Statistical Approach to A Leaner Sinter Hardening P/M Alloy
Gerald A. Gegel
Engineering Specialist-Research
Caterpillar Inc.
Michael A. Pershing
Sr. Manufacturing Project Specialist
Caterpillar Inc.
Thomas F. Murphy
Scientist, Laboratory Services
Hoeganaes Corporation
Gerard J. Golin
Metallographer
Hoeganaes Corporation
Abstract. A popular sinter-hardening alloy is based on pre-alloyed Fe-Ni-Mo-Mn powder to which 2%
copper and 0.9% graphite is added. Data in the literature suggests that reduced copper and graphite
contents may provide equal hardenability and higher tensile properties. Experimental results demonstrate
that a fully martensitic microstructure and higher tensile properties may be obtained with leaner alloy
chemistry. Reduced additive (copper and graphite) content improves pore free density. Response Surface
Methodology (RSM) is used to illustrate the results of the statistically based experimental design.
Introduction. Sinter-hardening is the process of cooling a powder metal (P/M) part from the sintering
temperature at a rate sufficient to transform a significant portion of the material matrix to martensite.
Since this is accomplished in the cooling zone of the sinter furnace, the process offers good
manufacturing economy by eliminating the austenitize and quench operations. Tempering to obtain the
desired hardness level is still accomplished.
In 1999(1), Hoeganaes Corporation introduced a Fe-Mn-Mo-Ni prealloyed powder (Ancorsteel 737SH)
that has both high hardenability and good compressibility. This alloy powder blended with 2.0% copper
and 0.9% graphite has been used to sinter harden numerous production parts(2, 3). A plot (see Figure 1) of
the tensile data presented by Baran, et al(1) suggests that there may be lower copper and graphite contents
that may result in higher mechanical properties. To test this hypothesis, an experiment was designed to
determine the effect of lower copper and graphite additions on the mechanical properties of the
commercial Fe-0.42Mn-1.24Mo-1.49Ni sinter-hardening material.
Experimental Procedure. A commercial statistical design software package was used to calculate
copper and graphite addition levels made to the pre-alloyed sinter-hardening powder. This same software
was also used to evaluate the responses. The coded mix compositions needed to evaluate the effects of
copper additions in the 0.9 to 1.5 weight percent range and graphite additions in the 0.4 to 0.7 weight
percent range for the central composite design (CCD) design of experiments (DOE), are shown in Table
1. Responses evaluated included tensile strengths, percent elongation, apparent and particle hardness, and
percent martensite.
Figure 1. Effect of Copper and Graphite Content on Tensile Strength of Ancorsteel 737SH. (Data from
Reference 1)
Table 1. Coded Copper and graphite additions used for Design of Experiments
Experiment
% Copper*
% Graphite*
1
1.00
1.00
2
0.00
-1.41
3
-1.00
-1.00
4
0.00
0.00
5
0.00
0.00
6
1.41
0.00
7
0.00
1.41
8
1.00
-1.00
9
0.00
0.00
10
0.00
0.00
11
-1.41
0.00
12
-1.00
1.00
13
0.00
0.00
* “0” is center-point of the design matrix.
Ten pound (4.5 kg) mixes for each of nine compositions were blended from the Fe-0.42Mn-1.24Mo1.49Ni pre-alloyed powder, copper powder (AcuPowder 8081), graphite powder (Asbury 3203) and
0.75% Acrawax. Ten standard MPIF tensile specimens were compacted at 50 TSI (690 MPa) for each
experiment. After determination of green density values, the specimens were sintered under production
conditions in a commercial sintering furnace equipped with Convecool. The cooling rate achievable with
the Convecool settings noted below was characterized separately in a test designed to standardize cooling
measurements between furnaces. No thermocouple measurements were made with the particular samples
discussed in this paper. The average cooling rate between 1380º and 1020ºF (750ºC and 550ºC) was
2.45ºF/s (1.36ºC/s) at the center of a fully sintered 1”x1”x1” cube of density 6.80-6.90 g/cm3. The
surface heat transfer coefficient, hc, which gave this cooling rate, was found to be 29.0 Btu/h/ft2/ºF
(164.63 W/m2/oC)(4). All specimens were sintered in the same production run using the following cycle
parameters:
Sintering Temperature:
2050ºF (1121ºC)
Atmosphere:
Belt Speed:
Convecool Setting:
endogas
6.3 inches/minute
2.0 inches of water plenum
The parts were in the high heat zone for 30 minutes. The sintered-hardened samples were tempered at
375°F (190°C) in air for 1 hour prior to testing.
