Effect of Small Additions of Boron on the
Mechanical Properties and Hardenability of Sintered P/M Steels
Michael Marucci*, Alan Lawley**, Robert Causton*, and Suleyman Saritas***
*
Hoeganaes Corporation, Cinnaminson, NJ 08077
Department of Materials Engineering, Drexel University, Philadelphia, PA 19104
***
Department of Mechanical Engineering, Gazi University, Maltepe/Ankara, 06570, Turkey
**
Abstract:
Low levels of boron (0.01-0.15w/o) may induce sufficient hardenability and
strength in powder metallurgy steels to permit a decrease in the level of the alloying
elements, increase powder compressibility and reduce the as-sintered hardness. These
lean alloys may be sufficiently ductile to coin and be hardened by subsequent heattreatment. The goal of this study was to identify the boron level in FLN2-4400 (Fe +
0.85w/oMo, 2.0w/oNi, 0.3w/oC) which yields the optimal combination of strength,
ductility, and hardenability. Tensile, transverse rupture, hardness, and Jominy end
quench tests were performed on this alloy with six different levels of boron added
Sintered strength and ductility increase up to 0.05w/oB, but decrease beyond this level,
even though sintered density increases significantly. Jominy hardness traces show that
the hardenability is not increased substantially until the concentration of boron reaches
0.05w/o. The microstructures of the Jominy bars show that with an increase in boron
level, the depth to which martensite is retained increases, but that grain boundary
segregation occurs. A level of boron ~ 0.05w/o gives the optimum combination of
strength, ductility, and hardenability in FLN2-4400.
Introduction:
Ferrous P/M has been a major metalforming technology for more than 40 years
during which time data have been developed to identify mechanical properties as a
function of sintered density. Heat-treatment response, in particular hardenability, has not
been studied extensively and data have been "borrowed" from the literature on wrought
steels. These data are useful, but only as a first approximation, since sintered P/M parts
contain pores. These pores affect the thermal properties of the steel with an attendant
effect on hardenability.
Hardenability is a measure of how much martensite is formed at a given cooling
rate in a steel. The hardenability of a steel is defined as the maximum (critical) diameter
of a cylinder that has a microstructure that is 50% at its center after quenching from the
austinitizing temperature1. Thus, a steel with a larger critical diameter has a higher
hardenability.
The work of Saroop2 and Fuhrer3 has shown that additions of boron to
Ancorsteel® 85HP improve the degree of sinter and hardenability. It is not clear,
however, which effect (sintering or hardenability) dominates at different boron levels. In
wrought steels there is evidence that at very low (trace) levels, boron is a potent element
with respect to hardenability, and at higher levels (0.10w/o) it is a sintering aid because a
liquid phase, Fe2B, forms. It is important to understand what is taking place
metallurgically at specific concentrations of boron, so that these alloys can be tailored for
specific applications.
There is a need for a powder metallurgy (P/M) alloy for machine parts that can be
coined following sintering to improve surface characteristics and which can be heattreated subsequently resulting in a wear resistant part that possesses high strength.
Current P/M steels that are sufficiently hardenable contain ferrite hardeners such as Cr
and Cu, which makes the steel too hard and brittle to coin. Removal of these ferrite
hardeners results in higher compressibility, reduced tool wear and improved green
density. Previous work by Saroop2, Fuhrer3, and Causton4 has shown that small levels of
boron increase hardenability, without the use of large quantities of carbon and/or nickel.
These hardenable lean P/M alloy steels are relatively soft (~ 80 HRB) and can be coined
in the as-sintered state. Previous hardenability data for similar P/M grades of steel were
obtained by sintering in a laboratory furnace with a slow attendant cooling rate2. The
present study utilized a furnace with a cooling rate comparable to that in production
furnaces with water-jacketed cooling zones. The goal of this study was to identify the
boron content that results in optimum hardenability and mechanical properties.
Experimental Procedure:
Alloy Test Matrix
The test matrix, shown in Table I, was established to explore the effects of boron
additions to a common P/M alloy. The test matrix holds all alloying elements constant
except for boron. Thus only the change in boron content should affect the resulting
properties.
