The practical application of the hottest Leeb hard

2022-08-01
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The practical application of Leeb hardness tester in the heat treatment industry

heat treatment is one of the important foundations to improve and ensure the quality and reliability of mechanical products and improve the market competitiveness of the manufacturing industry. The level of its industry plays an important role in the development of the manufacturing industry. At present, except for a few manufacturers, the overall level of China's heat treatment industry is far from being able to adapt to the more intense market competition environment faced by the heat treatment industry after China's entry into WTO. Among them, China's heat treatment testing equipment is more backward. It seriously restricts and hinders the development of process technology and causes the instability of product quality. More importantly, it can not effectively prevent unnecessary quality accidents. The following is an example of the most commonly used hardness test for mechanical parts, tools and dies after heat treatment

hardness is an index to measure the hardness and softness of metal materials. In fact, the hardness value is not a physical quantity of a unit. It is a comprehensive performance index that represents the combination of a series of different physical quantities such as elasticity, plasticity, deformation, strengthening rate, strength and toughness of materials. Generally, it can be considered that hardness refers to the ability to resist deformation and fracture in a small volume on the metal surface. The practical value of using portable Leeb hardness tester for hardness testing lies in that it does not need to damage the workpiece and can inspect the parts in batches. It has become one of the important experimental methods for product quality inspection, formulating reasonable process and analyzing product quality

I have been working at the production site of heat treatment. According to the use status and use mode of Leeb hardness tester, I think that Leeb hardness tester is a test method combining the indentation and rebound hardness values. From the structure, it has its unique and wide applicability, especially for testing the hardness values and failure analysis of large and medium-sized workpieces and tools and dies with complex shapes. It has irreplaceable advantages over other types of hardness testers

actual use of structural parts and mold testing

I. analyze and verify the pretreatment process and carburizing quenching process of large gear shaft and inner gear ring by hardness test with Leeb hardness tester:

1. Test after pretreatment process

the quality after this process directly affects the uniformity of hardness distribution of carburized layer of gear shaft and inner gear ring and the deformation of gear part after carburizing process operation. The more important thing to close the oil return valve is the strength of the core (impermeable layer). When using the Leeb hardness tester to test the tooth and shank of the gear shaft, the inner diameter and end face of the outer circle of the inner gear ring, if it is found that there is a large difference between the highest and lowest hardness in the same area tested, under the premise that the instrument furnace temperature, charging mode and process cooling mode are normal, it can be inferred that there is segregation or banded structure in the structure of the workpiece according to the difference and distribution of the hardness value. For example, the strip segregation shown in the metallographic picture

is that the hardness of the test point is higher in the black pearlite belt and lower in the white ferrite belt. This kind of structure defect can not be eliminated in the subsequent carburizing and quenching process, but can be retained. Once the large gear shaft and inner gear ring have such structural defects, it will not only cause the problem of short service life, but also often cause major equipment accidents. When such banded segregation can be inferred from forgings in developed countries and regions such as Europe and the United States through hardness test, defects can be eliminated immediately by using solution treatment process or other processes to eliminate banded segregation, so as to prevent equipment accidents and reduce losses

2. After the carburizing process, test

the final heat treatment process of the gear shaft and the inner gear ring. Generally, the inspectors only test the HRC hardness of the sample, as shown in the figure:

