高周波焼入れした鋼材の腐食疲労強度

Abstract

Effect of induction hardening on corrosion fatigue properties of steels is studied, especially in respect to the recent development of high tension steels.Specimens, such as in Fig. 1, made of four kinds of steels as in Tables 1 and 2, are induction-hardened as shown in Table 3, and tested on rotary bending fatigue testing machines (1500rpm) in the mist of well water (as in Table 4) in the chambers shown in Fig. 2.Results of corrosion fatigue tests are shown in Figs. 3, 4, 5 and 6, and Fig. 8 inclusively. As shown in these figures, the corrosion fatigue strength of any hardened steel is higher than the fatigue limit in air of non-hardened steel, accordingly this induction hardening being a superior protection against corrosion fatigue. Especially, the strength of S 45 C steel for 107 cycles is as high as 50kg/mm2 and this stress level looks like a fatigue limit. This property is believed to be one of the highest of the steels suffered with corrosion fatigue that have ever been reported.As well known, the metallurgical improvement of quality of steel by ordinary quenching does not contribute by itself to improve corrosion fatigue strength. For example, as shown in Fig. 7, the rail steel, induction-heated but quenched in oil at 25°C, is very weak under corrosion fatigue conditions. (cf. Fig. 6)As shown in Fig. 9, on the surface of induction-hardened specimens after corrosion fatigue tests, there are many lines of corrosion products rising along the periphery of specimen (perpendicular to the direction of stress). And the fracture surfaces of induction-hardened specimens, Fig. 10 (a) and (b), appear to be flat plane with shallow cracks on their edges, and that of non-induction-hardened, (c) and (d), appear to be the cutter-like fracture surface. From these facts, induction hardening can not prevent the initiation of corrosion fatigue cracks, but can arrest their further propagation near the surface of specimens. It is supposed that this crack-propagation-arresting ability mainly attributes to the compressive residual stress near the surface of specimens that has withstood many repetitions of stresses. For better understanding, the effect of mean stress on the corrosion fatigue strength of high carbon steel for some given cycles is shown in Fig. 11. As shown in this figure, time strength diagrams for corrosion fatigue are also limited by the fatigue limit in air. Also in this respect, induction-hardened steels have excellent properties as well known.