The mold goes through a heat treatment process that consists of preheating, terminal heating and surface hardening.
Heat treatment defects refer to various problems that occur during the final stage of mold heat treatment or in subsequent processing and use. These defects may include temper cracks, poor dimensional stability, insufficient hardness, electromachining cracks, grinding cracks, and early mold failure.
A more in-depth analysis is provided below.
Heat Treatment Defects
1. Extinguishing crack
The causes of crack extinction and preventive measures are as follows:
- Shape Effect: This is mainly caused by design factors such as small rounded corners, poor hole placement, and poor cross-section transitions.
Preventative measures: Check and improve the design, including rounded corners, hole placement, and cross-section transitions.
- Overheating: This is mainly caused by inaccurate temperature control, high process temperature settings, and uneven oven temperature.
Preventive measures: Maintain and review the temperature control system, adjust the process temperature and add iron between the part and the furnace floor.
- Decarburization: This is caused by overheating or excessive burning, unprotected heating in an air furnace, small machining margins, and residual decarburized layers from forging or preheat treatment.
Preventive measures: Use controlled atmosphere heating, salt bath heating, vacuum furnace, box furnace with box protection or apply antioxidant coating and increase machining tolerance by 2-3 mm.
- Inadequate cooling: This is mainly caused by improper coolant selection or excessive cooling.
Preventive Measures: Understand the cooling characteristics of the quenching medium or tempering treatment and select the appropriate coolant.
- Poor mold steel structure: This can be caused by severe carbide segregation, poor forging quality and inadequate preheat treatment.
Preventative measures: Use the correct forging process and implement a reasonable preheat treatment system.
2. Insufficient hardness
The reasons and precautions for insufficient hardness are as follows:
- Inadequate oven or cooling tank: This is caused by incorrect process temperature or errors in the temperature control system.
Preventive Measures: Correct the process temperature and review and verify the temperature control system. When installing the oven, workpieces should be evenly spaced and not stacked or grouped together for cooling.
- High quenching temperature: This is caused by incorrect process temperature or errors in the temperature control system.
Preventive Measures: Correct the process temperature and review and verify the temperature control system.
- Overtempering: This is caused by high tempering temperature, errors in the temperature control system, or entering the furnace at a high temperature.
Preventive Measures: Correct the process temperature and review and verify the temperature control system. Enter the oven at a temperature no higher than the set temperature.
- Inadequate cooling: This may occur if the pre-cooling time is too long, the cooling medium is not selected correctly, the cooling medium temperature increases while cooling performance decreases, agitation is poor, or the cooling medium outlet temperature tank is too high.
Preventive measures: Quickly enter the tank from the furnace, understand the cooling characteristics of the quenching medium, add or cool the quenching medium if necessary, strengthen the agitation of the coolant, and remove at a temperature of Ms + 50°C.
- Decarburization: It is caused by residual layers of decarburization of raw materials or during the quenching and heating process.
Preventative measures: Use controlled atmosphere and salt bath heating, vacuum furnace and box furnace with box protection or antioxidant coating and increase machining tolerance by 2-3 mm.
3. Floor deformation
In the field of mechanical manufacturing, the occurrence of deformation during heat treatment is considered absolute, while the absence of deformation is relative. In other words, it all depends on the size. This is mainly due to the surface relief effect caused by the transformation of martensite during heat treatment.
Preventing deformation (changes in dimensions and shape) during heat treatment is a challenging task and often requires experience to solve. This is because several factors, such as the type of steel, shape of the mold, inadequate distribution of carbides, and the method of forging and heat treatment, can contribute to or worsen the problem.
Furthermore, changes in any of the various conditions during heat treatment can greatly impact the degree of deformation of the steel part.
For a long time, solving the problem of heat treatment deformation was done mainly through experiment and heuristics. However, it is crucial to have a complete understanding of the relationship between mold steel forging, module orientation, mold shape, heat treatment method and heat treatment deformation. This understanding can be obtained by analyzing accumulated data and establishing heat treatment deformation files.
