Mold material selection must abide by two principles
(1) the mold meets the requirements of working conditions
1. When the abrasion resistant blank is plastic deformed in the mold cavity, it flows and slides along the surface of the mold cavity, causing severe friction between the mold surface and the blank, which leads to the failure of the mold due to wear.Therefore, one of the most basic and important properties of the material wear - resistant mold.Hardness is the main factor that affects wear resistance.Generally, the higher the hardness of the mold parts, the smaller the wear amount and the better the wear resistance.In addition, wear resistance is also related to the type, quantity, shape, size and distribution of carbide in the material.
2. The working conditions of the strong and ductile molds are mostly very harsh, and some of them usually bear a large impact load, resulting in brittle fracture.In order to prevent the mold parts from breaking suddenly when working, the mold should be of high strength and toughness.The toughness of the mold mainly depends on the carbon content, grain size and microstructure.
3. Fatigue fracture performance during the working process of the mold, under the action of cyclic stress, usually leads to fatigue fracture.Its forms include repeated impact fatigue fracture of small energy, contact fatigue fracture of tensile fatigue fracture and bending fatigue fracture.The fatigue and fracture properties of the mold mainly depend on the strength, toughness, hardness and the content of inclusions in the material.
4. High-temperature performance when the working temperature of the mold is higher, the hardness and strength will be reduced, leading to early wear or plastic deformation of the mold and failure.Therefore, the mold material should have higher tempering stability to ensure that the mold at the working temperature has a higher hardness and strength.
Some molds are in the state of repeated heating and cooling during working process, which makes the surface of the cavity subject to tension and pressure change stress, causing the surface crack and spalling, increasing the friction force, preventing the plastic deformation and reducing the dimensional accuracy, thus leading to the failure of the molds.Cold - heat fatigue is one of the main forms of failure of hot - working die.
6. Corrosion resistance some molds, such as plastic molds, decompose and precipitate strong corrosive gases such as HCI and HF after heat treatment due to the presence of chlorine, fluorine and other elements in the plastics while working, which will erode the mold cavity surface, increase the surface roughness and aggravate the failure of wear.
(ii) the mold meets the process performance requirements
Die manufacturing generally through forging, cutting processing, heat treatment and other processes.In order to guarantee the quality of mold manufacturing and reduce the production cost, the materials should be of good forgability, machinability, hardenability, hardenability and grindability.There should also be small oxidation, decarburization sensitivity and quenching deformation cracking tendency.
1. Low malleability of hot forging, good plasticity, wide range of forging temperature, low tendency of cold cracking and precipitated mesh carbide.
2. The annealing technology has a wide range of spheroidizing annealing temperature, low annealing hardness and small fluctuation range, and high spheroidization rate.
3. Large cutting capacity, low tool loss and low roughness.
4. Oxidative and decarburizing sensitivity: good antioxidant capacity, slow decarburizing speed, insensitive to heating medium, small tendency of pitting.
5. Uniform and high surface hardness after hardening.
6. Deep quenching layer can be obtained after hardenability and hardenability can be achieved by using tempered quenching medium.
7. The normal quenching volume changes little, the shape warps and distortion is slight, and the abnormal deformation tendency is low.Conventional quenching is not sensitive to quenching temperature and workpiece shape.
8. Relatively small wear and tear of grindable grinding wheel, large amount of grinding without burn, insensitive to quality and cooling conditions of grinding wheel, and less prone to grinding injuries and grinding cracks.