Table I: Alloy Test Matrix
Base Powder
1
2
3
4
5
6
*
Ancorsteel 85HP
Ancorsteel 85HP
Ancorsteel 85HP
Ancorsteel 85HP
Ancorsteel 85HP
Ancorsteel 85HP
Mo (w/o)
0.85
0.85
0.85
0.85
0.85
0.85
*
Ni (w/o)
2.00
2.00
2.00
2.00
2.00
2.00
Admixed
C (w/o) (Graphite)*
0.30
0.30
0.30
0.30
0.30
0.30
B (w/o)*
0.00
0.01
0.03
0.05
0.10
0.15
nominal composition
The alloyed powders were premixed using Hoeganaes’ water atomized Ancorsteel
85HP (dm = 95µm) as the base powder. Nickel and graphite were admixed using Inco
123 (3-13 µm) and Asbury Graphite 3201 (90 w/o <10µm), respectively. Boron was
added as a gas atomized ferroalloy containing 12.0w/oB (dm = 18µm). Without boron,
the mixes are referred to as FLN2-4400 throughout this paper. Two sets of mixes were
made, one set with 0.75w/o Acrawax C lubricant for uniaxial compaction into
mechanical test pieces, and one set without lubricant for the cold isostatic pressing (CIP)
of Jominy bars.
Compaction
The mechanical test samples were uniaxially compacted at 619 MPa (45 tsi) to
give a green density of 7.12 g/cm3 ±0.05 in a rigid die. Jominy bars were compacted by
cold isostatic pressing (CIP) at 414 MPa (60,000 psi) to a green density of 6.9 g/cm3
±0.1 in a cylindrical rubber mold.
Sintering
All test pieces were sintered in a high temperature Hayes furnace at the
Hoeganaes’ R&D facility, Cinnaminson, NJ. This is a pusher-type furnace with a
cooling capacity from >1100°C (>2000°F) to room temperature in 40 min in a pure H2
atmosphere. Sintering was done at 1120°C (2050°C), 1175°C (2150°F), and 1230°C
(2250°C). These temperatures were chosen because they fall below, at, and above the
Fe-Fe2B eutectic temperature of approximately 1175°C (2150°F). All sintering was
performed in pure H2 to avoid nitriding caused by sintering atmospheres that contain a
mixture of hydrogen and nitrogen. The samples were sintered for 30 min at temperature
(45 min total time, laboratory standard).
Mechanical Testing
Tensile and transverse rupture (TRS) tests were conducted on all samples for all
sintering temperatures. Five tensile and five transverse rupture specimens were tested for
each condition for a total of 90 tensile and 90 TRS pieces. The properties obtained from
the tensile and TRS specimens include: green and sintered density, dimensional change,
transverse rupture, yield strength, tensile strength, elongation, and hardness (HRA).
Hardenability
Sintered Jominy bars were austinitized at 870°C (1600°F) for 30 min., and
quenched according to ASTM Standard A2555. Hardness traces (HRA) were performed
on the quenched bars. Density measurements on the Jominy bars were made in both the
green and the as-sintered conditions. The microstructures of the end-quenched Jominy
samples were characterized on a representative number of samples.
Chemical Analysis
Chemical analysis was performed on the sintered P/M steels to characterize the
level/uniformity of carbon, sulfur, nitrogen, oxygen, and boron. Carbon and sulfur were
measured by infrared oxide detection. Nitrogen and oxygen were determined by enthalpy
measurements and CO2 detection, respectively. Boron was measured by inductively
coupled plasma-mass spectroscopy (ICP/MS). Chemical analysis was also performed on
the Jominy bars following heat-treatment.
Results and Discussion:
Mechanical Testing
Green density was in the range of 7.11-7.13 g/cm3 for all alloys examined. Figure
1 shows how sintered density changes as the boron level is increased; sintered density
increases as the concentration of boron increases, and as the sintering temperature
increases. This is expected because the liquid Fe2B phase forms, wetting the grain
boundaries and filling the pores. Figure 2 shows representative optical micrographs of
samples with 0.00w/oB and 0.10w/oB, respectively sintered at the highest temperature.