and then cut the sample for metallographic analysis: the martensite grade of the carburized layer, the carbide grade, the morphology distribution, the morphology of the residual austenite volume, and the martensite grade of the center, The microhardness tester is used to test the hardness of the infiltrated layer from the surface layer to hv550, and the operation above the deep depth is taken as the final inspection. However, from the accident analysis of the early failure and fracture of the gear shaft and serious wear of the gear, it is found that the hardness of the real object is lower than that of the sample, especially in the case of deep carburization. The reason is that the real gear shaft, inner gear ring and the sample are not cut from the same piece of material, nor are they pretreated in the same furnace. In this way, the original structure before carburizing is different from that of the sample, and the structure and hardness after carburizing are different to a certain extent. What is more important is that in the whole carburizing process, Due to the huge differences in the placement of workpieces and carburized surface area (there is an infiltration medium in the phase interface reaction and main factors in the absorption process of chemical heat treatment. Each dress is made of materials equivalent to 27 plastic bottles. The concentration of each component and the surface state, surface shape, surface area and surface energy of the workpieces have an extremely important impact on chemistry. 2. the types of instigation completed by measurement mainly include digital display and pointer; the heat treatment process has an extremely important impact) Comparing the hardness measured by Leeb hardness tester with the hardness measured by Rockwell hardness tester and microhardness tester on the sample, and combining the metallographic structure analysis of the infiltrated layer of the sample, we can quickly and accurately find out the causes of the quality problems. In addition, the necessary process adjustment can eliminate the potential accident before the assembly of the workpiece

II. Analyze and verify the process of medium and large-scale mold with complex structure by measuring the hardness with Leeb hardness tester

this is a feed mold made of martensitic stainless steel, which is mainly used to produce feed. There are 10000 ~ 27000 holes in the mold, and 10 tons of mixed feed powder are extruded from the hole area of the mold into granules every hour. Therefore, the key control points of ring mold quality and service life are the hardness and infiltration layer of the hole area on the ring mold. The following is the physical picture of die hole wear:

due to the large size of the die, the die holes are distributed on the outer circle, the test points are a curved surface, and the spacing between the holes is generally 3 ~ 4mm. Under this condition, it just gives full play to the diversified applicability and small and convenient carrying characteristics of the Leeb hardness tester, and can accurately test the actual hardness at the die holes. According to the measured hardness and metallographic analysis of the test block, a more reasonable modification and formulation of the process can be made to give full play to the potential of the material and improve the service life. For example, the ring mold can produce 10 tons of feed per hour, and when the service life is extended by 10 hours, it can produce 100 tons of feed, which will bring huge economic benefits to the enterprise

III. The role of Leeb hardness tester in failure analysis of tools and dies

large hot forging dies made of 5CrMnMo material often fail due to early wear and collapse of the cavity. As shown in the following figure:

when the hardness of the wear area is tested with a Leeb hardness tester, it is found that the hardness value of the grinding is lower than the hardness (38 ~ 42hrc) tested before use, which is in the range of 32 ~ 34hrc. This indicates that in the second stage of the cavity heating by the red hot blank (>10000c) (the blank heating in the cavity can be roughly divided into four stages), when the heat received is about 80% ~ 85%, the temperature in some points can reach more than 6500c due to the slow heat dissipation of the cavity wall thickness. When the temperature of the mold cavity exceeds the tempering temperature of the mold due to conduction heating, the mold is in the stage of continuous tempering process during the working process, and the changes in structure and properties will continue to occur, resulting in the reduction of the hardness value in the wear area and thermal wear, which indicates that 5CrMnMo, a hot work die steel, is not suitable for large molds. It is more suitable to use hot work die steels with thermal stability, thermal strength and yield strength, such as 3Cr2W8V and 3Cr3Mo3VNb, which are in service in the temperature range of 600 ~ 6500c than 5CrMnMo

under the condition that China's industrialization process is far from complete, it is urgent to make efforts to shorten the gap with the international advanced manufacturing level. As an enterprise, it must recognize the heat treatment industry from a strategic height, not only from the equipment, process, especially the testing equipment, but also from the actual production and application in the heat treatment enterprises. This is a very wise move, because there is a five-year transition period according to the WTO rules after China's entry into the WTO. For China's manufacturing industry, this is a very critical period. The key reason is that after the transition period, China's manufacturing enterprises will basically be in the same competitive position as large foreign companies, and during this transition period, we can be fully prepared to improve our competitiveness, so this is a very urgent requirement for enterprises

note: the banded segregation sample shown in the metallographic picture is intercepted from the planetary gear provided by XX Heavy Forging Co., Ltd.

the Leeb hardness tester is time group th140 (end)

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