The salt-bath nitriding is used widely today because it has an advantage of shorter treating time than the conventional nitrding process. We studied the effect of this process on the improvement in fatigue properties of high Cr steels, and clarified various features of hardened case by metallographic and X-ray diffraction tests.
The results obtained are as follows:
(1) The extremely hard compound layer produced in the surface of austenitic high Cr steel (SEH 4 or 21-4N) during the salt-bath nitriding at 570°C contains a large amount of Cr-nitrides and Cr-carbides. In ferritic high Cr steel (SEH 3), however, the formation of Cr-compound is less than in austenitic one, and it is found by X-ray diffraction analysis that the lattice in surface zone is distorted considerably after this treatment.
(2) The salt-bath nitriding improves remarkably the rotating bending fatigue strength of SEH 3. But this process is not so effective for the improvement of fatigue strength of SEH 4 or 21-4 N.
(3) In the welds of SEH 3 and SEH 4, in which the rupture occurs usually in SEH4 side during the fatigue test, the fatigue behavior of the welds before and after the nitiriding is almost equal to that of SEH 4 base material. On the other hand, it is found that this process is not always effective for improving fatigue characteristics in the welds of SEH 3 and 21-4 N, in which the rupture occurs usually in the weld zone.
(4) Tufftride process which employs the salt-bath consisting mostly of potassium cyanide and cyanate has a more pronounced effect on the improvement in fatigue strength of high Cr steels than the domestic process.
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