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The Influence of Silicon on the Mechanical Properties and Hardenability of PM Steels

Chris Schade & Tom Murphy

Hoeganaes Corporation

Cinnaminson, NJ 08077

Alan Lawley & Roger Doherty

Drexel University

Philadelphia, PA 19104

ABSTRACT

The effects of silicon additions on the microstructures, mechanical properties and

hardenability of powder metallurgy (PM) alloy systems have been investigated. It has

been demonstrated that the addition of silicon can increase strength in both the sintered

and heat treated conditions. In the sintered condition silicon strengthens the ferrite of the

pearlitic microstructure by solid solution hardening. The heat treated properties of various

silicon containing alloy systems were examined and the hardenability was compared for

various alloys by means of microindentation hardness measurements. The effectiveness

of accelerated cooling during sintering on sinter-hardening response of silicon-containing

alloys was demonstrated. The addition of silicon to high carbon (>0.80 w/o) PM steels

suppresses the formation of grain-boundary carbides which permits the use of carbon

concentrations that are significantly higher than those commonly used for commercial

PM applications.

INTRODUCTION

With the increasing use of high temperature sintering, manufacturers of alloy powders

used in the PM industry are now evaluating the use of elements such as chromium,

manganese, and vanadium in relation to alloy performance [1-3]. The higher sintering

temperatures allow for the reduction of oxides from these elements in atmospheres that

are predominately nitrogen with small amounts of hydrogen (90 v/o nitrogen / 10 v/o

hydrogen). One alloying element that has not been examined significantly is silicon.

Silicon is an abundant alloying element; it is the second most available element in the

earth’s crust [4]. In wrought steels, silicon is used primarily as a deoxidant in the

steelmaking process and is typically added at levels between 0.15 w/o and 0.30 w/o. The

high deoxdizing power of silicon allows for the production of steels with low sulfur

levels (sulfur removal is favored by low dissolved oxygen in the liquid steel) and also

aids in the recovery of highly oxidizable elements such as boron, manganese, titanium

and zirconium. Since the cost of silicon is low compared with other alloying elements, it

is used in a sacrificial manner to improve the recovery of other alloying elements. Higher

levels of silicon (~1.60 w/o) are used in some special high strength low alloy steels

(HSLA) but it’s main use is in electrical steels where the silicon content can be as high as

5.0 w/o. Silicon is used in these materials because it leads to high magnetic permeability,

high electrical resistivity and low core loss which are beneficial properties in the

manufacture of electric motors. Steels containing this level of silicon are extremely brittle

and require special processing to produce sheet steels for motor laminates.

Although silicon has been utilized in a few PM products, an extensive study on the

effects of silicon as an alloying element has not been undertaken [6]. In sintered PM

steels, which have a predominately pearlitic microstructure, silicon can strengthen the

ferrite by solid solution strengthening. Figure 1 shows the effect of the concentration of

various alloying elements on the hardness of ferrite in various PM iron alloys with no

carbon additions. The microstructure in each of the alloys was ferritic. The results

indicate that silicon has the potential to significantly increase the strength of the ferrite in

a pearlitic microstructure when compared with other alloying elements typically used in

PM steels.

15

20

25

30

35

40

0 0.5 1 1.5 2 2.5

Ap

pa

ren

t H

ard

ne

ss (

HR

A)

Alloy Content (w/o)

Si

Mn

Cu

Ni

Cr

Mo

Figure 1: Effect of concentration of various alloying elements on apparent hardness of

PM ferritic steels.

It has also been shown that factors such as the proportions of ferrite and pearlite, the

interlamellar spacing, size of the pearlite nodules, and ferrite grain size, affect the

mechanical properties [7-9]. Figure 2 shows that the interlamellar spacing controls the

strength and hardness of wrought steels with pearlitic microstructures. Hyzak and

Bernstein [8] found that the yield strength and hardness were controlled primarily by the

interlamellar spacing and that impact energy was dependent on the prior austenite grain

size, increasing with finer grain size, and to some extent on the pearlite colony size. The

effect which silicon has on the pearlitic microstructure is unclear as Takahashi et al. have

shown that silicon has only a small influence on the interlamellar spacing while Anya has

found that silicon inhibits the prior austenite grain size, leading to finer pearlite which is

beneficial to tensile and impact properties [10-11].

