EFFECT OF MICROSTRUCTURAL INHOMOGENEITIES
ON THE MECHANICAL PROPERTIES OF HYBRID P/M STEELS
Suleyman Saritas*, Robert Causton**, W. Brian James**, and Alan Lawley***
*Gazi University, Department of Mechanical Engineering, Maltepe/Ankara, 06570, TURKEY
(Currently, Visiting Department of Materials Engineering, Drexel University, Philadelphia, PA 19104)
**Hoeganaes Corporation, Cinnaminson, NJ 08077
****Department of Materials Engineering, Drexel University, Philadelphia, PA 19104
ABSTRACT
The effect of microstructural inhomogeneities on the tensile and impact response of a prealloyed (FL-4405) and
two hybrid (FLC2-4405 and FLN2-4405) P/M steels was investigated. Tensile and impact response, microstructures, pore characteristics and fracture modes were determined in the sintered, quenched + tempered and sinterhardened conditions. Sintering temperatures of 1120°C (2050°F) and at 1260°C (2300°F) were utilized and
densities in the range 7.0 - 7.4 g/cm3 were achieved by single and double pressing and sintering. Over this sintered
density range, tensile strength increases by >30%. In the quenched + tempered condition tensile strength exceeds
1000 MPa. Tensile properties are rationalized in terms of the attendant microstructures and modes of fracture.
INRODUCTION
Over the last two decades there has been an increasing demand for high performance powder metallurgy
(P/M) steels, prompted primarily by the automotive industry. This has resulted in the development of
several commercially viable alloy compositions [1-24].
The mechanical properties of P/M steels are related directly to their microstructure and the level of
porosity [25-35]. In addition, the alloying mode has a significant effect on the microstructure and pore
morphology in sintered steels. There are four major and distinct alloying methods used for formulating
P/M steels and these establish a classification for the steels: admixed, partially alloyed, prealloyed and
hybrid [1, 9, 14, 16, 20]. Admixed - Alloying additions are made to the base iron powder in the form of
elemental or ferroalloy powders. This is the least expensive and most commonly used alloying method.
Partially Alloyed (Diffusion Alloyed) - Alloying additions are diffusion alloyed to the base iron particles
in order to retain most of the compressibility of the base iron. Prealloyed - Alloying elements (except
carbon) are added to the melt before atomization. This results in a homogeneous microstructure. Prealloyed powders, with molybdenum as the principal alloy addition, have been developed with a comp-
1
ressibility approaching that of iron powder. Hybrid Alloys – They are based on a highly compressible
prealloyed powder which includes alloying additions to improve strength and hardenability.
P/M steels with high hardenability develop microstructures containing a significant fraction of martensite
in the as-sintered condition [6, 20, 36-39]. Accelerated cooling rates can be achieved in the sintering furnace (> 5 ºC/s), which permits larger parts to be sinter-hardened. In this way problems related to the oil
quenching of porous P/M steels, primarily oil penetration, are eliminated and there is less distortion in the
parts due to the lower cooling rate.
The sinter-hardening response of commercial ferrous powder alloys may be estimated from Jominy
hardenability data. Recent work by Skena et al [40], carried out in support of the MPIF P/M materials
standards development program, provides a basis for comparing the relative hardenabilities of different
steels. In order to make a direct comparison between various P/M steels, the comparison must be made at
the same sintered density - because the hardness values obtained in the Jominy end-quench test are a
function of sintered density. Although prealloyed steels exhibit homogeneous microstructures, they do not
possess particularly high hardenabilities. The addition of copper and/or nickel to prealloyed steels
enhances hardenability significantly, as observed in FLC-4608 [20]. In fact, for many years sinterhardened parts have been made by sintering FLC-4608 in conventional sintering furnaces. There is,
however, a need for commercial P/M steel with a higher hardenability than those that are currently
available so that larger P/M parts may be sinter-hardened.
In the present study, the mechanical properties of one prealloyed and two hybrid P/M steels based on
Ancorsteel 85HP are reported. Ancorsteel 85HP was developed in the early 1990s as a water-atomized
prealloyed powder for high performance applications. The prealloyed 0.85w/o molybdenum addition
provides high compressibility coupled with acceptable response to heat treatment. In previous studies,
some mechanical properties of medium to high density P/M steels based on this powder have been
reported [4, 5, 8, 21]. The results presented here establish a base for static and impact properties as a
precursor to ongoing research on cyclic tensile testing, rotating bending fatigue and fatigue crack
propagation. Three compositions, four densities, two sintering temperatures and three heat treatment
conditions are under investigation. Here results on tensile and impact response at two density levels, are
reported with attendant microstructures, pore characteristics and fracture morphologies.
EXPERIMENTAL PROCEDURE
Materials and Processing
Three P/M steels based on Hoeganaes Ancorsteel 85HP were examined. The compositions and properties
of the premixes (FL-4405, FLC2-4405 and FLN2-4405) are given in Table I. The source and mean size of
each constituent powder are summarized in Table II. After sintering, the carbon level was 0.57w/o in each
steel; oxygen levels were in the range 0.009 – 0.055w/o.
