Note for ME 350 with Professor Wei at UA-Static Machine Components
Note for ME 350 with Professor Wei at UA-Static Machine Components
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This 6 page Class Notes was uploaded by an elite notetaker on Friday February 6, 2015. The Class Notes belongs to a course at University of Alabama - Tuscaloosa taught by a professor in Fall. Since its upload, it has received 15 views.
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Date Created: 02/06/15
mnrrgrrm TimeTempnatureTransformation T TT Diagram T Time TTemperature TTransformatr39on diagram is a plot oftemperature versus the logarithm oftime for a steel alloy of de nite composition It is used to determine when transformations begin and end for an isothermal constant temperature heat treatment of a previously austenitized alloy When austenite is cooled slowly to a temperature below LCT Lower cntr39cal Temperature the structure that is formed is Pearlite As the cooling rate increases the pearlite transformation temperature gets lower The microstructure ofthe material is signi cantly altered as the cooling rate increases By heating and cooling a series of samples the history of the austenite transformation may be recorded TTT diagram indicates when a speci c transformation starts and ends and it also shows what percentage of transformation of austenite at a particular temperature is achieved Figures below show the microstructure of different phases Martensite Cooling rates in the order of increasing severity are achieved by quenching from elevated temperatures as follows furnace cooling air cooling oil quenching liquid salts water quenching and brine If these cooling curves are superimposed on the TTT diagram the end product structure and the time required to complete the transformation may be found Martensite transformation rapid cooling from above eutectoidtemperature to room T 7 FCC to Martensite Body Centered Tetragonal involves collective motion ofa lot of atoms TTT diagram In Figure l the area on the left of the transformation curve represents the austenite region Austenite is stable at temperatures above LCT but unstable below LCT Left curve indicates the start of a transformation and right curve represents the finish of a transformation The area between the two curves indicates the transformation of austenite to different types of crystal structures Austenite to pearlite austenite to martensite austenite to bainite transformation Isothermal Transform Diagram shows that y to transformation a is rapid at speed of sound b the percentage of transformation depends on Temperature only Flown 1013 The 800 I I com lete isothermal A transforEnation diagram A EL6Ct mtem it 1400 for an iron carbon alloy of eutectoid composition A austenite B bainite M martensite P pearlite 1200 1000 800 Temperature C Temperature F 600 400 200 10 1 1 10 102 103 104 105 Time s Figure 1 TTT Diagram Figure 2 represents the upper half of the TTT diagram As indicated in Figure 2 when austenite is cooled to temperatures below LCT it transforms to other crystal structures due to its unstable nature A specific cooling rate may be chosen so that the transformation of austenite can be 50 100 etc If the cooling rate is very slow such as annealing process the cooling curve passes through the entire transformation area and the end product of this the cooling process becomes 100 Pearlite In other words when slow cooling is applied all the Austenite will transform to Pearlite If the cooling curve passes through the middle of the transformation area the end product is 50 Austenite and 50 Pearlite which means that at certain cooling rates we can retain part of the Austenite without transforming it into Pearlite mangam Low R CRITICIALTEMPERATURE TEMPERATURE TIME Figure 2 Upper half or TTT DiagramAustenitePearlite Transformation Area Figure 3 indicates the types oftransformation that can he found at higher cooling rates If a cooling rate is Very high the cooling curve will rernain on the lett hand side of the Transformation Start curve In this case all Austenite will transform to Martensite If there is no interruption in cooling the end product will he rnartensite lxl39l39 l TEMPERATURE in mm TIME Figure 3 Lower half of TTT Diagram AusteniteMartensite and Bainite Transformation Areas In Figure 4 the cooling rates A and B indicate two rapid cooling processes In this case curve A will cause a higher distortion and a higher internal stresses than the cooling rate B The end product ofhoth cooling rates will he rnartensite Cooling rate B is also known as the Critical Cooling Rate which is represented by a cooling curve that is tangent to the nose ofthe TTT diagrarn Critical Cooling Rate is de ned as the lowest cooling rate which produces 100 Martensite while rninirnizing the internal stresses and distortions TEMYERATURE u it sign n ml m lwun sln mum in l in Illlr Vs lgtHl Figure 4 Rapid Quench milligram In Figure 5 a rapid quenching process is interrupted horizontal line represents the interruption by immersing the material in a molten salt bath and soaking at a constant temperature followed by another cooling process that passes through Bainite region ole39l diagram The end product is Bainite which is not as hard as Martensite As a result ofcooling rate D more dimensional stability less distortion andless internal stresses are created TEMPERATURE H mm imm m lvlumu lunmllmillullltl mm Figure 5 Interrupted Quench In Figure 5 cooling curve c represents a slow cooling process such as furnace cooling An example for this type of cooling is annealing process where all the Austenite is allowed to transform to Pearlite as a result of slow cooling TEMPERATURE H mm umm Figure 6 Slow cooling process Annealing Sometimes the cooling curve may pass through the middle ofthe AustenitePearlite transformation zone in Figure 7 cooling curve E indicates a cooling rate which is not high enough to produce 100 martensite This can he observed easily by looking at the TTT diagram Since the cooling curve E is not tangent to the nose of the transformation diagram austenite is transformed to 50 peanite curve E is tangent to 50 curve Since curve E leaves the transformation diagram at the Martensite zone the remaining 50 ofthe Austenite will he transformed to Martensite TEMPERATURE Figure 7 Cooling rate that permits both peariite and niartensite iorination Imiu um mmr muons nm m m muuln at Hmnwlll n39r diagram mm 7 LOWER cm39rcuL TEMI FKATURF lsnu 120 ll mm m sun 700 am 500 400 Jon um AILSTI39ENIT wumns TEMPERATURE or TIME gure 8 TTT Diagram and microstructures ohmined by different types of cooling rates Martensite Tempered martensite Bainlte Strength Ductility Fine pearlite Coarse pearlite gure 9 Dependence ofmechanical proper es on microstructures