In addition to the hardness value required to improve the quality of gear forgings, other mechanical performance indicators must also be specified:
1. Reasonable combination of strength and toughness. Usually the strength and toughness of steel materials are trade-offs. For structural forgings, the primary impact toughness is often used as a safety criterion, and high toughness is pursued without sacrificing strength, resulting in bulky and heavy mechanical products and short service life. On the contrary, for tools and dies, in order to improve wear resistance, high hardness and high strength (torsional strength) are pursued, while the effect of toughness on reducing die chipping and breakage is ignored, and the service life is not long. Therefore, the working conditions and failure modes of the forgings should be investigated and analyzed, and the strength and toughness indicators that should be selected for the forgings should be determined according to the reasonable combination of strength and toughness.
2. Correctly handle the relationship between material strength, structural strength and system strength. Various material strength indicators are measured with standard samples, which depend on the organizational state of the material (including surface state, residual stress and stress state). Structural strength of forgings is affected by dimensional factors and notch effects, while system strength is related to the interaction of other forgings. There is a big difference between the three, for example, the smooth bar of the material has high fatigue strength, but the fatigue strength of the physical object may be very low. Therefore, for some important parts, it is more appropriate to determine the mechanical performance index according to the simulation test results.
3. The strength matching of the assembly should be reasonable. A large number of tests and practical use have shown that when the assembly (such as worm gear, chain sprocket, ball and ferrule and transmission gear, etc.) achieves strength matching, the service life can be extended. For example, the hardness of the ball should be 2HRC higher than that of the ferrule, and the surface hardness of the driving gear of the rear axle of the car should be 2-5HRC higher than that of the driven gear seat. The same kind of steel is processed by the same method into friction pairs with the same hardness, and the wear resistance is relatively poor.
4. For surface-strengthened forgings, the core and surface strengths should be reasonably matched. When surface strengthening parts (such as carburizing quenching, carbonitriding quenching, nitriding, induction quenching, etc.), when the depth of the hardened layer is certain, the core should have appropriate strength, so that the core and surface strengths reach a good matching state, In order to ensure that the forgings have a high service life. If the strength of the core is too low, the transition zone is prone to fatigue sources, resulting in a decrease in fatigue performance; if the strength of the core is too high, the surface residual compressive stress is small, and the fatigue life is not long.