Modeling the Heat Treatment Response of P/M Components

Transcription

1 Modeling the Heat Treatment Response of P/M Components Research Team: Makhlouf M. Makhlouf, Professor Richard D. Sisson, Jr., Professor Virendra S. Warke, Ph.D. Student Focus Group Members: Ian Donaldson John Fulmer Chaman Lall Jean Lynn Stephen Mashl (Chair) Sim Narasimhan Reinaldo Soave Rocco Petrilli GKN Sinter Metals Worcester Nichols Portland Metal Powder Products Co. DaimlerChrysler Corporation. Bodycote IMT, Inc. Hoeganaes Corporation Mahle Metal Leve S.A. Sinterstahl G.m.b.H.

2 Objectives Develop and verify a computer simulation software and strategy that enables the prediction of the effect of heat treatment on P/M components Simulation predictions will include: Dimensional changes and distortion Residual stresses Type and quantity of metallurgical phases Hardness

3 Background Need Model provides insight and control of processing conditions to meet - Dimensional tolerances. - Mechanical properties. Model can be used to design a process. Model can be used to optimize an existing process.

4 Project outline. Develop model and modeling strategy. Select software Test predictive ability of software Develop necessary input for software: Heat transfer coefficients during quenching (quenching experiments/measurements) Phase transformation kinetics (quench dilatometry ORNL) Phase specific, temperature dependant mechanical properties (high temperature mech. properties measurement)

5 Project outline (cont.) 2. Validate model predictions. Develop/adopt measurement procedures for: Residual stresses Type and quantity of metallurgical phases Dimensional changes

6 Project outline (cont.) 3. Use the model to predict the heat treatment response of select P/M components.

7 Project outline : DANTE (cont.)

8 Schedule: Project duration: July 2003 July Develop model and modeling strategy. Select software DANTE Test predictive ability of software Develop necessary input for software: (for FL-4605 PM alloy) Heat transfer coefficients during quenching (quenching experiments/measurements) Phase transformation kinetics (quench dilatometry ORNL) Full density (powder forged) samples 95 % theoretical density samples 90 % theoretical density samples Transformation induced plasticity measurements (three levels of applied stress) Full density (powder forged) samples 95 % theoretical density samples 90 % theoretical density samples o Phase specific/temperature dependant mechanical properties (high temperature mech. properties measurement) 2. Validate model predictions 3. Use model to predict heat treatment response of industrial PM part.

9 Determination of phase transformation kinetics parameters using quench dilatometry Using a quench dilatometer Theta Industries, Inc. 26 Valley Road Port Washington NY 050 USA

10 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Matrix for developing the isothermal transformation data sets. Phases Ferrite Ferrite + Pearlite Bainite Isothermal holding temperature ( C) Number of tests

11 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Full Density samples Isothermal & continuous cooling measurements. Data analysis & fitting for kinetics parameters. Generation of TTT diagram. 95% Density samples Isothermal & continuous cooling measurements. Data analysis & fitting for kinetics parameters. Generation of TTT diagram. 90% Density samples. Isothermal & Continuous cooling measurements. Data analysis & fitting for kinetics parameters. Generation of TTT diagram.

12 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Strain vs. time for Bainite transformation in full density FL-4605 PM steel.

13 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Test matrix for the Martensitic transformation Test matrix for generating the continuous cooling transformation datasets Cooling rate ( C/s) Number of tests Cooling rate ( C/s) Number of tests

14 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Cooling rate curves for which the dilatation data in continuous cooling tests is acquired

15 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Strain vs. temperature for continuous cooling transformation in full density FL-4605 PM steel

16 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) CTE, Transformation strain, Tested time-temperature data Updated kinetics parameters Initial Phase kinetics parameters Predicted strain curves Objective function: Sensitivity Difference between analysis predicted and tested strain curves Search optimum kinetics parameters No Satisfied? Yes Output optimized kinetics parameters Kinetics checking: Jominy predictor TTT diagram predictor

17 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Data fitting to estimate coefficient of thermal expansion as a function of temperature Pearlite Bainite Austenite Martensite

18 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Predictive model for Austenite decomposition : This model is based on kinematics information i.e. data that relates to the external shape change is used to predict internal microstructure change. The model is based on the assumption that differential shape change can be related to differential microstructure change provided the base state is known. The strain vs. time and/or strain vs. temperature data from dilatometery is fitted into kinetic parameters to predict the change in phase fraction with time.

19 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Diffusive Kinetics State variable approach has been extended to include multiphase decomposition of austenite (Ф a ) to ferrite (Ф f ), pearlite (Ф p ), and bainite (Ф b ) " " 2a f! 2a f a f a f = k f 2 " f, eq " f < " f, eq!" f > " a 2a p! 2a p a p a p = k p 2 (!" f, eq) " p (!" p) " a f p!! 2ab " 2ab ab ab b = k b 2 ( "! f, eq) (! b +! En, b! a! ) a,f, & p Subscript for austenite, ferrite, and pearlite, respectively. Ф i Phase fraction of a phase. Ф f,eq Temperature dependent equilibrium atomic fraction of ferrite. K i Temperature dependent mobility functions. <.> McCalley bracket; gives the value if argument is positive, else gives zero. φ En,b = γ b (Ф f + Ф p ), reflects influence of existing ferrite and pearlite on the rate of formation of bainite.

