Issue 13

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 13 (2010) 3-16; DOI: 10.3221/IGF-ESIS.13.01 3 Ductile cast irons: microstructure influence on fatigue crack propagation resistance Francesco Iacoviello, Vittorio Di Cocco Università di Cassino, Di.M.S.A.T., via G. Di Biasio 43, 03043 Cassino (FR), iacoviello@unicas.it Mauro Cavallini Università di Roma “Sapienza”, DICMA, via Eudossiana 18, Roma R IASSUNTO . In questo lavoro è analizzata l’influenza della microstruttura sulla resistenza alla propagazione della cricca di fatica in cinque differenti ghise sferoidali. Sono state considerate quattro ghise sferoidali caratterizzate da differenti frazioni volumetriche di ferrite e di perlite, effettuando prove di propagazione della cricca di fatica in accordo con la normativa ASTM E647 (R pari a 01, 0.5 e 0.75, rispettivamente). I risultati ottenuti sono stati quindi comparati con il comportamento di una ghisa sferoidale austemprata. I micromeccanismi di danneggiamento sono stati investigati utilizzando le seguenti procedure: - Analisi delle superfici di frattura “tradizionale” effettuata mediante un microscopio elettronico a scansione (SEM); - Ricostruzione quantitativa 3D delle superfici analizzate con il SEM; - Analisi SEM dell’evoluzione del profilo longitudinale della cricca; - Analisi al microscopio ottico (LOM) del profilo trasversale della cricca. A BSTRACT . Microstructure influence on fatigue crack propagation resistance in five different ductile cast irons (DCI) was investigated. Four ferrite/pearlite volume fractions were considered, performing fatigue crack propagation tests according to ASTM E647 standard (R equals to 0.1, 0.5 and 0.75, respectively). Results were compared with an austempered DCI. Damaging micromechanisms were investigated according to the following procedures: - “traditional” Scanning Electron Microscope (SEM) fracture surfaces analysis; - SEM fracture surface analysis with 3D quantitative analysis; - SEM longitudinal crack profile analysis - Light Optical Microscope (LOM) transversal crack profile analysis; K EYWORDS . Ductile irons; Microstructure; Fatigue crack propagation. I NTRODUCTION p to the first half of the last century, only malleable irons were able to partially offer a combination of grey iron castability and steel mechanical properties (first of all, toughness). These cast irons were obtained as a result of extended annealing treatment of white iron, with a matrix microstructure that was characterized by different ferrite and pearlite volume fractions, as a function of the cooling cycle. The main problems of this procedure were the high costs and the difficulty to cast sound white iron components. In 1943, in the International Nickel Company Research Laboratory, a magnesium addition allowed to obtain a cast iron containing not flakes but nearly perfect graphite spheres. In 1948, at the American Foundryman Society Convention, it was announced that a small amount of cerium allowed to obtain the same result [1, 2]. U