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Engineering 45 Lab 4

Engineering 45 Lab 4 - 1 Introduction The purpose of this...

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1 Introduction : The purpose of this lab was to examine how heat processing of steel can affect its microstructure and its mechanical properties, such as hardness. Steel is an alloy composed of iron and carbon with weight percents of carbon ranging from .1 to 2.0. When austentite (when carbon is soluble in the FCC phase of Fe, also called gamma-Fe) is cooled below the eutectoid temperature of 727 ° C, the molecules inside the steel start becoming unstable and most of the carbon precipitates as Fe 3 C (cementite) whereas most of the Fe comes out as pure ferrite. If the austentite is rapidly cooled such that no nucleation or growth can take place in Fe 3 C, a transformation called martensite is made. Furthermore, the ferrite that comes out of the Fe can be transformed to two phases just below the eutectoid temperature: coarse and fine pearlite.
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3 All these different forms and phases are accurately represented by a time-temperature-transformation (TTT) diagram and the equilibrium phase diagram. The TTT diagram shows which phases are present at a certain time and temperature and at which temperature or time each transformation starts to begin and end. Figure 1: TTT diagram for eutectoid steel Figure 2: Phase diagram for Fe-Fe 3 C system The mechanical properties of steel, such as hardness, vary with the different transformations due to the dispersion of carbon atoms or of cementite particles in the Fe matrix. The hardness usually increases with the fineness of dispersion of the phases. Usually the hardest structure possible for a steel alloy is when the precipitation of carbon as Fe 3 C can be suppressed together, where the carbon atoms are distributed at random; this is the phase martensite. Procedure : First, a 1040 cold rolled sample of steel was hung from a nichrome wire into the furnace and above a bowl of water. A thermocouple was inserted into the steel specimen. The thermostat was turned on to heat the sample to 1050 ° C. The data acquisition program started to
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