Table I. Compositions and Properties of Premixes
Premix*
Apparent
Density
(g/cm3)
FL-4405
3.11
FLC-4405
3.11
FLN2-4405
3.13
* MPIF Standard 35
Flow
(s/50g)
29.5
29.9
29.1
Composition (w/o)
Ancorsteel 85HP
Copper
Nickel
Graphite
Lubricant
98.65
96.65
96.65
0
2
0
0
0
2
0.6
0.6
0.6
0.75
0.75
0.75
2
Table II. Powder Source and Characteristics
Powder
Ancorsteel 85HP
Nickel (123)
Copper (8081)
Graphite (3203HS)
Lubricant (Acrawax C)
Source
Hoeganaes
Inco
Acupowder
Asbury
Lonza
Type
Water atomized
Carbonyl
Water atomized
Milled
Atomized
Mean size, d50, (m)
100
5
60
10
-
A batch size of 227 kg (500 lb) was mixed for each of the three alloys in the Hoeganaes pilot plant using
the ANCORBOND process. To achieve a green density of 7.0 g/cm3 the powder was pressed uniaxially at
550 MPa (80 ksi) pressure. A higher green density of 7.4 g/cm3 was obtained by double pressing at
550/550 MPa pressure and double sintering (DPDS). The dimensions of the tensile and Charpy test pieces
were 12.7x12.7x100 mm and 10x10x55 mm, respectively.
Sintering was carried out in an Abbott ceramic belt high temperature furnace in an atmosphere of 75v/o
hydrogen - 25v/o nitrogen for 30min. For DPDS, the compacts were sintered first at 790 °C (1450 °F).
The furnace was equipped with a Varicool cooling zone to provide accelerated cooling from the sintering
temperature. The sintering and heat treating conditions employed in the present study are summarized in
Table III. For heat treatment, test pieces were austenitized in a Lindberg sealed quench furnace at 900 ºC
(1650 ºF) in an atmosphere of 75v/o hydrogen - 25v/o nitrogen for 30 min. The blanks were quenched
into oil preheated to 70 ºC (160 ºF). Excess oil was wiped from the quenched blanks before tempering in
a Blue M oven in air. Tempering was carried out at a temperature of 190 ºC (375 ºF) for 60 min. The
sinter-hardened test pieces were tempered under the same conditions as the heat-treated test pieces.
Table III. Sintering and Heat Treating Conditions
Process
Condition
Sintered
Sinter-Hardened
Quenched +
Tempered
Sintered
Sintering
Temperature
ºC (ºF)
1120 (2050)
1120 (2050)
1120 (2050)
1260 (2300)
Standard
100% Varicool
Standard
Austenitization
Temperature
ºC (ºF)
900 (1650)
Tempering
Temperature
ºC (ºF)
190 (375)
190 (375)
Standard
-
-
Cooling After
Sintering
Round threaded tensile test pieces were prepared according to MPIF Standard 10 [41]. The test pieces
were machined from the sintered blanks, except for the heat-treated test pieces; the latter were machined
oversize prior to heat treatment. The machined, sinter-hardened and heat treated test pieces were stress
relieved at 190 ºC (375 ºF) to remove any residual stresses from the machining and grinding operations.
No machining was applied to the Charpy test pieces [42]. Densities of the green and sintered steels were
determined by the gravimetric immersion method outlined in MPIF Standard 42 [43].
Mechanical Testing
Tensile tests were performed using a Tinius Olsen machine at a crosshead speed of 0.635 mm (0.025”)/min.
Elongation was measured with a 25.4 mm (1”) Epsilon extensometer. The load applied and the strain were
recorded continuously throughout the test. The resulting stress-strain curves were analyzed to identify the 0.2%
offset yield strength, tensile strength and elongation at fracture. The Charpy impact test pieces were broken using
a Baldwin impact tester. The hardness (HRA) of the test pieces was determined by means of a Mitutoyo ARK
hardness tester. For each type of test, the mean of five measurements was reported for each processing condition.
3
Metallography
Metallographic specimens of all test materials were analyzed by optical microscopy in the polished and
etched conditions. Polished metallographic samples were also utilized to evaluate the stereological
parameters of the pores (shape, size and distribution). This was accomplished with a Clemex 1024
automated image analysis system. Analysis of the pore shape was conducted to determine the average
degree of circularity, where circularity is determined from the equation [44]:
Circularity = 4A/(P )2
(1)
where A = pore area and P = pore circumference.
A form factor of unity represents a circular pore in the plane of analysis; as the number decreases from
unity, the degree of irregularity increases. The aspect ratio, which is the ratio of maximum chord length to
minimum chord length of pores, was also measured for comparison purposes.
Fracture surfaces of the tensile specimens were examined by scanning electron microscopy (SEM).
Fracture morphologies were identified and quantified for each processing condition.
EXPERIMENTAL RESULTS
Mechanical Properties
The mechanical properties of the P/M steels are summarized in Table IV and Table V as a function of
processing history. In the as-sintered condition, the tensile strengths of the alloys at a sintered density of
7.0 g/cm3 are 408 MPa, 580 MPa and 495 MPa for FL-4405, FLC2-4405 and FLN2-4405 respectively.
