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Unformatted text preview: Lab 05 Heat Treatment of Steel Objectives 1. To understand the effect of thermal processing (heat treatment) on both the microstructure and properties (hard- ness) of steel 2. To understand the application of time-temperature-transformation (TTT) curves in ferrous metallurgy Overview Steel is one of the most versatile engineering materials, used in a wide range of applications from structural components in the construction and transportation in- dustries to specialty tools in the medical industry. Its properties vary widely, depending upon the thermome- chanical processing cycles to which steel components are subjected, and their exposure to in-service condi- tions. This lab offers a survey of the role of heat treat- ment in the development of microstructures and proper- ties to optimize engineering performance in ferrous alloys. Figure 1 The Fe-C phase diagram, to 6.67wt.% C, from Metals Handbook , 8 th edition, Volume 8 , ASM, Metals Park, Ohio, (1973). 1 2 3 4 5 6 1100 1200 1300 1400 1500 1600 1000 900 800 700 600 500 400 300 200 100 wt % C T (C) Fe Fe 3 C L Cementite 1148 1227 1495 1394 1538 727 910 0.02 0.77 4.30 2.11 6.67 6.67 E 45 Laboratory Manual Copyright 2010, Berkeley Professor Ronald Gronsky Equipment Heat Treatment 1. ASTM 1045 (0.45 wt.% C) cold-rolled steel specimen 2. Furnace and wire for suspending specimen in hot zone 3. Thermocouple [chromel + alumel] 4. Computer data acquisition system Metallography 5. Polishing clamp, wet belt grinder , and polishing papers (240, 320, 400, 600) 6. Electropolisher, with polishing solution consisting of 60 ml distilled water + 50 ml butyl cellulose + 350 ml ethyl alcohol, 95%, (and just before using, add)+ 40 ml perchloric acid, 60% 7. Glass slide, specimen press 8. Optical microscope Hardness Testing 9. Rockwell hardness tester 10. Measurement ruler Background Steel is an alloy of iron (Fe) and carbon (C), with concentrations of carbon from 0.1 to 2.0 weight percent. Carbon is soluble in Fe because the C atoms are small enough to fit into interstitial locations between Fe atoms without too much distortion of the lattice. Carbon is soluble in the FCC phase of Fe (called austenite or gamma-Fe, or ) up to approximately 2%. However, in the BCC phase of Fe (called ferrite or alpha-Fe or ), the maximum solubility is only about 0.02%. These phases are shown on the equilibrium phase diagram in Fig. 1. When austenite is cooled below a critical temperature called the eutectoid temperature (727C), it becomes unstable. Most of the Fe tends to precipitate as nearly pure ferrite and most of the C tends to aggregate in the intermetallic com- pound Fe 3 C (cementite). The transformation of austenite requires redistribution of C atoms from a random solid solu- tion to one in which nearly all C is contained in the Fe 3 C precipitates....
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- Fall '08