Preventive measures for deformation and cracking of die heat treatment

Because of many kinds and specifications, complex shape and low surface roughness, it is difficult to manufacture the die. The deformation of the die after heat treatment will seriously affect the quality and service life of the die. Once the die is cracked during heat treatment, it will lead to the scrapping of the die. Therefore, reducing and preventing the die heat treatment deformation and avoiding its cracking is an important research topic for the majority of die heat treatment workers. This paper briefly expounds the common deformation and cracking defects of die in the process of heat treatment, analyzes the causes, and puts forward preventive measures.

Rational design and correct material selection

Rational design

The mold is mainly designed according to the use requirements, and its structure sometimes can not be completely reasonable, uniform and symmetrical. This requires the designer to take some effective measures when designing the mold without affecting the service performance of the mold, and try to pay attention to the manufacturability, the rationality of the structure and the symmetry of the geometry.

Try to avoid sharp corners and sections with great thickness difference

Sections, thin edges and sharp corners with great thickness differences shall be avoided. Smooth transition shall be made at the thickness junction of the die. This can effectively reduce the temperature difference of the die section and reduce the thermal stress. At the same time, it can also reduce the different timing of tissue transformation on the section and reduce the tissue stress. Figure 1 shows that the die adopts transition fillet and transition cone.

Figure 1

Fig. 2 reasonable wall thickness of die

Properly increase process holes

For some molds whose cross-section is really unable to ensure uniformity and symmetry, the through holes shall be changed into through holes or some process holes shall be added appropriately without affecting the service performance.

Fig. 3

Fig. 3A shows a die with narrow cavity, which will produce deformation as shown by the dotted line after quenching. If two process holes can be added in the design (as shown in Fig. 3b), the temperature difference of the section during quenching is reduced, the thermal stress is reduced, and the deformation is obviously improved.

Fig. 4 also shows an example of adding a process hole or changing a through hole into a through hole, which can reduce the increased cracking sensitivity due to uneven thickness.

Figure 4

Closed and symmetrical structure shall be adopted as far as possible

When the die shape is open or asymmetric structure, the stress distribution after quenching is uneven and easy to deform. Therefore, for the grooved die that is easy to deform, the reinforcement shall be reserved before quenching and then removed after quenching. The grooved workpiece shown in Fig. 5 was deformed at R after quenching. After reinforcement (shaded line in Fig. 5), the quenching deformation can be effectively prevented.

Figure 5

Adopt combined structure

For the large concave die with complex shape and size > 400mm and the punch with small thickness and large length, it is best to adopt the combined structure to simplify the complexity and change the large into small, and change the inner surface of the die into the outer surface, which is not only convenient for cold and hot processing, but also can effectively reduce deformation and cracking.
When designing the combined structure, it shall generally be decomposed according to the following principles without affecting the matching accuracy:

• (1) Adjust the thickness so that the die with wide difference in section is basically uniform after decomposition.
• (2) Decompose in the place where stress concentration is easy to occur, disperse its stress and prevent cracking.
• (3) Cooperate with the process hole to make the structure symmetrical.
• (4) It is convenient for cold and hot processing and assembly.
• (5) The most important thing is to ensure usability.

Fig. 6 large die

As shown in Figure 6, it is a large female die. If the integral structure is adopted, it will not only be difficult to heat treat, but also the shrinkage of the cavity after quenching is inconsistent, and even cause concave convex and plane distortion of the edge, which is difficult to remedy in future processing. Therefore, the combined structure can be adopted. It is divided into four pieces according to the dotted line in Figure 6. After heat treatment, it is assembled and formed, grinded and matched, which not only simplifies the heat treatment, but also solves the problem of deformation.

Correct material selection

Heat treatment deformation and cracking are closely related to the steel used and its quality, so they should be based on the service performance requirements of the die. Comprehensively consider the factors such as die accuracy, structure and size, as well as the nature, quantity and processing mode of processing objects. If there are no deformation and accuracy requirements for general molds, carbon tool steel can be used to reduce the cost; For easily deformed and cracked parts, alloy tool steel with high strength and slow critical quenching cooling speed can be selected; Figure 7 shows an electronic component die. The original T10A Steel has large water quenching and oil cooling deformation and is easy to crack, and the alkali bath quenching cavity is not easy to harden. Now 9mn2v steel or CrWMn steel is used, and the quenching hardness and deformation can meet the requirements.

