Comparative study on the fatigue crack growth behavior of 316L and 316LN stainless steels: effect of microstructure of cyclic plastic strain zone at crack tip

The fatigue crack growth rates in 316L and 316LN stainless steel were measured to compare the resistance to fatigue crack growth of the alloys and to evaluate the effects of nitrogen addition to the alloys. Fatigue crack growth tests were carried out at 1 Hz with a sinusoidal tension stress wave form (R=0.2) between 25°C and 600°C in air. Crack lengths were monitored by direct current potential drop method. 316LN stainless steel showed better crack growth resistance than 316L stainless steel did. The characteristics of the cyclic plastic strain zone of 316L and 316LN stainless steel at the crack tip were analyzed with micro hardness test, the X-ray diffraction method and transmission electron microscopy. The improvement of the fatigue crack growth resistance of 316LN stainless steel is discussed in relation to the degree of crack closure, strain induced martensite transformation and the dislocation structure in the cyclic plastic stain zone at the fatigue crack tip.