E45 lab5

E45 lab5 - Heat Treatment of Steel Dustin Chen Section 103...

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Heat Treatment of Steel Dustin Chen 4/13/09 Section 103 Lab Partner: Emmanuel Chao 1
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Abstract This lab explores the effects of heat treatment on both the mechanical properties of steel, and also on the microstructure. To better understand the effects, a TTT (time-temperature- transformation) curve is analyzed in ferrous metallurgy. In this experiment, a cold-worked, steel alloy with an inducted temperature gradient was used. It was found that smaller grains create a harder substance, thus making martensite the hardest phase, then pearlite, and then ferrite. These phases were obtained by cooling austenite differently, holding it at different temperatures. Introduction Steel, an alloy of iron and carbon, is widely used in engineering for many applications, from transportation, to household materials. This versatility is partially due to the many variation of its properties, which are in a large part due to the thermomechanical processes which are imposed on steel. A very important necessity, then, is to understand how to use this heat treatment to optimize performance in ferrous alloys. Carbon is soluble in iron because the carbon atoms are able to fit into the interstitial sites of the iron atom without distorting it too much. In the FCC phase of iron, austenite, approximately 2% carbon can dissolve, whereas only around .02% of carbon can dissolve in the BCC phase (ferrite). By cooling austenite below the eutectoid temperature, it becomes unstable, precipitating the Fe as ferrite, and distributing the carbon atoms into Fe 3 C (cementite). The nucleation rate of ferrite and cementite is low, and transformation occurs when the specimen is held at a temperature for a long time. As the temperature becomes increasing lower, the nucleation rate increases, until below 540 ° C where it becomes slower again, as the carbon atoms lose mobility in austenite, and must diffuse to Fe 3 C regions. 2
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The driving force is driven by diffusion at higher temperatures, and nucleation at lower temperatures. Therefore, at higher temperatures, coarse pearlite is formed, the coarsest dispersion of Fe 3 C and ferrite. At lower temperatures, nucleation is the driving force and finer dispersions are produced. Hardness of steel increases with the fineness of dispersion, and is the hardest when martensite is formed, as the carbon atoms are distributed at random. Martensite is formed when the alloy is cooled so quickly, it skips the diffusion process altogether. Procedure
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E45 lab5 - Heat Treatment of Steel Dustin Chen Section 103...

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