Abstract: Due to large variety of specifications, complex shape and low surface roughness value of mold, it is very difficult to manufacture. Deformation after heat treatment of mold will seriously affect quality and service life of mold. Once it cracks during heat treatment, it will cause mold to be scrapped. Therefore, reducing and preventing mold heat treatment deformation and avoiding its cracking are important research topics for mold heat treatment workers. This paper briefly expounds common deformation and cracking defects of molds during heat treatment, analyzes their causes, and puts forward preventive measures.

Mold is mainly designed according to requirements of use, its structure sometimes cannot be completely reasonable and evenly symmetrical. This requires designer to take some effective measures when designing mold without affecting performance of mold, try to pay attention to manufacturing process, rationality of structure and symmetry of geometric shape.

Sections with great disparity in thickness, thin edges and sharp corners should be avoided. There should be a smooth transition at junction of thinness and thickness of mold. This can effectively reduce temperature difference of cross-section of mold, reduce thermal stress, and at the same time reduce non-simultaneity of tissue transformation on cross-section, reduce stress of tissue. Figure 1 shows that mold adopts transition fillet and transition cone.

 

For some molds that cannot guarantee a uniform and symmetrical cross section, it is necessary to change non-through hole into a through hole or increase some process holes appropriately without affecting performance.

 

Figure 3a shows a die with a narrow cavity, which will be deformed as shown by dotted line after quenching. If two process holes can be added in design (as shown in Figure 3b), temperature difference of cross-section during quenching process is reduced, thermal stress is reduced, and deformation is significantly improved.
Figure 4 is also an example of adding process holes or changing non-through holes into through holes, which can reduce increased cracking sensitivity due to uneven thickness.

 

When shape of mold is open or asymmetrical, stress distribution after quenching is uneven and it is easy to deform. Therefore, for general deformable trough molds, ribs should be left as much as possible before quenching, then cut off after quenching. Trough workpiece shown in Figure 5 was originally deformed at R after quenching and reinforced (hatched part in Figure 5 ), can effectively prevent quenching deformation.

 

For large dies with complex shape and size >400mm and punch dies with small thickness and long length, it is best to adopt a combined structure, simplifying complex, reducing large to small, and changing inner surface of mold to outer surface, which is not only convenient for cold and hot processing, but also can effectively reduce deformation with cracking.
When designing a combined structure, it should generally be decomposed according to following principles without affecting matching accuracy:
(1) Adjust thickness so that cross-section of mold with a large cross-section difference is basically uniform after decomposition.
(2) Decompose in places prone to stress concentration, disperse its stress, and prevent cracking.
(3) Cooperate with process hole to make structure symmetrical.
(4) It is convenient for cold and hot processing and easy to assemble.
(5) The most important thing is to ensure usability.

 

As shown in Figure 6, it is a large die. If integral structure is adopted, not only heat treatment will be difficult, but also cavity will shrink inconsistently after quenching, even cause unevenness and plane distortion of cutting edge, which will be difficult to remedy in subsequent processing. Therefore, a combined structure can be used. According to dotted line in Figure 6, it is divided into four parts, after heat treatment, they are assembled and formed, then ground and matched. This not only simplifies heat treatment, but also solves problem of deformation.

Heat treatment deformation and cracking are closely related to steel used and its quality, so it should be based on performance requirements of mold. Considering precision, structure and size of mold, as well as nature, quantity and processing methods of processed objects, it is reasonably selected. If general mold has no deformation and precision requirements, carbon tool steel can be used in terms of cost reduction; for easily deformed and cracked parts, alloy tool steel with higher strength, slower critical quenching and cooling speed can be used; Figure 7 shows a stamping die for electronic components. Originally used T10A steel, water quenching oil cold deformation is large and easy to crack, and alkali bath quenching cavity is not easy to harden. Now use 9Mn2V steel or CrWMn steel, quenching hardness and deformation can meet requirements.

