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            Mechanical properties of die - conventional mechanical properties

            Author: ComeFrom: Date:2018/7/31 13:32:32 Hits:406
            The properties of the die material are determined by the composition of the mold material and the microstructure after heat treatment. The basic structure of die steel is composed of martensite matrix and carbide and intermetallic compound distributed on the matrix.
            The performance of die steel should meet the performance of a certain mold to complete the rated workload, but the requirements of the mould performance are different because of the different use conditions and the rated workload indicators. Because the chemical composition and microstructure of different steels have different effects on various properties, even the same grade of steel can not obtain the best value of all kinds of properties at the same time, the general performance improvement will lose other performance. Therefore, mould workers often choose die steel and the best treatment process according to the working conditions and quota of the mould, so that the main performance is best and the other performance is minimal.
            The performance requirements for various die steels include hardness, strength, plasticity and toughness.
            Mechanical properties of mould -- hardness
            Hardness shows the resistance of steel to deformation and contact stress. The hardness test is easy to prepare, the workshop and the laboratory are usually equipped with a hardness tester. Therefore, the hardness is a very easy to measure, and the hardness is also related to the strength, and the hardness value of the material can be obtained through the conversion of hardness and strength. The mold category defined by the hardness range, such as high hardness (52 ~ 60HRC), is generally used for cold working die, medium hardness (40 ~ 52HRC), generally used for hot working die.
            The hardness of steel is closely related to its composition and microstructure. A wide range of hardness can be obtained by heat treatment. For example, the new mold steel 012Al and CG - 2 can be treated by low temperature tempering respectively with the hardness of 60 ~ 62HRC and the hardness of 50 ~ 52HRC after the high temperature tempering treatment, so it can be used to make the cold and hot die with different hardness requirements. Therefore, this type of die steel can be called cold work and hot working die steel.
            In addition to martensite in mold steels, other phases with higher hardness exist, such as carbides and intermetallic compounds. Table L is the hardness of common carbides and alloy phases.
            Phase hardness HV
            About 100 ferrite
            Martensite: Omega C 0.2% about 530
            Martensite: Omega C 0.4% about 560
            Martensite: Omega C 0.6% about 920
            Martensite: Omega C 0.8% about 980
            Carburized carburized body (Fe 3 C) 850~1100
            Nitrides 1000~3000
            Intermetallic compound 500
            The hardness of die steel mainly depends on the dissolved carbon content (or nitrogen content) in martensite, and the carbon content in martensite depends on austenitizing temperature and time. When the temperature and time increase, the martensite hardness in martensite increases with the increase of carbon content in martensite, but the austenite grain increases with the high quenching temperature, and the residual austenite volume increases after quenching, which will lead to the decrease of hardness. Therefore, in order to select the best quenching temperature, the quenching temperature, grain size hardness relationship curve of the steel must be first made.
            The content of carbon in martensite is related to the alloying degree of steel to a certain extent, especially when tempering. With the increase of tempering temperature, the carbon content in martensite decreases, but when the alloy content in the steel is higher, the more obvious the two hardening effect is, the higher the peak of the hardening effect.
            Mechanical properties of moulds -- plastic properties
            The hardened die steel is of poor plasticity, especially the cold deformed die steel, which occurs brittle fracture at very small plastic deformation. To measure the plasticity of mould steel, it is usually indicated by two indexes of elongation after failure and shrinkage of section.
            The elongation after failure is the relative percentage of the length of the tensile specimen after being broken, expressed in Delta. The greater the elongation after failure, the better the plasticity of steel. The plasticity of hot die steel is obviously higher than that of cold mode steel.
            The section shrinkage ratio is the ratio of the reduction of the fracture section to the original section after tensile deformation and fracture, expressed as_. Plastic material has obvious necking after breaking, so the value is larger. When the brittle material is broken, the cross section almost does not shrink, that is, there is no necking, and the value is very small, which means that the ductility is very poor.
            Mechanical properties requirements of die - toughness
            Toughness is an important performance index of die steel. Toughness determines the fracture resistance of material under impact test force. The higher the toughness of the material is, the smaller the risk of brittle fracture and the higher the thermal fatigue strength. It is important to measure the brittle fracture tendency and impact toughness test.
