Abstract: 

Due to the large variety and specifications, complex shape, and low surface roughness value, the manufacturing of molds is difficult. The deformation generated after mold heat treatment will seriously affect the quality and service life of the mold. Once it cracks during heat treatment, it will cause the mold to be scrapped. Therefore, reducing and preventing mold heat treatment deformation and avoiding its cracking is an important research topic for mold heat treatment workers. This article briefly elaborates on the common deformation and cracking defects of molds during heat treatment, analyzes the reasons for their occurrence, and proposes preventive measures.

1.Reasonable design and correct material selection

Part.1

rational design

The mold is mainly designed according to usage requirements, and its structure often cannot achieve complete rationality and uniform symmetry. This requires designers to take effective measures while designing molds without affecting their performance, and to pay attention to the manufacturing process, structural rationality, and geometric symmetry as much as possible.

(1) Try to avoid sharp corners and sections with significant differences in thickness

Sections, thin edges, and sharp corners with varying thickness should be avoided. The transition should be smooth at the junction of thickness and thickness of the mold. This can effectively reduce the temperature difference of the mold cross-section, reduce thermal stress, and also reduce the different temporal changes in the microstructure transformation on the cross-section, thereby reducing the microstructure stress. Figure 1 shows the mold using transition fillets and transition cones.

(2) Increase process holes appropriately

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

Figure 3a shows a concave mold with a narrow cavity, which undergoes deformation as shown by the dashed line after quenching. If two process holes can be added during the design (as shown in Figure 3b), the temperature difference of the cross-section during the quenching process is reduced, the thermal stress is reduced, and the deformation situation is significantly improved.

Figure 4 is also an example of increasing process holes or converting non pass holes into through holes, which can reduce the increased cracking sensitivity due to uneven thickness.

(3) Try to use closed and symmetrical structures as much as possible

When the mold shape is an open or asymmetric structure, the stress distribution after quenching is uneven and easily deformed. Therefore, for groove shaped molds that are prone to deformation, it is advisable to leave ribs before quenching and then cut them off after quenching. The groove shaped workpiece shown in Figure 5, which was originally deformed at R after quenching, can effectively prevent quenching deformation by adding ribs (the shaded part in Figure 5).

(4) Adopting a modular structure

For the large concave die with complex shape and size>400mm and the punch with small thickness and large length, it is better to adopt a combined structure, which can simplify the complexity, reduce the size, and change the inner surface of the die to the outer surface, which is not only convenient for cold and hot processing, but also can effectively reduce deformation and cracking.

When designing a combined structure, it is generally necessary to decompose it according to the following principles without affecting the fitting accuracy: (1) Adjust the thickness to ensure that the cross-section of the mold with significant differences is basically uniform after decomposition.

(2) Decompose in areas prone to stress concentration, disperse stress, and prevent cracking.

(3) Coordinate with process holes to ensure symmetrical structure.

(4) It is convenient for cold and hot working and assembly.

(5) The most important thing is to ensure usability.

As shown in Figure 6, it is a large concave mold. If an integral structure is used, not only is it difficult to heat treat, but also the shrinkage of the mold cavity is inconsistent after quenching, which can even cause concave and convex edges and plane distortion, and it is difficult to remedy in future processing. Therefore, a combined structure can be used. According to the dashed line in Figure 6, it is divided into four parts, which are assembled and formed after heat treatment, ground, and then matched. This not only simplifies the heat treatment, but also solves the problem of deformation.

Part.2

Correct Material Selection

The deformation and cracking during heat treatment are closely related to the steel used and its quality, so it is necessary to meet the performance requirements of the mold. Taking into account factors such as mold accuracy, structure and size, as well as the nature, quantity, and processing method of the processed object, a reasonable selection should be made. If there are no deformation and accuracy requirements for general molds, carbon tool steel can be used to reduce costs; For parts that are prone to deformation and cracking, alloy tool steel with higher strength and slower critical quenching cooling rate can be selected; Figure 7 shows an electronic component stamping die. The original T10A steel was used, and the water quenched oil cold deformation was large and easy to crack, while the alkali bath quenched cavity was not easy to harden. We are now using 9Mn2V steel or CrWMn steel, which can meet the requirements for quenching hardness and deformation.