The tensile strength increases to 647 MPa, 788 MPa and 826 MPa, respectively with an increase in
sintered density to 7.4 g/cm3. In the quenched + tempered condition the tensile strengths at a sintered
density of 7.0 g/cm3 are 944 MPa, 1010 MPa and 1020 MPa for FL-4405, FLC2-4405 and FLN2-4405,
respectively. The strength levels increase to 1486 MPa, 1432 MPa and 1644 MPa, respectively by
increasing the sintered density to 7.4 g/cm3. High temperature sintering reduces the tensile strength of
FLC2-4405 and FLN2-4405. Sinter-hardening results in strength values between those of the as-sintered,
and quenched + tempered conditions. Elongation to fracture is in the range of 0.9% to 4.6%. Impact
energy values are in the range of 18 J to 39 J.
Microstructures and Fracture Morphologies
Figures 1 to 6 show representative microstructures of the three alloys for each of the processing
conditions. As-sintered microstructures consist primarily of unresolved pearlite in the three steels. FLC24405 and FLN2-4405 exhibit some martensitic areas. In addition, some nickel-rich areas are seen in
FLN2-4405. Quenching produced a structure of more than 90% martensite in the steels. Sinterhardening produced about 50% martensite in FLC2-4405 and FLN2-4405.
Figures 7 to 10 show the pore size distribution and shape analysis results. The numbers in the legend in
each figure refer to sintered density and sintering temperature (ºF). Pore size distributions are given in
terms of pore area rather than pore size. The definition and measurement of pore size is generally
misleading but the definition and measurement of pore area is simple and accurate. Pores < 6 m are
removed from the distribution in order to improve the measurements; these data are given separately in
Figure 8. For assessment of pore shape, the circularity (Figure 9) and aspect ratio (Figure 10), are plotted
on a frequency basis.
Representative fractographs of the tensile test pieces at the same processing conditions are shown in
Figures 11 to 13. The quenched + tempered fracture surfaces showed more than 50% dimpled rupture.
4
The as-sintered and sinter-hardened fracture surfaces contained more than 50% cleavage facets in the
steels.
Table IV. Mechanical Properties as a Function of Composition and Processing History
Material
FL-4405
FLC2-4405
FLN2-4405
Sintering
Temp. °F
(°C)
2050
(1120)
2300
(1260)
2050
(1120)
2300
(1260)
2050
(1120)
Density
(g/cm3)
Heat
Treatment
Hardness
(HRA)
7.0
AS*
QT**
AS
QT
AS
7.39
7.35
7.35
7.0
7.28
7.33
7.33
7.25
7.0
7.44
7.39
7.39
7.39
2300
(1260)
*AS: As sintered,
**QT: Quenched + tempered,
***SH: Sinter-hardened
Tensile
Strength
(MPa)
408
944
647
1486
659
Elongation
(%)
43
65
52
53
53
Yield
Strength
(MPa)
344
943
465
1410
458
AS
QT
AS
QT
SH***
AS
48
66
55
61
61
56
460
1004
603
1425
662
584
580
1010
788
1432
822
745
2.0
1.0
3.6
1.0
1.7
2.5
AS
QT
AS
QT
SH
AS
52
68
58
59
60
58
390
967
526
1316
643
535
495
1020
826
1644
982
535
1.4
1.0
3.5
1.7
4.6
3.3
1.3
0.9
4.3
1.1
4.2
Table V. Impact Energy as a Function of Composition and Processing History
Material
FL-4405
Sintering Temp.
°F (°C)
2050 (1120)
FLC2-4405
2300 (1260)
2050 (1120)
FLN2-4405
2300 (1260)
2050 (1120)
2300 (1260)
Density
(g/cm3)
7.34
7.34
7.36
7.32
7.33
7.32
7.35
7.43
7.42
7.42
7.46
Heat
Treatment
AS*
QT**
AS
AS
QT
SH***
AS
AS
QT
SH
AS
*AS: As sintered,
**QT: Quenched + tempered,
***SH: Sinter-hardened
5
Hardness
(HRA)
51
51
52
55
55
56
55
58
58
59
58
Impact Energy
Ft-lb (J)
19 (26)
13 (18)
29 (39)
23 (31)
14 (19)
24 (33)
27 (37)
27 (37)
15 (20)
24 (33)
28 (38)
(a)
(b)
Figure 1. Microstructures of FL-4405 at 7.0 g/cm3 sintered density:
(a) as sintered at 2050 ºF, (b) sintered at 2050 ºF and quenched + tempered.
(a)
(b)
(c)
Figure 2. Microstructures of FL-4405 after DPDS: (a) as sintered at 2050 ºF,
(b) sintered at 2050 ºF and quenched + tempered, (c) as sintered at 2300 ºF
6
(a)
(b)
Figure 3. Microstructures of FLC2-4405 at 7.0 g/cm3 sintered density:
(a) as sintered at 2050 ºF, (b) sintered at 2050 ºF and quenched + tempered.