Complete chemical analysis was accomplished on at least two specimens from each experimental group.
Apparent hardness measurements were performed on the surface of the specimens using a Rockwell
hardness tester. Microhardness (particle) measurements were accomplished on polished cross-sections of
the specimens using a Knoop indenter and a 100-gram load (MPIF Standard 51). Tensile testing was
performed on a 56,000-pound (250 KNt) Instron universal testing machine at a strain rate of 6% per
minute. Elongation values were determined using an extensometer with a range of 0 to 20%. The
extensometer was removed before specimen failure.
Experimental Results. The results for tensile strength properties, particle hardness and percent
martensite were input to the statistical analysis software. The copper and carbon contents used for the
analysis reflect average chemical analysis results of at least two specimens from each experiment.
Tensile data are the averages from five specimens from each experiment. Particle hardness was used to
evaluate the different chemistries instead of apparent hardness because it is not a function of sintered
density. The average sintered densities of the specimen as a function of measured chemistry are plotted in
Figure 2. Because the total range of these data is only 0.03 g/cm3 we treated the data from the tensile
tests as coming from specimens having the same density.
Figure 2. Average Sintered Density as a Function of Copper and Carbon Content.
Quantitative metallography was used to determine the amount of each phase present in representative
microstructures. At least two tensile specimens from each experiment were sectioned, planar ground
using 120 grit SiC abrasive paper and then finally ground with 15-µm diamond on a composite disk.
Polishing was performed in three steps using diamond abrasives. The particle sizes for the polishing steps
were 6, 3, and 1 µm. The microstructure was revealed by etching with a 50/50 combination of 4 v/o picral
with either 1 or 2 v/o nital.
The phases or constituents in the microstructures were quantified using a manual point count on the
etched surfaces. In practice, an image of the microstructure was projected onto a transparent, rectangular,
20-point grid for a total of 20 applications. The determination of the relative amounts of each phase or
constituent was calculated by counting the number of points falling on each constituent, dividing that total
by the total number of applied points, then multiplying that total by 100 to obtain a percentage. This
procedure conforms to the Standard Test Method for Determining Volume Fraction by Systematic
Manual Point Count, ASTM 562. Typical micrographs are shown in Figures 3 thru 6. The black areas in
the micrographs are pores.
Figure 3. 100% martensite structure. 1% Nital and 4% Picral etch.
Figure 4. Martensite plus small amount of retained austenite. 1% Nital and 4% Picral etch.
Figure 5. Microstructure containing 96% martensite and 4% bainite. 1% Nital and 4% Picral etch.
Figure 6. Microstructure containing 74% martensite and 26% bainite. 1% Nital and 4% Picral etch.
Results – Statistical Models. The statistical design software analyzes the input data and generates
models for each response. These are provided as equations, contour plots and three-dimensional plots
(response surfaces). Examples of these graphical models are shown in Figures 7 and 8 for ultimate tensile
and percent martensite.
Tensile Strength
908.249
785.904
0.61
1.50
%
Gr
ap
hit
e
0.40
%
0.90
p
Co
per
Figure 7. Graphical representation of ultimate tensile strength model for effect of copper and carbon.
1000
per
1.50
Co
p
rdness
Particle Ha
300
%G
rap
%
0.70
h it e
0.40
0.90
Figure 8. Graphical representation of particle hardness model for effect of copper and carbon.
Discussion.
Hardenability. Several of the chemistries evaluated with these experiments readily achieved microstructures containing 100% martensite during air quenching in the cooling zone of the sinter furnace. All
mixes containing carbon contents greater than 0.6% and copper contents greater than 1.27% were fully
martensitic. As the carbon content is reduced, the degree of hardenability decreases as evidenced by the
increasing presence of bainite in the microstructure.
Property Prediction. First, a cautionary note; the samples used for these experiments were small and thus
readily quenched. Thus, the predicted chemistries may not have sufficient hardenability to achieve the
same properties in a part. The DOE software uses the generated models to predict the copper and carbon
contents needed to attain specific properties for this particular sinter hardening alloy chemistry. This is
done by applying constraints on the model; i.e., the response is maximum; minimum; meets target;
etcetera. Two examples using different criteria and their predicted results are shown in Tables 2 and 3.
