Influence of shrinkage porosity on fracture under tensile stress in ductile cast iron

MARCOS G. LOPEZ; DIEGO O. FERNANDINO; ROBERTO E. BOERI; JUAN M.MASSONE

Cracovia

Congreso; 73rd World Foundry Congress "Creative Foundry"; 2018

Wolrld Foundry Organization/ Polish Foundrymen's Association

One of the major problems affecting the quality of cast parts are shrinkage defects that occur during solidification. When high strength pieces are produced, it becomes critical to ensure that they are structurally sound. Typical classification of shrinkage defects [1] includes three different types of shrinkage cavities: a) the concentrated shrinkage; b) the dispersed macroshrinkage, and c) the dispersed microshrinkage. Concentrated shrinkage forms because of metal contraction during cooling of the liquid and the phase change. It can be open or closed to the atmosphere. The dispersed macroshrinkage consists of cavities of millimeters size resulting from the lack of feeding during solidification. It appears in heavy sections or at hot spots of the casting and can be prevented by an optimization of the riser design. The microshrinkage appears usually dispersed along the whole volume of the casting. It consists of micron-size cavities that are formed between the eutectic grains or dendrite arms. The presence of these micro cavities, could affect the mechanical answer of the part. The macroscopic deformation of ferritic SGI is generally considered to induce a homogeneous local deformation at the microscopic level; however, the experimental evidence shows that the mechanical properties and deformation behavior at microscopic level are heterogeneous. [2-3]. This is caused by the heterogeneity in the mechanical properties of the microconstituents. However, a lack of knowledge about the effect of the microsegregated zones of the metallic matrix and the dispersed microshrinkage on the occurrence and sequence of damage mechanisms is evident. This work focuses on the study of shrinkage porosity in SGI, identifying the morphology and location of porosity in cast samples especially developed, and assessing the influence of dispersed microshrinkage cavities as a preferential site for cracks initiation and propagation, by means of in-situ tensile tests and Digital Image Correlation (DIC) analysis.