The boron-free sample has pores that are completely empty. In contrast, in the boxed area
of the 0.10w/oB sample, pores that have been filled with liquid are evident.
Sintered Density
(g/cm3)
7.45
7.40
1120°C (2050°F)
7.35
1175°C (2150°F)
7.30
1230°C (2250°F)
7.25
7.20
7.15
7.10
7.05
7.00
0.00
0.01
0.03
0.05
0.10
0.15
w/oB
Figure 1: Sintered density of FLN2-4400 + boron
(a.)
(b.)
Figure 2: (a.) Microstructure of FLN2-4400 + 0.00 w/oB, (b.) FLN2-4400 + 0.10w/oB
both sintered at 1230°C (2250°F), optical micrograph (2% nital/4% picral
etch).
Figures 3 and 4 display the tensile strength (TS) and elongation respectively, of
the base alloy for all the boron levels evaluated. Strength and elongation increase but
then plateau at about 0.05w/oB. At higher concentrations of boron, strength decreases;
this occurs at 1120°C (2050°F) and 1230°C (2250°F). At 1175°C (2150°F), the Fe-Fe2B
TS
(ksi)
TS
(103 psi)
800
116
1120°C (2050°F)
1175°C (2150°F)
1230°C (2250°F)
750
109
700
102
650
94
600
87
550
80
500
73
450
65
400
58
0.00
0.01
0.30
0.05
0.10
0.15
w/oB
eutectic temperature, the strength continues to increase but elongation falls dramatically
at 0.10w/oB. This fall in elongation is attributed to segregation of boron to the grain
boundaries, which causes embrittlement of the steel.
Figure 3: Tensile Strength (TS) of FLN2-4400 as a function of sintering temperature and
boron content.
Figure 4: Elongation to failure of FLN2-4400 as a function of boron content and
sintering temperature.
Reviewing Figure 3, the tensile strength of the samples with 0.10 and 0.15w/oB
sintered at 1175°C (2150°F) is much higher than that of the samples sintered at the other
sintering temperatures. This is also reflected in the hardness levels. The test pieces
sintered at 1175°C (2150°F) have Rockwell hardness values (HRA) of 62 and 60 whereas
the pieces sintered at the other sintering temperatures have values around 50 HRA.
However, the sintered carbon level is consistent for all conditions. This indicates that the
test specimens sintered at 1175°C (2150°F) were subjected to a different cooling rate
than the specimens sintered at 1120°C (2050°F) and 1230°C (2250°F). This anomaly is
attributed to breakage of the ceramic sintering boat in the cooling zone of the sintering
furnace resulting in a faster cooling rate.
Elongation
(%)
5.0
1120°C (2050°F)
1175°C (2150°F)
1230°C (2250°F)
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
0.00
0.01
0.03
0.05
w/oB
0.10
0.15
Figure 4 shows elongation values as a function of nominal boron level and
sintering temperature. The elongation of the samples with 0.10 and 0.15w/oB sintered at
1120°C (2050°F) is > 4.0%. This is three times the elongation of the samples with the
same concentration of boron sintered at the higher temperatures. Comparison of Figure
5(a) and 5(b) explains what is happening. Both micrographs are for the same level of
boron, however the microstructures are different. The sample sintered at 1120°C
(2050°F) consists of ferrite with dispersed carbides and an average grain size of 25 µm.
The sample sintered at 1230°C (2250°F) consists of coarse divorced pearlite with an
average grain size of 50 µm and with Fe2B at the grain boundaries. The ferritic
microstructure resulting from the lower sintering temperature has lower strength because
of the absence of well-defined pearlite. It has a higher elongation because of the small
grain size. Strength and elongation at the higher sintering temperature are reduced
because of the presence of coarse pearlite, combined with the larger grain size and grain
boundary segregation, even though the sintered density is higher.