Figure 2: Hardness and yield strength of wrought steels as a function of interlamellar

spacing in fully pearlitic microstructures [8].

The ability of an alloy to transform to martensite during heat treatment is affected by the

various alloying elements. Hardenability is generally accepted as a qualitative measure

describing the ease and depth to which steel is able to transform to martensite upon

cooling from the austenitizing temperature. The mechanical properties of a heat treated

steel depend primarily on its hardenability. Sokolowski et al. [12] have reviewed various

factors influencing the hardenability of PM steels. Certain alloying elements, in particular

chromium, manganese, molybdenum and nickel, have a strong influence on hardenability

and have been utilized extensively in PM steels. It is also well known that elements such

as molybdenum and nickel can have a synergistic effect leading to enhanced

hardenability when alloyed together. In relation to hardenability, silicon has not been

used as extensively as these other elements in PM steels. Figure 3 shows the multiplying

factors for wrought steels developed by Jatczak [13] for carbon contents in the range of

0.60 to 1.10 w/o and an austenitizing temperature of 926 oC (1700

oF). At these high

carbon contents, which are typical for PM steels, silicon has a hardenability factor similar

to that of chromium and manganese. It has also been found by the author that a

synergistic hardenability effect exists between silicon and molybdenum, namely when

silicon is used in the presence of molybdenum its hardenability is much higher than what

is shown in Figure 3.

1

2

3

4

5

0 0.2 0.4 0.6 0.8 1

Hard

en

ab

ilit

y F

ac

tor

Alloy Addition (w/o)

MnCr

Ni

Si

Mo

Figure 3: Hardenability factors for alloy elements in wrought steels.

A number of authors studying high carbon pearlitic steels for rail applications have found

that the addition of silicon suppresses the formation of continuous grain boundary

carbides, allowing these materials to have a high strength while maintaining adequate

ductility for cold drawing. The utilization of high carbon levels without embrittling grain

boundary carbides is attractive in the PM industry since graphite is a relatively

inexpensive alloying element.

In the present study, a development program was undertaken in which silicon was added

to several ferrous PM systems. Mechanical properties were measured and

microstructures characterized in the sintered and heat treated conditions to assess the

effectiveness of silicon on improving the mechanical properties and hardenability of PM

alloy systems.

ALLOY PREPARATION AND TESTING

Mixtures of base powders and master-alloys containing silicon were utilized to prepare

test specimens. The mean particle size (d50) of the additive was 8-15 m. Ancorsteel

1000B was used for the iron base alloy systems, and Ancorsteel 30HP, Ancorsteel 50HP

and Ancorsteel 85HP were used for the iron-molybdenum alloys (nominal molybdenum

levels of 0.30 w/o, 0.50 w/o and 0.85 w/o respectively).

The powders were mixed with Acrawax C lubricant and graphite. Graphite additions

were 0.70 w/o (unless otherwise noted) resulting in a sintered carbon level of

approximately 0.65 w/o. Samples for transverse rupture (TR) and tensile testing were

compacted uniaxially at a pressure of 690 MPa. The test pieces were sintered in a high

temperature Abbott continuous-belt furnace at 1260 C (2300 F) for 30 min in an

atmosphere of 90 v/o nitrogen / 10 v/o hydrogen. Sinter-hardening experiments were

conducted using the same furnace but utilizing a gas quench to provide accelerated

cooling at a rate of 1.9 oC/s. Test pieces that were sinter-hardened were tempered at 204

oC (400

oF) for 1 h.

For heat treatment, samples were austenitized at 900 °C (1650 °F) for 60 min at

temperature in a 75 v/o nitrogen / 25 v/o hydrogen atmosphere prior to quenching in oil.

Prior to mechanical testing, green and sintered densities, dimensional change (DC), and

apparent hardness were determined on the tensile and TR samples. Five tensile

specimens and five TR specimens were evaluated for each composition. The densities of

the green and sintered steels were determined in accordance with MPIF Standard 42.