            The impact toughness refers to the impact absorption work on the cut area of the impact specimen, and the impact absorption work refers to the work absorbed by the specimen with the specified shape and size when the impact test force is broken once. The impact test includes the Charpy U - shaped notch impact test (the sample U - shaped notch), the Charpy V - shaped notch impact test (the sample of the V - shaped notch) and the AI impact test.
            There are many factors affecting the impact toughness. The impact toughness of die steel with different materials is very different. Even the same material, the impact toughness is different because of the different microstructure, grain size and internal stress state. Usually the coarser the grain is, the more serious the carbide segregation (band, network, etc.), and the coarser the martensite structure will make the steel brittle. The impact toughness is not the same as the temperature is different. Generally speaking, the higher the temperature, the higher the impact toughness, and some steels at room temperature toughness is very good, when the temperature drops to minus 20 ~ 40 degrees Celsius will become brittle steel.
            In order to improve the toughness of steel, reasonable forging and heat treatment processes must be adopted. The carbides should be broken as much as possible and the carbide segregation should be reduced or eliminated. When the heat treatment is quenched, the grain should be prevented from overgrowing and the cooling speed should not be too high to prevent the internal stress from producing. Some measures should be taken to reduce internal stress before or during the use of dies.
            Mechanical properties requirements of die - special performance requirements
            Because of various kinds of mould and great difference of working conditions, the normal performance and mutual matching requirements of the die are different, and the actual performance of some mold is not consistent with the data measured under the specific conditions. Therefore, in addition to the routine performance of the material, it is necessary to measure the use characteristics of the mould according to the actual working conditions, and put forward the requirements for the special performance of the die, and establish a system for the correct evaluation of the die performance.
            The hardness, strength and impact toughness of hot working die must be tested at high temperature. Because the hot working die is in service at a certain temperature, the performance data measured at room temperature will change when the temperature rises. The difference of performance trend and rate is also great, for example, the hardness of A material is higher than that of material B at room temperature, but with the temperature rising, the hardness decreases significantly. After reaching the temperature, the hardness value will be lower than that of the material B. Then, when the wear resistance is high under high temperature conditions, A materials can not be selected, but the material B, which is low in hardness at room temperature but with a slow decrease of hardness with the increase of temperature, is required.
            In addition to the hardness, strength and toughness of the hot working die, the hot working die also requires some special properties.
            Mechanical properties of die - thermal stability
            Thermal stability characterizing the stability of steel in maintaining its microstructure and properties during heating. In general, the thermal stability of steel is tempered by 4h, and the highest heating temperature when hardness drops to 45HRC. This method is related to the original hardness of the material, and the data will be heated to the steel of the predetermined strength level. The heat preservation is 2H. The maximum heating temperature of the hardness to the failure hardness of the general hot forging die is determined to be the stability index of the steel. For hot working moulds that fail to accumulate due to insufficient heat resistance, the service life of dies can be predicted according to thermal stability.
            Mechanical property requirements of die - tempering stability
            Tempering stability refers to the degree to which the strength and hardness of a material decrease with the increase of tempering temperature, also known as tempering resistance or tempering softening energy.
            Thermal fatigue resistance represents the working life and the rate of expansion after thermal fatigue crack initiation. Thermal fatigue is usually determined by the number of cycles of cracks that occur during repeated heating and cooling at the temperature of 20 750 C or by measuring the length of cracks after a certain number of cycles. The material with high thermal fatigue resistance is not easy to generate thermal fatigue crack, or when the crack initiates, the expansion volume is small and the expansion is slow. Fracture toughness is characterized by crack propagation instability, high fracture toughness, and unstable crack propagation.
            Requirements for mechanical properties of dies -- high temperature wear and oxidation resistance
            High temperature wear is one of the main failure modes of hot working dies. Under normal conditions, most hammer forging dies and press dies are invalid due to wear. Thermal wear resistance is a requirement for the performance of hot working dies, and is a comprehensive embodiment of various high temperature mechanical properties. At present, some domestic units have made the die hot wear test on the self-made thermal wear machine, and have received an ideal test result.
            The actual application shows that the oxidation resistance of mould materials has great influence on the service life of dies. The oxidation will aggravate the wear and tear of the die during the working process, resulting in the mold cavity size being oversized and scrapped. Oxidation will also cause corrosion groove on the surface of the mold and become the origin of thermal fatigue crack, which will aggravate the initiation of thermal fatigue cracks.
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