(a)
(b)
(c)
(d)
Figure 4. Microstructures of FLC2-4405 after DPDS: (a) as sintered at 2050 ºF,
(b) sintered at 2050 ºF and quenched + tempered, (c) as sintered at 2300 ºF,
(d) sinter-hardened from 2050 ºF
7
(a)
(b)
Figure 5. Microstructures of FLN2-4405 at 7.0 g/cm3 sintered density:
(a) as sintered at 2050 ºF, (b) Sintered at 2050 ºF and quenched + tempered.
(a)
(b)
(c)
(d)
Figure 6. Microstructures of FLN2-4405 after DPDS: (a) as sintered at 2050 ºF,
(b) sintered at 2050 ºF and quenched + tempered, (c) as sintered at 2300 ºF,
(d) sinter-hardened from 2050 ºF
8
Frequency (%)
100
90
80
70
60
50
40
30
20
10
0
7.0 (2050)
(a)
DPDS (2050)
DPDS (2300)
100
200
300
400
500
600
700
800
900 >1000
Pore Area (µm²)
80
Frequency (%)
7.0 (2050)
(b)
70
DPDS (2050)
60
DPDS (2300)
50
40
30
20
10
0
100
200
300
400
500
600
700
800
900 >1000
Frequency (%)
Pore Area (µm²)
100
90
80
70
60
50
40
30
(c)
7.0 (2050)
DPDS (2050)
DPDS (2300)
20
10
0
100
200
300
400
500
600
700
800
Pore Area (µm²)
Figure 7. Pore distribution by area (pores > 6 m):
(a) FL-4405, (b) FLC2-4405, (c) FLN2-4405
9
900
>1000
30
Frequency (%)
7.0 (2050)
(a)
25
DPDS (2050)
DPDS (2300)
20
15
10
5
0
2.5
5
7.5
30
10
12.5
Pore Area
15
(b)
>20
7.0 (2050)
25
Frequency (%)
17.5
DPDS (2050)
20
DPDS (2300)
15
10
5
0
2.5
5
7.5
10
12.5
15
17.5
>20
Pore Area
30
7.0 (2050)
(c)
Frequency (%)
25
DPDS (2050)
20
DPDS (2300)
15
10
5
0
2.5
5
7.5
10
12.5
15
Pore Area
Figure 8. Pore distribution by area (pores < 6 m):
(a) FL-4405, (b) FLC2-4405, (c) FLN2-4405
10
17.5
>20
30
7.0 (2050)
Frequency (%)
25
(a)
DPDS (2050)
DPDS (2300)
20
15
10
5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Circularity
30
7.0 (2050)
DPDS (2050)
DPDS (2300)
Frequency (%)
25
20
(b)
15
10
5
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0.7
0.8
0.9
1
Circularity
30
frequency (%)
25
20
7.0 (2050)
(c)
DPDS (2050)
DPDS (2300)
15
10
5
0
0.1
0.2
0.3
0.4
0.5
0.6
Circularity
Figure 9. Circularity of pores: (a) FL-4405, (b) FLC2-4405, (c) FLN2-4405
11
30
7.0 (2050)
(a)
Frequency (%)
25
DPDS (2050)
DPDS (2300)
20
15
10
5
0
1 1.25 1.5 1.75
2 2.25 2.5 2.75
3 3.5
4 4.5
>5
Aspect Ratio
30
7.0 (2050)
(b)
Frequency (%)
25
DPDS (2050)
DPDS (2300)
20
15
10
5
0
1
1.25 1.5 1.75
2
2.25 2.5 2.75
3
3.5
4
4.5
>5
Aspect Ratio
30
7.0 (2050)
DPDS (2050)
DPDS (2300)
(c)
Frequency (%)
25
20
15
10
5
0
1
1.25 1.5 1.75
2
2.25 2.5 2.75
3
3.5
4
4.5
>5
Aspect Ratio
Figure 10. Aspect ratio of pores: (a) FL-4405, (b) FLC2-4405, (c) FLN2-4405
12
(a)
(b)
(c)
(d)
(e)
Figure 11. Tensile fractographs of FL-4405: (a) sintered density 7.0 g/cm3; as sintered at 2050 ºF,
(b) sintered density 7.0 g/cm3; sintered at 2050 ºF and quenched + tempered, (c) DPDS, as sintered at
2050 ºF, (d) DPDS, sintered at 2050 ºF and quenched + tempered, (e) DPDS, as sintered at 2300 ºF
13
(a)
(b)
(c)
(d)
(e)
(f)
Figure 12. Tensile fractographs of FLC2-4405: (a) sintered density 7.0 g/cm3; as sintered at 2050 ºF,
(b) sintered density 7.0 g/cm3; sintered at 2050 ºF and quenched + tempered, (c) DPDS, as sintered at
2050 ºF, (d) DPDS, sintered at 2050 ºF and quenched + tempered, (e) DPDS, as sintered at 2300 ºF,
(f) DPDS, sinter-hardened from 2050 ºF
14
(a)
(b)
(c)
(d)
(e)
(f)
Figure 13. Tensile fractographs of FLN2-4405: (a) sintered density 7.0 g/cm3; as sintered at 2050 ºF,
(b) sintered density 7.0 g/cm3; sintered at 2050 ºF and quenched + tempered, (c) DPDS, as sintered at
2050 ºF, (d) DPDS, sintered at 2050 ºF and quenched + tempered, (e) DPDS, as sintered at 2300 ºF,
(f) DPDS, sinter-hardened from 2050 ºF.
