### Transcription of Continuous Cooling Transformation (CCT) Diagrams

1 **Continuous** **Cooling** **Transformation** (CCT) DiagramsR. MannaAssistant ProfessorCentre of Advanced StudyDepartment of Metallurgical EngineeringInstitute of Technology, Banaras Hindu UniversityVaranasi-221 005, Steel-TRAERF Faculty Fellowship Visiting ScholarDepartment of Materials Science and MetallurgyUniversity of Cambridge, Pembroke Street, Cambridge, CB2 **Cooling** **Transformation** (CCT) diagramThere are two types of CCT diagramsI) Plot of (for each type of **Transformation** ) **Transformation** start, specific fraction of **Transformation** and **Transformation** finish temperature against **Transformation** time on each **Cooling** curveII) Plot of (for each type of **Transformation** ) **Transformation** start, specific fraction of **Transformation** and **Transformation** finish temperature against **Cooling** rate or bar diameter for each type of **Cooling** medium2 Definition.

2 Stability of phases during **Continuous** **Cooling** of austeniteDetermination of CCT diagram type , ( )isaustenitisedinaspeciallydesignedfurna ce( ) 1: Sample and fixtures for dilatometric measurements Fig. 2 : Dilatometer equipment4 Coolingdataareplottedastemperatureversus time( ).Dilationisrecordedagainsttemperature( ). TemperatureacbdFor a **Cooling** schedule TSTFFig. 3: Schematic **Cooling** curvesFig. 4: Dilation-temperature plot for a **Cooling** (orheating) (s). ,however, *100whereX,YandZareintersectionpointofte mperatureTlinetoextendedconstantslopecur veofaustenite(ba),transformationcurve(bc )andextendedconstantslopecurveoflowtempe raturephase(cd) (10%,20%,30%etc.) ,locusofstartpoints,isopercentagepointsa ndfinishpointsgivethetransformationstart line, (a)to(e) ,P,B,Mstandforferrite,pearlite, Sand F a schedulemartensitestartsatMSandfinishesa tMFandtherefore100% b b timeHVHVHVHVHVMM+BF+BF+BF+PFig.

3 5: Schematic dilatometric plots for five different **Cooling** rates where F, P, B and M stands for ferrite, pearlite, bainite and martensite respectively and subscript S and F stands for **Transformation** start and **Transformation** finish for respective products for a hypoeutectoid steelFig. 6: Schematic CCT diagram constructed from data of Fig 3(for the hypoeutectoid steel). Dotted line is 25% of total **Transformation** . abcde10 Incoolingschedule c c d e e two **Cooling** curves are considered equivalent if (i) the times to cool from Ae3to 500 C are same.(ii) the times to cool from Ae3 to a temperature halfway between Ae3and room temperature , are same.(iii) the **Cooling** rates are same.(iv) the instant **Cooling** rates at 700 C are same. Therefore to make it useful different types of CCT **Diagrams** need to be made following any one of the above schedule that matches with heat treatment **Cooling** test method for type I CCT diagramAnumberofJominyendquenchsamplesar efirstend-quenched( )foraseriesofdifferenttimesandtheneachof them(wholesample) , ,finishorspecificpercentageoftransformat iongenerateCCTdiagram( ).

4 131 (29 mm)diameter1 2 ( mm)1 ( mm) diameter ( mm) ( mm)4 (102 mm) long2 (64 mm)Free height of water jet ( mm) ( mm) diameter Fig 7(a): Jominy sample with fixture and water jet Water : Figures show (b) experimental set up, (c ) furnace for austenitisation, (d) end quenching process. Courtesy of DOITPoMS of Cambridge , Martensite finish temperatureM50,50% MartensiteMS,Martensite start temperatureMetastable austenite +martensiteMartensiteHardness, HRCT emperatureLog timeAe1to=Minimum incubation period at the nose of the TTT diagram, t o=minimum incubation period at the nose of the CCT diagramt0 AFEDCBD istance from quench endABCDEFJ ominy sampleMartensitePearlite+MartensiteFine pearlitepearliteCoarse :CCTdiagram()projectedonTTTdiagram()ofeu tectoidsteelt ,B,C,D,E, ,B,C,D,E, , s concept of fractional **nucleation** /progressive (1) (2)Thetransformationatatemperatureisnoti ndependenttocoolingaboveit.