(a.)
(b.)
Figure 5: Microstructure of FLN2-4400 + 0.10 w/oB sintered at (a.) 1120°C (2050°F)
and (b.) sintered at 1230°C (2250°F), optical micrograph (2% nital/4% picral
etch).
Hardenability
Figure 6 shows representative Jominy hardness traces of FLN2-4400 for each
boron concentration at the three sintering temperatures. There is an error of ±2 in
hardness readings and this yields a trace with noise. To decrease the noise level the
traces were plotted using a moving average given by:
(1)
where N is the number of periods including the actual value, A is the actual value at a
distance j, and F is the resulting value. A period of 3 was used for the data presented.
(a.)
HRA
75
70
65
60
55
50
45
40
35
30
25
20
75
70
65
60
55
50
45
40
35
30
25
20
1120°C (2050°F)
1175°C (2150°F)
1230°C (2250°F)
0.0
0.5
1.0
(b.)
HRA
1.5
2.0
1120°C (2050°F)
1175°C (2150°F)
1230°C (2250°F)
0.0
2.5
0.5
HRA
(c.)
75
70
65
60
55
50
45
40
35
30
25
20
70
65
60
55
50
45
40
1120°C (2050°F)
1175°C (2150°F)
1230°C (2250°F)
35
30
25
20
0.5
1.0
1.5
2.0
0.0
2.5
0.5
0.5
1.0
1.0
1.5
Depth (in.)
1.5
2.0
2.5
(f.)
HRA
75
70
65
60
55
50
45
40
35
30
25
20
1120°C (2050°F)
1175°C (2150°F)
1230°C (2250°F)
0.0
2.5
Depth (in.)
(e.)
75
70
65
60
55
50
45
40
35
30
25
20
2.0
1120°C (2050°F)
1175°C (2150°F)
1230°C (2250°F)
Depth (in.)
HRA
1.5
(d.)
HRA
75
0.0
1.0
Depth (in.)
Depth (in.)
2.0
2.5
1120°C (2050°F)
1175°C (2150°F)
1230°C (2250°F)
0.0
0.5
1.0
1.5
2.0
2.5
Depth (in.)
Figure 6: Jominy hardness traces for FLN2-4400 (a.) 0.00w/oB, (b.) 0.01 w/oB, (c.)
0.03w/oB, (d.) 0.05w/oB, (e.) 0.10w/oB, and (f.) 0.15w/oB.
Examination of the traces shows that the sintering temperature does not affect
hardenability except for the samples with 0.10 and 0.15w/o B. The samples sintered at
and above the Fe-Fe2B eutectic temperature [1175°C (2150°F)] exhibit traces that are of
similar shape but are shifted upward on the plot, as seen in Figure 6(e) and 6(f). Thus,
there is an increase in overall hardness. This increase in hardness as sintering
temperature increases can be attributed to the fact that FLN2-4400 with boron levels
greater than 0.10w/o sintered above the Fe-Fe2B eutectic temperature forms a liquid
phase that fills the pores. The resulting increase in sintered density increases the
apparent hardness of the steel.
Figure 7: Jominy hardness traces for FLN2-4400 sintered at 1230°C (2250°F).
HRA
75
70
65
60
55
50
0.00 w/o B
0.01 w/o B
0.03 w/o B
0.05 w/o B
0.10 w/o B
0.15 w/o B
45
40
35
30
0.0
0.5
1.0
1.5
Depth (in.)