Tensile testing followed MPIF Standard 10 and apparent hardness measurements were

made on the tensile and TR specimens, in accordance with MPIF Standard 43.

The effect of silicon on quenched hardenability was also studied. 25 mm (1 in) dia x 25

mm high compacts were pressed to a green density of 6.95 g/cm3 at a nominal pressure of

690 MPa (50 tsi). After sintering these samples were reheated to 900 C (1650 F) and

oil quenched. The samples were cross-sectioned at mid-height and mounted for

microstructural analysis. In addition, microindentation hardness traverses were made

from the edge to the center of the compacts. In this way the hardenability could be

measured in a similar fashion to the Jominy test.

Specimens for microstructural characterization were prepared using standard

metallographic procedures. Subsequently, they were examined by optical microscopy in

the polished and etched (2 v/o nital / 4 w/o picral) conditions.

RESULTS AND DISCUSSION

Sintered Alloys

It is recommended that silicon-containing PM alloys be sintered at high temperatures in

order to reduce the oxides that form at the grain boundaries. Figure 4 shows a 0.50 w/o

Si pre-alloy with the addition of 0.70 w/o graphite sintered at various temperatures. The

light optical photomicrographs shows the oxide located at particle boundaries for a range

of sintering temperatures from 1120 oC to 1260

oC (2050

oF – 2300

oF). The amount of

oxide at prior particle boundaries is still significant up to 1204 oC (2220

oF). The

corresponding TR strength properties do not exhibit any significant increase nor was a

decrease in oxygen content seen until the powder mix is sintered > 1163 oC (2125

oF).

Oxygen levels approaching those in traditional PM alloys are not achieved until the

sintering temperature reaches ~ 1260 oC (2300

oF). Therefore property data was

developed at 1260 oC (2300

oF) in the rest of this study.

(a) 1120 (

oC) (b) 1163 (

oC)

(c) 1204 (

oC) (d) 1260 (

oC)

Figure 4: Oxides at particle boundaries in a 0.50 w/o Si alloy as a function of sintering

temperature.

The effect of silicon level on mechanical properties is shown in Table I. A mixture of

Ancorsteel 1000B and a silicon master-alloy and 0.70 w/o graphite was used to determine

the effects of silicon concentration on mechanical properties. As the silicon content

increased, the apparent hardness, yield strength and ultimate tensile strength all increase

with only a slight decrease in the elongation. The impact energy increases with

increasing silicon content notwithstanding the increase in hardness and strength. It is

clear from microindentation hardness of the pearlite (Figure 5) that silicon has increased

the strength of the pearlite through solid solution strengthening of the ferrite.

Table I: Mechanical Properties of Sintered PM Steels with Silicon Additions

Sintered

Density

Apparent

HardnessElongation

Alloy (g/cm3) (J) (ft.lbf) (HRA) (10

3 psi) (MPa) (10

3 psi) (MPa) (%)

0.0 w/o Si 7.18 20 15 40 55.1 379 36.3 250 4.2

0.30 w/o Si 7.16 20 15 44 61.5 423 39.2 270 4.1

0.60 w/o Si 7.14 22 16 46 66.9 460 42.7 294 3.8

0.90 w/o Si 7.12 23 17 47 71.2 490 45.8 315 3.7

1.20 w/o Si 7.10 23 17 48 73.1 503 48.1 331 3.5

Impact

EnergyUTS

0.20% Offset

Yield

Figure 5 also shows that the pearlite spacing decreased as the silicon content increased.

The increase in impact energy is presumably due to the reduction in grain size as a result

of the silicon addition. Table II shows the results of 0.60 w/o addition of silicon to a

number of common ferrous PM material systems. In the copper system the silicon

addition leads to only slight increases in strength while systems such as the Fe-C and Fe-

Mo-C lead to more significant increases in strength and hardness (~ 15 to 20%). The

iron-copper-carbon alloy was the only system investigated which did not show a

significant increase (>10%) in tensile strength with the addition of silicon. Since copper

is an austenite stabilizer, the amount of austenite that is available to transform to pearlite

is increased. It may also be possible that, at this level of copper, the amount of pearlite is

already maximized and the pearlite spacing is already fine so that the use of silicon does

not contribute to an increase in properties. Since copper does not form carbides, the

copper is in solution in ferrite and Figure 1 shows that copper additions also harden the

ferrite to a similar level as silicon.

