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1、IRON IRON CARBON DIAGRAM IRON IRON CARBON DIAGRAM Ferrite Austenite SteelCast iron Pearlite Pearlite and Cementine Pearlite and Carbide Eutectic eutectoid Outline Introduction Cooling curve for pure iron Definition of structures Iron Carbon equilibrium phase diagram Sketch The Iron Iron Carbide Diag
2、ram Explanation The Austenite to ferrite cementite transformation Nucleation qElongation 40 in 2 in qHardness Rockwell C 0 or Rockwell B 90 Definition of structures nPearlite is the eutectoid mixture containing 0 80 C and is formed at 723 C on very slow cooling nIt is a very fine platelike or lamell
3、ar mixture of ferrite and cementite nThe white ferritic background or matrix contains thin plates of cementite dark Definition of structures Pearlite nAverage properties are qTensile strength 120 000 psi qElongation 20 in 2 in qHardness Rockwell C 20 Rock well B 95 100 or BHN 250 300 Definition of s
4、tructures nAustenite is an interstitial solid solution of Carbon dissolved in F C C iron nMaximum solubility is 2 0 C at 1130 C nHigh formability most of heat treatments begin with this single phase nIt is normally not stable at room temperature But under certain conditions it is possible to obtain
5、austenite at room temperature Definition of structures Austenite nAverage properties are qTensile strength 150 000 psi qElongation 10 percent in 2 in qHardness Rockwell C 40 approx and q toughness high Definition of structures nCementite or iron carbide is very hard brittle intermetallic compound of
6、 iron responsible for the hardness of quenched steel The Iron Iron Carbide Diagram nA map of the temperature at which different phase changes occur on very slow heating and cooling in relation to Carbon is called Iron Carbon Diagram nIron Carbon diagram shows qthe type of alloys formed under very sl
7、ow cooling qproper heat treatment temperature and qhow the properties of steels and cast irons can be radically changed by heat treatment Various Features of Fe C diagram Peritectic L g Eutectic L g Fe3C Eutectoid g a Fe3C Phases present L Reactions BCC structure Paramagnetic g austenite FCC structu
8、re Non magnetic ductile a ferrite BCC structure Ferromagnetic Fairly ductile Fe3C cementite Orthorhombic Hard brittle Max solubility of C in ferrite 0 022 Max solubility of C in austenite 2 11 Three Phase Reactions nPeritectic at 1490 deg C with low wt C alloys almost no engineering importance nEute
9、ctic at 1130 deg C with 4 3wt C alloys called cast irons nEutectoid at 723 deg C with eutectoid composition of 0 8wt C two phase mixture ferrite cementite They are steels How to read the Fe C phase diagram Ferrite Austenite SteelCast iron Pearlite Pearlite and Cementine Pearlite and Carbide Eutectic
10、 eutectoid The Iron Iron Carbide Diagram The diagram shows three horizontal lines which indicate isothermal reactions on cooling heating nFirst horizontal line is at 1490 C where peritectic reaction takes place Liquid austenite nSecond horizontal line is at 1130 C where eutectic reaction takes place
11、 liquid austenite cementite nThird horizontal line is at 723 C where eutectoid reaction takes place austenite pearlite mixture of ferrite cementite Delta region of Fe Fe carbide diagram Liquid austenite Ferrite region of Fe Fe Carbide diagram Simplified Iron Carbon phase diagram austenite pearlite m
12、ixture of ferrite cementite The Austenite to ferrite cementite transformation in relation to Fe C diagram The Austenite to ferrite cementite transformation in relation to Fe C diagram In order to understand the transformation processes consider a steel of the eutectoid composition 0 8 carbon being s
13、low cooled along line x x nAt the upper temperatures only austenite is present with the 0 8 carbon being dissolved in solid solution within the FCC When the steel cools through 723 C several changes occur simultaneously The Austenite to ferrite cementite transformation in relation to Fe C diagram nT
14、he iron wants to change crystal structure from the FCC austenite to the BCC ferrite but the ferrite can only contain 0 02 carbon in solid solution nThe excess carbon is rejected and forms the carbon rich intermetallic known as cementite Pearlitic structure nThe net reaction at the eutectoid is the f
15、ormation of pearlitic structure n Since the chemical separation occurs entirely within crystalline solids the resultant structure is a fine mixture of ferrite and cementite Schematic picture of the formation and growth of pearlite Ferrite Cementite Austenite boundary Nucleation growth of pearlite Th
16、e Austenite to ferrite cementite transformation in relation to Fe C diagram nHypo eutectoid steels Steels having less than 0 8 carbon are called hypo eutectoid steels hypo means less than nConsider the cooling of a typical hypo eutectoid alloy along line y y nAt high temperatures the material is entirely austenite nUpon cooling it enters a region where the stable phases are ferrite and austenite nThe low carbon ferrite nucleates and grows leaving the remaining austenite richer in carbon The Aust