15
DISCUSSION
The mechanical properties summarized in Tables IV and V show clearly that the P/M steels investigated
demonstrate strength levels suitable for high performance applications (tensile strength > 1000 MPa).
Tensile strength is improved dramatically by increasing the nominal sintered density from 7.0 g/cm3 to 7.4
g/cm3. In the as-sintered condition the increases in tensile strength are 58%, 36% and 67% for the FL4405, FLC2-4405 and FLN2-4405 steels, respectively. In the quenched + tempered condition the
corresponding increases are 57%, 30% and 61%, respectively. At a nominal sintered density of 7.4 g/cm3
high temperature sintering did not produce any change in tensile strength for FL-4405 but it did reduce
tensile strength in the FLC2-4405 and FLN2-4405 steels. Sinter-hardening of the FLC2-4405 and FLN24405 steels from 1120 ºC (2050 ºF) resulted in tensile strength levels which are higher than the as-sintered
values, but much lower than the quenched + tempered values. These strength levels, coupled with the
microstructures, confirm that the cooling rate of the furnace in 100% Varicool is not sufficient for the
complete transformation of austenite to martensite. Elongation to fracture values are relatively low for the
steels, even at a sintered density of 7.4 g/cm3.
The metallographic samples were cut from the shoulders of the broken tensile test pieces. Since the
shoulder area was more than twice the area in the gauge length, it is believed that no deformation had
taken place in the shoulders during tensile testing. Thus the samples should reflect the true pore
morphology and microstructure present in the P/M steels.
Microstructures of the FL-4405 steel in the as-sintered conditions (Figures 1(a), 2(a) and 2(c)) are similar,
except for the pore size and pore distribution. The microstructure consists primarily of unresolved pearlite
and some ferrite. The quenched + tempered microstructure consists of almost 100% decomposed martensite (Figures 5(b) and 6(b)). The Jominy distance for 50% martensite was 8 mm, based on concurrent
work by the authors [45]. The heat treated section is about 12x12 mm; thus with this geometry, the P/M
steel should through-harden.
Microstructures of the FLC2-4405 steel in the as-sintered condition (Figures 3(a), 4(a) and 4(c)) are
comparable. The brownish etching areas are more pronounced in the DPDS microstructures than in the
single press and sinter conditions. The authors believe that these areas consist of fine martensite. The
presence of more than 50% cleavage facets on the tensile fracture surfaces confirms this observation. The
microstructure consists primarily of unresolved pearlite. The quenched + tempered microstructure is
almost 100% decomposed martensite (Figures 3(b) and 4(b)). In the paper cited [45], the Jominy distance
for 50% martensite was 12 mm. The sinter-hardened microstructure is about 50% martensite, bainite (or
unresolved pearlite) and some retained austenite.
Microstructures of the FLN2-4405 steel in the as-sintered condition (Figures 5(a), 6(a) and 6(c)) are
comparable. The brownish etching areas are more pronounced in the DPDS microstructures, for example
FLC2-4405. These areas consist of fine martensite. The fracture surfaces again exhibit more than 50%
cleavage facets. The microstructure is composed primarily of unresolved pearlite with white etching
nickel-rich areas. The quenched + tempered microstructure is composed of 90% decomposed martensite
(Figures 5(b) and 6(b)). The remaining 10% is composed of nickel-rich areas and retained-austenite. The
sinter-hardened microstructure consists of about 50% martensite, 40% unresolved pearlite (or bainite)
with the balance comprising nickel-rich areas and retained-austenite.
Figure 7 shows that most of the pores (>60%) in the P/M steels investigated are in the size range (by area)
<100 m 2 which corresponds to pores < 11 m dia. The pore distribution shown in Figure 2 for pores <
6 m was not included in these values. Thus, it can be concluded that a majority of the pores are <11
m. The number of small pores increases as the sintered density increases from 7.0 g/cm 3 to 7.4 g/cm3.
An increase in the sintering temperature from 1120 ºC (2050 ºF) to 1260 ºC (2300 ºF) resulted in a similar
effect, except for the steel alloyed with copper where the number of smaller pores decreases as the
sintering temperature increases. The frequency of pores in the 100 to 200 m 2 range (corresponding to 11
16
to 16 m dia) is about 10% and is much less for larger pores. In contrast, there are few pores > 1000 m 2
corresponding to a pore dia > 35 m dia in the steels, which is true primarily for the copper steel. Pores <
6 m dia show almost a normal distribution and the mean value is about 10 m 2 in area, corresponding
to 3.5 m dia (Figure 8).