Figure.7 electronic component die
It can be seen that when the deformation of the die made of carbon steel can not meet the requirements, alloy steels such as 9mn2v steel or CrWMn steel are used. Although the material cost is slightly higher, the problems of deformation and cracking are solved, which is still cost-effective on the whole.
While correctly selecting materials, we should also strengthen the inspection and management of raw materials to prevent mold heat treatment cracking due to raw material defects.

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 use requirements. Try not to quench as a whole. For the overall quenching die, if the requirements can be relaxed locally, try not to force consistency. For molds with high cost or complex structure, when the heat treatment is difficult to meet the technical requirements, the technical conditions shall be changed and the requirements that have little impact on the service life shall be appropriately relaxed to avoid scrapping due to repeated repair.
For the selected steel, the maximum hardness it can achieve cannot be used as the technical conditions specified in the design. Because the highest hardness is often measured by small samples with limited size, which is very different from the hardness that can be achieved by molds with larger actual size. Because the pursuit of the highest hardness often needs to improve the quenching cooling speed, so as to increase the quenching deformation and cracking tendency, using higher hardness as the technical condition, even the die with smaller size will bring some difficulties to the heat treatment operation. In short, the designer should reasonably formulate practical technical conditions according to the service performance and the selected steel grade. In addition, when putting forward hardness requirements for the selected steel grades, the hardness range that produces tempering brittleness should also be avoided.

Reasonably arrange the process flow

The effective measures to reduce the deformation of die heat treatment are to correctly deal with the relationship between machining and heat treatment, reasonably arrange the process flow and make the cold and hot processing cooperate closely.

The key to reasonably arrange the process flow

The deformation of some molds can not be solved only from the perspective of heat treatment, but unexpected results can often be obtained by changing the way of thinking and starting from the whole process flow. Fig. 8 shows a semicircular die, which will produce significant distortion during quenching due to asymmetric shape. If an integral ring is processed before quenching and then cut into two pieces with a saw blade grinding wheel after heat treatment, it can not only reduce the cost, but also reduce the deformation.

Fig. 8 semi circular die

Reserve machining allowance according to characteristics

Deformation is inevitable during heat treatment. If the deformation characteristics can be mastered and the machining allowance can be reasonably reserved, it can not only simplify the heat treatment operation, but also reduce the workload of subsequent machining, especially grinding. Figure 9 shows a 45 steel forming die. The inner hole will tend to swell after heat treatment. Therefore, negative tolerance should be reserved in advance during machining to make it meet the design requirements after heat treatment.

For those molds whose deformation size and direction cannot be predicted in advance, a trial quenching can be carried out before the cavity is machined to the design size, and the corresponding machining allowance can be reserved according to its deformation characteristics.

Figure.9 forming die

Necessary stress relief annealing or aging treatment

For precision molds, the stress generated by cutting or grinding will cause deformation and cracking. Therefore, if stress relief annealing or aging treatment is added in the process flow, it can often significantly reduce deformation and prevent cracking. For example, for slender shafts and molds with complex shapes, a stress relief annealing is carried out after rough machining and forming to eliminate cutting stress, which is very effective to reduce quenching deformation. For another example, for some molds requiring fine grinding, after heat treatment and rough grinding, an aging treatment process can be arranged to eliminate grinding stress, stabilize size and prevent deformation and cracking.

Reasonable forging and pre heat treatment

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

Reasonable forging

Practice has proved that reasonable forging is the key to reduce heat treatment deformation and ensure high die life. It is particularly important for alloy steels (such as CrWMn, Cr12 and Cr12MoV steels). The premise for this kind of steel to achieve low deformation is to fully forge to minimize the degree of carbide segregation in the steel. Therefore, the forging process must be correctly controlled in the following five links:
(1) Forging method
It needs to be forged for many times before forming. Generally, high alloy steel shall not be less than three times to ensure that carbides are broken and evenly distributed.
(2) Forging ratio
There must be a certain forging ratio. For example, the total forging ratio of high alloy steel is generally 8-10.
(3) Heating speed
Slowly raise the temperature to about 800 ℃, and then slowly heat it to 1100-1150 ℃. During the heating process, the blank shall be turned over frequently to heat evenly and burn thoroughly.
(4) Control final forging temperature
If the final forging temperature is too high, the grain is easy to grow, the performance becomes worse (if the final forging temperature is too low, the plasticity decreases, it is easy to form banded structure and fracture).