Table II: As Sintered Chemistry
0.00w/oB 1120oC (2050oF)
0.00w/oB 1175oC (2150oF)
0.00w/oB 1230oC (2250oF)
0.01w/oB 1120oC (2050oF)
0.01w/oB 1175oC (2150oF)
0.01w/oB 1230oC (2250oF)
0.03w/oB 1120oC (2050oF)
0.03w/oB 1175oC (2150oF)
0.03w/oB 1230oC (2250oF)
0.05w/oB 1120oC (2050oF)
0.05w/oB 1175oC (2150oF)
0.05w/oB 1230oC (2250oF)
0.10w/oB 1120oC (2050oF)
0.10w/oB 1175oC (2150oF)
0.10w/oB 1230oC (2250oF)
0.15w/oB 1120oC (2050oF)
0.15w/oB 1175oC (2150oF)
0.15w/oB 1230oC (2250oF)
Carbon*
Sulfur*
0.28
0.28
0.29
0.29
0.29
0.30
0.31
0.31
0.30
0.31
0.31
0.31
0.30
0.31
0.32
0.30
0.33
0.33
0.005
0.005
0.003
0.004
0.003
0.004
0.004
0.003
0.003
0.006
0.006
0.007
0.006
0.005
0.006
0.006
0.007
0.005
w/o
Oxygen** Nitrogen*** Boron****
0.036
0.030
0.029
0.045
0.043
0.040
0.058
0.044
0.044
0.062
0.060
0.072
0.077
0.100
0.097
0.084
0.085
0.110
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.0001
<0.001
<0.001
<0.001
0.007
0.008
0.008
0.023
0.019
0.020
0.038
0.029
0.036
0.094
0.150
0.104
0.156
0.151
0.158
*Infrared oxide detection, ** CO2 Detection, ***Enthalpy measurement, ****ICP/MS
A comparison of how different concentrations of boron affect hardenability shows
that as the boron level increases, so does hardenability. This can be seen in Figure 7.
However, little change in hardenability is observed in the steel with less than 0.03w/oB at
all sintering temperatures. Steels with boron levels ≥0.05w/o exhibit a significant
increase in hardenability. At both temperatures the hardness does not begin to decrease
up to a distance of 11.1 mm (7/16 in.) from the water-quenched end. In the lower level
boron steels, hardness drops at a distance between 4.0-6.0 mm (3/16-1/4 in.) from the
water-quenched end. The boron concentrations quoted are nominal. Measurement of the
boron concentrations in the sintered condition by inductively coupled plasma-mass
spectroscopy (ICP/MS) confirms that the boron levels are close to the nominal additions
(Table II). This table also gives the C, S, O, and N levels after sintering.
Figures 8-10 show microstructures of the Jominy bars sintered at 1230°C
(2250°F). These micrographs illustrate how increasing boron concentration affects the
microstructure. Figure 8 shows the sample that contains no boron. At the quenched end
there is a mixture of martensite and ferrite forming as grain boundary allotriomorphs
(GBA); there are also light etching Ni-rich regions. At 25.4 mm (1 in) from the waterquenched end, the microstructure consists of pearlite with Ni-rich regions; the grain size
is about 30 µm. The microstructure at 50.8 mm (2 in) from the water-quenched end is the
same as at 25.4 mm (1 in); however, the pearlite is coarser and the grain size is 40 µm.
The pores are irregular with dimensions in the range of 10-15 µm. Figure 9 shows the
microstructure with 0.05w/oB. The water-quenched end consists entirely of lath
martensite. At 25.4 mm (1 in) from the water-quenched end, the microstructure consists
of a combination of bainite, fine divorced pearlite, Ni-rich regions, and small ferrite
GBA’s. The structure at 50.8 mm (2 in) from the water-quenched end is pearlite with
some bainite and Ni-rich regions. The pores at this boron level are rounded with an
average size of about 15-25 µm. Figure 10 shows the microstructure of a steel containing
0.10w/oB. At all distances from the water-quenched end, the structures are the same as
in the 0.05w/oB steel except that segregation of Fe2B at the grain boundaries and in the
pores is evident. The other difference is that the degree of sinter is enhanced
significantly; the pores are rounded and are 25-35 µm.
(a.)
(b.)
(c.)
Figure 8: Microstructure of FLN2-4400 + 0.00 w/oB following Jominy quenching, (a.) at
quenched end, (b.) at 25.4 mm (1.0 in.), (c.) at 50.8 mm (2.0 in.), sintered at
1230°C (2250°F), optical micrograph (2% nital/4% picral etch).
