Effect of Cooling Rates During Sinter-Hardening
George Fillari, Robert Causton, Alan Lawley
Hoeganaes Corporation
Cinnaminson, NJ 08077
Presented at PM2TEC 2003, Las Vegas, NV
ABSTRACT
Sinter-hardening is becoming a more widely used process for the production of high strength P/M parts.
The ability to produce martensite directly from the sintering furnace enables the process to produce parts,
with properties close to those of quenched and tempered steels, more efficiently by omitting a separate
heat treatment operation. The success of sinter-hardening depends upon the ability to produce
microstructures of high martensite content consistently during accelerated cooling after sintering. This
paper will examine and illustrate the effects of changes in cooling rate from sintering temperature upon
the microstructure, hardness and properties of a hybrid P/M steel. It will show how comparison of
cooling curves of instrumented Jominy and sintering furnace can be used to improve the sinter-hardening
process.
INTRODUCTION
Sinter-hardening is a process in which the cooling rate in the cooling zone of a sintering furnace is high
enough that a significant portion of the steel is transformed to martensite. Interest in sinter-hardening has
increased because it offers manufacturing economy by providing a one-step process that increases the
strength, toughness, and hardness of steels [1,2].
The properties of steels are dependent on microstructure and it is important to understand how the
microstructure develops in a P/M part during heat treatment. Differing microstructures can be obtained
by varying the cooling rate. Figure 1 illustrates a continuous cooling transformation (CCT) curve for an
eutectoid steel onto which cooling curves have been superimposed at four specific distances along the
Jominy bar. The corresponding microstructures are also included. [3] The conventional cooling rate (D)
will result in complete transformation of austenite to coarse pearlite. At a higher cooling rate such as air
cooling (C), a fine pearlitic microstructure develops. Accelerated cooling (B) usually results in a mixture
of martensite and fine pearlite. When the cooling rate (A) is high enough to avoid the nose of the CCT
curve (A), martensite forms.
The hardenability of steel is a critical property in dictating the microstructure that will be produced upon
cooling. When evaluated using the Jominy End Quench Test, hardenability is defined as the capacity of
the steel to harden “in depth” under a given set of heat treatment conditions. When evaluated using
microstructure as a monitor, hardenability is defined as the capacity of the steel to transform partially or
completely from austenite to martensite at a given depth, when cooled under known conditions [4].
Figure 1. Correlation of hardenability and continuous cooling rate for an iron-carbon alloy of eutectoid
composition [3]
Alloying elements are also used in P/M steels to promote hardenability and to increase strength.
Elements such as molybdenum, copper, and nickel shift the nose of the transformation curve to the right,
thus allowing the phase transformation to occur at lower cooling rates. For this study the level of the
alloying elements was held constant except for carbon content.
TEST PROGRAM
The test program was intended to investigate how cooling rates and different levels of graphite affect
microstructure, hardenability and properties of a hybrid P/M steel. This program was divided into three
parts:
1. To investigate the effect of graphite upon hardenability for a specific hybrid P/M alloy.
2. To control the cooling rates within a sinter-hardening furnace and using these cooling rates to predict
desirable microstructure and properties.
3. To examine the effects cooling rates on microstructure and mechanical properties.
Alloy Test Matrix
The level of alloying elements was held constant so that only the change in graphite content could affect
the resulting properties. The base powder was water atomized and pre-alloyed with 1.50 w/o
molybdenum with a mass median particle size (dm) of 66 µm. The 150HP is premixed with 2.0 w/o
copper and 0.75 w/o Lonza Acrawax-C. Two sets of mixes were made, one set containing lubricant for
the mechanical test pieces. (uniaxially pressed) The other set of premixes, without lubricant, was used for
the Jominy bars. (cold isostatic pressed) All materials are listed in Table I.
