Characterization of shrinkage porosity and fatigue properties of cast CA-6NM low carbon martensitic stainless steels

Abstract

Cast CA-6NM low carbon martensitic stainless steel is used for hydraulic turbine runner manufacturing since the sixties of the last century owing to its high strength, toughness, cavitation-erosion resistance and corrosion to withstand high cycle loads in long lifetime operation. The ideal microstructure consists of a martensitic matrix with some dispersed austenite after appropriate tempering which gives these steels their unique properties. However, these castings contain several types of defects which may have detrimental effects on the performance of the produced parts. Several aspects of the casting defect characterization in these steels have not been extensively studied in particular. The main objectives of this investigation include the characterization of the casting defects described in the following three sections. In the first article, the relation between radiography mapping and actual macro-defect morphologies in several severity levels was studied destructively using the “Salami” crosssectioning to document the real shape and morphology of defects. Results of these experiments represented defects as macro-shrinkage porosities with extremely sharp endings. The distribution of porosity size was measured via Murakami’s geometrical parameter (V aera) and Feret’s diameter. Higher severity levels of macro-shrinkage porosities demonstrated defects with sharper endings and larger Murakami’s parameter. Metallographic characterization of cast microstructure was also performed to study both chemical composition and microstructure around macro-defects. In the second part, uniaxial tension-tension low cycle fatigue testing (LCF) was conducted on the cast CA-6NM fatigue samples. The impact of defects position and sizing parameter (i.e., Murakami’s geometrical parameter (V area) on the fatigue life was studied. To this end, a two-dimensional (2D) SEM fractographic examination was performed on the fracture surfaces. The fatigue life generally diminished when the defect size increased. However, the position of the defect was also of critical importance. Moreover, the evolution of the shrinkage porosities during fatigue crack propagation and variation of geometrical factor Y(a) (considering linear elastic fracture mechanics (LEFM)) were investigated. In the third part, non-destructive three-dimensional (3D) X-ray tomographic evaluation was performed before (i.e., pristine sample) and after (i.e., broken sample) fatigue failure. The results were compared with the two-dimensional (2D) fractographic observations. The 3D analysis provided the distribution of shrinkage porosities precisely and thoroughly. The defects possessed a tortuous and intricate morphology. Results after fatigue testing displayed that the position of shrinkage porosity was more effective than the size in initiating the fatigue crack.

Date	16 Dec 2021
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Martin Viens (Supervisor)