Pearlitic ductile cast iron: damaging micromechanisms at crack tip

F. Iacoviello et alii, Frattura ed Integrità Strutturale, 25 (2013) 102-108; DOI: 10.3221/IGF-ESIS.25.15 Special Issue: Characterization of Crack Tip Stress Field Pearlitic ductile cast iron: damaging micromechanisms at crack tip F. Iacoviello, V. Di Cocco, A. Rossi Università di Cassino e del Lazio Meridionale, DiCeM, via G. Di Biasio 43, 03043 Cassino (FR), Italy M. Cavallini Università di Roma “La Sapienza”, DICMA, via Eudossiana 18, Rome, Italy ABSTRACT. Ductile cast irons (DCIs) are characterized by a wide range of mechanical properties, mainly depending on microstructural factors, as matrix microstructure (characterized by phases volume fraction, grains size and grain distribution), graphite nodules (characterized by size, shape, density and distribution) and defects presence (e.g., porosity, inclusions, etc.). Versatility and higher performances at lower cost if compared to steels with analogous performances are the main DCIs advantages. In the last years, the role played by graphite nodules was deeply investigated by means of tensile and fatigue tests, performing scanning electron microscope (SEM) observations of specimens lateral surfaces during the tests (“in situ” tests) and identifying different damaging micromechanisms. In this work, a pearlitic DCIs fatigue resistance is investigated considering both fatigue crack propagation (by means of Compact Type specimens and according to ASTM E399 standard) and overload effects, focusing the interaction between the crack and the investigated DCI microstructure (pearlitic matrix and graphite nodules). On the basis of experimental results, and considering loading conditions and damaging micromechanisms, the applicability of ASTM E399 standard on the characterization of fatigue crack propagation resistance in ferritic DCIs is critically analyzed, mainly focusing the stress intensity factor amplitude role. KEYWORDS. Ductile cast irons (DCIs); Fatigue crack propagation; Graphite nodules; Damaging micromechanisms. INTRODUCTION D uctile cast irons (DCIs) have been relatively recently developed and they are characterized by the presence of free graphite with a nodule shape (instead of lamellae as in grey cast iron): this allows to combine the more peculiar cast iron property (castability) with mechanical properties that are similar to those of carbon steels [1] (first of all, toughness). DCIs are used in the form of ductile iron pipes (for transportation of raw and tap water, sewage, slurries and process chemicals), in safety related components for automotive applications (gears, bushings, suspension, brakes, steering, crankshafts) and in critical applications as containers for storage and transportation of nuclear wastes [12]. Matrix controls mechanical properties and matrix names are used to designate spheroidal cast iron types. Many different DCIs grades are commercially available. Among them, ferritic-pearlitic DCIs offer a wide range of mechanical properties, with ferritic grades that are characterized by good ductility and a tensile strength (more or less equivalent to a low carbon steel), pearlitic DCIs that show higher strength values, good wear resistance and moderate ductility and, finally, ferritic–pearlitic grades properties that are intermediate between ferritic and pearlitic ones, at least considering tensile strength (Fig. 1). In fact, considering the fatigue crack propagation resistance (Fig. 2), the ferritic-pearlitic DCI seems to be characterized by the best behaviour, at least for higher K and R values. 102 da/dN [m/cycle] F. Iacoviello et alii, Frattura ed Integrità Strutturale, 25 (2013) 102-108; DOI: 10.3221/IGF-ESIS.25.15 10 -6 10 -7 10 -8 10 -9 10 Figure 1: Stress –strain behaviour for carbon steel, grey iron and ferritic and pearlitic DCIs [1]. 100% F GJS350-22 R = 0.1 R = 0.5 R = 0.75 50% F + 50% P GJS500-7 R = 0.1 R = 0.5 R = 0.75 100% P GJS700-2 R = 0.1 R = 0.5 R = 0.75 -10 3 10 50 1/2 K [MPa m ] Figure 2: Ferritic-pearlitic DCIs. Microstructure and stress ratio influence on fatigue crack propagation [8]. Considering the fatigue resistance, and considering both the initiation and propagation of micro- and macro- cracks, the role played by graphite nodules is not univocally determined. Different mechanisms are proposed for the graphite nodules [3-7]: - “rigid spheres” not bonded to the matrix and acting like voids under tension; - “crack-arresters”, due to their peculiar shape that minimizes the stress intensification at the crack tip; - “crack closure effect raisers”, due to the role they play at the lower values of the applied Kmin. Other research activities allowed to conclude that graphite nodule cannot be regarded as voids with no strength and that they don’t cause micro-notch stress concentration by itself [8]. It has been proposed [9]that the role played by the graphite nodules in DCIs fatigue crack propagation is more complex, suggesting the presence of a mechanical properties gradient inside the graphite nodules, probably due to the different graphite nodules solidification and growth mechanisms. Considering the fracture mechanics principles, stress intensity factor (“K”) is used to quantify the stress state ("stress intensity") near the crack tip caused by a remote load or residual stresses and, considering fatigue crack propagation, stress intensity factor amplitude (e.g. K = Kmax-Kmin) is the main parameter used to characterize the stress conditions at the crack tip. Both K and K usefulness is confirmed only considering an homogeneous and linear-elastic body: obviously, a crack tip plastic zone is always present, but, if its radius is negligible, the K parameter is still valid. Under monotonic loading, plastic zone size is usually estimated as follows: 1 K ry    2   y  1 K ry    6   y  2 (plane stress conditions) (1) (plane strain conditions) (2) 2 Considering a fatigue crack propagation problem, Eqs. (1) and (2) represent the crack tip plastic zone corresponding to the upward excursion of the load cycle (up to K(t) = Kmax). Fatigue crack propagation is characterized by the presence of a “reversed” or “cyclic” plastic zone, rrpz (four times lower than the monotonic value corresponding to Kmax): the tensile load reduction from the max, and the presence of the surrounding elastic body, imply a compression condition at the crack tip. Considering that, for R = 0.1, applied K value ranges between 9 and 32 MPam (Fig. 2), assuming the investigated pearlitic DCI as an homogeneous material, and according to relationships (1) and (2), crack tip plastic zone ( rpzK max , for K = Kmax) and reversed plastic zone radii range respectively: Crack tip plastic zone radius, rpzK max : - between 0.099 and 1.258 mm (plane stress conditions); - between 0.033 and 0.419 mm (plane strain conditions). 103 F. Iacoviello et alii, Frattura ed Integrità Strutturale, 25 (2013) 102-108; DOI: 10.3221/IGF-ESIS.25.15 Reversed plastic zone radius, r (...truncated)