The mold undergoes a heat treatment process consisting of preheating, terminal heating, and surface hardening.
Heat treatment defects refer to various problems that occur during the final stage of the mold’s heat treatment or in subsequent processes and usage. These defects may include quenching cracks, poor dimensional stability, insufficient hardness, cracks from electromachining, grinding cracks, and early mold failure.
A more in-depth analysis is provided below.
The causes of quenching cracks and preventive measures are as follows:
- Shape Effect: This is primarily caused by design factors such as small rounded corners, improper hole placement, and poor cross-section transitions.
Preventive 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 furnace temperature.
Preventive Measures: Maintain and proofread the temperature control system, adjust the process temperature, and add iron between the workpiece and the furnace floor.
- Decarburization: This is caused by overheating or overburning, 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 anti-oxidation coating, and increase the machining allowance by 2-3mm.
- Improper Cooling: This is mainly caused by improper selection of the coolant or overcooling.
Preventive Measures: Understand the cooling characteristics of the quenching medium or tempering treatment and select the appropriate coolant.
- Poor Structure of the Mold Steel: This can be caused by severe segregation of carbides, poor forging quality, and improper preheat treatment.
Preventive Measures: Use the correct forging process and implement a reasonable preheat treatment system.
The reasons and precautions for insufficient hardness are as follows:
- Improper Furnace or Cooling Tank: This is caused by incorrect process temperature or errors in the temperature control system.
Preventive Measures: Correct the process temperature and overhaul and check the temperature control system. When installing the furnace, workpieces should be spaced evenly and not stacked or bundled 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 overhaul and check 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 overhaul and check the temperature control system. Enter the furnace at a temperature no higher than the set temperature.
- Improper Cooling: This can occur if pre-cooling time is too long, the cooling medium is not selected properly, the temperature of the quenching medium increases while cooling performance decreases, stirring is poor, or the outlet temperature of the tank is too high.
Preventive Measures: Quickly enter the tank from the oven, understand the cooling characteristics of the quenching medium, add or cool the quenching medium if necessary, strengthen the stirring of the coolant, and remove at a temperature of Ms + 50°C.
- Decarburization: This is caused by residual decarburization layers from raw materials or during the quenching and heating process.
Preventive Measures: Use controlled atmosphere and salt bath heating, vacuum furnace and box furnace with box protection or anti-oxidation coating, and increase the machining allowance by 2-3mm.
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 martensite transformation 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 various factors, such as the type of steel, the shape of the mold, the improper distribution of carbides, and the method of forging and heat treatment, can all contribute to or worsen the problem.
Additionally, changes in any of the various conditions during heat treatment can greatly impact the degree of deformation in the steel piece.
For a long time, solving the issue of heat treatment deformation was mainly done through experience and heuristics. However, it is crucial to have a thorough understanding of the relationship between mold steel forging, module orientation, mold shape, heat treatment method, and heat treatment deformation. This understanding can be gained by analyzing the accumulated data and establishing archives of heat treatment deformation.
Decarburization is a phenomenon and reaction in which carbon on the surface layer of steel is completely or partially lost due to the effect of the surrounding atmosphere during heating or insulation.
The decarburization of steel parts can result in insufficient hardness, quench cracking, heat treatment deformation, and chemical heat treatment defects. Additionally, it can significantly impact fatigue strength, wear resistance, and mold performance.
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, the 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 electric discharge. This process forms a white EDM deteriorative layer on the machining surface and generates a tensile stress of about 800 MPa. As a result, deformation or cracks may occur during the electrical processing of the mold.
Therefore, when using EDM molds, it’s crucial to understand the impact of EDM on the mold steel and take preventive measures to avoid defects:
- Prevent overheating and decarburization during heat treatment, and temper the steel fully to reduce or eliminate residual stress.
- To fully 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 the relaxation treatment.
- Set reasonable process holes and slots.
- Fully eliminate the re-solidified layer to ensure it is in a sound state.
- Using the principle of vector translation, cut through and disperse the internal stress of the cutting outpost through drainage.
The insufficient toughness can be attributed to either excessively high quenching temperature and prolonged holding time, leading to grain coarsening, or failing to avoid tempering in the brittle zone.
The presence of a large amount of retained austenite in the workpiece can result in structural stress and cracking of the workpiece when the tempering transformation occurs during grinding heat. To prevent this, two preventive measures can be taken: performing a cryogenic treatment after quenching or repeating the tempering process (typically 2-3 times for low-alloy tool steels in cold working) to minimize the amount of retained austenite.