(a) 0.0 w/o Si (b) 0.30 w/o Si

(c) 0.60 w/o Si (d) 0.90 w/o Si

(e) 1.20 w/o Si (f) Microindentation Hardness of Pearlite

Figure 5: Microstructure of Ancorsteel 1000B + 0.70 w/o Graphite at various silicon

levels and microindentation hardness of the pearlite. Light optical microscopy.

Table II: Mechanical Properties of Sintered PM Steels With and Without Silicon

Additions.

Sintered

Density

Apparent

HardnessElongation


3 psi) (MPa) (10

3 psi) (MPa) (%)

Fe-CNo Si 7.18 20 15 40 55.1 379 36.3 250 4.2

0.60 w/o Si 7.14 22 16 46 66.9 460 42.7 294 3.8

Fe-2 w/o Ni-CNo Si 7.21 27 20 48 76.4 526 44.9 309 4.6

0.60 w/o Si 7.16 27 20 52 86.0 592 52.7 363 4.1

Fe-2 w/o Cu-CNo Si 7.10 19 14 54 88.2 607 67.5 464 2.6

0.60 w/o Si 7.05 19 14 54 90.4 622 69.2 476 2.6

Fe-0.30 w/o Mo-CNo Si 7.12 19 14 45 65.6 451 48.0 330 3.3

0.60 w/o Si 7.09 20 15 50 75.2 517 54.9 378 2.8

Impact

EnergyUTS

0.20% Offset

Yield

Heat Treated Alloys

The hardenability of PM steels is an important measure of how readily certain alloy

systems can be heat treated. One of the most widely used tests for hardenability is the

Jominy end-quench test, where samples are heated into the austenite range and water

quenched on one end of the sample, producing a wide range in cooling rate within one

sample. This technique requires the compaction of a large specimen and multiple

machining steps to produce the final test specimen. Lindsley et al have developed a

simplified technique in which 25 mm dia x 25 mm high compacts, were austenitized and

oil quenched. Subsequently the microindentation hardness was measured through the

thickness of the specimen [14]. This technique gives a measure of hardenability similar to

that derived from the Jominy end quench test.

300

400

500

600

700

800

900

0.0 2.0 4.0 6.0 8.0 10.0 12.0

0 w/o Si

Mic

roin

den

tati

on

Hard

nes

s (H

V 5

0 g

f)

Distance (mm)

300

400

500

600

700

800

900

0.0 2.0 4.0 6.0 8.0 10.0 12.0

0.30 w/o Si

Mic

roin

den

tati

on

Hard

nes

s (H

V 5

0 g

f)

Distance (mm) (a) (b)

300

400

500

600

700

800

900

0.0 2.0 4.0 6.0 8.0 10.0 12.0

0.60 w/o Si

Mic

roin

den

tati

on

Ha

rdn

ess

(HV

50 g

f)

Distance (mm)

Silicon rich area's

300

400

500

600

700

800

900

0.0 2.0 4.0 6.0 8.0 10.0 12.0

0.90 w/o Si

Mic

roin

den

tati

on

Ha

rdn

ess

(HV

50 g

f)

Distance (mm)

Silicon rich area's

(c) (d)

Figure 6: Microindentation hardness profiles from the surface (distance = 0) toward the

center of oil quenched 25 mm dia compacts. Ancorsteel 1000B with 0.70 w/o graphite:

(a) 0.0 w/o Si; (b) 0.30 w/o Si; (c) 0.60 w/o Si; (d) 0.90 w/o Si.

