Materials Lab Procedures
Materials Lab Procedures MSE 300
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This 33 page Class Notes was uploaded by Eudora Blick on Monday October 26, 2015. The Class Notes belongs to MSE 300 at University of Tennessee - Knoxville taught by Philip Rack in Fall. Since its upload, it has received 11 views. For similar materials see /class/229813/mse-300-university-of-tennessee-knoxville in Materials Science Engineering at University of Tennessee - Knoxville.
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Date Created: 10/26/15
MSE 300 Materials Laboratory Procedures IronCarbon Phase Diagram a review see Callister Chapter 9 University of Tennessee Dept of Materials Science and Engineering MSE 300 Materials Laboratory Procedures The Iron Iron Carbide Fe Fe3C Phase Diagram In their simplest form steels are alloys of Iron Fe and Carbon C The FeC phase diagram is a fairly complex one but we will only consider the steel part of the diagram up to around 7 Carbon 1538 1200 f g E y Austemie 2 14 43930 7 m g E 1000 m c g 912 C y Fe3C a 800 L 7 o y 727 c 075 0022 600 a Femte 0 F930 Cemenme F930 7 University of Tennessee Dept of Materials Science and Engineering 3 4 Composition man a 5 6 67 MSE 300 Materials Laboratory Procedures Phases in Fe Fe3C Phase Diagram ocferrite solid solution of C in BCC Fe Stable form of iron at room temperature The maximum solubility of C is 0022 wt Transforms to FCC yaustenite at 912 oC yaustenite solid solution of C in FCC Fe The maximum solubility of C is 214 wt Transforms to BCC 5ferrite at 1395 oC Is not stable below the eutectic temperature 727 C unless cooled rapidly Chapter 10 8ferrite solid solution of C in BCC Fe The same structure as ocferrite Stable only at high T above 1394 oC Melts at 1538 oC Fe3C iron carbide or cementite This intermetallic compound is metastable it remains as a compound indefinitely at room T but decomposes very slowly within several years into ocFe and C graphite at 650 700 oC FeC liquid solution MSE 300 Materials Laboratory Procedures A few comments on Fe Fe3C system C is an interstitial impurity in Fe It forms a solid solution with or y 5 phases of iron Maximum solubility in BCC ocferrite is limited max 0022 wt at 727 C BCC has relatively small interstitial positions Maximum solubility in FCC austenite is 214 wt at 1147 oC FCC has larger interstitial positions Mechanical properties Cementite is very hard and brittle can strengthen steels Mechanical properties also depend on the microstructure that is how ferrite and cementite are mixed Magnetic properties or ferrite is magnetic below 768 C austenite is nonmagnetic Classification Three types of ferrous alloys Iron less than 0008 wt C in oc ferrite at room T Steels 0008 214 wt C usually lt 1 wt ocferrite Fe3C at room T Chapter 12 Cast iron 214 67 wt usually lt 45 wt University of Tennessee Dept of Materials Science and Engineering 4 Temperature C MSE 300 Materials Laboratory Procedures Eutectic and eutectoid reactions in Fe Fe3C Eutectic 430 wt C 1147 C L y Fe3C 1200 y Austemte yFe3C 1000 727 C 7 0022 a Femte a FeSC Cemenme F930 i i i i i 1 2 3 A 5 5 Composmon era c Eutectoid 076 wtC 727 C y076 Wtoo C 0c 0022 Wtoo C Fe3C Eutectic and eutectoid reactions are VCIy important in heat treatment of steels MSE 300 Materials Lzbnmtnry Prncedures Development of Microstructure in Iron Carbon alloys Microstructure depends on composition carbon content and heat treatment In the discuss1on below we consider slow cooling in which equilibrium is maintained Microstructure of eutectoid steel I 1 i 1000 7 y Fejc 800 Y Temperaiuve 07 7m 500 7 a FE3C 1quot 1 o Camposmon wt c 6 MSE 300 Materials Laboratory Procedures Microstructure of eutectoid steel 11 When alloy of eutectoid composition 076 wt C is cooled slowly it forms perlite a lamellar or layered structure of two phases ocferrite and cementite Fe3C The layers of alternating phases in pearlite are formed for the same reason as layered structure of eutectic structures redistribution C atoms between ferrite 0022 wt and cementite 67 wt by atomic diffusion Mechanically pearlite has properties intermediate to soft ductile ferrite and hard brittle cementite In the micrograph the dark areas are Kgu g Fe3C layers the light phase is oc 1 ferrite n A Austemte gram I boundary 5 a 