04E45 Heat Treatment of Steel Lab Report

04E45 Heat Treatment of Steel Lab Report - E45 Lab #4...

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Figure 4 – test locations gure 3: Setup for heat-gradient of steel E45 Lab #4 (March 31, 2008) Norbert Wang Heat Treatment of Steel Abstract The purpose of this laboratory experiment is to explore how heat treatment can generate specific phases in steel. Rockwell type C standard tests will be performed to measure the hardness of the steel at various phases. These phases are established by first creating a heat gradient in the steel and the quenching it in cold water. The order from hardest to softest phases was found to be: martensite, fine pearlite, coarse pearlite, and ferrite. Relative to the bar, the hardest regions were on the ends (martensite and martensite + ferrite) and the softest was in the middle (pearlite). When austenite containing steel is quenched quickly in water, martensite, a very hard meta-stable phase, is formed. Hardness is associated with how many dislocations the material has. The more dislocations it has the harder it is for an indentation to create more dislocations in the material, making the material hard. Introduction Steel is a very versatile and abundant material applicable in a variety of engineering projects. Its strength, resistance to corrosion, and relatively low cost makes it a very profitable industry. Steel is an alloy of iron and carbon. Its phase diagram is shown below
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in Figure 1. The amount of carbon in a typical steel piece is quite low, ranging from 0.1 to 2.0 percent by weight. The carbon atoms fit into the interstitial sites in iron. Iron has a somewhat odd crystal structure. It has a eutectoid temperature of 727 C. At temperatures below 727 C, the iron is typically is a mixture of alpha phase (called ferrite) with Fe 3 C (called cementite). Together, this mixture is called pearlite. This structure is a body- centered-cubic (BCC) crystal structure. Above 727 C but below the liquidus line, the phase is called austenite, or gamma phase. In the region the crystal structure is primarily face- centered-cubic (FCC). It turns out that carbon is more likely to be soluble in the FCC structure rather than BCC because FCC provides bigger interstitial sites. Therefore carbon atoms in FCC will distort the structure less than carbon atoms in BCC. When austenite is cooled to form pearlite, the structure becomes more unstable because it reconfigures to form BCC. The carbon atoms that were initially in the interstitials of FCC austenite are forced to diffuse out of the structure. Carbon-rich areas that precipitate out will become the cementite regions. The increasing iron areas become the ferrite regions. The kinetics of this reaction depends on how far below the eutectoid line the steel is cooled to. At temperatures just shy of 727 C, the reaction of austenite to pearlite is slow. At temperatures significantly lower than 727 C but higher than 540 C, the reaction is much faster. Equilibrium takes time to complete. If the steel is cooled at a steady slow rate, there is enough time to form equilibrium between ferrite and cementite. If the steel is cooled very quickly, ferrite and cementite cannot equilibrate well. The FCC austenite is immediately
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04E45 Heat Treatment of Steel Lab Report - E45 Lab #4...

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