Fracture Analysis

DIEGO O. FERNANDINO; ROBERTO E. BOERI

Cast Iron Science and Technology ASM Handbook

ASM International

Año: 2017; p. 1 - 13

The technology involved in the production of castings has advanced significantly in recent decades. Many industries, such as agriculture, automotive, wind power, machines and tools, make extensive use of cast iron parts. Cast iron, in its various forms, continues to successfully replace other materials such as steel castings, machined or forged steel products, and even aluminum alloys. However, this development takes place in a very competitive environment, between several metallic alloys and composite materials. The knowledge base plays a very relevant role at the time a given material is selected for a specific application. Without continued efforts in research, any material will gradually lose its comparative advantages. In this context, as cast iron parts are extensively applied, fracture events will eventually take place. Consequently, it becomes essential to carry out failure analyses to identify the cause of fracture, and to provide corrective actions that allow guarantees of safe operation. Every fracture, in principle, contains all the evidence about its cause, although this information is sometimes hard to extract from the fracture surfaces. It is generally accepted that in the fracture process of a metallic material, the predominant fracture mechanisms can be revealed by identifying some characteristic features on the fracture surface (Ref 1-4).The fracture surface must be examined macro- and microscopically. Since its development, scanning electronic microscopy (SEM) has been applied to the fractographic analysis of parts that failed in service. SEM observations have allowed the identification of fractographic characteristics that can be related to the external stresses and the microstructure of various metal alloys. The most common fracture mechanisms involved in metal fracture are ductile fracture, cleavage fracture, intergranular fracture and fatigue (Ref 4-6). Their particular features are distinguishable by means of SEM observations. In consequence, fractographic study is a fundamental tool for the failure analysis of a metallic part. When the materials used to make the parts are steel and other common alloys, there is usually plenty of information about the correlation between the fractographic observations and the kind of loads that caused the failure (Ref 2). However, when the fractured material is cast iron, there is a noticeable lack of knowledge. This is generally associated with the intrinsic difficulty in the analysis of the fracture morphology, which is typically generated on a metallic matrix that may be composed by various phases and that include free graphite precipitates of different size and morphology. This analysis can be further complicated by the usual heterogeneities present in the metallic matrix, such as microsegregation, inclusions and microshrinkage. This lack of knowledge may condition the application of cast iron, in certain cases and could be considered a strategic problem, because a design engineer would prefer to use the best-characterized material (e.g. steel) before another that has a more limited information base. Then, the problem becomes relevant to the dissemination of cast iron applications in high stress applications.