 

It can be seen that when deformation of mold made of carbon steel does not meet requirements, it is still cost-effective to use alloy steel such as 9Mn2V steel or CrWMn steel. Although material cost is slightly higher, problem of deformation and cracking is solved. .
While selecting materials correctly, it is also necessary to strengthen inspection and management of raw materials to prevent mold heat treatment cracking due to raw material defects.
Part.3 Reasonably formulate technical conditions
Reasonably formulating technical conditions (including hardness requirements) is an important way to prevent quenching deformation and cracking. Local hardening or surface hardening can meet requirements of use, try not to quench the whole. For the overall quenching mold, requirements can be relaxed locally, and try not to insist on consistency. For molds with high cost or complex structure, when heat treatment is difficult to meet technical requirements, technical conditions should be changed, and those requirements that have little effect on service life should be appropriately relaxed, so as to avoid scrapping due to repeated repairs.
For selected steel type, the highest hardness it can achieve cannot be used as technical condition specified in design. Because the highest hardness is often measured with a small sample of limited size, which is very different from hardness that can be achieved by a mold with a larger actual size. Since pursuit of the highest hardness often needs to increase quenching cooling rate, thereby increasing tendency of quenching deformation and cracking, so using higher hardness as technical condition, even a smaller mold will bring certain difficulties to heat treatment operation. In short, designer should reasonably formulate feasible technical conditions according to performance and selected steel type. In addition, when hardness requirements are raised for selected steel grades, hardness range that produces temper brittleness should also be avoided.

Correctly handling relationship between mechanical processing and heat treatment, rationally arranging process flow, making cold and hot processing closely cooperate are effective measures to reduce deformation of mold heat treatment.

Deformation of some molds cannot be solved only from perspective of heat treatment, but if you change your way of thinking and start from the whole process, you can often get unexpected results. Figure 8 shows a semi-circular mold. Due to asymmetric shape, significant distortion will occur during quenching. If it is processed into a whole ring before quenching, then cut into two pieces with a saw blade and grinding wheel after heat treatment, it will not only reduce cost, but also reduce deformation.

 

It is inevitable that there will be deformation during processing. If deformation characteristics can be grasped and machining allowance can be reserved reasonably, it can not only simplify heat treatment operation, but also reduce subsequent machining, especially workload of grinding. Figure 9 shows a forming die of 45 steel. After heat treatment, inner hole will tend to expand. Therefore, during machining, a negative tolerance should be reserved in advance to make it meet design requirements after heat treatment.
For those molds whose deformation size and direction cannot be predicted in advance, a test quenching can be carried out before cavity is machined to design size, and a corresponding machining allowance is reserved according to its deformation characteristics.

 

For precision molds, stress generated by cutting or grinding can cause deformation and cracking, so adding stress relief annealing or aging treatment in process can often significantly reduce deformation and prevent cracking. For example, for slender shafts and molds with complex shapes, a stress relief annealing is performed after rough machining to eliminate cutting stress, which is very effective in reducing quenching deformation. For another example, for some molds that need to be finely ground, after heat treatment and rough grinding, an aging treatment process can be arranged to eliminate grinding stress, stabilize dimensions, prevent deformation and cracking.

Banded structure and component segregation in steel often cause uneven deformation of mold, matrix structure before quenching will also affect specific volume difference before and after quenching. Under certain conditions, quality of original structure in steel becomes main factor affecting heat treatment deformation. In order to reduce quenching deformation, in addition to taking effective measures during quenching process, microstructure in steel before quenching should also be properly controlled.

Practice has proved that reasonable forging is the key to reducing heat treatment deformation and ensuring a higher life of die. It is especially important for alloy steels (such as CrWMn, Cr12 and Cr12MoV steels). Premise that this type of steel can achieve low deformation is to be fully forged to minimize degree of carbide segregation inside steel. Therefore, forging process must be correctly controlled in the following five links:
(1) Forging method. It needs to be formed after forging for many times, generally not less than three times for high-alloy steel, so as to ensure that carbides are broken and evenly distributed.
(2) Forging ratio. There must be a certain forging ratio, such as the total forging ratio of high alloy steel is generally 8-10.
(3) Heating speed. Slowly heat up to about 800℃, and then slowly heat up to 1100-1150℃. During heating process, blank should be turned over frequently, strive to heat evenly and burn through.
(4) Control final forging temperature. If final forging temperature is too high, grains are easy to grow, and performance is deteriorated (final forging temperature is too low, plasticity is reduced, it is easy to form a banded structure, and it is easy to break.