Pre heat treatment

Die deformation and cracking are not only related to the stress produced in the quenching process, but also related to the original structure and residual internal stress before quenching. Therefore, necessary pre heat treatment must be carried out on the die blank.
Generally speaking, the die with smaller size made of T7 and T8 steel is easy to swell during quenching. IF quenching and tempering treatment is carried out in advance to obtain tempered sorbite structure with larger specific volume, the quenching deformation can be reduced. While the die with larger size made of high carbon steel T10 and T12 steel is easy to shrink during quenching, spheroidizing annealing should be adopted to achieve better effect than quenching and tempering treatment.
For low alloy tool steel, a quenching and tempering treatment is arranged after machining to make the alloy carbides evenly distributed, which 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, increase the specific volume of original structure, improve the mechanical properties of steel and reduce deformation. For high alloy tool steel (such as high chromium steel) dies, 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.
Alloy structural steel can obtain higher hardness by pre quenching and tempering treatment, and can reduce the specific volume change during quenching, which is conducive to reducing quenching deformation and cracking. Using low-temperature annealing to eliminate the cold working stress of the die is simpler, shorter cycle and less oxidation than quenching and tempering treatment, and different materials can be treated by the same process.
In order to eliminate the network carbide caused by poor forging and increase the depth of quenching layer, normalizing treatment can be adopted.
To sum up, various pre heat treatments should adjust the original structure and eliminate machining stress in advance according to the expansion and contraction law of the die, so as to reduce deformation and cracking.

Adopt reasonable heat treatment process

In order to reduce and prevent quenching deformation of workpieces, in addition to reasonably designing workpieces, selecting materials, formulating technical requirements for heat treatment, and correctly performing hot processing (casting, forging and welding) and pre heat treatment on workpieces, it is more important to pay attention to the following problems in heat treatment:

Reasonable selection of heating temperature

On the premise of ensuring hardening, generally lower quenching temperature should be selected as far as possible, but for some high carbon alloy steel molds (such as CrWMn and cr12mo steel) , the MS point can be reduced and the residual austenite volume can be increased by appropriately increasing the quenching temperature to control the quenching deformation. In addition, for the high carbon steel die with large thickness, the quenching temperature can also be appropriately increased to prevent quenching cracks. For the die easy to deform and crack, stress relief annealing should be carried out before quenching.

Reasonable heating

Uniform heating shall be achieved as far as possible to reduce the thermal stress during heating. Generally, high alloy steel molds with large section, complex shape and high deformation requirements shall be preheated or the heating speed shall be limited.

Correctly select the cooling mode and cooling medium

Precooling quenching, step quenching and step cooling shall be selected as far as possible. Precooling quenching has a good effect on reducing the deformation of slender or thin molds. For molds with great thickness difference, it can reduce the deformation to a certain extent. For molds with complex shape and different sections, step quenching is better. For example, step quenching at 580-620 ℃ is basically avoided for high-speed steel Quenching deformation and cracking were.

Correctly master the quenching operation method

Properly select the way of quenching the workpiece into the medium to ensure that the mold gets the most uniform cooling and enters the cooling medium along the direction of minimum resistance, and move the slowest cooling surface towards the liquid. When the mold cools below the MS point, it shall stop moving. For example, for the mold with uneven thickness, the thick part shall be quenched first; for the workpiece with large cross-section change, it can be quenched by adding process holes, reserving reinforcing ribs and holes Plug asbestos and other methods to reduce heat treatment deformation; for workpieces with concave and convex surfaces or through holes, the concave surfaces and holes shall be quenched upward to discharge bubbles in the through holes.

Conclusion

Heat treatment is one of the indispensable processing technologies in the mold manufacturing process. It has a great impact on the quality and cost of the mold and is one of the important measures to improve the service life of the mold. Deformation and cracking are two difficult problems in the mold heat treatment, and the causes are complex. However, as long as we master its law, carefully analyze and study it, the remedy can be applied to the case, and the mold deformation can be solved And its cracking can be controlled.

Source: China Flanges Manufacturer – Yaang Pipe Industry (www.epowermetals.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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