Stable and Unstable Crack Growth in Chromium Pre-Alloyed Steel

Abstract

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. Prismatic specimens were pressed at 7.0 g/cm3 density from Astaloy CrM powder and sintered in different conditions, varying the sintering temperature and the cooling rate after sintering. Optical observations allowed to evaluate the dimensions and the morphology of the porosity and the microstructural characteristics for all the investigated conditions. Fatigue tests were performed at R-ratio equal to 0.1 to investigate the threshold zone and to calculate the Paris law coefficients. All the tests were carried out according with the compliance method, and the crack length has been evaluated during whole the test. Moreover KIc tests were performed in order to complete the investigation. Both on fatigue and KIc samples a fractographic analysis was carried out to investigate the crack path and the fracture surface features. The results show that the Paris law crack growth exponent is around 6.0 for 1120°C sintered and around 4.7 for 1250°C sintered materials. The same dependence to process parameters is not found for KIth. Values close to 6 MPa√m are here found for all variants.