3quot Austem te W IzK v a 39 Growth direction earhte 0D Carbon dlffusmn MSE 300 Materials Lzbnmtury Pmcedures Microstructure of hypoeutectoid steel 1 Compositions to the left of eutectoid 0022 076 wt C hypoeutectoid less than eutecmid GTeek alloys y ay gt aFe3C 1000 900 Temperature quotc1 quot jgtPeamte 600 F630 Prueulectoid a Eutectowd Ll a F239 10 20 Co Cumpusmun W c 8 MSE 300 Materials Laboratory Procedures Microstructure of hypoeutectoid steel II Hypoeutectoid alloys contain proeutectoid ferrite formed above the eutectoid temperature plus the eutectoid perlite that contain eutectoid ferrite and cementite Pea rl ite Fe3C Proeutectoid a Eutectoid x MSE 300 Materials Lab nmtnry Pmcedures Microstructure of hypereutectoid steel 1 Compositions to the right of eutectoid 076 214 WT C hypereutectoid more than eutectoid Greek alloys 7 7Fe3C 0LFe3C 1000 7 y 7 g 900 3 E sun A g v a m y E 2 mo 0 7 Pe me son a 7 Pro le Emectmd FESC so 500 7 a v F638 4 x z x a 1 20 c Composmon WWn c MSE 300 Materials Laboratory Procedures Microstructure of hypereutectoid steel 11 Hypereutectoid alloys contain proeutectoid cementite formed above the eutectoid temperature plus perlite that contain eutectoid ferrite and cernentite Pearlite a I Pro cute ctoid MSE 300 Materials Lab nmtnry Prncedures How to calculate the relative amounts of proeutectoid phase on or Fe3C and pearlite Application of the lever rule with tie line that extends from the eutectoid composition 076 Wt C to on 7 on Fe3C boundary 0022 Wt C for hypoeutectoid alloys and to on Fe3C 7 Fe3C boundary 67 Wt C for hypereutectoid alloys Temperature Composition Wt C Fraction of on phase is determined by application of the lever rule across the entire on Fe3C phase field MSE 300 Materials Lab nmtnry Prncedures Example for hypereutcctoid alloy with composition C1 Fraction of pearlite WP X VX 67 C1 67 076 Fraction of proeutectoid cem entite WFesc V VX C1 076 67 076 Temperatuve Composition mm C MSE 300 Materials Laboratory Procedures Phase Transformations of FeC a review see Callister Chapter 10 University of Tennessee Dept of Materials Science and Engineering 14 MSE 300 Materials Laboratory Procedures Phase transformations Kinetics Phase transformations change of the microstructure can be divided into three categories gt Diffusiondependent With no change in phase composition or number of phases present eg melting solidification of pure metal allotropic transformations recrystallization etc gt Diffusiondependent With changes in phase compositions andor number of phases 6 g eutectoid transformations gt Diffusionless phase transformation produces a metastable phase by cooperative small displacements of all atoms in structure 6 g martensitic transformation discussed in later in this chapter Phase transformations do not occur instantaneously Diffusiondependent phase transformations can be rather slow and the final structure often depend on the rate of coolingheating We need to consider the time dependence or kinetics of the phase transformations University of Tennessee Dept of Materials Science and Engineering 15 MSE 300 Materials Lab oratory Procedures Kinetics of phase transformations Most phase transformations involve change in composition 2 redistribution of atoms via diffusion is required The process of phase transformation involves gt Nucleation of of the new phase formation of stable small particles nuclei of the new phase Nuclei are o en formed at ain oundaries and other defects gt Growth of new phase at the expense of the original phase y l eXp ktquot Avrami Equation lu S sha e curve percent of material transform ed vs the logarithm of time 3 Fraction of transformation 0 m t05 Nucleation Growth I Logarithm of healing time t MSE 300 Materials Laboratory Procedures Superheating supercooling gt Upon crossing a phase boundary on the composition temperature phase diagram phase transformation towards equilibrium state is induced gt But the transition to the equilibrium structure takes time and transformation is delayed gt During cooling transformations occur at temperatures less than predicted by phase diagram supercooling gt During heating transformations occur at temperatures greater than predicted by phase diagram superheating gt Degree of supercoolingsuperheating increases with rate of coolingheating gt Metastable