TABLE I – Alloy Test Matrix
BASE POW DER
PREMIX
1
ANCORSTEEL 150HP
2
ANCORSTEEL 150HP
3
ANCORSTEEL 150HP
4
ANCORSTEEL 150HP
PREMIX ADDITION
Mo w/o Cu w/o C w/o
1.5
2.0
0.25
1.5
2.0
0.50
1.5
2.0
0.75
1.5
2.0
1.00
TEST SPECIMENS
Instrumented Jominy Bars
Jominy samples were compacted from each premix for a total of 4 samples. The bars were compacted by
cold isostatic pressing (CIP) at a pressure of 345 MPa (50,000 psi). The bars were pressed oversize and
machined to size after sintering. Bulk density measurements on the Jominy bars were made in both the
green and the sintered condition. The Jominy bars were sintered, austenatized and quenched according to
ASTM – A255.[5] Figure 2 illustrates the instrumented Jominy bar.
To record the cooling rates of the Jominy samples, four K-type thermocouples were used. The
thermocouple wires were insulated in a ceramic fiber and placed inside an inconel sheath. Data from the
thermocouples was recorded by a 16 channel RUSTRAK data-logger, with a receiver capacity of 100 ms
intervals. To mount the thermocouples four holes were drilled radially into the specimen at distances of
5, 25, and 45, 65 mm (0.2, 1.0, 1.8, and 2.6 in., respectively) from the water quenched end. [6] Hardness
measurements were made on diametrically opposite ground flats as a function of distance from the
quenched end. Jominy bars were sectioned at the thermocouple locations for metallography.
Mechanical Properties
Tensile tests (ASTM E-8) were conducted on all four premixes. The samples were compacted at 625
MPa (45 tsi). A single compaction pressure was used because microstructures obtained from the different
cooling rates were used to examine the mechanical properties. Tensile strength, yield strength, and
maximum elongation were obtained from the average of five dog-bone tensile samples. Apparent
hardness measurements were performed on the surface of the tensile bars using a Rockwell hardness
tester. All measurements were conducted using the HRA scale for ease of comparison. Tensile testing
were performed on a 267,000 N (60,000 lb) Tinius Olsen universal testing machine with a cross head
speed of 0.635 mm/min (0.025 in/min) Elongation values were determined by utilizing an extensometer
with a range of 0 - 20 %. The extensometer was attached to the samples up to failure.
Figure 2. Geometry of Jominy specimen and position of thermocouples [6]
Temperature Time Profiles
The furnace that is being used for this study is a continuos belt furnace equipped with a post sintering
cooling system, which is illustrated in Figure 3. It combines radiant and convection cooling to accelerate
cooling rates. High cooling rates in the cooling zone are achieved by circulation of the atmosphere within
the cooling zone through heat exchangers. The cooled gases are then circulated over the sintered parts at
a set velocity. The cooling system uses an A.C. frequency inverter to vary the speed of the motor that
runs the blower. It can be varied from 0-60 Hz. At 30 Hz the blower is running at half of its capacity,
corresponding to 50% cooling capacity. Three green compressibility cylinder samples were compacted
from each premix. The cylinder pieces were of 2.54 cm. dia x 3.175 cm long (1 in. x 1.25 in) and
weighed approximately 100g. The cylinders were pressed to density to correspond to the Jominy density
shown in Table II.
Table II. Green and Sintered Densities for Jominy and Profile Samples.
PREMIX
1
2
3
4
JOMINY
Gr
Sint
Density
Density
(g/cm3)
(g/cm3)
6.85
6.89
6.91
7.03
6.81
6.92
6.84
6.90
PROFILE
Sint
Density
(g/cm3)
6.90
6.95
6.90
6.95
Delube
Sintering Zone
Accelerated Cooling
Zone
Slow cool
Figure 3. Diagram of continuos belt furnace equipped with a post sintering cooling unit.
A temperature profile was performed for each of the sintering condition. Each profile contained one
cylinder test piece from each premix to monitor the cooling rate. The test pieces were placed on the belt
with routine production parts to maintain normal belt loading conditions. The temperature profiles were
measured by inserting a K-type thermocouple into the center of a test piece. To mount the thermocouple
one hole was drilled into the bottom of the specimen at distances of 1.5 cm (0.59 in) from the top surface.
