Important Most phase transforma0ons occur by di usion Recall that di usion involves the movement of atoms at the atomic scale and that di usivity increases with temperature A di usive phase transforma0on can be prevented by rapidly cooling the material to make di usion prohibi0vely slow e g the transforma0on takes years instead of minutes Rapid cooling to minimize di usion is called quenching We ve seen how quenching is used to form ne precipitate par0cles in aluminum alloys Now we will examine di usive phase transforma0ons in iron carbon alloys carbon steel We will also learn about phase transforma0ons that occur without di usion These are displacive phase transforma0ons more commonly referred to as martensi0c transforma0ons 1 Let s examine the TTT diagram for iron with 0 80 wt carbon 2 No0ce that at roughly 550 C we can form pearlite from austenite in about 10 seconds 3 Analogous to Al Cu alloys a lower isothermal temperature results in a ner microstructure Coarse Pearlite Fine Pearlite 4 This is re ected in the mechanical behavior of the material The hardness increases with increasing wt C because the weight frac0on of Fe3C is increasing The ne pearlite results in more Fe3C plates per unit mass of Fe3C hence the greater hardness Eutectoid Composi0on 5 Spheroidite is a microstructure produced by prolonged hea0ng of pearlite It is much so er because the Fe3C is in the form of spheres rather than plates making disloca0on mo0on easier Spheroidite is rela0vely easy to machine 6 Heat Trea0ng for fun and pro t Goal want a material with the easy machinability of spheroidite but I want the nal component to have the mechanical proper0es of ne pearlite Machine the shape you want from a carbon steel heat treated to produce spheroidite When nished heat the component above the eutectoid temperature to make austenite then cool such that ne pearlite forms 7 Each alloy composi0on has a unique TTT diagram The TTT diagram below re ects the forma0on of proeutectoid cemen0te in hypereutectoid alloys In hypoeutectoid alloys the TTT diagram would show the forma0on of proeutectoid ferrite Hypereutectoid Composi0on 8 Martensi0c Transforma0ons Quenching Al Cu alloys freezes in the high temperature microstructure Something quite di erent happens when Fe C alloys are quenched Rather than freezing in the high temperature structure quenching causes a displacive di usionless phase transforma0on commonly known as a martensi0c transforma0on The resul0ng microstructure called martensite has a characteris0c needle like morphology that results in a very hard and bridle material 9 Martensite has a Body Centered Tetragonal BCT structure resul0ng from the very localized shi of atoms from the FCC structure of austenite Martensite is very hard because the large amount of carbon in austenite does not have 0me to di use away and form Fe3C Instead it is trapped in the BCT structure forming a supersaturated solid solu0on of carbon that is very e ec0ve at inhibi0ng disloca0on mo0on 10 FCC to BCT Transforma0on x carbon inters00al site No0ce that the BCT structure resides within the FCC structure allowing the transi0on to occur with only a localized shi of the atoms 11 The martensite needles grow as lense like shapes that pop across the austenite grains as the temperature drops below the martensite start temperature 12 Martensite forma0on occurs over a short range of temperature bounded by the start and nish temperatures 13 When cooling there is a compe00on between pearlite and martensite forma0on Slower cooling forms pearlite while rapid cooling forms martensite 14 Cooling rate e ects are accounted for using Con0nuous Cooling Transforma0on CCT diagrams For this eutectoid 0 80 wt C alloy a cooling rate in excess of 140 C s is required to avoid the forma0on of pearlite and form only martensite 15 Martensite is much harder than pearlite so hard that it is bridle and not very useful in prac0ce However hea0ng below the eutectoid temperature will allow di usion to release some of the carbon from solid solu0on that forms small Fe3C par0cles This is called tempering 16 Tempered Martensite Spheroidite 17 By choosing the appropriate tempering temperature and 0me one can reach the full range of hardness values between martensite and pearlite 18 Bodom Line There is a large range of mechanical proper0es accessible using iron carbon alloys See the Steel Processing le in the Blackboard Handouts folder 19
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