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Chapter 11: Problem 5

On the basis of microstructure, briefly explain why gray iron is brittle and weak in tension.

Short Answer

Expert verified
Answer: Gray iron's brittleness and weak tensile strength can be attributed to the presence of graphite flakes in a pearlite/ferrite matrix. Graphite flakes act as stress concentrators and have weak bonds with the matrix, which reduces ductility and tensile strength. The weak bond between graphite flakes and the matrix also facilitates crack initiation and propagation, leading to brittle fracture.

Step by step solution

01

Understand the microstructure of gray iron

Gray iron is a type of cast iron that gets its name due to the gray color it displays when fractured. This gray color comes from the presence of graphite flakes in its microstructure. Gray iron is composed of graphite flakes dispersed in a pearlite/ferrite matrix. Both the amount and morphology of the graphite flakes, as well as the composition of the matrix, have a strong influence on the mechanical properties of gray iron.
02

Discuss the role of graphite flakes in brittleness

Graphite flakes present in gray iron have a very low tensile strength and are brittle. When subjected to tensile loads, these graphite flakes act as stress concentrators, meaning they cause stress to build up locally around the flakes. As the load is applied, the stress concentration increases at the tips of the graphite flakes, causing the material to fracture.
03

Examine the mechanism of brittleness in gray iron

The presence of graphite flakes in gray iron leads to a drastic reduction in the material's ductility, making it brittle. The cracks in gray iron initiate at the tips of the graphite flakes, which have a weak bond with the surrounding matrix. When gray iron is subjected to tensile stress, the bond between the graphite flakes and the matrix weakens. As the tensile stress increases, cracks propagate easily along the graphite-matrix interface, eventually leading to brittle fracture.
04

Observe the weak tensile strength of gray iron

Gray iron displays weak tensile strength due to the weak bonds between graphite flakes and the matrix. The graphite flakes can't bear much tensile load, so when the material is subjected to tension, the load is mainly borne by the surrounding matrix. However, the presence of graphite flakes also weakens the overall strength of the matrix, thus reducing gray iron's overall tensile strength.
05

Conclude the explanation

To summarize, gray iron is brittle and weak in tension due to its unique microstructure characterized by the presence of graphite flakes in a pearlite/ferrite matrix. Graphite flakes act as stress concentrators, reduce ductility, and weaken the overall tensile strength of the material. The weak bond between graphite flakes and the matrix contributes to crack initiation and propagation, leading to brittle fracture.

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Compute the volume percent of graphite, \(V_{\mathrm{Gr}}\), in a \(2.5\) wt \(\%\) C cast iron, assuming that all the carbon exists as the graphite phase. Assume densities of \(7.9\) and \(2.3 \mathrm{~g} / \mathrm{cm}^{3}\) for ferrite and graphite, respectively.
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