Analyzing Gray Cast Iron

Topics: Cast iron, Iron, Diffraction Pages: 15 (3894 words) Published: February 22, 2012
To: Professor Lyman
From: Taylor McManus
Date: April 29th, 2011
Subject: Analysis of a 1912 Ford Model-T rear drum brake failure and composition

Executive Summary
The analysis of the 1912 Ford Model-T rear drum brake, Figures 1-2, has resulted in the conclusion that the brake material is best described as gray cast iron. The material has been proven to contain the phases of ferrite (α-Fe), cementite (Fe3C), graphite (C), iron phosphide (Fe3P), manganese sulfide (α-MnS), and titanium oxide (TiO), which are all phases commonly found in gray cast iron [9]. The probable casting method is gravity permanent mold casting, commonly used to cast gray iron. The brake failure is due to a brittle fracture occurring in the weak interdendritic regions along the edges containing type D graphite. Crack propagation most likely took place along the edge and then spread through the material along the graphite flakes, which have a much lower tensile strength than the iron matrix. Background

The process that was most likely used to cast the 1912 Ford Model-T rear drum brake would be gravity permanent mold casting. This process was commonly used to cast gray iron. Molten metal is poured into what is commonly a graphite mold. For gray iron, the liquid metal is usually poured at approximately 1370˚C [3]. This method of casting will lead to a higher cooling rate on the sections of the casting that are touching the mold and a lower cooling rate for the center of the cast. Five types of graphite are found in the sample using light optical microscopy. Different types of graphite form due to differences in solidification rate, section thickness, and pouring temperature [4]. Figure 3 is a microphotograph of Type A and C graphite found in the radial cut sample. Type A and C graphite form when undercooling is slight [4]. This type of graphite is the most commonly desired structure if the mechanical properties are to be optimized in terms of wear applicants. Type B graphite, shown in Figure 5, also known as the rosette pattern, forms in iron with near-eutectic composition that undergo a greater amount of undercooling than Types A and C graphite [4]. Type B graphite is commonly found in thin sections (about 10mm thick) and along the surfaces of thicker sections. Type D graphite, imaged in Figure 4, is found in interdendritic regions and is promoted by rapid solidification of thin sections and high superheating temperatures [4]. This type of graphite is formed near rapidly cooled surfaces or in thin sections and is frequently surrounded by ferrite, resulting in soft spots in the casting. Type D graphite is characterized by a fine graphite pattern sharply delineating the primary dendrites. Type E graphite, seen in Figure 6, forms when the amount of undercooling is high and is also found in interdendritic regions [4]. Unlike Type D graphite, however, type E graphite can be associated with a pearlitic matrix and enables the casting to have wear properties as good as those in type A [4].

Dendrites tend to form on the outside edges of the cut since dendrites form due to rapid cooling. Since the edges cool faster than the middle during cooling, dendrites are more prominent along the edges [4].

The density of the sample was measured and found to be 6.99 g/cm3. The sample was then examined under a light optical microscope, and a hypothesis was formed that the material is gray cast iron. The known density of gray cast iron ranges from 7.0-7.3 g/cm3 [1]. In order to determine the unknown material, all phases must first be identified within the sample.

Ferrite (α-Fe)
An X-ray map, shown in Figure 7, taken with a Scanning Electron Microscope (SEM) was taken of the sample at the location of the image in Figure 8. From the map it is clear that iron is heavily present in the material. Figure 9 shows an X-ray spectrum taken with the SEM of the sample. The spectrum shows that iron is the most prominent element in the sample.

A Powder X-ray...
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