Effect of inclusions on very high cycle behaviour in a ferritic corrosion resisting steel

Abstract

The number of structural components which must operate frequently at repeated loading higher as N ≈ 106 – 107 cycles increases. This fact is serious problem in engineering practice. Fatigue lifetime in the ultra high cycle region (108 < N< 1011 cycles) is therefore studied very intensively with the aim to correctly select the structural material. The paper is devoted to an experimental examination of the fatigue lifetime of X12Cr13 corrosion resisting steel used in the power industry. The high frequency fatigue testing (frequency f ≈ 20 kHz, temperature T = 20 ± 10°C, sinus push-pull loading with load ratio R= -1) was used with the aim to obtain experimental data in the high cycle and ultra high cycle region (from N ≈ 106 to N ≈ 1011 cycles). The obtained S–N curve revealed a continuous downward slope without any indication for plateau region, whereby surface (in the region 106 < N< 109 cycles) and subsurface (N > 109 cycles) fatigue crack initiation occurs in the used range of loading amplitudes. The inclusions of type Al4Ca and Al2Ca were the places of subsurface crack initiation; their role is discussed with regard on the fatigue lifetime of the X12Cr13 steel with ferritic matrix. Obtained results confirm that occurrence of those inclusions must be taken in consideration in application of the X12Cr13 steel for very high cycle service in power industry.