Phase Transformation: Lecture Review of Phase Diagrams

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Transcript of Phase Transformation: Lecture Review of Phase Diagrams

Materials Engineering MM-501 Phase Transformation in Solids

MM-501 Phase Transformation in Solids

Fall Semester-2015

Engr. Muhammad Ali SiddiquiLecturer, Metallurgical Engineering Department, NED UETBE: Mehran UET, 2007ME: NED UET, 2011

Important InstructionClass as per schedule Inshallah; should maintain 75% attendance.

LECTURES: soft copy can be taken after the class or from my Office.

Make your OWN NOTES and read the books for in-depth understanding.

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Books for Understanding the subject & for Examination PreparationPhase Transformations in Metals and Alloys, Porter and Easterling, 2nd ed (Library call No 669 POR)Chapters, 1,2,3,5 and 6 2) Steels Microstructure and Properties, HKDH Bhadeshia(Library call No 669.142 BHA)chapters 5, 6 & 10

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Sessional Marks Distribution..40Nos02 Test ..20 Nos01 Assignment.. 10 NosAssignment Presentation/Viva......05 NosAttendance05 NosImportant DatesTest No:01 Sep 17,2015Test No:02 Nov 5 or 12, 2015Session Assignment Oct 1, 2015 (Last Date)Viva /presentation on Last Lecture day. (Expected Nov 19,2015)..InshallahType of Test:MCQs/BCQs/ Descriptive/Problems4

Assignment Criteria In-Time SubmissionLate Submission will cost -1 marks/day and there will be seven daysStyle and Format:No: of pages > 30 but < 45, at least 25 pages contain full text.A-4 size paper, Typed in a single space,1 Margin all around the text,Font Time New Roman #12 for text and 12Bold sub heading, 14 Main heading, Reference Harvard University Style.All figures and Tables must be captioned and discussed in the text.

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Topics for Session Assignment and Presentation Topic 1: Diffusionless Phase Transformation-IDiffusionless Character, Nucleation and Growth of Martensite, The Habit Plane, Orientation Relationships., Athermal Nature of Transformation.

Topic 2: Diffusionless Phase Transformation-IIStructure of the Interface b/w & , The Shape Deformation, Bain Distortion Model, Phenomenological Theory of Martensite, Morphology and crystallography of (bcc or bct).6

Topic 3: Reconstructive Phase TransformationAustenite to Allotriomorphic ferrite, Idomorphic ferrite, Massive ferrite and Pearlite transformation.

Topic 4: Displacive Phase TransformationBainite (upper & lower), Acicular Ferrite and widmanstatten ferrite transformation.

Topic 5: Precipitation and Age Hardening PhenomenaStrengthening of Non Ferrous Alloy. E.g Al, Cu and Mg

Topic 6: Solid Solution and Phase Diagram Solid solution, Binary and Ternary phase diagram7

Introduction LectureIron - Phase Diagram.Effect of Alloying Elements on Fe-Fe3C Diagram.TTT Diagrams. (Construction )8

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Phases Observed in Fe-C Diagram 1. Ferrite Ferrite is the interstitial solid solution of carbon in alpha iron. It has B.C.C. Structure. It has very limited solubility for carbon (maximum 0.022%/0.025 at 727C and 0.008% at room temperature). Ferrite is soft and ductile. 2. Austenite Austenite is the interstitial solid solution of carbon in gamma () iron. It has FCC structure. Austenite can have maximum 2.14% carbon at 1143C. Austenite is normally not stable at room temperature. Austenite is non-magnetic and soft. 3. Cementite Cementite or iron carbide (Fe3C) is an intermetallic compound of iron and carbon. It contains 6.67% carbon. It is very hard and brittle. This intermetallic compound is a metastable phase and it remains as a compound indefinitely at room temperature.4. -ferrite It is a solid solution of carbon in -iron. It is stable at high temperatures. It has BCC structure.10

Phase Mixtures Observed in Fe-C Diagram 1. Pearlite The pearlite consists of alternate layers of ferrite and cementite. It has properties somewhere between ferrite and cementite. The average carbon content in pearlite is 0.76%2. LedeburiteLedeburite is an eutetcic mixture of austenite and cementite in the form of alternate layers. The average carbon content in ledeburite is 4.3%.11

If alloying elements are added to the iron-carbon alloy (steel), the position of the A1, A3, and Acm boundaries and the eutectoid composition are changed.all important alloying elements decrease the eutectoid carbon content, the austenite-stabilizing elements manganese and nickel decrease A1, and the ferrite-stabilizing elements chromium, silicon, molybdenum, and tungsten increase A1.

