plastically
Hardened die steel has poor plasticity, especially cold-deformed die steel, which is brittle with little deformation. The plasticity of die steel is usually measured by two indexes: elongation after fracture and area shrinkage.
Elongation after fracture refers to the relative percentage of length increase of tensile samples after fracture. Express delivery. Elongation after fracture? The greater the value, the better the plasticity of steel. The plasticity of hot die steel is obviously higher than that of cold die steel.
Area shrinkage refers to the ratio of the reduction of the broken section to the original section after the tensile deformation and fracture of the tensile test bar. Express delivery. Plastic materials have obvious necking after tensile fracture, then? Great value. However, after the brittle material is broken, the cross section hardly shrinks, that is, there is no necking. The value is very small, indicating that the plasticity is very poor.
toughness
Toughness is an important performance index of die steel, which determines the fracture resistance of materials under impact test force. The higher the toughness of the material, the smaller the risk of brittle fracture and the higher the thermal fatigue strength. This is of great significance for measuring the brittle fracture tendency and impact toughness test of dies.
Impact toughness refers to the impact absorption work on the cross-sectional area at the notch of the impact sample, and the impact absorption work refers to the work absorbed when the sample with a specified shape and size breaks under an impact test force. Impact tests include Charpy U-notch impact test (sample notch is U-shaped), Charpy V-notch impact test (sample notch is V-shaped) and Ehrlich impact test.
There are many factors that affect impact toughness. The impact toughness of die steels made of different materials varies greatly. Even the same material has different impact toughness due to different microstructure, different grain size, different internal stress state. Generally, the coarser the grain, the more serious the carbide segregation (banded, reticulated, etc.). ), and the coarser the martensite structure. This will make steel brittle. Impact toughness is different at different temperatures. Generally speaking, the higher the temperature, the higher the impact toughness value, while some steels have good toughness at room temperature, and will become brittle steel when the temperature drops to MINUS 20 ~ 40℃.
In order to improve the toughness of steel, reasonable forging and heat treatment processes must be adopted. When forging, carbide should be broken as much as possible to reduce or eliminate carbide segregation. In the process of heat treatment and quenching, the grain growth should be prevented from being too large and the cooling rate should not be too high to prevent internal stress. Some measures should be taken to reduce the internal stress before or during the use of the mold.
Special performance requirements
Because there are many kinds of molds, the working conditions are very different, and the conventional performance and matching requirements of molds are also different. The actual performance of a mold is not consistent with the data measured by the sample under specific conditions. Therefore, in addition to measuring the conventional properties of materials, it is also necessary to measure the service characteristics of the mold according to the simulated actual working conditions, and put forward requirements for the special properties of the mold, and establish a system for correctly evaluating the performance of the mold.
The hardness, strength and impact toughness of hot working dies 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 trend and speed of performance changes are also very different. For example, although the hardness of material A is higher than that of material B at room temperature, the hardness decreases obviously with the increase of temperature, reaching? After reaching a certain temperature, the hardness value will be lower than that of material B. Therefore, when high wear resistance is required at high temperature, material A should not be selected, but material B with lower hardness at room temperature, but the hardness decreases slowly with the increase of temperature.
In addition to the hardness, strength and toughness at high temperature, hot working dies also require some special properties.
thermostability
Thermal stability means the ability of steel to keep its metallographic structure and properties stable during heating. The thermal stability of steel is usually expressed by the highest heating temperature when the tempering temperature is 4 hours and the hardness drops to 45 hours RC. This method is related to the original hardness of the material. It is reported that the highest heating temperature of the steel that reaches the predetermined strength level and keeps the temperature for 2 hours to reduce the hardness to the failure hardness of the general hot forging die of 35HRC is regarded as the stability index of the steel. For hot working dies that fail due to insufficient heat resistance, the life level of the dies can be predicted according to the thermal stability.
Tempering stability
Tempering stability refers to the degree to which the strength and hardness of materials decrease with the increase of tempering temperature, which is also called tempering resistance or tempering softening resistance. It is usually expressed by tempering temperature-hardness curve of steel, and slow decrease of hardness indicates high tempering stability or high tempering resistance. Tempering stability is also related to the microstructure change during tempering, which, together with the thermal stability of steel, represents the microstructure stability of steel at high temperature and the deformation resistance of die at high temperature.
Fracture resistance
In addition to the one-time fracture resistance indexes of conventional mechanical properties such as impact toughness, compressive strength and bending strength, the low-energy multiple impact fracture resistance is more suitable for the practical use of cold-working dies. As the performance index of mold materials, it also includes compression fatigue strength, contact fatigue strength and so on. The fatigue fracture index is characterized by the number of fracture cycles measured under a certain cyclic stress or the load that leads to fracture under a certain cycle number. Whether fracture toughness is an important performance index of cold working die materials needs to be studied and discussed.
Anti-occlusion ability and anti-softening ability
Anti-occlusion ability and anti-softening ability respectively characterize the appearance of mold pairs. Cold welding? As well as hardness and wear resistance caused by temperature rise during loading.
Thermal fatigue resistance and fracture toughness
Thermal fatigue resistance represents the working life of the material before thermal fatigue crack initiation and the growth rate after initiation. Thermal fatigue is usually at 20℃? The number of cycles or the length of cracks during repeated heating and cooling at 750℃ shall be determined after a certain number of cycles. Materials with high thermal fatigue resistance are not prone to thermal fatigue cracks, or the expansion amount is small and slow when cracks are initiated. Fracture toughness represents resistance to crack instability and propagation. If the fracture toughness is high, the crack is not easy to be unstable and propagate.
High temperature wear and oxidation resistance
High temperature wear is one of the main failure forms of hot working dies. In general, most hammer forging dies and press dies fail due to wear. Thermal and wear resistance is a requirement for the performance of hot working dies, and it is a comprehensive embodiment of various high-temperature mechanical properties. At present, some domestic units have carried out the thermal wear test of molds on the self-made thermal wear machine, and received ideal test results.
The practical application shows that the oxidation resistance of die materials has a great influence on the service life of the die. Because oxidation will aggravate the wear and tear in the working process of the die, the die cavity size is out of tolerance and it is scrapped. Oxidation will also cause corrosion grooves on the die surface, which will become the source of thermal fatigue cracks and aggravate the initiation and propagation of thermal fatigue cracks in the die. Therefore, the mould is required to have certain oxidation resistance.
In addition to the conventional mechanical properties, cold-working die steel usually needs the following properties:
Wear resistance, fracture resistance, bite resistance and oxidation resistance.
abrasive resistance
During the use of the cold working die, the formed blank will slide and flow along the die surface, resulting in great friction between the die and the blank. This kind of friction causes the die surface to be subjected to shear stress, and concave and convex marks are carved on its surface, which bite with the uneven surface of the blank, gradually causing mechanical damage or wear on the die surface. Cold working molds, especially those that fail normally, are mostly scrapped due to wear. Therefore, one of the most basic requirements for cold working dies is wear resistance. Under normal circumstances, the higher the hardness of the material, the better the wear resistance. However, wear resistance is also closely related to the shape and distribution of hard spots on soft matrix.
The wear of cold working dies includes abrasive wear, adhesive wear, corrosion wear and fatigue wear.