First, the characteristics of the hardness test Now take the hardness of the press-in method as an example, the characteristics are summarized as follows: 1. When the hardness of the press-in is tested, the stress state of the loading mode is the softest (α>2), that is, the maximum shear stress is much larger than Maximum normal stress. Any metal material will be plastically deformed under such stress conditions, and the test sample will not completely destroy even if it is very brittle. Therefore, in principle, any metal material can be subjected to the indentation hardness test and ensure that the material to be tested is not damaged. That is, the hardness test can be regarded as a non-destructive test. This is very important for many parts. 2. The hardness test is the only mechanical property test in the case where the part cannot be processed into other mechanical properties. For some large parts, a hardness tester can also be used for hardness testing. 3. The hardness test can be used to identify the metal composition or phase composition of the metal (such as microhardness, etc.) and can be used to inspect the sheet or surface layer (such as chemical heat to download the surface layer of missing parts, metal plating or coating, etc.) Quality. This is difficult to do with other mechanical properties tests. 4. Hardness test equipment and operation are relatively simple, convenient and easy to master. 5. It is particularly useful that, as described in this chapter, there is a certain relationship between hardness and other properties. Therefore, in the production, research and selection of materials, hardness testing can be used more quickly to judge and inspect metal materials and their systems. Other mechanical properties of the part, as well as control and optimization of the process during production and research. It is precisely because of the hardness test that it has been widely used in production and research. Hardness testing has become the most basic, first, and indispensable important test method in the mechanical properties test of materials.
Second, the limitations of the hardness test The above mentioned the characteristics and importance of the hardness test. However, the hardness test is by no means a panacea. Since the hardness only reflects the deformation resistance and the breaking force in the local small volume of the surface layer of the material, it also has certain limitations in use. 1. Hardness values alone do not reflect the mechanical properties of different materials. For different materials, if tensile strength (σb), yield strength (σb or σ0.2), elongation (δ), reduction of area (ψ) and When the mechanical properties such as impact toughness (αk) are the same, there are many cases where they can be considered to have the same mechanical behavior and interchangeability under various bearing conditions. For example, in mechanical manufacturing, 40 MnB can be used instead of 40 Cr because the σb, σs, δ, ψ, and Ak of the two are the same or similar (see Table 1). However, if two steel parts are only of the same hardness, their other mechanical properties may vary greatly. For example, the two alloys with the same carbon content but the alloy elements contain the first two different steels. The mechanical properties after quenching and tempering are shown in Figure 1. The hardness of the two is the same, but other mechanics. The performance is very different: steel shafts with low alloying elements (low hardenability), other mechanical properties (especially impact toughness) are much lower than steel shafts with more alloying elements (high hardenability). 3. Hardness is not representative of the mechanical properties under certain conditions. It is precisely because hardness can not fully reflect the other mechanical properties of the material, especially the overall mechanical properties. Therefore, in many working conditions, only the hardness is far from enough. Characterize the performance of the material. For example, although there is a certain approximation between hardness and fatigue limit, stress fatigue or strain fatigue performance cannot be fully reflected, because fatigue performance is not only related to the overall mechanical properties of the part (this is the most important), but also The surface is related to the surface stress caused by surface treatment (such as whether it has been shot peened). Hardness also does not indicate the performance of the part at high and low temperatures and the performance under corrosive conditions. Of course, the high and low temperature hardness values can be used to qualitatively compare the high and low temperature mechanical properties of the material, but the hardness cannot account for creep and permanent strength, because the hardness is only a mechanical property index measured in a short time. As for the mechanical properties of the material in a corrosive environment, it is not only related to stress, but also closely related to the corrosion resistance of the material. Only the hardness is used to explain the performance of the material. Hardness alone does not fully reflect the wear resistance of the material. As mentioned above, wear parts such as gauges, cutting tools and grinding balls generally have higher hardness and better wear resistance. But this is only a general, rough situation. Because wear resistance is a relatively complex mechanical property, in addition to hardness, it is also related to the organization of the material. For example, steel grinding is a relatively complex mechanical property, in addition to hardness, it is also related to the organization of the material. For example, in steel grinding balls, if the hardness (including surface hardness and volume hardness) is the same, if there is some austenite in the structure, austenite may undergo "deformation-induced phase transformation" or work hardening during use. Its wear resistance will be better. Some steels (such as high manganese steel), even if the hardness is not high (the hardness of Mn 13 after "water tough treatment" is only about HB 200), but the surface hardness will increase rapidly due to a phase transformation hardening process during use. To HB 450~600, its wear resistance (mainly due to strong impact or abrasion resistance under high pressure) is quite excellent. 4. It is difficult to comprehensively measure the quality of the material after heat treatment using hardness alone, so it is difficult to judge whether the heat treatment process is proper. For example, hypoeutectic steel parts that are quenched to be superheated have the same hardness as normal quenched parts, but the impact toughness of the former is much lower than the latter. This is because the hardness of the hardened steel mainly depends on the carbon content of the martensite, and the impact toughness is closely related to the thickness of the martensite structure. The bainite of the superheated steel is coarse, so its impact toughness is greatly reduced. Another example is the hard aluminum that is obviously quenched and burnt. Its hardness is the same as that of the same alloy that is not burnt, but the strength (especially fatigue strength), elongation and impact toughness of the former are significantly lower than the latter. The hardened aluminum has undergone grain boundary melting and oxidation. Due to the above limitations of the hardness test, in the production and research work of materials and their parts, in addition to the hardness test, other performance tests must be carried out according to the specific conditions. Of course, this does not affect the wide application of hardness testing.