Tomoyosi Kawada1, Yuwakiti Sawano1, Matuhei Kisi1
It is known that fatigue limit of steel is lowererd by the hard chromium plating, but the details of this phenomenon are not fully tested and the cause is not explained. Using the rotating beam fatigue machine, we tested the fatigue limit of the test pieces of the following four types, each of which are Ni-Cr steel test pieces electrically plated with hard chromium of different thickness. Four types are as follows: (1) plated, (2) annealed, (3) with the plated surface finished with emery paper and (4) with the surface finished after annealing. of the above four cases the relations between the fatigue limit and the thickness of plating are representediin Fig. 3. The photographs of electron diffraction pattern of the hard chromium plating before and after (broken and not broken) the fatigue test are shown in Fig. 5, and are found no change in its pattern. From this fact we concluded that the plated chromium layer itself has no relation to the lowering effect. It is expected that the graininess of the plated surface will act as notch to the test piece and lower the limit. This expectation is verified by the following two facts: (i). seeing the curve (1) of Fig. 3, we find the following: the fact that the thinner the plating is, the lower the limit becoines, corresponds to the fact that the thinner the plating is, the sharper the graininess becomes, and (ii) comparing the curve (1) with (3), (2) with (4) of Fig. 3, we see that the finished smooth one has the higher limit than the plated grainy one Also by the same reason the annealed crack of the plating lowers the limit, which is clearly seen by comparing the curve (1) with (2), (3) with (4) of Fig. 3. From the fact that the deplated test pieces have the same fatigue limit as the paper finished ones and that at the static test the deplated one shows hydrogen brittleness, we conclude that the hydrogen absorbed into the steel during plating will lower the limit at a few percent.The internal stress between plating and steel is considered to act as the mean stress to the fatigue test and may lower the limit. But the part attributed to this effect will be small, if exist, because the fatigue limit of the annealed one, which has no internal stress,is lower than that of non-annealed one. Thus we attribute the lowering effect of fatigue limit of steel by the hard chromium plating to the graininess of the plated surface, the annealed crack, and the absorbed hydrogen.
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