The Effects of Alloying Elements on Iron-Carbon Alloys12

Effect on the Eutectoid Point 13

Changes eutectoid TemperatureChanges eutectoid Composition

Effect of alloying additions on the -phase field: (a) Mn;

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Effect of alloying additions on the -phase field: (b) Mo;

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Effect of alloying additions on the -phase field: (c) Cr;

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Effect of alloying additions on the -phase field: (d) Ti

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The Effects of Alloying Elements on Iron-Carbon AlloysFor analyzes the effects of alloying elements on iron-carbon alloys would require analysis of a large number of ternary alloy diagrams over a wide temperature range.Wever pointed out that iron binary equilibrium systems fall into four main categories: open and closed -field systems, and expanded and contracted -field systems.This approach indicates that alloying elements can influence the equilibrium diagram in two ways: by expanding the -field, and encouraging the formation of austenite over wider compositional limits. These elements are called -stabilizers.by contracting the -field, and encouraging the formation of ferrite over wider compositional limits. These elements are called -stabilizers.

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Fig. 1 Classification of iron alloy phase diagrams:

open field;

expanded field;

closed field;

contracted field 19

Open -fieldThis group belong the important steel alloying elements nickel and manganese, as well as cobalt and the inert metals ruthenium, rhodium, palladium, osmium, iridium and platinum. Both nickel and manganese, if added in sufficiently high concentration, completely eliminate the bcc -iron phase and replace it, down to room temperature, with the -phase. So nickel and manganese depress the phase transformation from to to lower temperatures , i.e. both Ac1and Ac3are lowered. It is also easier to obtain metastable austenite by quenching from the -region to room temperature, consequently nickel and manganese are useful elements in the formulation of austenitic steels.20

Fe-Mn Phase Diagram21

Fe-Ni Phase Diagram22

Closed -fieldMany elements restrict the formation of -iron, causing the -area of the diagram to contract to a small area referred to as the gamma loop. This means that the relevant elements are encouraging the formation of bcc iron (ferrite), and one result is that the - and -phase fields become continuous. Alloys in which this has taken place are, therefore, not agreeable to the normal heat treatments involving cooling through the /-phase transformation. Silicon, aluminium, beryllium and phosphorus fall into this category, together with the strong carbide forming elements, titanium, vanadium, molybdenum and chromium.23

Fe-Cr Phase Diagram24

Fe-Ti Phase Diagram25

Fe-Mo Phase Diagram26

Fe-V Phase Diagram27

Expanded -fieldCarbon and nitrogen are the most important elements in this group. The -phase field is expanded, but its range of existence is cut short by compound formation. Copper, zinc and gold have a similar influence. 28

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Fe-Cu Phase Diagram30

contracted y-fieldBoron is the most significant element of this group, together with the carbide forming elements tantalum, niobium and zirconium.The -loop is strongly contracted, but is accompanied by compound formation.31

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The Effect on the Formation and Stability of CarbidesSome alloying elements form very stable carbides when added to steel (Fig. 13.2)33

Alloy Carbides

The periodic table showing the positions of strong carbide-forming elements 34

In steel six kinds of carbides can be formed as shown in Table 1, where M denotes a sum of carbide-forming (metal) elements. The carbides placed in group I posses a complicated crystal structure; an examples is cementite (Fe3C), or Cr23C6. A specific structural feature of the carbides of group II as interstitial phases is a simple crystal lattice (e.g., TiC, WC, NbC and Mo2C).

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Isothermal Transformation (I.T) / Time Temperature Transformation (TTT) Diagram

Today we will discuss about its Construction

36What is TTT?T (Time) T (Temperature) T (Transformation) diagram is a plot of temperature versus the logarithm of time for a steel of definite composition.

It is used to determine when transformations begin and end for an isothermal (constant temperature) heat treatment of a previously austenitized alloy.

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Step: 4 After Cooling, check sample hardness and microstructure.

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39Step 2,3 and 4 are shown in the following Figure

If we draw a curve b/w %age of Pearlite Transformed and Time

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41Step: 5 all steps are repeated at different sub-critical temperature below Ac1 in order to get sufficient points for plotting the curve.

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Composition of AISI 1080 Steel

C= 0.75-0.88, Mn= 0.60-0.90 , P= 0.040, S= 0.050 Fe= balance

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Example: Effect of Alloying Elements

48A cooling curve is determined experimentally by placing a thermocouple at a definite location in a steel sample and then measuring the variation of temperature with time.

Since the coordinates of the I-T diagram are the same as those for a cooling curve,

it is possible to superimpose various cooling curves o