The changes observed in microstructure as the boron level increases show that
boron improves the degree of sinter which improves hardness. Hardenability studies on
wrought steels have confirmed that boron inhibits the onset of ferrite nucleation, allowing
the bainite reaction to take place6. It is reasonable to assume that boron behaves in a
similar manner in sintered steels. In combination, the effect of boron on sintering
response and hardenability produces a harder steel. This results in the development of a
martensitic/banitic microstructure.
(a.)
(b.)
(c.)
Figure 9: Microstructure of FLN2-4400 + 0.05 w/oB following Jominy quenching, (a.) at
quenched end, (b.) at 25.4 mm (1.0 in.), (c.) at 50.8 mm (2.0 in.), sintered at
1230°C (2250°F), optical micrograph (2% nital/4% picral etch).
(a.)
(b.)
(c.)
Figure 10: Microstructure of FLN2-4400 + 0.10 w/oB following Jominy quenching, (a.)
at quenched end, (b.) at 25.4 mm (1.0 in.), (c.) at 50.8 mm (2.0 in.), sintered at
1230°C (2250°F), optical micrograph (2% Nital/4% Picral Etch).
Conclusions:
Boron improves the degree of sinter in the P/M steels evaluated. Above the
eutectic temperature of 1175°C (2150°F) liquid Fe2B forms which fills the pores. Above
0.05w/oB the boron segregates to the grain boundaries where Fe2B forms improving the
degree of sinter. The mechanical properties are improved by the increase in sintered
density; however, once grain boundary segregation occurs the steel becomes weak and
brittle. Boron improves hardenability by inhibiting the nucleation of ferrite at the
austenite grain boundaries; this allows bainite to form, increasing the depth to which the
steel hardens.
The goal of this study was to identify the level of boron that produces the
optimum combination of strength, ductility, and hardenability, utilizing a lean alloy
composition. Strength and elongation decrease beyond 0.05w/oB. This is explained by
Fe2B segregation to the grain boundaries. The hardness traces show that at 0.05w/oB the
hardenability is comparable to, or greater than that of, steels containing 0.10 and
0.15w/oB. Since the strength of FLN2-4400 + 0.05w/oB does not decrease (as it does at
higher boron levels), and this alloy has a markedly improved hardenability than the steels
with a lower boron level, FLN2-4400 steel with a boron level of about 0.05w/o is
optimal.
References:
1. R. E. Reed-Hill, and R. Abbaschian, 1994 Physical Metallurgy Principles. Third
Edition, PWS Publishing Company, Boston, MA.
2. G. Saroop, “Microstructure, Mechanical Properties, and Hardenability of Sintered FeB and Fe-B-C Alloys”, 2000, M.S. Thesis, Drexel University, Philadelphia, PA.
3. J. Fuhrer, “Hardenability of Sintered Fe-Ni-C-B Alloys”, 2000, Senior Design Final
Report, Drexel University, Philadelphia, PA.
4. R. J. Causton, J-S., Oh, and A. Lawley, “Processing Microstructure and Mechanical
Properties of Fe-B and Fe-B-C Alloys”, Advances in Powder Metallurgy and
Particulate Materials – 1999, Compiled by C.L. Rose and M.H. Thibodeau, Metal
Powder Industries Federation, Princeton, NJ, Vol. 2, Part 7, p.3, 1999
5. “Standard Test Method for End-Quench Test for Hardenability of Steel (A255-96).”
ASTM Book of Standards-1997, Vol. 3.01, p. 25, American Society for Testing and
Materials, West Conshohocken, PA, 1997.
6. D.A. Porter, and K.E. Easterling, 1992, Phase Transformations in Metals and Alloys.
Second Edition, Chapman & Hall, London.
Acknowledgments:
The authors are indebted to M. Shiber and D. VonRohr, Drexel University, and G. Golin,
T. Murphy, and P. Meidunas, Hoeganaes Corporation for assistance with specimen
preparation, material characterization, and chemical analysis.