Deformation and cracking of mold are not only related to stress generated during quenching process, but also related to original structure and residual internal stress before quenching. Therefore, necessary pre-heat treatment must be carried out on mold blank.
Generally speaking, smaller size molds made of T7 and T8 steel are easy to expand in volume during quenching. If quenching and tempering is performed in advance to obtain a tempered sorbite structure with a larger specific volume, quenching deformation can be reduced. For larger molds made of high carbon steel T10 and T12 steel, volume is easy to shrink during quenching, so spheroidizing annealing should be used to obtain better results than quenching and tempering.
For low-alloy tool steel, arrange a quenching and tempering treatment after machining to make alloy carbides evenly distributed, which has a good effect on improving structure and eliminating adverse effects of forging and original structure. Quenching and tempering treatment can obtain evenly distributed carbide and fine-grained sorbite structure, which increases specific volume of original structure, which can not only improve mechanical properties of steel, but also help reduce deformation. For high-alloy tool steel (such as high chromium steel) molds, after quenching and tempering, different degrees of shrinkage will occur during quenching, so if high-temperature tempering in quenching and tempering is changed to annealing treatment, better results can be obtained after quenching.
Alloy structural steel can obtain higher hardness by pre-quenching and tempering treatment, and can reduce specific volume change during quenching, which is conducive to reducing quenching deformation and cracking. Use of low temperature annealing to eliminate cold working stress of mold is simpler than quenching and tempering treatment, cycle is shorter, oxidation is less, and different materials can be treated by same process.
In order to eliminate network carbides caused by poor forging and increase depth of hardened layer, normalizing treatment can be used.
To sum up, all kinds of pre-heat treatment should be in accordance with expansion and contraction laws of mold, pre-adjust original structure and eliminate machining stress, so as to reduce deformation and cracking.

In order to reduce and prevent quenching deformation of workpiece, in addition to rationally designing workpiece, selecting materials, formulating heat treatment technical requirements, correctly performing thermal processing (casting, forging, welding) and pre-heat treatment on workpiece blank, it is more important to pay attention to following issues in heat treatment:

Under premise of ensuring hardening, generally lower quenching temperature should be selected as much as possible. But for some high-carbon alloy steel molds (such as CrWMn, Cr12Mo steel), Ms point can be reduced by appropriately increasing quenching temperature, and amount of retained austenite can be increased to control quenching deformation. In addition, for high-carbon steel molds with large thickness, quenching temperature can also be appropriately increased to prevent quenching cracks. For molds that are easy to deform and crack, stress relief annealing should be carried out before quenching.

It should be heated as evenly as possible to reduce thermal stress during heating. For high-alloy steel molds with large cross-sections, complex shapes, and high deformation requirements, they should generally be preheated or heating rate should be limited.

Pre-cooling quenching, graded quenching and graded cooling methods should be used as much as possible. Pre-cooling and quenching has a good effect on reducing deformation of slender or thin molds. For molds with great disparity in thickness, it can reduce deformation to a certain extent. For molds with complex shapes and widely different cross-sections, it is better to use graded quenching. For example, high-speed steel is quenched in stages at 580-620℃, which basically avoids quenching deformation and cracking.

Correctly choose the way workpiece is quenched into medium to ensure the most uniform cooling of mold and enter cooling medium along direction of least resistance, and move the slowest cooling surface towards liquid. When mold cools below Ms point, it should stop moving. For example, for molds with uneven thickness, thick part should be quenched first; for workpieces with large cross-sectional changes, heat treatment deformation can be reduced by adding process holes, reserving reinforcing ribs, and plugging asbestos in the holes; Or for workpieces with through holes, concave surface and holes should be quenched upwards to discharge air bubbles in through holes.

Heat treatment is one of indispensable processing techniques in mold manufacturing process. It has a great impact on quality and cost of mold, and is one of important measures to improve service life of mold. Deformation and cracking are two major problems in heat treatment of molds. Reasons for them are complex, but as long as you master rules, carefully analyze and study it, and prescribe right medicine, deformation of mold can be reduced, its cracking can also be controlled.