A series of 25 mm dia compacts were produced with increasing levels of silicon followed

by austenitizing and oil quenching. Microindentation hardness profiles from the surface

to the center of the compacts were then measured. Figure 6 shows the results of these

hardenability traces. As the silicon level increases the hardenability (distance towards the

center of the compact at which the surface hardness is maintained) increases. One point

becomes obvious, namely that since the silicon is not prealloyed, hardness level depends

on the diffusion of the silicon from the additive to the base powder. Maxima and minima

in the hardness measurements occur over short distances corresponding to areas where

silicon has and has not diffused into the base powder. However, it is evident that as

silicon is increased in the matrix the hardenability is increased. Figure 3 indicates that in

high carbon steels (carbon levels typically used in PM) silicon has a hardenability factor

similar to that of chromium at concentrations <1.0 w/o.

In his work on the hardenability of wrought high carbon steels (0.60 to 1.0 w/o carbon),

Jatczak discovered that the hardenability factor for silicon was higher when used in the

presence of molybdenum, particularly when austenitized at 927 oC. When used in

conjunction with molybdenum the hardenability of silicon exceeded that of chromium.

300

400

500

600

700

800

900

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Without SiliconWith Silicon

Distance (mm)

Mic

roin

den

tati

on

Ha

rdn

ess

(HV

50 g

f)


center of oil quenched 25 mm diameter compacts in an Ancorsteel 30HP (~ 0.30 w/o Mo)

with 0.70% graphite: Solid line - 0.0 w/o Si; Dashed Line- 0.60 w/o Si.

Figure 7 shows the hardenability traces of Ancorsteel 30HP with and without the addition

of silicon. With no silicon present the hardness drops off as the center of the compact is

approached. With the addition of silicon the compact through hardens (i.e. the hardness

level is maintained from the surface to the core of the 25 mm dia compact). Unlike the

iron based alloy the hardness in the molybdenum-containing alloy does not exhibit the

variation in hardness (the minima’s and maxima’s) seen with the iron based system. The

reason for this is unclear, but may be related to enhanced diffusion of the silicon in

combination with molybdenum, or the overall increase in hardenability masks the effect.

300

400

500

600

700

800

900

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Cr Only

Distance (mm)

Mic

roin

den

tati

on

Hard

nes

s (H

V 5

0 g

f)

300

400

500

600

700

800

900

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Cr+Si

Mic

roin

den

tati

on

Hard

nes

s (H

V 5

0 g

f)

Distance (mm) (a) (b)

300

400

500

600

700

800

900

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Mn Only

Mic

roin

den

tati

on

Ha

rdn

ess

(HV

50 g

f)

Distance (mm)

300

400

500

600

700

800

900

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Mn+Si

Mic

roin

den

tati

on

Ha

rdn

ess

(HV

50 g

f)

Distance (mm) (c) (d)


center of oil quenched 25 mm dia compacts with Ancorsteel 1000B + 0.70% graphite

with: (a) 0.35 w/o Cr; (b) 0.35 w/o Cr + 0.60 w/o Si; (c) 0 .75 w/o Mn; (d) 0 .75 w/o Mn

+ 0.60 w/o Si.

Figure 8 shows the effects of adding silicon to a 0.35 w/o Cr alloy and a 0.75% Mn alloy.

These alloys were produced by the addition of high carbon ferrochromium and high

carbon ferromanganese to Ancorsteel 1000B. The mean particle size of the ferroalloy

additives was 10 um. In each case the addition of the alloying element by itself had little

effect on hardenability. When silicon was added at a concentration of 0.60 w/o the

hardenability of the alloy systems increased dramatically. In both cases the large maxima

and minima in microindentation hardness were not significant indicating that all the

elements were in solution at 1260 oC.

Sinter-hardening

Sinter-hardening is a process in which hardening of a component is accomplished during

an accelerated cooling from the sintering temperature. Traditional sinter-hardening PM

steel compositions utilize high levels of copper, molybdenum and nickel along with high

carbon contents to achieve martensitic microstructures in the as-sintered condition. The

preceding discussion has shown that the use of silicon imparts an increased level of

hardenability when used in combination with chromium, manganese and molybdenum.