When the results indicate chemistries that are near or at the edge of the test area it is good practice to
exclude them, as the error is much larger in these regions. In the tables these results have a gray
background. Note that setting target value criteria result in multiple results that meet the criterion (see
Tables 4 and 5) while using a maximization selection criteria usually results in a single result. For
example a target particle hardness target of Rockwell C55 (659 HK100) may be obtained with a large
range of copper and carbon content combinations. Similarly, a microstructure that is 100% martensite is
predicted to be attainable with a range of copper and carbon contents (see Table 5). Setting two maxima
criteria and a 100% martensite target gives a single prediction that will provide a solution within the
constraints selected. This sinter-hardening alloy contains less copper and less carbon and has higher
Table 2. Example 1 – Target Particle Hardness Level
Model Constraints
Name
Copper
Carbon
Tensile Strength
Yield Strength
% Elongation
Particle hardness
Martensite
Goal
in range
in range
in range
in range
in range
target = 659
in range
Model Predictions
Copper
%
1.0
1.5
1.6
1.6
1.0
1.1
1.1
1.2
1.5
Carbon
%
0.5
0.5
0.5
0.5
0.6
0.6
0.6
0.6
0.6
Particle
Hardness
HK100
659
659
659
659
706
706
708
710
718
Martensite
%
97
97
99
99
100
100
100
100
100
At edge of test area.
OK
tensile properties, and equivalent particle hardness as the same base alloy containing 2.0% copper
and 0.8% carbon.
Table 3. Example 2 – Target Martensite Content in Microstructure
Model Constraints
Name
Copper
Carbon
Tensile Strength
Yield Strength
Elongation
Particle hardness
Martensite
Goal
in range
in range
in range
in range
in range
in range
target = 100
Model Predictions
Copper
%
1.6
1.6
1.0
1.6
1.7
0.9
0.9
1.6
1.6
Carbon
%
0.5
0.5
0.6
0.5
0.5
0.4
0.5
0.5
0.5
Particle
Hardness
HK100
646
647
706
648
658
577
636
655
645
Martensite
%
100
100
100
100
100
100
100
100
100
At edge of test area.
OK
Conclusions
1. The use of statistical design of experiments (DOE) reduced the number of alloy chemistries
needed to predict copper and carbon chemistries that could achieve higher tensile properties.
2. The results of the DOE confirmed our preposition that lower copper and carbon contents may
provide higher tensile properties. Specifically, the lower copper and carbon contents provide a
leaner sinter-hardening alloy with predicted ultimate tensile strength values that are significantly
higher than those obtained with the generally used Fe-0.42Mn-1.24Mo-1.49Ni-2.00Cu-0.80C
alloy.
3. Additional experiments focused on a smaller range of chemistries and applied to larger specimen
sizes are needed to confirm the predicted properties.
References:
(1) M.C. Baran, A.H. Graham, A.B. Davala, R.J. Causton & C. Schade, “A Superior SinterHardenable Material”, Advances in Powder Metallurgy and Particulate Materials, compiled by
V. 2, Part 7, Metal Powder Industries Federation, Princeton, NJ, 1999, p. 1185.
(2) T.J. Miller, J. Groark, R.J. Causton & A. Davala, “Improved Efficiency by Use of SinterHardened P/M Automotive Components”, SAE Paper No. 2000-01-0406, SAE International,
Warrendale, PA, 2000.
(3) T.E. Harberberger, T.J. Carnelio, M.C. Baran, & P.J. Winterton, “Field Experience in a New
Sinter-Hardening Material”, Advances in Powder Metallurgy and Particulate Materials-2000,
compiled by H. Ferguson and D. T. Whychell, Sr., Part 10, Metal Powder Industries Federation,
Princeton, NJ, p.10-101.
(4) M.A. Pershing and H. Nandi, “A Methodology for Evaluating Sinter-Hardening Capability”,
Advances in Powder Metallurgy and Particulate Materials-2001, compiled by W. B. Elsen and S.
Kassam, Metal Powder Industry Federation, Princeton, NJ, p. 5-26.
(5) W. Crafts and J. Lamont, “Affect of Some Elements on Hardenability”, Trans. AIME, V. 154,
American Institute of Mining, Metallurgical and Petroleum Engineers, New York, NY, 1943, p.
386.
(6) M. A. Grossmann, “Hardenability Calculated from Chemical Composition“, Trans. AIME, V.
150, American Institute of Mining, Metallurgical and Petroleum Engineers, New York, NY,
1942, p. 227.