Fatigue and corrosion fatigue crack growth resistance of RQT501 steel.

This project is concerned with the fatigue response of roller quenched and tempered (RQT50I) steels. Three closely related aspects are considered: (i) the relationship between the microstructure and fatigue properties, (ii) the influence of the environment on fatigue properties, (iii) theoretical modelling of fatigue crack growth. The material used is an RQT501 grade steel. Two steels have been chosen: (A) without Mo and V additions, (B) with Mo and V additions. The second condition is chosen to evaluate the role of these elements, as hydrogen traps, on the resistance to corrosion fatigue crack growth. In order to evaluate these effects, tests have been carried out under freely corroding conditions and applied cathodic polarisation (-9 . SO mV/SCE). Metallographic examination has been carried out to evaluate grain size and inclusion distribution, size and morphology. To enable a comparison between short and long fatigue crack growth, smooth shallow hour-glass specimens have been used to study initiation and growth of short fatigue cracks, and compact tension specimens were used to monitor propagation of long fatigue cracks. The fatigue response (S - Nf curve) and fatigue crack growth rate behaviour has been studied in air (5 - 10 biz) and in 3.5%NaCI solution (0.2 Hz) or artificial seawater (0.2 Hz) with an applied stress ratio of 0. I and sinusoidal waveform. The generation and growth behaviour of cracks from smooth hour-glass specimens has been investigated through successive observation using the plastic replication method. Long fatigue crack lengths were measured using a Direct Current Potential Drop (DCPD) technique and monitored via a chart recorder. Based upon the results of these tests a fatigue crack growth model was developed based on non-equilibrium statistical theory incorporating a consideration of the influence of microstructure, e. g. grain boundaries and hydrogen embrittlement. A stochastic theory of corrosion has also been developed for the use with corrosion fatigue test data.