From this, it can be seen that when the deformation of the mold made of carbon steel cannot meet the requirements, switching to alloy steel such as 9Mn2V steel or CrWMn steel, although the material cost is slightly higher, solves the problems of deformation and cracking, and overall, it is still cost-effective.

While selecting materials correctly, it is also necessary to strengthen the inspection and management of raw materials to prevent mold heat treatment cracking caused by raw material defects.

Part.3

Reasonably formulate technical conditions

Reasonable formulation of technical conditions (including hardness requirements) is an important way to prevent quenching deformation and cracking. Local hardening or surface hardening can meet the usage requirements, and overall quenching should be avoided as much as possible. For overall quenching molds, local requirements can be relaxed and consistency should be avoided as much as possible. For molds with high costs or complex structures, when heat treatment is difficult to meet technical requirements, technical conditions should be changed and requirements that have little impact on service life should be appropriately relaxed to avoid scrapping due to multiple repairs.

For the selected steel grade, the maximum hardness it can achieve cannot be used as the technical conditions specified during design. Because the highest hardness is often measured using small specimens of limited size, which differs greatly from the hardness that can be achieved by larger molds of actual size. Due to the fact that the pursuit of the highest hardness often requires increasing the quenching cooling rate, thereby increasing the tendency for quenching deformation and cracking, using higher hardness as a technical condition can bring certain difficulties to heat treatment operations even for smaller molds. In short, designers should reasonably develop practical and feasible technical conditions based on their performance and the selected steel grade. In addition, when proposing hardness requirements for the selected steel grade, the hardness range that produces temper brittleness should also be avoided.

2.Reasonably arrange the process flow

Correct handling of the relationship between machining and heat treatment, reasonable arrangement of process flow and close cooperation between cold and hot working are effective measures to reduce the deformation of die heat treatment.

Part.1

The Key to Reasonably Arranging Process Flows

The deformation of some molds cannot be solved solely from the perspective of heat treatment, but if we change our thinking mode and start from the entire process flow, we can often achieve unexpected results. Figure 8 shows a semi circular mold, which exhibits significant distortion and deformation during quenching due to its asymmetric shape. If a circular ring is machined into a whole before quenching, and then cut into two pieces with a saw blade grinding wheel after heat treatment, not only can it reduce costs but also reduce deformation.

Part.2

Reserve machining allowance based on characteristics

Deformation is inevitable during processing. If the deformation characteristics can be grasped and machining allowances can be reasonably reserved, not only can heat treatment operations be simplified, but also the workload of subsequent mechanical processing, especially grinding, can be reduced. Figure 9 shows a forming mold made of 45 steel. After heat treatment, the inner hole tends to expand, so negative tolerances should be reserved in advance during mechanical processing to ensure that the heat treatment meets the design requirements.

For molds that cannot predict the size and direction of deformation in advance, a trial quenching can be carried out before the cavity is machined to the design size, and corresponding machining allowances can be reserved based on their deformation characteristics.

Part.3

Necessary stress relieving annealing or aging treatment

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

3.Reasonable forging and pre heat treatment

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

Part.1

Reasonable forging

Practice has proven that reasonable forging is the key to reducing heat treatment deformation and ensuring a longer lifespan of the mold. Especially important for alloy steels such as CrWMn, Cr12, and Cr12MoV steels. The premise for this type of steel to achieve low deformation is that it has been fully forged to minimize the degree of carbide segregation inside the steel. Therefore, it is necessary to correctly control the forging process in the following five steps:

(1) The forging method requires multiple forgings to form, and generally high alloy steel should be formed no less than three times to ensure that the carbides are broken and evenly distributed.

(2) The forging ratio must have a certain forging ratio, such as the total forging ratio of high alloy steel, which is generally 8-10.

(3) The heating rate slowly heats up to around 800 ℃, and then slowly heats up to 1100-1150 ℃. During the heating process, the blank should be frequently flipped to ensure uniform heating and thorough burning.

