热处理力学性能指标与检测方法
The mechanical properties of metallic materials mainly include strength and plasticity, hardness, impact toughness and fatigue strength. The strength index of the mechanical property test of metallic materials can better help us select materials in the field of mechanical processing and better meet the requirements for the performance of components.
I. Strength and Strength Indicators
Strength refers to the ability of metallic materials to resist plastic deformation or fracture when subjected to static loads. The strength of metallic materials is expressed by stress, which can accurately reflect the internal force state of the metallic materials. The internal force per unit area (the force within a material that opposes the external force) is called stress.
The strength index of metallic materials is usually expressed by yield strength. When metallic materials exhibit the yield phenomenon, the stress at which plastic deformation occurs without an increase in load is the yield strength. Yield strength is further divided into upper yield strength and lower yield strength, which are respectively represented by Reh and ReL. Generally, the components we use are not allowed to undergo obvious plastic deformation. The higher the yield strength of the material, the stronger its ability to resist plastic deformation will be, and the better the performance of the material in use will be. Therefore, yield strength is the main basis for us to design components and select materials.
Ii. Plasticity and Plasticity Indicators
Plasticity refers to the ability of metallic materials to withstand the maximum plastic deformation before fracture under load. Plasticity indicators are usually expressed by elongation after fracture and reduction of area.
The elongation after fracture is denoted by A, which is the percentage of the difference between the gauge length of the sample after breaking and the original gauge length to the original gauge length.
The reduction of area is denoted by Z, which is the percentage of the difference between the original cross-sectional area of the specimen and the minimum cross-sectional area at the fracture to the original cross-sectional area.
Among them, the reduction of area and the elongation can more accurately reflect the plasticity of metallic materials. Because the reduction of area is not affected by the size of the specimen. The greater the reduction of area and elongation, the better the plasticity of this metal material will be, making it easier to undergo shaping processing and more likely to be processed into complex-shaped components. Among them, materials with good plasticity will undergo plastic deformation first when subjected to relatively large impact forces without suddenly breaking, and their performance in use is relatively safe.
Iii. Hardness and Hardness Testing
Hardness refers to the ability of metallic materials to resist local plastic deformation and failure. There are many types of hardness testing methods, among which the Brinell hardness test method, Rockwell hardness test method and Vickers hardness test method are the most commonly used.
(1) Brinell hardness test method ① Principle of Brinell hardness test. A hard alloy ball is used as the indenter, and it is simultaneously pressed into the metal surface under a certain test pressure. After maintaining for a specified period of time, the test force is eliminated. Finally, the diameter of the indentation surface is measured, and the hardness value is calculated through the calculation formula. Experiments show that the magnitude of the Brinell hardness value is only related to the size of the indentation diameter. The larger the D, the larger the diameter of the indentation, which means the lower the hardness.
② Characteristics and application scope of Brinell hardness. The test force and indentation diameter of Brinell hardness are both relatively large, so the indentation is also relatively large, which is a more intuitive and accurate way to measure the hardness value. However, due to the relatively large indentation, the degree of damage to the metal surface is also considerable. This requires that the Brinell hardness test method is not suitable for measuring components with high surface quality requirements or thin-walled components.
(2) Rockwell Hardness test Method ① Principle of Rockwell hardness test. The principle of the Rockwell hardness test is similar to that of the Brinell hardness test. The difference lies in that the spherical indenter of the hard alloy is changed to a diamond conical indenter, and the hardness value is measured by the depth of the indentation. There are three commonly used Rockwell hardness scales: A, B, and C, with the C scale being the most frequently used. Hardness values measured on different scales cannot be compared with each other; they must be converted to the same scale for comparison.
② Characteristics and application scope of Rockwell hardness. Rockwell hardness is different from Brinell hardness. Its indentation is relatively small, causing less damage to components. Therefore, it can measure some thin-walled and components with high surface quality requirements. However, due to the small indentation, there are certain limitations, and the measured hardness values are not accurate enough.
(3) Vickers Hardness test method The difference between the Vickers hardness test and the Brinell hardness test lies in that the indenter is a diamond regular tetrahedral pyramid, and the hardness value is calculated based on the length of the diagonal of the indentation. The test force applied in the Vickers hardness test is relatively small and the indentation is not obvious, so it can measure thin-walled parts. However, the measurement of the diagonal of the indentation is rather complicated.
Iv. Impact Toughness and Fatigue Strength
During the actual processing of metallic materials, the loads they bear are not only static loads, but also impact loads and alternating loads, and may be the superposition of multiple loads. It is far from sufficient to study static loads only through tensile tests and hardness tests. This requires us to study the mechanical properties of metallic materials under various loads. Therefore, the research on impact loads and fatigue loads is of great significance.
Impact toughness, as the name suggests, refers to the ability of a metallic material to resist impact loads without being damaged. Impact toughness is mainly measured through bending tests. For the measurement of impact force, we mainly rely on the magnitude of impact toughness. When metallic materials are subjected to large energy impact loads, the size of impact resistance mainly depends on the magnitude of impact toughness. However, under multiple repeated impact actions, the impact resistance is mainly determined by the strength and plasticity of the material.
(2) Fatigue strength: In our daily life, we often encounter this phenomenon. Many mechanical components, although they bear forces lower than the yield strength of the material, will still break after long-term operation. This phenomenon is metal fatigue, and fatigue failure is one of the main causes of component damage. The fatigue strength of metallic materials is determined by multiple factors, such as the working conditions of components, the internal structure of the material, its organizational composition, and the stress it bears. Therefore, improving the structure of components and adopting some surface strengthening methods can both enhance the fatigue strength of metallic materials
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