Hardness tester applied in heat treatment industry

- Jan 10, 2019-

Heat treatment is one of the important foundations for improving and guaranteeing the quality and reliability of mechanical products and improving the competitiveness of the manufacturing market. 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 fierce market competition environment faced by the heat treatment industry after joining the WTO. Among them, China's heat treatment testing equipment is even more backward. Severe restrictions and hinder the development of process technology and instability of product quality, and more importantly, can not effectively prevent products from having unnecessary quality accidents. Below I will give an example of the most commonly used hardness test for mechanical parts and tooling after heat treatment. Hardness is an indicator of the hardness of a metal material. The hardness value is not actually a unit of physical quantity. It is a comprehensive performance that characterizes a combination of different physical quantities such as elasticity, plasticity, deformation, strength, strength and toughness of the material. The index, generally speaking, refers to the ability of the metal to resist deformation and cracking in a small volume on the surface of the metal. The practical value of using the portable Leeb hardness tester for hardness testing is that it does not have to damage the workpiece and can inspect parts in batches. It has become one of the important experimental methods for product quality inspection, rational process development and analysis of product quality. Structural parts and molds are tested for actual use. The hardness is measured by the Leeb hardness tester to analyze and verify the large-toothed shaft, the inner ring gear pretreatment process and the carburizing and quenching process. 1. After the pretreatment process, the quality of the process is tested. It directly affects the uniformity of the hardness distribution of the carburizing layer of the pinion and the inner ring gear after the operation of the carburizing process, and the deformation amount of the tooth portion. More important is the strength of the core (unpermeable layer). When using the Leeb hardness tester to test the tooth portion and the shank of the pinion, the inner diameter and the end surface of the inner ring of the ring gear, if the difference between the highest and lowest hardness of the same area is found to be large, the temperature of the instrument and the method of loading the furnace are examined. Under the premise that the process cooling method is normal, it can be inferred that there is segregation or band structure in the workpiece according to the difference of the hardness value and the distribution. For example, the metallographic image shows the band segregation because the hardness test is the test point in the black pearlite belt hardness is too high, the white ferrite band hardness is low. This tissue defect is not likely to be eliminated in subsequent carburizing and quenching process operations, but is retained. Once the large pinion and the ring gear have such a structural defect, not only the problem of short service life is caused when the machine is installed, but also a major equipment accident is often caused. When we can infer that there is such banding segregation in the forgings by the test hardness method, the solution can be eliminated immediately by the solution treatment process or other processes for eliminating the band segregation, thereby preventing equipment accidents and reducing losses. 2. After the carburizing process, the final heat treatment process of the toothed shaft and the inner ring gear of the process is tested. Usually, the inspector only tests the HRC hardness of the sample and then cuts the sample for metallographic analysis: the martensite level of the layer, Carbide grade, morphology distribution, retained austenite amount morphology, and core martensite grade, and then the microhardness tester was used to test the seepage layer from the surface layer to the HV550 hardness as the deep depth. However, it is found from the accident analysis of the early failure of the gear shaft and the severe wear of the tooth. The hardness of the object is lower than the hardness of the sample, especially in deep carburizing. The reason is the physical pinion and the inner ring gear. The sample is not taken from the same block, nor is it pretreated in the same furnace. In this way, the original structure is different between the physical object and the sample before infiltration, and the microstructure and hardness after the infiltration are different. More importantly, in the whole carburizing process operation, due to the difference in the placement of the workpiece and the surface area of carburizing (chemical heat treatment absorption) The phase interface reaction of the process and one of the main factors are the concentration of each component of the infiltrated medium and the surface state, surface shape, surface area and surface energy of the workpiece have an extremely important influence on the chemical heat treatment process. The hardness measured by the Leeb hardness tester Compared with the hardness measured by the Rockwell hardness tester and the microhardness tester on the sample, combined with the metallographic analysis of the sample layer, the cause of the quality problem can be found quickly and accurately. In addition to making the necessary process adjustments, accidents can be eliminated before the workpiece is assembled. Use the hardness tester to test and verify the medium and large complex structure mold process. This is a feed mold made of martensitic stainless steel, mainly used for the production of feed. There are 10,000 to 27,000 holes in the mold, and 10 tons of mixed feed powder per hour is extruded from the die hole area into pellets. Therefore, the key control point of the mass and service life of the ring mold is the hardness and permeability of the die hole area on the ring die. Due to the large size of the mold and the distribution of the die holes on the outer circle, the test point is a curved surface. The spacing between the holes and the holes is generally 3 to 4 mm. Under such conditions, the various applicability and compact carrying of the Leeb hardness tester are just played. Convenient features enable accurate testing of the actual hardness at the die hole. According to the measured hardness and the metallographic analysis of the test block, the process can be corrected and formulated more reasonably, and the material potential can be fully utilized to improve the service life. If the ring mold produces 10 tons of feed in one hour, when the service life is extended for 10 hours, 100 tons of feed can be produced, which will bring huge economic benefits to the enterprise. The role of the Leeb hardness tester in the failure analysis of tooling The material 5CrMnMo large hot forging die often fails in the early use of the cavity due to wear and collapse. When the hardness was tested on the wear zone using a Leeb hardness tester, it was found that the hardness value of the grinding was lower than the hardness (38 to 42 HRC) tested before use, in the range of 32 to 34 HRC. This shows that in the second stage of heating the red hot blank (>10000C) to the cavity (the blank can be divided into four stages in the cavity heating), the heat is about 80% to 85%, due to the cavity wall. The heat dissipation is slow, and the temperature in some spot areas is as high as 6500C or more. When the temperature of the mold cavity exceeds the tempering temperature of the mold due to conduction and heat, the mold is in the process of continuing the tempering process during the working process, and the change of the structure and the performance will continue to occur, resulting in the decrease of the hardness value of the wear zone and the heat generation. Wear and tear, it shows that 5CrMnMo is not suitable for large molds. When using hot working die in the temperature range of 600~6500C, such as 3Cr2W8V, 3Cr3Mo3VNb, hot work die steel with thermal stability, heat strength and yield strength is more suitable than 5CrMnMo. In the case that China's industrialization process is still far from being completed, there is an urgent need to work hard to shorten the gap with the international advanced manufacturing level. As a company must recognize the heat treatment industry from a strategic perspective, not only from equipment, technology, and especially inspection equipment. Drawing on new technologies at home and abroad, it depends on the actual production and application in heat treatment enterprises. This is a very sensible move, because after the accession to the WTO, there is a five-year transition period according to the WTO rules. This is a very critical period for China's manufacturing industry. The reason is that it is because of the post-transition period. Enterprises will basically be in the same competitive position as large foreign companies, and in this transition period, we can fully prepare to enhance competitiveness, so this is a very urgent requirement for enterprises.