pearlite - austenite
2, preventive measures
(1) appropriate to reduce the quenching temperature. As described earlier quenching heating the higher the temperature, the greater the amount of retained austenite, so select the appropriate quenching temperature is to reduce the mold an important measure to reduce. In general, to ensure the technical requirements of the mold, the mold to consider the overall performance, appropriate to reduce the mold quenching temperature.
(2) Some data show that after quenching the Cr12MoV steel mold, the amount of retained austenite after tempering at 500 ℃ is higher than that at 200 ℃, so the tempering temperature should be properly raised under the premise of ensuring the technical requirements of the mold. Production practice shows that: Cr12MoV steel mold 500? C tempering mold deformation is the smallest, while the hardness decreased less (2HRC ~ 3HRC).
(3) After quenching the mold to take cold treatment is to reduce the amount of retained austenite the best technology, but also reduce the mold deformation, the use of dimensional stability occurs when the best measures, so precision complex mold generally should be used cryogenic treatment.
Sixth, the impact of cooling medium and cooling method
Mold heat treatment deformation is often shown after quenching and cooling, although there are all the above factors, but the impact of the cooling process can not be ignored.
1, the causes of deformation
When the mold is cooled to below the Ms point, the steel phase change occurs, in addition to the thermal stress due to inconsistent cooling has, as well as the structural stress caused by the unequal time of phase transition, the cooling rate is faster, the cooling Uneven, the greater the stress generated, the greater the deformation of the mold.
2, preventive measures
(1) In the premise of ensuring the hardness of the mold, try to use precooling, for carbon steel and low alloy die steel can be precooled to the edges of the black (720 ℃ ~ 760 ℃). For pearlite transformation zone overcooled austenite more stable steel can be pre-cooled to about 700 ℃.
(2) The use of hierarchical cooling quenching can significantly reduce the mold quenching heat stress and tissue stress, is to reduce some of the complex deformation of the effective method.
(3) for some sophisticated and complex mold, the use of isothermal quenching can significantly reduce the deformation.
Seven, improve the heat treatment process, reduce the deformation of the mold heat treatment
Mold deformation after quenching, no matter what method is taken, the deformation is inevitable, but to strictly control the deformation of the complex precision molds can take the following methods to control.
1, using heat treatment
On the basic hardness is not demanding, while the surface hardness of the sophisticated high precision molds can be taken after the rough mold roughing heat treatment, finishing low temperature nitriding treatment (500 ℃ ~ 550 ℃, due to mold low nitriding temperature , There is no matrix phase change, the other furnace cooled to room temperature baked, cooling stress less, smaller mold deformation.
2, using pre-heat treatment
The precision of complex mold, such as its hardness is not too high, pre-heat treatment can be pre-hardened steel, die steel (such as 3Cr2Mo, 3CrMnNiMo steel) pre-heat treatment, so that the hardness reaches the use of (lower hardness 25HRC ~ 35HRC , A higher hardness of 40HRC ~ 50HRC), and then the mold is no longer heat treatment molding, precision molds to ensure the accuracy.
3, aging hardening mold steel
For precision and sophisticated molds can be hardened steel, such as PMS (1Ni3Mn2CuA1.Mo) steel is a new type of aging die steel, hardened at 870 ?? C solution hardness of about 30HRC, easy machining, mold forming Aging heat treatment at about 500 ℃, you can get 40HRC ~ 45HRC higher hardness, mold deformation is small, only need to be polished, is the ideal precision steel mold complex.
The reasons for the deformation of precision and complicated molds are often complex. However, as long as we grasp the law of deformation and analyze the causes, we can reduce the deformation of molds by using different methods and can control them. In general, the heat treatment of precision complex mold deformation can take a preventive approach.
(1) reasonable selection. The precision complex die should choose a good material micro-deformation mold steel (such as empty quenching steel), severe segregation of carbide die steel should be reasonable forging and quenching and tempering heat treatment of large and can not be forged steel can be dissolved in the solution double Refined heat treatment.
(2) Die structure design to be reasonable, the thickness should not be too poor, the shape should be symmetrical, deformation of large mold to master the deformation laws, allowance for processing allowance, for large, complex and complex molds can be combined structure.
(3) Precise and complicated molds should be pre-heat treated to eliminate the residual stress generated in the machining process.
(4) A reasonable choice of heating temperature, control the heating rate, for complex precision molds can take slow heating, preheating and other balanced heating method to reduce the mold heat treatment deformation.
(5) In the premise of ensuring the hardness of the mold, try to use pre-cooling, graded cooling quenching or warm quenching process.
(6) For precision and complicated molds, under the condition of permitting, vacuum quenching and quenching after cryogenic treatment shall be adopted as much as possible.
(7) For some sophisticated molds can be pre-heat treatment, aging heat treatment, tempering nitriding heat treatment to control the accuracy of the mold.
In addition, the correct heat treatment process operations (such as plugging, tied hole, mechanical fixation, suitable heating method, the correct choice of the mold cooling direction and the direction of movement in the cooling medium, etc.) and reasonable tempering heat treatment process is also to reduce the complexity Mold deformation effective measures [Suzhou precision mold manufacturers]