The data from the thermocouples were recorded by the RUSTRAK data-logger.
Initial experiments have shown when the sintered parts are subjected to different cooling settings, from a
frequency setting of 30 Hz and at 50 Hz, the change in the cooling rate of the sintered part is negligible.
This lack of cooling in the cooling zone is due to the sintered part exiting the hot zone and experiencing a
high amount of radiant cooling before it enters the cooling unit.
For this study the cooling setting on the furnace was held constant at 30 Hz. In order to decrease the
amount of radiant cooling between the sintering and cooling zone, the belt speed was indexed so that the
sintered part would spend minimal time within this section of the furnace. By indexing the furnace, the
sintered part enters the cooling unit at a higher temperature, achieves different cooling rates, and still
maintains the required time for a complete sinter.
Metallographic evaluation was taken at the center of each profile sample. The percentages of phases are
determined by using the point counting method. Point counting is used to obtain the volume fraction of
uniform phases. A large number of points distributed at random on a superimposed grid that is placed
over the structure. The number of points or nodal points, which coincide with the phase, in comparison
with the total number of points, then indicates the surface proportion of the phase and thus its volume
fraction in the structure. If 500 points of a total number of 2500 grid points coincide with phase, the
proportion by volume of this phase is thus 500/2500 = 0.2 or 20 percent. Note there is an estimated +/5% error using the point counting method.
SINTERING
All test pieces were sintered under production conditions at the Hoeganaes R&D facility, Cinnaminson,
NJ. Three sintering cycles were examined in order to evaluate the effect of cooling rate on the resulting
microstructure and mechanical properties. Listed below are the three sintering cycles:
SINTER 1 - Jominy bars, and Tensile (set 1)
Sintering Temperature: 1140 °C (2050 °F)
Atmosphere:
75 v/o H2 / 25 v/o N2
Belt Speed:
7.0 cm/min. (2.75 in/min)
SINTER 2 - Tensile (set 2)
Sintering Temperature: 1140 °C (2050 °F)
Atmosphere:
75 v/o H2 / 25 v/o N2
Belt Speed:
Index 1
SINTER 3 – Tensile (set 3)
Sintering Temperature: 1140 °C (2050 °F)
Atmosphere:
75 v/o H2 / 25 v/o N2
Belt Speed:
Index 2
The sintered specimen were tempered at 204 °C (400 °F) in air for 1hr. prior to testing.
RESULTS
Instrumented Jominy Samples Hardenability
Figure 4 shows the Jominy hardness traces for each of the premixes. 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 using the formula:
F(t +1) ) =
1
N
N
∑A
i +1
t − i +1
where N is the number of periods including the actual value, A is the actual value at distance j, and F is
the resulting value. A period of 3 was used for the data presented. Examination of the hardness profiles
indicates that the samples containing graphite contents ≥ 0.50 w/o were considerably hardenable. A
comparison of how different concentrations of graphite affect hardenability, resulted in hardness profiles
of similar shape but are shifted upward. The hardness of the alloy containing 0.25 w/o graphite, is
approximately 54 HRA (7 HRC) at its quenched end. Increasing the graphite to 0.50 w/o resulted in a
hardness of 59 HRA (18 HRC) Increasing to 0.75 w/o graphite increased its hardness to 68 HRA (35
HRC) at its quenched end. Its hardness remained above 66 HRA (30 HRC) to a distance of 2.54 cm (1.0
in). The hardness alloy containing 1.0 w/o graphite, is approximately 72 HRA (44 HRC) at the quenched
end. Its hardness remained above 66 HRA (30 HRC) to a distance of 2.7 cm (>1.0 in). It is interesting to
note that the samples containing graphite contents ≥ 0.50 w/o resulted in hardness traces that were almost
parallel with each other. And for alloy 4 containing 1.0 w/o graphite, at a distance of 65 mm (2.6 in) the
microstructure results in a martensite content of 34%.
