Metal Processing Institute
Particulate Materials Research Center

Modeling the Heat Treatment Response of P/M Components

Makhlouf M. Makhlouf
Richard D. Sisson, Jr.
Virendra S. Warke

Introduction

P/M components experience considerable changes during heat treatment that include changes in mechanical properties, in dimensions, in magnitude and sense of residual stresses, and in metallurgical phase composition. Since the quality assurances criteria that all heat-treated P/M components must meet include prescribed minimum mechanical properties and compliance with dimensional tolerances, it is necessary for P/M producers to be able to accurately predict these changes in order to take appropriate measures to prevent their harmful effects and insure the production of good quality parts. Satisfactory response to heat treatment is often gauged by the ability of the component to be heat treated to a desired microstructure, hardness and strength level without undergoing cracking, distortion or excessive dimensional changes.

Other than the reversible changes caused by thermal expansion and contraction, metallic components experience permanent dimensional changes during heat treatment. These permanent changes can be classified into three groups based on their origin:

  1. Dimensional changes with mechanical origins, these include dimensional changes caused by stresses developed by external forces, dimensional changes arising from thermally induced stresses, and dimensional changes caused by relaxation of residual stresses.
  2. Dimensional changes with metallurgical origins, these include dimensional changes caused by recrystallization, solution and precipitation of alloying elements, and phase transformations.
  3. Dimensional changes due to quenching, these are dimensional changes that occur during quenching or that result from stresses induced by quenching.

Residual stresses often adversely affect the mechanical properties of P/M components. They are caused by differing rates of cooling during quenching and depend on the differential rate of cooling, section thickness, and material strength. Decreasing the severity of the quench results in a lower level of residual stresses but with a correspondingly reduced material strength of solution heat-treated materials. Residual stresses may also arise from phase transformations during heat treatment that result in differential volumetric changes in the material.

Objective

The main objective of this project is to develop and verify a computer simulation software and strategy that enables the prediction of the effects of heat treatment on powder metallurgy components. The simulation will accurately predict dimensional change and distortion, residual stresses, type and quantity of metallurgical phases in the microstructure, and hardness.

Methodology

Commercially available software, Dante, which is a finite element based CAE tool for analyzing metal heat treatment processes and is marketed by DCT, Inc. will be used (Figure 1). This software can perform all the required simulations, but its materials properties database was not designed for P/M. Consequently, Phase I of the project will focus on assessing the capabilities of Dante and the possibility of adapting it to the specifics of powder metallurgy. Once, this is accomplished, Phase II of the project will commence and will focus on using the modified software to predict the heat treatment response of powder metallurgy components. The predicted responses will be compared to experimentally measured responses and a modeling/prediction strategy will be formulated and recommendations will be made to the consortium members.

Figure 1: DANTE analysis approach

The predicting ability of this software mainly depends on the database for phase transformation kinetics, mechanical behavior and thermal properties for the alloy of interest. In order to adapt Dante to modeling P/M components, these data sets will be generated for P/M alloys and compiled into the model databases. Porosity will be introduced as a state variable to account for its effect on various aspects of thermal, mechanical, and kinetics behavior of the alloy.

The kinetics data for FL-4605 P/M alloy has been generated for three levels of porosity using an MMC quech dilatometer. Figure 2 shows the TTT diagram for Austenite to Bainite and Austenite to Martensite transformations, generated from the dilatation data using special fitting routines. The dilatation curves show that porosity has a significant effect on the transformation kinetics of the alloy (Figure 2).

Figure 2: TTT diagram for 90% and 100% density material plotted for the austenite to banite and the austenite to martenisite transformations.

The necessary heat transfer coefficients were measured using CHTE quench probes that were quenched in Houghton-G oil. Figure 3 shows the effect of porosity on the surface heat transfer coefficient for FL-4605 P/M alloy. Results of quenching tests show that the 90% density material cools faster than the 95% density material.

Figure 3: Surface heat transfer coefficients as a function of temperature for P/M parts at two levels of density (or porosity)

Publications

  1. Virendra S. Warke, Jack Yuan, Mohamed Maniruzzaman, Makhlouf M. Makhlouf, and Richard D. Sisson, Jr., "Modeling the Heat Treatment of Powder Metallurgy Steels," Proceedings of the 2004 International Conference On Powder Metallurgy and Particulate Materials, part-1, pp.39-53, Chicago, IL, June 2004.
  2. Richard D. Sisson, Jr., Mohamed Maniruzzaman, Shuhui Ma, Virendra S. Warke, and Makhlouf M. Makhlouf, "Quenching Powder Metallurgy Products," Proceedings of the 2004 International Conference On Powder Metallurgy & Particulate Materials, part-6, pp. 1-11, Chicago, IL, June 2004.
  3. V.S. Warke, R.D. Sisson, Jr., and M.M. Makhlouf, "Effect of Porosity on the Transformation Kinetics of P/M Steels," Proceedings of PM2Tech 2005, pp. 85-99, Montreal, Canada, June 2005.
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Last modified: October 23, 2007 08:40:17