浸炭焼入れした鋼材の疲労強度と残留応力

抄録

Carburizing is a favored process to improve the fatigue strength and wear resistance of automobile gears and many other machine parts. Despite many years of studies and reports on the properties of carburized hardened steels, there still remain many problems to be solved in this respect where production is concerned.
The values of surface hardness, core hardness and case depth are known as main factors which influence the strength of carburized hardened steels. Among these, case depth and core hardness are variable extensively according to the differences in chemical composition, hardenability, and methods of carburizing and quenching, even if the parts are of the same quality. Moreover, those values vary widely depending on the hardenability of steels, even when the same type of steel is treated at the same temperature.
There have been many reports in which comparisons are made among the types of steels and the methods of carburizing and hardening but very few reports are obtainable concerning the investigations that dealt with these two factors at a time, especially on one type of steel.
Therefore, we investigated the influences of case depth, core hardness and residual stress on the fatigue strength of carburized hardened steels. The materials tested are chrome-molybdenum case hardening steels (SCM 21 & 22) of various hardenabilities.
The following points were made clear as the results of these tests. The fatigue strength of carburized hardened steel is influenced by the case depth and core hardness, both of which have optimum values to cause the maximum endurance. When the core hardness is low and the case depth is thin, crack initiates from case-core boundary (fish-eye). The depth of this fish-eye from the surface is almost equal to the total case depth. The decrement of residual stress during the fatigue test is rather small, and comparatively large residual stress is observed even after the test.
The surface residual stress of carburized hardened specimens first increases, then tends to decrease as the case depth is increased, and this can be considered to be one of the main causes for the behavior of maximum endurance of carburized hardened steels.
From those results, it seems necessary that further studies are pursued to ascertain the effect of residual stress, in order to attain high fatigue strength even for production parts.