Introducing graphite contents from 0.75 – 1.0 w/o increases the hardness values considerably. Maximum
hardness values of 72 HRA (44 HRC) are achieved, while hardness values above 30 HRC are obtained at
2.7 cm (<1.0 in) from the quenched end. Figures 5 and 6 shows the cooling curves for the instrumented
Jominy sample for alloy 4 along with the resulting microstructure at the quenched end and at the
thermocouple locations.
At the water-quenched end the structure consist of a fine martensite with retained austenite. At the 5mm
(0.2 in) distance the structure is mostly martensite with some formation of some pearlite and retained
austenite. The structure at the 25 mm (1.0 cm) distance is mostly martensite, some pearlite and bainite.
At the 45 and 65 mm (1.8 and 2.6 cm) distance the structures are similar containing less martensite, more
pearlite and some areas of bainite.
58
57
80
70
48
39
20
38
50
40
28
30
18
20
0.23 carbon
0.46 carbon
0.68 carbon
0.91carbon
10
Rockwell C
Rockwell A
60
8
0
-2
0
10
20
30
40
50
60
Distance from quenched end (mm)
70
Figure 4. Effect of graphite content upon Jominy hardness. 150HP + 2w/o Cu + 1.0 w/o Gr
Temp °C
1000
5 mm
25 mm
45 mm
65 mm
900
800
700
Temp °F
1832
1632
1432
1232
600
1032
500
832
400
632
300
200
432
100
232
0
32
0
200
400
600
800
1000
1200
time (sec)
Figure 5. Cooling curves for alloy 4, thermocouple locations at the 5mm, 25mm, 45mm, and 65mm (0.2,
1.0, 1.8, and 2.6 in., respectively) from the water quenched end.
2% Nital / 4%
Picral
40 µm
a) Quenched end
2% Nital / 4%
Picral
40 µm
d) 45 mm (1.8 in) from quenched end
40 µm
b) 5mm (0.2 in) from quenched end
2% Nital / 4%
Picral
2% Nital / 4%
Picral
2% Nital / 4%
Picral
40 µm
e) 65 mm (2.6 in) from the quenched end
40 µm
c) 25mm (1.0 in) from quenched end
Figure 6a-e. Microstructures illustrated at thermocouple locations at the 5 mm, 25 mm, 45 mm, and 65
mm (0.2, 1.0, 1.8, and 2.6 in., respectively) from the water quenched end. - 500X
It is interesting to note in Figure 5, the cooling curves for the thermocouples located at the 45 and 65 mm
(1.8 and 2.6 in) distance. At about 500 – 550 °C (930 – 1020 °F) there is an inflection that occurs. This
inflection becomes more apparent as the cooling rate is decreased. Metallographic evaluation confirms
that the inflection results from pearlitic or bainitic transformation taking place.
Mechanical Property
The tensile property data collected for all four compositions are presented in Table III. As expected
increasing the graphite content of the material resulted in an increase in strength. Figures 7 and 8
illustrate tensile strength and yield strength as a function of cooling rate and graphite content. As the
graphite content is increased, martensitic transformation increases and it strengthens and hardens the
martensitic microstructure. Beyond this increase at 0.75 w/o graphite, there is a decrease in strength.
Such decreases in strength are frequently observed by the formation of retained austenite. Cooling rate
also had an effect on the properties. For the alloy containing 0.75 w/o graphite, increasing the cooling
rate from 29 °C/min (85 °F/min) to 38 °C/min (100 °F/min) resulted in a 10 % increase in tensile strength
and a 6 % increase in yield strength. Increasing the rate to 71 °C/min (160 °F/min) resulted in a 15 %
increase in tensile strength and a 20 % increase in yield strength.
160
1000
140
800
100
600
80
60
UTS (MPa)
UTS (10 3 psi)
120
400
40
29 °C/min (85 °F/min)
38 °C/min (100 °F/min)
71 °C/min (160 °F/min)
20
0
0.10
200
0
0.30
0.50
0.70
Sintered Carbon
0.90
Figure 7. Tensile strength as a function of sintered carbon level and cooling rate.