(4) Control the final forging temperature. If the final forging temperature is too high, the grain size is easy to grow and the performance becomes poor (if the final forging temperature is too low, the plasticity decreases, and the banded structure is easy to form, and it is also easy to fracture).

Part.2

Pre heat treatment

The deformation and cracking of the mold are not only related to the stress generated during the quenching process, but also to the original structure and residual internal stress before quenching. Therefore, necessary pre heat treatment must be carried out on the mold blank.

Generally speaking, smaller molds made of T7 and T8 steel are prone to volume expansion during quenching. If quenching and tempering treatment is carried out in advance to obtain a larger tempered sorbite structure than the volume, quenching deformation can be reduced. For larger molds made of high carbon steel T10 and T12 steel, which are prone to volume shrinkage during quenching, spheroidizing annealing should be adopted to achieve better results than quenching and tempering treatment.

For low alloy tool steel, arranging a quenching and tempering treatment after mechanical processing to evenly distribute alloy carbides has a good effect on improving the structure and eliminating the adverse effects of forging and original structure. Quenching and tempering treatment can obtain uniformly distributed carbides and fine-grained sorbite structure, increasing the specific volume of the original structure, which not only improves the mechanical properties of the steel but also helps to 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. Therefore, if the high-temperature tempering in quenching and tempering is changed to annealing treatment, better results can be obtained after quenching.

The use of pre quenched and tempered alloy structural steel can achieve higher hardness and reduce the specific volume change during quenching, which is beneficial for reducing quenching deformation and cracking. The use of low-temperature annealing to eliminate cold working stress in molds is simpler than quenching and tempering treatment, with shorter cycles and less oxidation, and different materials can be treated using the same process.

In order to eliminate the network of carbides generated by poor forging and increase the depth of the quenched layer, normalizing treatment can be used.

In summary, various pre heat treatments should follow the expansion and contraction laws of the mold, pre adjust the original structure and eliminate mechanical processing stress to reduce deformation and cracking.

4.Adopting reasonable heat treatment processes

In order to reduce and prevent quenching deformation of workpieces, in addition to reasonable design of workpieces, material selection, formulation of technical requirements for heat treatment, and correct hot working (casting, forging, welding) and pre heat treatment of workpieces, more importantly, the following issues must be paid attention to in terms of heat treatment:

(1) Reasonable selection of heating temperature

Under the premise of ensuring quenching hardness, it is generally recommended to choose a lower quenching temperature as much as possible. However, for some high carbon alloy steel molds (such as CrWMn, Cr12Mo steel), the Ms point can be reduced and the residual austenite content can be increased by appropriately increasing the quenching temperature to control quenching deformation. In addition, for high carbon steel molds with larger thickness, the quenching temperature can also be appropriately increased to prevent the occurrence of quenching cracks. For molds that are prone to deformation and cracking, stress relieving annealing should also be performed before quenching.

(2) Reasonable heating

Uniform heating should be achieved as much as possible to reduce thermal stress during heating. For high alloy steel molds with large cross-section, complex shape, and high deformation requirements, preheating or limiting heating speed should generally be carried out.

(3) Correct selection of cooling method and cooling medium

Try to choose pre cooling quenching, graded quenching, and graded cooling methods as much as possible. Pre cooling quenching has a good effect on reducing deformation of slender or thin molds, and can play a role in reducing deformation to a certain extent for molds with significant thickness differences. For molds with complex shapes and significantly different cross-sections, it is better to use graded quenching. If high-speed steel is quenched in stages at 580-620 ℃, quenching deformation and cracking are basically avoided.

(4) Correctly grasp the quenching operation method

Correctly select the method of quenching the workpiece into the medium to ensure that the mold receives the most uniform cooling and enters the cooling medium along the direction of minimum resistance, moving the slowest cooling surface towards the liquid. When the mold cools below the Ms point, it should stop moving. For example, for molds with uneven thickness, the thicker 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; For workpieces with concave and convex surfaces or through holes, the concave surfaces and holes should be quenched upwards in order to remove bubbles from the through holes.