材料综合实验：Manual for Experiment-5 iron-carbon equilibrium diagram-【Lab 5】

1、Fundamentals of Metal MaterialsExperiment V: Iron-carbon equilibrium diagramObjectives:To grasp the iron-carbon equilibrium diagram, e.g., phases, eutectoid/eutectic points, and microstructures;To understand the change of microstructure as a function of carbon fraction; To understand seven equilibri

2、um-crystallization processes, especially, the microstructure evolution with temperature. Contents:To observe microstructures of typical samples in the iron-carbon equilibrium diagram;To estimate the concentration of different phases or microstructures in samples, and compare the experimental values

3、with calculated ones from the phase diagram.Basic concepts of iron-carbon equilibrium diagramIn their simplest form, steels are alloys of Iron (Fe) and Carbon (C). The Fe-C phase diagram is a fairly complex one, but we will only consider the steel part of the diagram, up to around 7% Carbon, i.e., F

4、eFe3C Phase Diagram.Phases in FeFe3C Phase Diagram-ferrite - solid solution of C in BCC FeStable form of iron at room temperature. The maximum solubility of C is 0.0218 wt%Transforms to FCC -austenite at 912 C-austenite - solid solution of C in FCC FeThe maximum solubility of C is 2.11 wt%. Transfor

5、ms to BCC -ferrite at 1395 C Is not stable below the eutectic temperature (727 C) unless cooled rapidly.-ferrite solid solution of C in BCC FeThe same structure as -ferriteStable only at high T, above 1394 CMelts at 1538 CFe3C (iron carbide or cementite)It contains 6.69 wt% of CThis intermetallic co

6、mpound is metastable, it remains as a compound indefinitely at room temperature, but decomposes (very slowly, within several years) into -Fe and C (graphite) at 650 - 700 CFe-C liquid solutionClassification. Three types of ferrous alloysIrons are alloys with less than 0.0218 wt% of carbonSteels are

7、carbon-iron alloys with carbon in the range 0.0218 wt.% to 2.11%. Cast irons contain 2.116.69 wt% of carbonMicrostructures at room temperatureFerrite: as mentioned above.PearlitePearlite is a two-phased, lamellar (or layered) structure composed of alternating layers of alpha-ferrite (88 wt%) and cem

8、entite (12 wt%) that occurs in some steels and cast irons. In an iron-carbon alloy, during slow cooling pearlite forms by a eutectoid reaction as austenite cools below 727 C (the eutectoid temperature). Pearlite is a common microstructure occurring in many grades of steels.The eutectoid composition

9、of austenite is approximately 0.77% carbon; steel with less carbon content will contain a corresponding proportion of relatively pure ferrite crystallites that do not participate in the eutectoid reaction and cannot transform into pearlite. Likewise steels with higher carbon contents will form cemen

10、tite before reaching the eutectoid point. The proportion of ferrite and cementite forming above the eutectoid point can be calculated from the iron/iron-carbide equilibrium phase diagram using the lever rule.Ledeburite In iron and steel metallurgy, ledeburite is a mixture of 4.3% carbon in iron and

11、is a eutectic mixture of austenite and cementite. Ledeburite arises when the carbon content is between 2.11% and 6.69%. The eutectic mixture of austenite and cementite is 4.3% carbon, with a melting point of 1147 C.Ledeburite-II (at ambient temperature) is composed of cementite-I with recrystallized

12、 secondary cementite (which separates from austenite as the metal cools) and (with slow cooling) of pearlite. The pearlite results from the eutectoid reaction of the austenite at 723 C. Cementite: as mentioned above. Experimental materials and equipment:Typical Fe-C alloys;Experimental items for met

13、allographic sample preparationExperimental items for metallographic observationExperimental procedure:To observe microstructures of typical samples in the iron-carbon equilibrium diagram;To estimate the concentration of different phases or microstructures in the samples, and compare the experimental

14、 values with calculated ones from the phase diagram.Experiment report:Please discuss the microstructure-property correlation of at least three types of Fe-C alloys. Calculate the concentrations of different phases and microstructures in selected typical Fe-C alloys (e.g., hypoeutectoid and hypoeutectic composition) based on the phase diagram,