160
1000
140
800
100
600
80
60
400
40
29 °C/min (85 °F/min)
20
38 °C/min (100 °F/min)
200
71 °C/min (160 °F/min)
0
0.10
0
0.30
0.50
0.70
Sintered Carbon
0.90
Figure 8. Yield strength as a function of sintered carbon level and cooling rate.
Yield (MPa)
Yield (103 psi)
120
Table III. Tensile Properties Sintered at 1140 °C (2050 °F)
Tempered in Air at 205 °C (400 °F)
w/o
Gr.
0.25
0.50
0.75
1.00
w/o
Gr.
0.25
0.50
0.75
1.00
w/o
Gr.
0.25
0.50
0.75
1.00
29 °C/min (85 °F/min)
Sintered
0.20%
Density
UTS
Offset
(g/cc) MPa / 103 psi MPa / 103 psi
7.00
558 / 80
412 / 59
7.00
747 / 107
558 / 80
7.00
879 / 126
747 / 107
7.00
907 / 130
845 / 121
38 °C/min (100 °F/min)
Sintered
0.20%
Density
UTS
Offset
(g/cc) MPa / 103 psi MPa / 103 psi
7.00
593 / 85
433 / 62
7.00
921 / 132
663 / 95
7.00
977 / 140
796 / 114
7.00
831 / 119
712 / 102
71 °C/min (160 °F/min)
Sintered
0.20%
Density
UTS
Offset
3
(g/cc) MPa / 10 psi MPa / 103 psi
7.00
614 / 88
468 / 67
7.00
873 / 125
691 / 99
7.00
1005 / 144
935 / 134
7.00
768 / 110
684 / 98
El
%
2.4
2.1
1.1
1.0
HR
A
48
57
65
71
El
%
2.5
1.9
1.2
1.0
HR
A
50
60
66
65
El
%
2.0
1.5
1.1
0.7
HR
A
51
61
69
69
Furnace Profile – Standard Cooling Settings
Temperature profiles were performed for each sintering cycle. Cylindrical samples 2.54 cm. dia x 3.12
cm long (1.0 in. x 1.25 in) from alloy 4 were evaluated for metallography. Initial experiments have shown
when the sintered parts are subjected to different cooling settings, from a frequency setting of 30 Hz and
at 50 Hz, the change in the cooling rate of the sintered part is negligible. This lack of cooling in the
cooling zone is due to the sintered part exiting the hot zone and experiencing a high amount of radiant
cooling before it enters the cooling unit. Figure 9. illustrates three different cooling profiles for different
cooling settings with a belt speed of 7.0 cm/min. (2.75 in/min). Profiles were performed at 1120 °C (2050
°F). In all cases the sintered part enters the accelerated cooling unit at a temperature around 600 °C (1100
°F). For the standard sinter, the part experiences a cooling rate of 9 °C/min. (48 °F/min). When the part is
subjected to a cooling setting of 30 Hz, (50 % cooling capacity) its cooling rate is increased to 29 °C/min
(85 °F/min). At a cooling setting of 50 Hz, (100 % cooling capacity) its cooling rate remains unchanged.
Note that all cooling rates were calculated from 980 °C to 427 °C (1600 °F – 800 °F).
1400
1200
2000
1000
1500
800
Cooling Zone
Start
600
1000
Temperature (°F)
Temperature (°C)
2500
0Hz
30Hz
50Hz
400
500
200
Hot Zone
End
0
0
90
100
110
120
time (min)
130
140
Figure 9. Initial cooling profiles: 0 Hz – standard sinter - 9°C/min. (48 °F/min), 30 Hz – 50 % cooling
capacity - 29 °C/min. (85 °F/min), 50 Hz – 100% cooling capacity - 29 °C/min. (85 °F/min)
Furnace Profile – Indexed Settings
Figures 10 – 12 illustrates the temperature profile for each sintering cycle along with the corresponding
microstructures. The first sinter cycle produced a cooling rate of 29 °C/min (85 °F/min). The structure
consists of 26 % bainite, 26 % martensite and 2 % divorced pearlite. The second sintering cycle (index
1), a cooling rate of 38 °C/min (100 °F/min) resulted in a structure of 49 % bainite, 42 % martensite and
4 % divorced pearlite. At the highest cooling rate (index 2) of 71 °C/min (160 °F/min) resulted in a
structure of 47 % martensite, 39 % bainite and 15 % divorced pearlite. Table IV lists the percentage of
structures with pores and metal only for alloy 4.