4. Decarbonization
Decarburization is a phenomenon and reaction in which the carbon in the surface layer of steel is completely or partially lost due to the effect of the surrounding atmosphere during heating or insulation.
Decarburization of steel parts can result in insufficient hardness, quenching cracks, heat treatment deformation, and chemical heat treatment defects. Furthermore, it can significantly affect the fatigue resistance, wear resistance and performance of the mold.
5. Cracks caused by electrical discharge machining
In mold manufacturing, electrical discharge machining (EDM) is becoming an increasingly common processing method. However, with its widespread use, defects caused by EDM have also increased.
EDM is a machining method that melts the surface of a mold using the high temperature generated by electrical discharge. This process forms a white EDM deteriorating layer on the machining surface and generates a tensile stress of about 800 MPa. As a result, deformations or cracks may occur during electrical processing of the mold.
Therefore, when using EDM molds, it is crucial to understand the impact of EDM on mold steel and take preventive measures to avoid defects:
- Avoid overheating and decarburization during heat treatment and fully quench the steel to reduce or eliminate residual stress.
- To completely eliminate the internal stress generated during quenching, high temperature tempering is necessary. Steels that can withstand high temperature tempering, such as DC53, ASP-23 and high-speed steel, should be used for processing under stable discharge conditions.
- After EDM processing, stabilize relaxation treatment.
- Define reasonable process holes and grooves.
- Completely eliminate the resolidified layer to ensure it is in good condition.
- Using the principle of vector translation, cut and disperse the internal stress of the cutting outpost through the drainage.
6. Insufficient resistance
Insufficient toughness may be attributed to excessively high quenching temperature and prolonged holding time, leading to grain coarsening, or failure to prevent tempering in the brittle zone.
7. Crack grinding
The presence of a large amount of austenite retained in the part can result in structural stress and cracks in the part when tempering transformation occurs during grinding heat. To avoid this, two preventative measures can be taken: carry out a cryogenic treatment after quenching or repeat the tempering process (typically 2-3 times for cold working low alloy tool steels) to minimize the amount of retained austenite.
Preventive measures for deformations and cracks in the heat treatment of molds
I. Rational Design and Correct Selection of Materials
Part 1. Rational Design
The mold design mainly depends on the intended use and its structure may not always be completely rational and symmetrical.
This requires designers to take effective measures during the mold design process. Without compromising the performance of the mold, they must pay attention to manufacturability, structural rationality and geometric symmetry.
(1) Avoid sharp corners and sections with large differences in thickness.
Sections with drastic differences in thickness, thin edges and sharp corners should be avoided.
Smooth transitions should be employed at the junctions of the thick and thin sections of the mold. This can effectively reduce temperature differences in the mold cross-section, minimizing thermal stress.
It can also reduce the timing disparity of structural transformations along the section, thus reducing structural stress. Figure 1 illustrates the use of transition fillets and cones in mold design.
(2) Incorporation of Additional Holes in the Process
For molds that are truly challenging to ensure uniform and symmetrical cross-sections, one must, without compromising their functionality, convert blind holes into through holes or properly integrate additional process holes.
Figure 3a illustrates a type of die with a narrow cavity, which after quenching undergoes deformation as represented by the dotted lines. If two process holes are added during the design phase (as shown in Figure 3b), this reduces the temperature difference across the section during quenching, thereby decreasing the thermal stress and significantly improving the deformation situation.
Figure 4 also demonstrates an example of adding process holes or changing blind holes to through holes, which can reduce the increased susceptibility to cracking caused by uneven thicknesses.
(3) Use closed and symmetrical structures as much as possible
When the mold shape is open or asymmetrical, the stress distribution after quenching is uneven, which easily leads to deformation. Therefore, for generally deformable grooved molds, it is advisable to leave the ribs before quenching and cut them after quenching.