states can be formed as a result of fast temperature change Microstructure is strongly affected by the rate of cooling gt Below we will consider the effect of time on phase transformations using ironcarbon alloy as an example University of Tennessee Dept of Materials Science and Engineering 17 Let us consider eutectoid reaction as an example eutectoid reaction g y076 wt C g a 0022 wt C 8 Fe3C ll Composmon wi C Percent pearhte w o l l 1 10 102 103 Time 5 The Sshaped curves are shifted to longer times at higher T showing that the transformation is dominated by nucleation nucleation rate increases with supercooling and not by diffusion which occurs faster at higher T Isothermal Transformation or TTT Diagrams Temperature Time and Transformation 100 n E g I I I l I I E SE Transformation TFEHSTDWBHW E 3 temperature 675 I ends on 4 E E 50 I E E a 39 3 E Transformatron a 3 begins at E I 0 I I T I I I 1 10 1 1nd I 103 113 1 105 1 Time 5239 i I 39 I 39 39 I l I I I I I Austenrte stable iEutactmd temperatur Austenite ma unstable Pearlite E 600 50 Completion curve 5 D E E 500 Completlon curve l pearlite Begin curve v D pearlite 100 1 10 1D2 103 104 105 Time 5 19 MSE 300 Materials Laboratory Procedures TTT Diagrams 15 1mm in 1dy i Eutectoid f temperature a ferrite Coarse pearlite 600 a E m a a E u 7 Austenite gt earlite 500 t p Denotes that atransi39orma on rans orma on is owning Time s The thickness of the ferrite and cementite layers in pearlite is N 81 The absolute layer thickness depends on the temperature of the transformation The higher the temperature the thicker the layers University of Tennessee Dept of Materials Science and Engineering 20 MSE 300 Materials Laboratory Procedures TTT Diagrams gt The family of Sshaped curves at different T are used to construct the TTT diagrams gt The TTT diagrams are for the isothermal constant T transformations material is cooled quickly to a given temperature before the transformation occurs and then keep it at that temperature gt At low temperatures the transformation occurs sooner it is controlled by the rate of nucleation and grain growth that is controlled by diffusion is reduced gt Slow diffusion at low temperatures leads to finegrained microstructure with thinlayered structure of pearlite fine pearlite gt At higher temperatures high diffusion rates allow for larger grain growth and formation of thick layered structure of pearlite coarse pearlite gt At compositions other than eutectoid a proeutectoid phase ferrite or cementite coexist with pearlite Additional curves for proeutectoid transformation must be included on TTT diagrams University of Tennessee Dept of Materials Science and Engineering 21 MSE 300 Materials Laburatury Pmcedures Formation of Bainite Microstructure I u i i i i i A Eutectoid temperature 1 700 7 A 1 600 7 A quot 1 3 N Upper j 500 7 baimte 7 5 E g 7 8 g 400 7 300 7 7 6 200 7 50 i 4 1 i i i i i 10 1 1 10 102 103 104 105 Time s If transformation temperature is low enough 54OOC bainite rather than fine pearlite forms University nf Tennessee Dept nf Materials Science and Engineering 22 Formation of Bainite lVIicrostructure II gt For T N 300540 C upper bainite consists of needles of ferrite separated by long cementite particles gt For T N ZOO300 C lower bainite consists of thin plates of ferrite containing very ne rods or blades of cementite gt In the bainite region transformation rate is controlled by microstructure growth diffusion rather than nucleation Since diffusion is slow at low temperatures this phase has a very ne microscopic microstructure gt Pearlite and bainite transformations are competitive transformation between pearlite and bainite not possible Without rst reheating to form austenite MS 3 Materials Lzhnrztnry Frncedllrzs Spheroidite I Annealing of pearlitic or bainitic microstructures at elevated temperatures just below eutectoid eg 24 h at 700 C leads to the formation of new microstructure 7 spheroidite spheres of cementite in a ferrite matrix I Composition or relative amounw of ferrite and cementite are not changing in this tmnsformation only shape of the cementite inclusions is changing I Transformation proceeds by C diffusion 7 needs high T I Driving force for the tmnsformation reduction in total ferrite cementite boundary area MSE 300 Materials Laboratory Procedures