Practical applications of the Rockwell hardness tester in the heat treatment industry

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Heat treatment is one of the key foundations for improving and ensuring the quality and reliability of mechanical products, as well as for enhancing the competitiveness of the manufacturing industry; the level of expertise in this field plays a crucial role in the development of the manufacturing sector. At present, aside from a few manufacturers, the overall level of China’s heat treatment industry is far from sufficient to cope with the more intense market competition it faces after joining the WTO. Among these, the heat treatment testing equipment in our country is even more backward. Severe restrictions hinder the development of processing technologies and lead to unstable product quality; more importantly, they fail to effectively prevent unnecessary quality defects in the products. Next, I will give examples using the most common hardness tests for mechanical parts and tooling after heat treatment. Hardness is an indicator used to measure the softness or hardness of metal materials. In fact, the hardness value is not a physical quantity with a specific unit; it represents a comprehensive performance indicator that reflects a combination of various physical properties such as the material’s elasticity, plasticity, deformation capacity, strengthening rate, strength, and toughness. Generally speaking, hardness can be considered as the ability of a metal surface to resist deformation and fracture over a small volume. The practical advantage of using a portable Rockwell hardness tester for hardness testing lies in the fact that it does not require damaging the workpiece, and it enables batch inspection of parts. It has become one of the important experimental methods for product quality inspection, developing appropriate manufacturing processes, and analyzing product quality. Based on the conditions and methods of use of the Rockwell hardness tester, it is a testing method that combines indentation and rebound hardness values. Structurally, it possesses unique and broad practicality, especially for measuring the hardness of large and medium-sized components with complex shapes, as well as for failure analysis; it offers advantages that other types of hardness testers cannot match. Actual application of testing for structural components and molds. I. Method of using a Rockwell hardness tester to measure hardness in order to analyze and verify the pre-treatment processes as well as the carburizing and quenching processes for large gear shafts and internal gear rings: 1. Testing after the pre-treatment process. The quality resulting from this process directly affects the uniformity of the hardness distribution of the carburized layer on the gear shafts and internal gear rings after the carburizing process, as well as the degree of deformation of the tooth surfaces. More importantly, it is the strength of the core (unpenetrated layer). When using a Rockwell hardness tester to measure the teeth and shank of the gear shaft, as well as the outer diameter, inner diameter, and end faces of the internal gear ring, if a large difference is found between the highest and lowest hardness values in the same area being tested, and assuming that the temperature of the testing apparatus, the way the parts are placed in the apparatus, and the cooling methods are all normal, it is possible to infer the presence of segregation or stratified structures within the material of the workpiece, based on the difference in hardness values and their distribution. The banding segregation shown in the metallographic images is due to the fact that, as determined by hardness testing, the hardness at the test points is higher in the black pearlite zones and lower in the white ferrite zones. This organizational defect cannot be eliminated during the subsequent carburizing and quenching process; instead, it remains. Once such structural defects exist in large gear shafts and internal gear rings, they not only lead to a shortened service life during operation but also often cause serious equipment failures. Once it can be determined through the hardness testing method that such banding segregation is present in the forgings, immediate application of solution treatment or other processes to eliminate banding segregation can remove this defect, thereby preventing equipment failures and reducing losses. 2. Testing after carburizing process: This process represents the final heat treatment step for spur gears and internal gear rings. Typically, inspectors only measure the HRC hardness of the samples, as shown in the figure. Afterwards, the samples are cut open to undergo metallographic analysis: the grade of martensite in the carburized layer, the grade and distribution of carbides, the amount and morphology of residual austenite, as well as the grade of martensite in the core area. Finally, a microhardness tester is used to determine the hardness of the carburized layer, with a target hardness of HV550, and this measurement serves as the final inspection criterion. However, analysis of the accidents that occurred on site, involving premature failure and shaft breakage of the gear shafts as well as severe wear of the teeth, revealed that the hardness of the actual components was lower than that of the test samples. This was especially common during deep carburizing, as the gear shafts and inner ring gears in use were not cut from the same piece of material as the test samples, nor were they subjected to the same preprocessing in the same furnace. In this way, since the original microstructure of the actual part differs from that of the specimen before carburizing, there are certain differences in the microstructure and hardness after carburizing. More importantly, throughout the entire carburizing process, due to variations in the positioning of the workpiece and the large difference in its surface area (one of the key factors in the phase boundary reactions during chemical heat treatment is the concentration of various components in the carburizing medium; the surface condition, shape, area, and energy of the workpiece also have a significant impact on this chemical heat treatment process), it is possible to compare the hardness measured by the Rockwell hardness tester on the specimen with the hardness values obtained using the Brinell hardness tester and microhardness tester. By combining this with an analysis of the microstructure of the carburized layer, it becomes possible to quickly and accurately identify the causes of quality issues. Furthermore, by making the necessary process adjustments, potential accident risks can be eliminated before the workpieces are assembled. This post was last edited by zzg5680 on 2008-12-13 16:27.]