As shown in Figure 5, the grooved part originally deformed at point R after quenching. By adding ribs (the shaded part in Figure 5), quenching deformation can be effectively avoided.
(4) Implementing a Composite Structure
For large-scale, complex-shaped concave molds exceeding 400mm in size, as well as thin and elongated convex molds, it is ideal to employ a composite structure.
This approach simplifies complexity, reduces size, and transforms the mold's inner surfaces into outer surfaces. This not only facilitates thermal processing, but also effectively minimizes deformation and cracking.
When designing a composite structure, the decomposition should generally follow these principles as long as they do not affect the accuracy of the fit:
(1) Adjust the thickness to ensure a uniform cross section after decomposition, especially for molds with marked differences in their initial cross sections.
(2) Decompose areas prone to stress concentration to distribute stress and prevent cracking.
(3) Incorporate aligned process holes to maintain structural symmetry.
(4) Facilitates thermal processing and ease of assembly.
(5) Above all, usability must be guaranteed.
Figure 6 illustrates a large concave matrix. Opting for a monolithic structure makes heat treatment challenging and results in inconsistent shrinkage throughout the die cavity after quenching.
This can even lead to uneven blade edges and flat distortion, which are difficult to remedy in subsequent processing. Therefore, a modular structure can be used. As indicated by the dashed lines in Figure 6, the structure is divided into four parts.
After heat treatment, these parts are reassembled, ground and fitted. This not only simplifies the heat treatment process, but also solves the problem of deformation.
Part 2: Proper Material Selection
Deformation and cracking from heat treatment are closely related to the steel used and its quality. Therefore, the material must be selected based on the die performance requirements.
Factors such as die accuracy, structure and size, as well as the nature, quantity and processing methods of the part must be considered.
Generally, if there are no deformation and precision requirements for the die, carbon tool steel can be used to reduce costs. For components prone to warping and cracking, high-strength alloy tool steel with slower critical quench cooling rates can be chosen.
Figure 7 shows a matrix for an electronic component. Originally, T10A steel was used, and the water quenching and oil cooling process led to significant deformation and susceptibility to cracking.
Furthermore, quenching in an alkaline bath made it difficult to harden the matrix cavity. Now, 9Mn2V steel or CrWMn steel is used, meeting the requirements of quenching, hardness and deformation.
It is evident that when molds made of carbon steel do not meet the deformation requirements, replacing them with alloy steel such as 9Mn2V or CrWMn can solve the problems of deformation and cracking.
Despite the slightly higher material cost, it is still economical in the grand scheme of things.
Simultaneously, in addition to making the right material choice, it is crucial to improve raw material inspection and management to prevent mold cracking due to heat treatment due to raw material defects.
Part.3 Rational Formulation of Technical Conditions
The rational formulation of technical conditions, including hardness requirements, is a crucial way to avoid deformations and quenching cracks.
If spot hardening or surface hardening can meet the usage requirements, try to avoid hardening the entire part.
For fully tempered molds, where localized requirements can be relaxed, uniformity should not be strictly pursued.
For high-cost molds or complex structures, when heat treatment does not meet technical requirements, conditions must be changed, appropriately relaxing those demands that have little impact on service life to avoid scrapping due to multiple reworks.
The type of steel chosen should not have its maximum achievable hardness defined as a technical design condition.
This is because maximum hardness is often measured with limited sample sizes, which can differ significantly from the hardness achieved by larger, full-size molds.
As seeking maximum hardness often requires increasing the quenching cooling rate, leading to a greater tendency for deformation and cracking during quenching, using a higher hardness as a technical condition can pose certain challenges, even for smaller-sized molds during heat treatment.
In summary, the designer must formulate feasible technical conditions based on usage performance and the selected steel type.
Furthermore, when defining the hardness requirements for the selected steel type, the hardness range that can cause tempering brittleness should be avoided.