5 (3) t1/Z1+ t2/Z2+ t3/Z3+..+ tn/Zn=1 Where (1935).Thereactionsforwhichtheadditiveru leisjustifiiedarecalledisokinetic, (atb), , curve E just touches the nose of CCT diagram and that produces almost fully martensite. **Cooling** curve F avoid nose of C curve in CCT but touches the nose of TTT gives entirely martensite. Notice the critical **Cooling** rate to avoid nose of CCT diagram diffusional transformations is lower than that to TTT features of CCT diagrams1. CCT diagram depends on composition of steel, nature of **Cooling** , austenite grain size, extent of austenite homogenising, as well as austenitisingtemperature and Similar to TTT **Diagrams** there are different regions for different **Transformation** ( cementite/ferrite, pearlite, bainiteand martensite). There are **Transformation** start and **Transformation** finish line and isopercentagelines.

6 However depending on factors mentioned earlier some of the **Transformation** may be absent or some **Transformation** may be In general for ferrite, pearliteand bainitetransformation start and finish temperature moves towards lower temperature and **Transformation** time towards higher timing in comparison to isothermal **Transformation** . **Transformation** curve moves down and right. ,however,itcanbeloweredatlowercoolingrat eifcoolingcurvessuchthatausteniteenriche swithcarbonduetobainiteorferriteformatio n(inhypoeutectoidsteel).OntheotherhandMS cangoupforlowercoolingratesuchthatausten itebecomeleanincarbonduetocarbideseparat ion(inhypereutectiodsteel).247. Large variety of microstructure like ferrite/cementite/carbide +pearlite+bainite+martensite can be obtained in suitable **Cooling** rate. It is not feasible or limited in case of isothermal of type II CCT , , , (usuallyat700 C)in (inHRC) (fromtoptobottom), diameter, in mmAir cooledOil quenchWater quenchTemperature, CHardness, HRCH ardness, HVCooling rate at 700 C, C per minMs0%50%90%100%M50 MfM90 Fig.

7 9: CCT diagram for hypoeutectoid steelHardness after **Transformation** at room temperature27 Conversion of TTT to CCT diagram, Scheil s method (1935) Scheil t1/Z1+ t2/Z2+ t3/Z3+..+ tn/Zn=1 Where of TTT to CCT, Grange and Kiefer Method (1941)Duringcontinuouscoolingalongagiven coolingcurvewhichinterceptstheTTTstartcu rveattemperatureT1,thetransformationwill startattemperatureT2,suchthatthetimeofco olingbetweenT1andT2isequaltothetimeforth estartoftransformationduringisothermalho ldingattemperatureT3=(T1+T2)/2( ).t3= ,however, timeT3=(T1+T2)/2and t3=t2-t1 or t2=(t1+t3)/2Ae3 Fig. 10: Graphic method of converting TTT diagram to CCT diagram [Grange and Kiefer method]30 Conversion of TTT to CCT, Avrami method (1939)Let TTT(T) be time required to obtain a given percentage of **Transformation** , X at temperature T during isothermal time required( CCT) to obtain the same percentage of **Transformation** , X, on **Continuous** **Cooling** at TCCTis given by the conditionX= Ae3 TCCTdX= Ae3 TCCT Ae3 TCCT g-dt-------1g-=time average **Transformation** rate (at any temperature T)=X/ IT(T).

8 Substituting this in equation 1 We get Ae3 TCCT dt/ TTT(T) =1--------2,By rewriting equation 2 we get Ae3 TCCT dT/( TTT(T) dT/dt)=1----------3 Both these integrals are called Avrami integral. Any one of these integrals has to be evaluated for each **Cooling** curve to get the CCT at TCCT31 Conversion of CCT to TTT diagram, Kirkaldy and Sharma method (1982)Let CCT(TCCT) be the time required to obtain a given percentage of **Transformation** , X at temperature TCCT during **Continuous** **Cooling** . If it is assumed that CCT diagram was constructed using constant **Cooling** rate(linear **Cooling** ),Then dT/dt=-(Ae3-TCCT)/( CCT(TCCT)----4 Substituting equation 4 in equation 3, cross multiplying and differentiating with respect to TCCTWe get TTT(TCCT)=1/(d/dTCCT[(Ae3-TCCT)/ CCT(TCCT)])---5 Where TTT is the time required for the given percentage **Transformation** , X, when carried out isothermally at rate of **Cooling** is not constant but **Cooling** rate can be expressed analytically or empirically as dT/dt=f1(x)f2(T)=f1(TCCT)f2(T) ---6(Exp.))

9 Jominy **Cooling** curve can be expressed in this form)where x is the distance from the surface of a continuouly cooled equation 6 in equation 3, cross multiplying and differentiatingWe get TTT (TCCT)=1/(f2(TCCT) df1/dTCCT)-----7 Equation 5 or 7 can be used for the conversion of CCT diagram to TTT diagram depending on constant **Cooling** rate or case of **Cooling** rate that can be expressed in analytical or empirical **Cooling** curves can be expressed in equation 6 form and the using equation 7, CCT diagram can be converted to TTT diagram. 33