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In order to control these factors in production, after years of summary, in the early 1980s, the ISO/TC60 gear working group proposed a standard discussion draft of ISO268 "Gear Steel Quality and Fatigue Limits", until the end of 1996. Formed ISO6336-5 "Gear Steel Quality and Strength" standard. In 1996, China adopted the GB/T8539 "General Regulations for Heat Treatment Quality Control of Gear Materials". The biggest feature of this standard is the close connection between gear material and heat treatment quality and fatigue strength.
A total of 19 factors affecting the fatigue strength of carburized and quenched gears are listed, completely changing the single-value relationship between hardness and stiffness in the past gear fatigue strength design; and, according to the importance of gears (ML-low gear, MQ-mid gear, ME-high-grade) puts different requirements on the influencing factors, which not only guarantees the performance quality, but also has the economics of production.
The following is an example of several important heat treatment quality influencing factors in the table: the effective hardened layer depth in the provisions of item 9 of Table 2 has such a paragraph: "When the minimum hardened layer depth is specified, attention should be paid to the bending fatigue strength. The optimum value of the surface load capacity is different.†According to the results obtained from a large number of tests, the optimum effective hardened layer depth is different for gear contact fatigue strength and bending fatigue strength, as shown in Figure 1. For the modulus 10 gears, from the bending fatigue strength, the best effective hardened layer depth is 1.0 ~ 1.5mm, the deepest is 1.5mm; and from the contact fatigue strength, the depth is 1.5 ~ 2.0mm, the shallowest is 1.5mm.
Surface non-martensitic gears tend to form non-martensitic structures (or black structures) due to surface oxidation during carburizing and quenching, which are detrimental to gear fatigue strength. And Table 3 is the relationship between non-horse gear contact fatigue strength and bending fatigue strength and effective hardened layer depth. The effect of non-martensitic structure on fatigue limit. The effect of HeatTreatment on the bending fatigue strength and contact fatigue. As seen from Table 3 and the data, this effect is exacerbated as the depth of the non-martensitic layer increases, so the standard specifies the allowable depth range of non-martensitic according to the strength grade of the gear and the depth of the hardened layer.
After surface carburizing of the retained austenitic gear steel, the surface layer will produce different amounts of retained austenite due to the alloy composition and carbon content of the steel. Since the retained austenite will lower the hardness and change the residual stress state, the content should be limited. . Table 4 shows the relationship between the retained austenite content and the fatigue strength. From the table, the optimum content is 10% to 20%.
C-N co-infiltration layer depth / mm black tissue layer depth / mm under the 360MPa stress, the occurrence of pitting points / N0.92 ~ 0.95055.9 × 1060.80.0257.7 × 1061.0 ~ 1.10.07 ~ 0.080.46 × 106 3 The influence of black tissue on contact fatigue life From the above analysis, ISO6336-5 and GB/T8539 standards provide us with a guideline and are operable. If carburst quenching in China's automobile gears is guided by this standard, Good results will be achieved for improving gear load carrying capacity and reliability.