Table IV. Percentages of Phases Produced From Varying Cooling Rates:
Alloy 4 - 150HP + 2.0 w/o Cu + 1.0 w/o Gr
(metal only / with pores)
Cooling Rate Martensite Bainite
Div.Pearlite
(°C/°F)/min
%
%
%
29 / 85
26 / 22
69 / 58
2.1 / 1.9
38 / 100
42 / 36
49 / 42
9.6 / 8.0
71 / 160
47 / 41
39 / 34
15 / 13
2500
1200
1000
1500
800
600
1000
400
Temperature (°C)
Temperature (°F)
2000
500
200
0
2% Nital /
4% Picral
0
0
20
40
60
80
100
120
40 µm
Time (min)
Figure 10. Temperature profile for sintering cycle 1: 150HP + 0.25 w/o Cu + 1.0 w/o Gr – cooling rate
of 29 °C/min. (85 °F/min) Microstructure 200X
2500
1200
1000
1500
800
600
1000
400
500
Temperature (°C)
Temperature (°F)
2000
200
0
2% Nital /
4% Picral
0
0
20
40
60
80
40 µm
Time (min)
Figure 11. Temperature profile for sintering cycle 2: 150HP + 0.25 w/o Cu + 1.0 w/o Gr – cooling rate
of 38 °C/min. (100 °F/min) Microstructure 200X
2500
1200
1000
1500
800
600
1000
400
Temperature (°C)
Temperature (°F)
2000
500
200
0
0
25
50
75
Time (min)
100
0
125
2% Nital /
4% Picral
40 µm
Figure 12. Temperature profile for sintering cycle 3: 150HP + 0.25 w/o Cu + 1.0 w/o Gr – cooling rate
of 71 °C/min. (160 °F/min) Microstructure 200X
It is interesting to note that when comparing the temperature profile samples to the instrumented Jominy
at the 65 mm (2.7 in) section, the cooling rate for the 2nd sinter (71 °C/min. (160 °F/min) and the Jominy
(66 °C/min. (150 °F/min) are close to each other. Figure 13 illustrates the cooling rates from the profile
samples to the instrumented Jominy. When comparing the percentages of martensite of the two samples
proved to be similar, the profiled sample contained 47 % whereas the Jominy contained 35%. Note there
is a +/- 5% error for this point count evaluation.
1800
1st Sinter: 29 °C/min (85 °F/min)
2nd Sinter: 38 °C/min (100
°F/min)
3rd Sinter: 71 °C/min (160
°F/min)
Instrumented Jominy 65mm: 66
°C/min (150 °F/min)
Temperature (°F)
1400
1200
900
800
700
600
1000
500
800
400
600
300
400
200
200
100
0
Temperature (°C)
1600
0
0
5
10
Time (min)
15
20
Figure 13. Temperature cooling profiles for the three sintering conditions along with the instrumented
Jominy sample at the 65 mm (2.7 in) section.
Prior to indexing the furnace the maximum percentage of martensite for alloy 4 (150HP + 0.25 w/o Cu +
1.0 w/o Gr) was 26%. By indexing, the cooling rate was increased and martensite content increased to
47%. This correlation between cooling rates and resulting microstructure proves that sinter-hardening is a
process that depends not only on alloy composition alone, but controlling the cooling rate of the sintering
furnace and part plays a major role in the process. By using instrumented Jominy end quench tests
coupled with controlling the cooling rates of sintered part, it is possible to understand the microstructures
observed in parts, then modify the process to improve martensite transformation.