II. Rational Arrangement of Technological Processes
Correct management of the relationship between mechanical processing and heat treatment, and the rational arrangement of the technological process, allowing close coordination between cold and hot work, are effective measures to reduce the deformation of mold heat treatment.
Part.1 The Key to Rationally Organizing Technological Processes
In some cases, mold deformation cannot be resolved from a heat treatment perspective alone. However, changing the mindset and approaching the entire technological process often produces unexpected results.
Figure 8 shows a semicircular mold which, due to its asymmetric shape, undergoes significant torsional deformation during tempering.
If it is machined into a complete ring before quenching and then cut into two parts with a saw blade grinding wheel after heat treatment, not only can costs be reduced, but deformation can also be minimized.
Part.2 Allocation of processing tolerances based on deformation characteristics
Distortion is inevitable during processing.
If its characteristics can be understood and appropriate processing allowances can be reasonably reserved, not only can the heat treatment operation be simplified, but subsequent mechanical processing, especially grinding work, can be reduced.
Figure 9 shows a 45# steel forming mold. After heat treatment, the inner hole tends to expand, so a negative tolerance must be reserved in advance during mechanical processing to meet the design requirements after heat treatment.
For molds where the size and direction of deformation cannot be anticipated, a quench test can be performed before the mold cavity has been machined to the designed dimensions.
Based on its deformation characteristics, corresponding mechanical processing margin can be reserved.
Part.3 Annealing required for stress relief or aging treatment
For precision molds, the stress generated by the cutting or grinding processes can cause deformation and cracking.
Therefore, incorporating stress relieving annealing or aging treatment into the processing flow can significantly reduce deformation and prevent cracking.
For example, for slender shaft and complex shaped molds, performing stress relief annealing after rough machining to eliminate cutting stress is highly effective in reducing quench deformation.
Likewise, for some molds that require precision grinding, an aging treatment can be scheduled after heat treatment and coarse grinding to eliminate grinding stress, stabilize dimensions, and prevent warping and cracking.
III. Rational Forging and Preliminary Heat Treatment
Band-like structures and compositional segregation in steel often lead to uneven deformation of molds. The state of die organization before quenching can also affect the difference in mold volume before and after quenching.
Under certain conditions, the quality of the original steel structure becomes an important factor affecting heat treatment deformation.
To minimize quenching deformation, in addition to taking effective measures during the quenching process, the internal structure of the steel before quenching must also be properly controlled.
Part 1. Rational Forging
Experience proves that rational forging is essential to minimize heat treatment deformation and ensure that the mold has a longer service life. This is especially important for alloy steels (such as CrWMn, Cr12 and Cr12MoV steels).
The premise for these types of steels to achieve low deformation is through sufficient forging, allowing the degree of carbide segregation within the steel to be minimized.
Therefore, the forging process must be correctly controlled in the following five aspects:
(1) Forging Method: The forming process requires multiple forging steps, typically no less than three for high-alloy steel, to ensure that the carbides are fractured and distributed evenly.
(2) Forging rate: A certain forging rate is required. For example, the total forging ratio for high alloy steels is generally between 8-10.
(3) Heating speed: Gradually heat to approximately 800°C, then slowly increase the temperature to 1100-1150°C. During the heating process, the workpiece must be turned regularly to ensure uniform heating and complete penetration.
(4) Controlling the final forging temperature: If the final forging temperature is too high, the grain size tends to increase, resulting in worse performance. On the other hand, if the final forging temperature is too low, the material becomes less ductile, prone to band-like structures, and can fracture easily.
Part.2 Pre-Thermal Treatment
The deformation and cracking of molds are not only associated with the stress generated during the tempering process, but also with the original structure and the residual internal stress before tempering. Therefore, it is essential to implement the necessary heat pretreatment for the molds.
Typically, smaller molds made from T7 and T8 steel tend to expand in volume during quenching. If previously tempered, a tempered sorbite structure larger than the original volume can be achieved, reducing deformation during quenching.
