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Powder metallurgy processing of steels typically results in a material characterized by residual porosity, whose dimension and morphology, together with the microstructure, strongly affect the fatigue crack growth behaviour of the material. In this paper, the behaviour of steels from chromium pre-alloyed Astaloy CrM powders and from diffusion bonded Distaloy AE and HP powders were considered. Astaloy CrM powders were sintered in different conditions, varying the sintering temperature and the cooling rate after sintering: 1120°C, 30 minutes with a cooling rate of 0.3°C/s (Cr-S steel); 1250°C, 30 minutes with a cooling rate of 1°C/s (Cr-SH steel). The latter steel was tempered at 200°C, 1 hour. Distaloy powders were sintered at 1120°C, 30 minutes with a cooling rate of 0.2°C/s. For all the powders the sintering atmosphere was Nitrogen-Hydrogen (90%-10%). Fatigue tests were performed at R-ratio equal to 0.1 in order to find the threshold values and to calculate the Paris laws in the stable crack propagation zone.