CONCLUSION
Examination of the hardness profiles indicates that the samples containing graphite contents ≥ 0.50 w/o
were considerably hardenable. For the alloy containing 1.0 w/o graphite resulted in a martensite content
of 35% at the 65 mm (2.7 in) section. Jominy samples containing graphite contents ≥ 0.50 w/o resulted in
hardness traces that were almost parallel with each other. A comparison of how different concentrations
of graphite affect hardenability resulted in hardness profiles of similar shape but shifted upward. The
alloy containing 0.25 w/o graphite has a relatively low hardenability. Introducing graphite contents from
0.50 – 1.0 w/o increases the hardness values considerably. Maximum hardness values of 72 HRA (44
HRC) are achieved, while hardness values above 30 HRC are obtained at 2.7 cm (<1.0 in) from the
quenched end.
Graphite additions were shown to influence the properties of 150HP. As the graphite content is
increased, it increased the martensite transformation and also strengthened the microstructure. The
tensile strength increases but then peaks at 0.75 w/o graphite. Beyond this increase of 0.75 w/o graphite,
there is a decrease in strength due to the lower Ms temperatures and the formation of retained austenite
along grain boundaries. The optimum amount of graphite was to be 0.75 w/o, 0.68 sintered carbon.
Controlling microstructure by controlling cooling rates from sintering temperature will produce
martensitic transformation and increase sintered strength. Prior to indexing the furnace, maximum
cooling rate within the cooling zone were 29 °C/min (85 °F/min). By indexing the furnace, the amount of
radiant cooling is decreased and cooling rates up to 71 °C/min (160 °F/min) were achieved. Increasing
the cooling rate had a significant impact on increasing the tensile properties. For the alloy containing
0.75 w/o graphite, increasing the cooling rate resulted in a 15 % increase in tensile strength and a 20 %
increase in yield strength.
In terms of microstructure, different percentages of martensite resulted from controlling the cooling rates.
For the alloy containing 1.0 w/o graphite a cooling rate of 29 °C/min (85 °F/min) resulted in a structure
of 26 % martensite. Increasing to 38 °C/min (100 °F/min) resulted in a structure of 42 % martensite. At
the highest cooling rate of 71 °C/min (160 °F/min) resulted in a structure of 47 % martensite.
ACKNOWLEGEMENTS
The author wishes to acknowledge Gerald Golin for his metallographic work and William Bentcliff for
his assistance in specimen preparation and data collection.
REFERENCES
1. Grange, M. and Trudel, Y., Effects of Post-Sintering Cooling On the Properties of Low Alloy
Sintered Materials”, Advances in Powder Metallurgy – 1991, Vol.4, p.115, Compiled by L.F.
Pease, R.J. Sonsoucy. Metal Powder Industries Federation, Princeton, NJ.
2. Causton, R.J. and Fulmer, J.J., “Sinter-Hardening Low-Alloy Steels”, Advances in Powder
Metallurgy – 1992, Vol.5, p.17, Compiled by J.M. Capus, R.M German. Metal Powder Industries
Federation, Princeton, NJ.
3. Atlas of Isothermal Transformation and Cooling Transformation Diagrams, American Society for
Metals, Metals Park, OH, 1977, p.376
4. Llewellyn, D. T., and Hudd, R, C., Steels: Metallurgy and Applications, 3rd Edition, Reed
Educational and Professional Publishing Ltd, 1998, p.207
5. “Standard Test Method for End-Quench Test For Hardenability of Steel (A255-96).” ASTM
Book of Standards-1997, Vol.301, p. 25-28, American Society for Testing and Materials, West
Conshohocken, Pa, 1997.
6. Saritas, S., Doherty, R. D., and Lawley, A. “Effect of Porosity on The Hardenability of P/M
Steels”, International Journal of Powder Metallurgy, 2002, vol.38, No.1, p.31