High-temperature use of carbon steel
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This post was last edited by yakui on 2009-9-23 09:04. When carbon steel is used at temperatures above 425°, cementite decomposes, followed by the graphitization of carbon. The material becomes brittle, with a decrease in strength and toughness~! But personally, I wonder: if the material’s strength decreases, how does it become brittle?
It’s just like quenching, you know. It is a structural transformation of the crystal
2# qcy1225 seems not to be it···
At high temperatures, cementite decomposes and graphitizes; due to the layered and loose structure of graphite, it can slide. This means that whether the graphitized structure appears at grain boundaries or within the grains, it has a highly destructive effect on the toughness of the material. When toughness is destroyed, the material becomes brittle.
“Graphitization is a phenomenon in which cementite in steel decomposes on account of high temperatures; it is also known as the ink separation phenomenon: Fe3C = 3Fe + C (graphite). Initially, graphite appears in the form of tiny dots, which then gradually aggregate. Graphite has extremely low strength; it is as if there are voids within the metal, and stress concentration occurs around these voids, which renders the material brittle and reduces its strength and ductility. If graphite particles aggregate into chains, it can lead to the material breaking. ”——Excerpt from Engineering Materials, Chemical Industry Press
To answer the question raised by the original poster in detail, it is necessary to delve into the iron-carbon alloy phase diagram and structure. For ordinary carbon steel at temperatures below 727 degrees Celsius, the phases of such alloys consist of a ferrite solid solution along with the intermetallic compound cementite (Fe3C). The entire iron-carbon alloy is primarily composed of a solid solution, with intermetallic compounds distributed throughout it; this arrangement helps to enhance the material’s strength, hardness, and wear resistance. When carbon steel is used at temperatures above 425° for an extended period, cementite decomposes, causing carbon in the Fe3C present between the grains of the solid solution to precipitate, resulting in graphitization. This leads to a decrease in the density between the solid solution phase and the cementite compounds, resulting in increased brittleness and a decline in strength and toughness of the material.