Principles of mold material selection
（1） Meet the requirements of working conditions
1. Wear resistance
When the billet undergoes plastic deformation in the mold cavity, it flows and slides along the surface of the cavity, causing severe friction between the cavity surface and the billet, resulting in the failure of the mold due to wear. So the wear resistance of materials is one of the most basic and important properties of molds.
Hardness is the main factor affecting wear resistance. In general, the higher the hardness of mold parts, the smaller the wear amount, and the better the wear resistance. In addition, wear resistance is also related to the type, quantity, morphology, size, and distribution of carbides in the material.
2. Strong resilience
The working conditions of molds are mostly very harsh, and some often bear large impact loads, leading to brittle fracture. To prevent sudden brittle fracture of mold parts during operation, the mold should have high strength and toughness.
The toughness of molds mainly depends on the carbon content, grain size, and microstructure of the material.
3. Fatigue fracture performance
During the working process of molds, long-term cyclic stress often leads to fatigue fracture. Its forms include low-energy multiple impact fatigue fracture, tensile fatigue fracture, contact fatigue fracture, and bending fatigue fracture.
The fatigue fracture performance of molds mainly depends on their strength, toughness, hardness, and the content of inclusions in the material.
4. High temperature performance
When the working temperature of the mold is high, it will cause a decrease in hardness and strength, leading to early wear or plastic deformation of the mold and failure. Therefore, the mold material should have high resistance to tempering stability to ensure that the mold has high hardness and strength at working temperature.
5. Cold and hot fatigue resistance performance
Some molds are in a state of repeated heating and cooling during the working process, causing the surface of the mold cavity to be subjected to tensile and compressive stress, resulting in surface cracking and peeling, increasing friction, hindering plastic deformation, reducing dimensional accuracy, and leading to mold failure. Cold and hot fatigue is one of the main forms of failure in hot work molds, and such molds should have high resistance to cold and hot fatigue.
6. Corrosion resistance
Some molds, such as plastic molds, may decompose and release highly corrosive gases such as HCI and HF due to the presence of chlorine, fluorine, and other elements in the plastic during operation. These gases can corrode the surface of the mold cavity, increase its surface roughness, and exacerbate wear and failure.
（2） Meet the requirements of process performance
The manufacturing of molds generally involves several processes such as forging, cutting, and heat treatment. To ensure the manufacturing quality of molds and reduce production costs, their materials should have good malleability, machinability, hardenability, hardenability, and grindability; It should also have low sensitivity to oxidation and decarburization, as well as a tendency towards quenching deformation and cracking.
1. Malleability
Has low resistance to hot forging deformation, good plasticity, wide forging temperature range, and low tendency for forging cracking, cold cracking, and precipitation of network carbides.
2. Annealing processability
The spheroidization annealing temperature range is wide, the annealing hardness is low and the fluctuation range is small, and the spheroidization rate is high.
3. Machinability
Large cutting amount, low tool wear, and low surface roughness during machining.
4. Sensitivity to oxidation and decarbonization
When heated at high temperatures, it has good antioxidant capacity, slow decarburization rate, insensitivity to the heating medium, and a small tendency to produce pitting.
5. Quenching hardening
After quenching, it has a uniform and high surface hardness.
6. Hardenability
After quenching, a deeper hardened layer can be obtained, which can be hardened by using a mild quenching medium.
7. Quenching deformation and cracking tendency
Conventional quenching has small volume changes, slight shape warping and distortion, and low tendency for abnormal deformation. Conventional quenching has low sensitivity to cracking and is insensitive to quenching temperature and workpiece shape.
8. Grindability
The relative wear of the grinding wheel is small, the burn free limit grinding amount is large, and it is insensitive to the quality and cooling conditions of the grinding wheel, making it less prone to grinding damage and cracks.
（3） Meet economic requirements
When selecting materials for molds, the principle of economy must be considered to minimize manufacturing costs as much as possible. Therefore, on the premise of meeting the performance requirements, the first choice is to choose lower priced ones. If carbon steel can be used, alloy steel is not needed, and if domestic materials can be used, imported materials are not needed