On the other hand, larger molds made from high carbon steel, such as T10 and T12 steel, tend to contract in volume when tempered. In this case, spheroidizing annealing should be adopted, which can produce better results than tempering.
For low-alloy tool steels, arranging a tempering process after mechanical machining can distribute the alloy carbides evenly, significantly improving the structure and mitigating the adverse effects of forging and original structures.
The tempering process results in evenly distributed carbides and a fine-grained sorbite structure, increasing the comparative volume of the original structure.
This not only improves the mechanical properties of the steel, but also helps to minimize deformation. For high-alloy tool steel molds (such as high-chromium steel), different degrees of shrinkage may occur during quenching after tempering.
Therefore, replacing high-temperature tempering with annealing during the tempering process can produce better results after quenching.
Alloy structural steel can achieve greater hardness through pre-tempering treatment, which also minimizes volume changes during quenching, reducing potential warping and cracking.
The use of low-temperature annealing to relieve the stress of cold working in molds is simpler than tempering, with a shorter cycle, less oxidation and applicability to different materials in the same process.
To eliminate lattice carbides caused by poor forging and increase the depth of the hardened layer, normalizing treatment can be applied.
In summary, all types of preheating treatment must be carried out in accordance with the mold expansion and contraction patterns, adjusting the initial structure and eliminating machining stresses to reduce deformation and cracking.
4. Implementing Reasonable Heat Treatment Processes
To minimize and avoid distortion of the workpiece, not only is it necessary to rationally design the workpiece, select materials, formulate technical requirements for heat treatment and correct heat processing (casting, forging, welding) and heat pretreatment of blanks, but it is also essential to pay attention to the following issues in heat treatment:
(1) Rational Heating Temperature Selection
To ensure hardening, the quenching temperature should generally be as low as possible. However, for certain high-carbon alloy steel molds (such as CrWMn, Cr12Mo steel), quenching distortion can be controlled by appropriately increasing the quenching temperature to lower the Ms point, increasing the amount of residual austenite.
In addition, for thicker high-carbon steel molds, the quenching temperature can also be slightly increased to prevent the occurrence of quenching cracks.
For molds prone to distortion and cracking, stress relieving annealing should be performed prior to tempering.
(2) Rational Heating Process
Uniform heating should be achieved as much as possible to minimize thermal stress during heating.
For high-alloy steel molds with large cross-sections and complex shapes with high distortion requirements, preheating or restricted heating rates should normally be applied.
(3) Correct Selection of Cooling Method and Medium
The quenching methods of pre-cooling, step-quenching and step-cooling should be selected as much as possible.
Pre-cooling quenching has a good effect on reducing distortion in long, thin or thin molds, and to a certain extent it can reduce distortion in molds with significant thickness variations.
For complex shapes or molds with significant sectional differences, step hardening is preferred. For example, using step quenching at 580-620°C for high-speed steel essentially avoids quenching distortions and cracks.
(4) Correct Mastery of Quenching Operation Methods
The correct choice of how to immerse the part in the medium must ensure more uniform cooling of the mold and entry into the cooling medium via the path of least resistance, with the slower cooling side facing the direction of liquid movement.
When the mold cools below point Ms, movement should stop. For example, molds with uneven thickness should be immersed first in the thickest part; parts with significant section changes can reduce heat treatment deformation by increasing process holes, reserving reinforcing ribs, plugging holes with asbestos, etc.
For parts with concave surfaces or through holes, the concave side and holes must be immersed upward to expel bubbles inside the through holes.
V. conclusion
Heat treatment is an essential manufacturing process in mold production, significantly impacting the quality and cost of the mold, and serves as a crucial measure to increase its useful life. Deformation and cracking are two major challenges during mold heat treatment.
The causes behind these problems are complex, but by understanding their patterns, performing thorough analysis and research, and accurately addressing the issues, it is possible to reduce mold deformation and effectively control cracking.