Martensite I Martensite forms when austenite is rapidly cooled quenched to room T It forms nearly instantaneously when the required low temperature is reached The austenitemartensite does not involve diffusion gt no thermal activation is needed this is called an athermal transformation Each atom displaces a small subatomic distance to transform FCC yFe austenite to martensite which has a Body Centered Tetragonal BCT unit cell like BCC but one unit cell aXis is longer than the other two Martensite is metastable can persist indefinitely at room temperature but will transform to equilibrium phases on annealing at an elevated temperature Martensite can coexist with other phases andor microstructures in FeC system Since martensite is metastable nonequilibrium phase it does not appear in phase FeC phase diagram University of Tennessee Dept of Materials Science and Engineering 25 MSE 3m Mataials Lzhnmtm39y Prncedurzs Wily 001quot 1001v 100 a The ma ensitic transformation involves the sudden reon39entation ofC and Fe atoms from the FCC solid solution of yFe austenite to a bodycentered tetragonal BCT solid solution ma ensite University nf Tmnzssee Dept nf Mataials Scimce and Engineering 25 TTT Diagram including Martensite 800 Tempevatuve quotCl 100 7 A Austenite P Pearlite B Bainite M Martensite 10 1 1 10 102 103 10 105 Txme sh Austenitetomartensite is dif lsionless and Very fast The amount of maItensite formed depends on temperature only Temperature PC 4 c D Timetemperature path microstructure l A E Eutectoid tern perature Mistart E39 x H quot 3 gquot 39i9 3 tr all all 100 50 Pearl ta 100 Martensite 50 Bainite Bainite l 10 1 11 10 102 103 104 105 Time a Cementite is harder and more brittle than ferrite MSE 3m Mataials Lahmztnry Prnca lurzs Mechanical Behavior of FeC Alloys 1 increasing cementite fraction therefore makes harder less ductile materr Weld and tensile snenzm 1200 1100 lO O i 3 Percent Feat 12 3 l lt7 Peavllte A femte 160 7 350 Peavhte F23C Tensvle snengm Vleld snengm amen namness Compasmnn wt C DE Bnnell hardness numbev MSE 300 Materials Laboratory Procedures Mechanical Behavior of FeC Alloys II The strength and hardness of the different microstructures is inversely related to the size of the microstructures fine structures have more phase boundaries inhibiting dislocation motion Mechanical properties of bainite pearlite spheroidite Considering microstructure we can predict that gt Spheroidite is the softest gt Fine pearlite is harder and stronger than coarse pearlite gt Bainite is harder and stronger than pearlite Mechanical properties of martensite Of the various microstructures in steel alloys gt Martensite is the hardest strongest and the most brittle The strength of martensite is not related to microstructure Rather it is related to the interstitial C atoms hindering dislocation motion solid solution hardening Chapter 7 and to the small number of slip systems University of Tennessee Dept of Materials Science and Engineering 30 EnneH hardness nu mber Mechanical Behavior of Fe C Alloys III Sphemme 280 240 200 120 7o 7 60 7 E 50 7 E Q Coalse pearhta E 40 3 a 30 7 PerceanE3C 3 12 15 Fme peavllle l J l 12 04 05 08 10 Cum pasmon me C Fme pearme Cuavse peZIhlE Sphevodle w l I l w l I 0 2 04 0 6 08 1 C Cumpcsmon mm c 31 MSE 300 Materials Laboratory Procedures Tempered Martensite I Martensite is so brittle that it needs to be modi ed for practical applications This is done by heating it to 25 0650 0C for some time tempering which produces tempered martensite an extremely negrained and well dispersed cementite grains in a ferrite matrix gt Tempered martensite is less hardstrong as compared to regular martensite but has enhanced ductility ferrite phase is ductile gt Mechanical properties depend upon cementite particle size fewer larger particles means less boundary area and softer more ductile material eventual limit is spheroidite gt Particle size increases with higher tempering temperature andor longer time more C diffusion therefore softer more ductile material University of Tennessee Dept of Materials Science and Engineering 32 Rockwell newness HNC MSE 300 Materials Laboratory Procedures Tem pered Martensite II 1mm 1h 35 Hi her temperature amp g time spheroidite soft h of tempered martensi p
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