Increasing the density by compacting at a higher pressure, or by sintering at a higher temperature,
decreases the number of pores < 3.5 m dia and increases the number of pores >3.5 m dia. As seen
from Figure 3, high temperature sintering is effective in increasing the circularity of the pores. The effect
of this is more dominant for pores which are near spherical initially with a circularity or shape factor of
0.8 to 0.9. For pores with a shape factor < 0.6, high temperature sintering is not as effective as it is for
pores with a shape factor of 0.9. The distribution of the aspect ratio is close to a normal distribution
compared to circularity. The mean aspect ratio of the pores for the P/M steels is about 1.5, corresponding
to an ellipsoidal shape of major diameter to minor diameter ratio of 1.5. The effects of increasing density
by double pressing and/or sintering at high temperature on pore shape and number are seen clearly in
micrographs in Figures 1 to 6.
The fractographs (Figures 11 to 13) are helpful in understanding the deformation behavior of the steels
during tensile testing. Low ductility in pressed and sintered P/M steels is well known [25-35]. This low
ductility is due mainly to poor bonding between powder particles. In the steels examined, the Charpy
impact energy values are relatively low. Areas of transgranular cleavage covering more than one powder
particle are evident on the tensile fracture surfaces. Some of the fracture surfaces of FLC2-4405 and
FLN2-4405 steels exhibit more than 80% cleavage (Figures 12(c), 12(e), 12(f), 13(c), 13 (e) and 13 (f)).
Clearly this is evidences of a higher hardenability and hence suitability in sinter-hardening. The cleavagetype fracture should be eliminated or reduced by a suitable tempering treatment.
CONCLUSIONS
1. FL-4405, FLC2-4405 and FLN2-4405 P/M steels are suitable for high performance applications
with a tensile strength > 1000 MPa in the qunched + tempered condition at a sintered density of
7.4 g/cm3. FLC2-4405 and FLN2-4405 exceed this value at a sintered density of 7.0 g/cm3.
2. Increase in nominal sintered density from 7.0 g/cm3 to 7.4 g/cm3 results in tensile strength
increases of more than 50% in the FL-4405 and FLN2-4405 steels, and about 30% in FLC2-4405.
3. FLC2-4405 and FLN2-4405 can be sinter-hardened but the cooling rate of the furnace was not
sufficient for complete transformation of austenite to martensite.
4. High temperature sintering reduces the tensile strengths of FLC2-4405 and FLN2-4405 but does
not produce any major change in the tensile strength of FLC2-4405.
AKNOWLEDGEMENTS
Professor Saritas is indebted to the Hoeganaes Corporation for financial support during a sabbatical leave
(2000/2001) at Drexel University. The authors express their gratitude to Tom Murphy for assistance with
the pore and microstructure analysis.
17
REFERENCES
1. W.B. James and R.C. O'Brien, “High Performance Ferrous P/M Materials: The Effect of Alloying
Method on Dynamic Properties”, Progress in P/M, Compiled by E.A. Carlson and G. Gaines, Metal
Powder Industries Federation, Princeton, NJ, vol.42, 1986, pp.353-372.
2. U. Engstrom, “A Newly Developed Sintered High Strength Material”, Horizons of P/M, Edited by
W.A. Kaysser and W.J. Huppmann, Proc. 1986 World P/M Con., EPMA, Dusseldorf, 1986, part II,
pp.1039-1048.
3. B. Lindqvist, “Influence of Microstructure and Porosity on Fatigue Properties of Sintered Steels”,
Modern Developments in P/M, Compiled by P.U. Gummeson and D.A. Gustafson, Metal Powder
Industries Federation, Princeton, NJ, vol. 21, 1988, pp.67-82.
4. B. Lindqvist, “A Molybdenum-Alloyed P/M Steel For Surface Hardening”, PM Into The 1990’s,
Proc. World Con. on P/M, London, 1990, vol.2, pp.170-177.
5. J.J. Fulmer and R.J. Causton, “Tensile, Impact and Fatigue Performance of a New Water Atomized
Low-Alloy Powder-Ancorsteel 85HP”, Advances in P/M & Particulate Materials, Compiled by E.R.
Andreotti and P.J. McGeehan, Metal Powder Industries Federation, Princeton, NJ, vol.2, 1990, pp
459-486.
6. R.J. Causton and W.B. James, “Performance Characteristics of a New Sinter-Hardening Low Alloy
Steel”, Compiled by L.F. Pease III and R.J. Sansoucy, Advances in P/M, Metal Powder Industries
Federation, Princeton, NJ, vol.5, 1991, pp 91-104.
7. W.B. James, “Effect of Alloying Methods on Thermal Processing and Properties of Ferrous
Materials”, Industrial Heating, vol.59, no.6, 1992, pp. 34-39.
8. W.B. James and R.J. Causton, “Surface-Hardenable Heat Treated P/M Steels”, Advances in P/M &
Particulate Materials, Compiled by J.M. Capus and R.M. German, Metal Powder Industries
Federation, Princeton, NJ, vol.5, 1992, pp 65-91.
9. U. Engstrom, C. Lindberg and J. Tengzelius, “Powders and Processes for High Performance PM
Steels”, Powder Metallurgy, vol.35, No.1, 1992, pp.67-72.