20 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Mobility functions as a function of temperature for diffusive kinetics: k f = e "# f " Q f 0.4g R( T + 273) n f 2 e ( Ae 3 " T ), Acm < T! Ae 3 & k f > k b k p = e "# p " Qp 0.32g R( T + 273) np 2 e ( Ae " T ), Bs < T! Ae k b = e " Qb "# b 0.29g R( T + 273) nb 2 e ( Bs " T ), M s < T! B s a,f,and p Q i n i α i B s M s Subscript for austenite, ferrite, and pearlite, respectively. Effective thermal activation energy. Critical exponent that relates driving force to the degree of undercooling. Mobility parameters. Bainite start temperature. Martensite start temperature.

21 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Martensitic Kinetics The martensitic transformation is assumed to be athermal but is written and solved in a rate form: " m = # here, U is the heavy-side step function: % dt m $ m!, m) (!" m) " a U ( Min( T, M s )! T ) dt m( " m + " En min where, T min is the lowest temperature attained, and α m,β m, and ν m are algebraic functions of the carbon level: # "! m m m = # = " =! m0 m0 m0 + # + " +! m m m C C C +! m2 C 2 +! 3 m3

22 Determination of phase transformation kinetics parameters using quench dilatometry (cont.) Kinetics data fitting for full density FL-4605 PM steel: Pearlite Bainite Martensite

23 Detemination of phase transformation kinetics parameters using quench dilatometry (cont.) TTT diagram for full density FL-4605 PM steel generated using the fitting routine.

24 Effect of porosity on the phase transformation of FL-4605 PM steel

25 Effect of porosity on the phase transformation of FL-4605 PM steel (cont.)

26 Effect of porosity on the phase transformation of FL-4605 PM steel (cont.)

27 Determination of transformation induced plasticity using low stress dilatometry Determination of transformation induced plasticity (TRIP) Plastic behavior of steels during metallurgical transformations can be attributed to: ) Classical plasticity - which is plastic flow arising from variation of the applied stress or the temperature cycle. 2) Transformation induced plasticity (TRIP) - which is plastic flow arising from variation of phase proportion due to phase transformation. TRIP has been observed by many researchers during Austenite to Martensite and Austenite to Bainite transformations. Low stress dilatometry can be used to determine TRIP by applying an external static compressive load just before the transformation begins. The applied stress must be lower than the flow stress of Austenite at the testing temperature.

28 Determination of transformation induced plasticity using low stress dilatometry (cont.) Matrix for determining TRIP for the Austenite to Martensite transformation. Cooling rate : 80 C/s Temperature for application of stress : 300 C Test Number Sample Density Stress, MPa Load, N Fully dense (A) % of theoretical density (B) % of theoretical density (C) * A negative sign indicates a compressive stress.

29 Determination of transformation induced plasticity using low stress dilatometry (cont.) Matrix for determining TRIP for the Austenite to Bainite transformation. Isothermal holding temperature : 425 C Test Number Sample Density Stress (MPa)* Load, N (lbf) Fully dense (A) % of theoretical density (B) % of theoretical density (C) * A negative sign indicates a compressive stress.

30 Determination of transformation induced plasticity using low stress dilatometry (cont.) Change in diameter vs. time during Austenite to Bainite transformation under different applied loads for fully dense samples. tp thm e! ( T ) =! ( T ) "! ( T ) "! ( T ) ε tp ε TRIP strain Total strain ε thm Thermal strain ε e Elastic strain

31 Determination of transformation induced plasticity using low stress dilatometry (cont.) 90% density 95% density 00% density

32 Determination of transformation induced plasticity using low stress dilatometry (cont.) 0 MPa 30 MPa 60 MPa 90 MPa

33 Summary Quench dilatometric measurements were performed on FL4605 PM steel samples of 90%,95% & 00% theoretical density at ORNL. Strain vs. time datasets from isothermal dilatational curves and strain vs. temperature datasets from continuous cooling dilatation curves were obtained from these measurements. The data for full density samples was fitted into the kinetics equations using a specialized routine developed by DCT. TTT diagram for full density FL-4605 powder forged steel has been developed. The effect of porosity on phase transformation of FL-4605 PM steel has been analyzed. Low stress dilatometric measurements were performed on FL-4605 PM steel for 3 porosity levels and 4 stress levels at ORNL.

34 Work planned for the next reporting period The data from dilatation curves for samples with 90% and 95% density will be fitted into the kinetics equations. The TTT diagrams for 90% and 95% theoretical density material will be constructed. This information will be used to characterize the effect of porosity on the various aspects of phase transformations in the alloy, including transformation strains, transformation kinetics, and transformation temperatures. TRIP data will be calculated from the measurements conducted at ORNL. Effect of porosity on TRIP will be characterized. Jominy end quench tests will be performed on samples that were pressed and sintered to 90% and 95% of their theoretical density in order to investigate the effect of porosity on the heat transfer characteristics of the alloy.