10. C. Lindberg, “Sintered High Strength Materials”, Advances in P/M & Particulate Materials,
Compiled by J.M. Capus and R.M. German, Metal Powder Industries Federation, Princeton, NJ vol.5,
1992, pp 107-114.
11. R.J. Causton, J.A. Hamil and S.O. Shah, “Properties of Heat Treated P/M Alloy Steels”, Advances in
P/M & Particulate Materials, Compiled by A. Lawley and A. Swanson, Metal Powder Industries
Federation, Princeton, NJ, vol.4, 1993, pp 61-96.
12. W.B. James and M.J. Koczak, “New Ferrous Powder Metallurgy Alloys: An Overview”, Proc. 1993
P/M World Congress, Edited by Y. Bando and K. Kosuge, JPMA, Kyoto, 1993, part 1, pp.483-488.
13. Y. Trudel and F. Chagnon, “Effects of Steel Powder Composition and Alloying Technique on
Properties of PM Materials”, PM’94, Proc. World Con. on PM, Paris, 1994, vol.2, pp.815-818.
14. W.B. James, “Recent Developments in Ferrous Powder Metallurgy Alloys”, Int. J. Powder
Metallurgy, vol.30, no.2, 1994, pp 157-162.
15. R.J. Causton, “Development of High Performance PM Steels”, Advances in P/M & Particulate
Materials, Compiled by T.M. Cadle and K.S. Narasimhan, Metal Powder Industries Federation,
Princeton, NJ, vol.4,1996, pp.13-391/13-411.
16. F. Chagnon and Y. Trudel, “Designing Low Alloy Steel Powders for Sinter Hardening Applications”,
Advances in P/M & Particulate Materials, Compiled by T.M. Cadle and K.S. Narasimhan, Metal
Powder Industries Federation, Princeton, NJ, vol.4, 1996, pp.13-211/13-220.
17. F. Chagnon and Y. Trudel, “Effect of Sintering Parameters on Mechanical Properties of Sinter
Hardened Materials”, Advances in P/M & Particulate Materials, Compiled by R.A. McKotch and R.
Webb, Metal Powder Industries Federation, Princeton, NJ, vol.2, 1997, pp 14-97/14-106.
18. T.M. Cimino, H.G. Rutz, A.H. Graham and T.M. Murphy, “The Effect of Microstructure on Fatigue
Properties of Ferrous P/M Materials”, Advances in P/M & Particulate Materials, Compiled by R.A.
McKotch and R. Webb, Metal Powder Industries Federation, Princeton, NJ, vol.2, 1997, pp 13137/13-149.
18
19. T.M. Cimino, A.H. Graham and T.F. Murphy, “The Effect of Microstructure and Pore Morphology
on Mechanical and Dynamic Properties of Ferrous P/M Materials”, Advances in P/M & Particulate
Materials, Compiled by J.H. Reinshagen and J.J. Oakes, Metal Powder Industries Federation,
Princeton, NJ, vol.3, 1998, pp 13-33/13-43.
20. W.B. James, “What is Sinter-Hardening?”, PM2TEC’98 International Conference on Powder
Metallurgy and Particulate Materials, May 31- June 4, 1998, Las Vegas, Nevada, Presented at
Special Interest Session but not published.
21. T.M. Cimino, A.H. Graham, T.F. Murphy and A. Lawley, “The Effect of Microstructure and Pore
Morphology on Mechanical and Dynamic Properties of Ferrous P/M Materials”, Advances in P/M &
Particulate Materials, Compiled by C.L. Rose and M.H. Thibodeau, Metal Powder Industries
Federation, Princeton, NJ, vol.2, 1999, pp.7-65/7-84.
22. C. Lindberg, “Mechanical Properties of Water Atomized Fe-Cr-Mo Powder and How to Sinter It”,
Advances in P/M & Particulate Materials, Compiled by C.L. Rose and M.H. Thibodeau, Metal
Powder Industries Federation, Princeton, NJ, vol.2, 1999, pp.7-229/7-243.
23. C. Lindberg, “Mechanical Properties of Warm Compacted Astaloy CrM”, Advances in P/M &
Particulate Materials, Compiled by H. Ferguson and D. Wychell, Metal Powder Industries Federation,
Princeton, NJ, 2000, part 6, pp.6-81/6-88.
24. W.B. James, R.C. Causton, M.C. Baran, T.F. Murphy and K.S. Narasimhan, “New High Performance
P/M Alloys to Replace Malleable Cast Irons”, Advances in P/M & Particulate Materials, Compiled by
H. Ferguson and D. Wychell, Metal Powder Industries Federation, Princeton, NJ, 2000, part 13, pp.
13-123/13-132.
25. R. Haynes, “A Study of The Effect of Porosity Content on The Ductility of Sintered Metals”, Powder
Metallurgy, vol.20, No.1, 1977, pp 17-20.
26. R. Haynes, The Mechanical Behavior of Sintered Metals, Freund Publishing, House, London, 1981.
27. F.J. Esper, C.M. Sonsino and G. Leuze, “Influence of Homogeneity on the Fatigue Properties of
Sintered Steels”, Proc. Int. Con., Florence, 1982, pp.117-125.
28. C.M. Sonsino, G. Schlieper and W.J. Huppmann, “Influence of Homogeneity on The Fatigue
Properties of Sintered Steels”, Int. J. Powder Metallurgy, vol.20, no.1, 1984, pp 45-50.
29. K.D. Christian, R.M. German and A.S. Paulson, “Statistical Analysis of Density and Particle Size
Influences on Micro-structural and Fatigue Properties of a Ferrous Alloy”, Modern Developments in
P/M, Proc. Int. Con, MPIF, Orlando, 1988, vol. 21, pp.23-39.
30. B. Lindqvist, “Influence of Microstructure and Porosity on Fatigue Properties of Sintered Steels”,
Metal Powder Report, 1989, vol.44, pp.443-448.
31. F. Chagnon, C. Gelinas and Y. Trudel, “Development of High Density Materials for P/M
Applications”, Advances in P/M & Particulate Materials, Compiled by C. Lall and A.J. Neupaver,
Metal Powder Industries Federation, Princeton, NJ, vol.3, 1994, pp.199-206.
32. K.D. Christian and R.M. German, “Relation Between Pore Structure and Fatigue Behavior in Sintered
Iron-Copper-Carbon”, Int. J. Powder Metallurgy, vol.31, No.1, 1995, pp 51-61.
33. H.G. Rutz and F.G. Hanejko, “High Density Processing of High Performance Ferrous Materials”, Int.
J. Powder Metallurgy, vol.31, no.1, 1995, pp.9-17.
34. F. Chagnon and Y. Trudel, “Effect of Density on Mechanical Properties of Sinter-Hardened
Materials”, Advances in P/M & Particulate Materials, Compiled by J.H. Reinshagen and J.J. Oakes,
Metal Powder Industries Federation, Princeton, NJ, vol.3, 1998, pp 12-119/12-125
35. A. Piotrowski and G. Biallas, “Influence of Sintering Temperature on Pore Morphology,
Microstructure and Fatigue Behavior of Mo-Ni-Cu Alloyed Sintered Steel”, Powder Metallurgy,
vol.41, no.2, 1998, pp.109-114.
36. H.G. Rutz, A.H. Graham and A.B. Davala, “Sinter-Hardening P/M Steels”, Advances in P/M &
Particulate Materials, Compiled by R.A. McKotch and R. Webb, Metal Powder Industries Federation,
Princeton, NJ, vol.1, 1997, pp 8-3/8-20.
37. G. L’Esperance, S. Harton, A. de Rege and S. Nigarura, “Evaluation of the Hardenability,
Microstructure and Properties of Various Sinter Hardening Alloys”. Advances in Powder Metallurgy
and Particulate Materials, Compiled by M. Philips and J. Porter, Metal Powder Industries Federation,
Princeton, NJ, 1995, part 8, pp.8-3/8-18
19
38. G. L’Esperance, E. Dushesne and A. de Rege, “Effect of Materials and Process Parameters on the
Microstructure and Properties of Sinter Hardening Alloys”. Advances in Powder Metallurgy and
Particulate Materials, Compiled by T.M. Cadle and K.S. Narasimhan, Metal Powder Industries
Federation, Princeton, NJ, 1996, part 11, pp.11-397/11-413
39. E. Dushesne, G. L’Esperance and A. de Rege, “Sinter-Hardening and Hardenability”, Int. J. Powder
Metallurgy, vol.36, no.1, 2000, pp.49-60.
40. C. Skena, T. Prucher and j.M. Jo, “Hardenability Characteristics of P/M Alloy Steels”, Int. J. Powder
Metallurgy, vol.33, no. 7, 1997, pp.25-35.
41. MPIF Standard 10, “Preparing and Evaluating Tensile Specimens of Powder Metallurgy Materials”,
Standard Test Methods for Metal Powders and Powder Metallurgy Products, Metal Powder Industries
Federation, Princeton, NJ, 2000.
42. MPIF Standard 40, “Determination of Impact Energy of Unnotched Powder Metallurgy Test
Specimens”, Standard Test Methods for Metal Powders and Powder Metallurgy Products, Metal
Powder Industries Federation, Princeton, NJ, 2000.
43. MPIF Standard 42, “Determination of Density of Compacted or Sintered Metal Powder Products”,
Standard Test Methods for Metal Powders and Powder Metallurgy Products, Metal Powder Industries
Federation, Princeton, NJ, 2000.
44. R.T. DeHoff and E.H. Aigeltinger, “Experimental Quantitative Microscopy with Special Application
to Sintering”, Perspectives in Powder Metallurgy, vol.5, Advanced Experimental Techniques in
Powder Metallurgy, Plenum Press, New York, 1970, pp. 81-137.
45. S. Saritas, R.D. Doherty and A. Lawley, “Effect of Porosity on the Hardenability of P/M Steels”,
PM2TEC 2001, MPIF, New Orleans, LA, May 13-17, 2001.
20