To investigate the effectiveness of silicon in relation to sinter-hardening a series of

molybdenum prealloys were alloyed with the silicon master alloy. The alloys were

sintered conventionally (no accelerated cooling) and with accelerated cooling and then

tempered at 204 oC (400

oF) for 1 h. Tempering after sinter-hardening is recommended to

increase strength through stress reduction and modification of the microstructure.

Table III: Mechanical Properties of Molybdenum (With and Without silicon) Containing

Alloys: Conventional Cooling versus Accelerated Cooling.

Silicon

Addition

Sintered

Density

Apparent

HardnessElongation

Alloy (w/o) (g/cm3) (J) (ft.lbf) (HRA) (10

3 psi) (MPa) (10

3 psi) (MPa) (%)

0.30 w/o MoConventional Cooling 0.00 7.16 23 17 45 66.9 460 48.9 336 3.3

Conventional Cooling 0.75 7.05 23 17 49 83.0 571 59.3 408 3.3

Accelerated Cooling 0.00 7.14 26 19 47 72.9 502 52.8 363 3.5










Impact

EnergyUTS

0.20% Offset

Yield

When examining the data in Table III, for conventional cooling, it is obvious that the

addition of silicon increases the mechanical properties (ultimate tensile strength, yield

strength and apparent hardness). As previously discussed, this is a result of stengthening

the ferrite and refining the pearlite. But the role of silicon in hardenability can be seen

when comparing the conventionally cooled material to the material in which accelerated

cooling was utilized. In the case where no silicon was added there was only small

increases in mechanical properties between the conventionally sintered material and the

material that was sinter-hardened (accelerated cooling). However, the molybdenum-

containing alloys with silicon show a significant increase in ultimate tensile, yield

strength and hardness; while surprisingly the ductility and elongation also increase. The

microstructure’s in Figure 9 (for the 0.50 w/o Mo alloy) show the reason for this increase

in mechanical properties. The addition of silicon has increased the hardenability utilizing

accelerated cooling resulting in a mixture of bainite and martensite whereas the

accelerated cooled material with no silicon added has a pearlitic microstructure. At the

lower molybdenum levels (0.30 w/o Mo), where pearlite is predominant in the

microstructure, the accelerated cooling in combination with silicon leads to a more

refined microstructure and the impact energy is increased notwithstanding an increase in

hardness. As the structure transforms more to marteniste and bainite, due to the higher

molybdenum levels, there is no increase in the impact energy and elongation but the

levels of each are still maintained despite the attendant increase in strength and hardness.

(a) (b)

Figure 9: Microstructure of Ancorsteel 50HP utilizing accelerated cooling: (a) with no

silicon addition, divorced pearlite and (b) with silicon addition, bainite/martensite.

Suppression of Carbides

Research on wrought steels used for rails and wire has shown that when silicon is added

to high carbon steels ( >0.80 w/o) it retards the formation of grain boundary carbides [15-

17 ]. The cost effectiveness of carbon has been exploited in PM steels for years with

levels of carbon typically higher than those in wrought steels. However, above 0.80 w/o

C a near continuous film of cementite precipitates tends to form on the grain boundaries

in PM steels leading to embrittlement and a decrease in mechanical properties. It has

been found that in wrought steels that silicon slows the diffusion of carbon to the

cementite, thereby surpressing the formation of the grain boundary cementite. These two

effects have resulted in wrought steels with carbon levels between 0.90 w/o C and 1.10

w/o C, with excellent mechanical properties.

Figure 10 shows the microstructure of a Fe-1.1 w/o C alloy with and without silicon

additions. The alloy without a silicon addition (Figure 10a) has an extensive amount of

iron-carbide located at grain boundaries. This grain boundary cementite network is

brittle and provides both crack nucleation sites and serves as an easy fracture path and it

is the reason why PM steels with this high carbon content are normally too brittle for

commercial use. Figure 10b shows the same alloy system but with the addition of 0.60

w/o Si. There is minimal embrittling grain boundary carbides and those that exist are

thinner and not continuous; consequently this leads to less deleterious effects on

mechanical properties as cited in Table III.

(a) (b)

Figure 10: Appearance of grain boundary carbides (light etched areas) in a Fe-1.1 w/o C

system (a) with no Si and (b) with 0.60 w/o Si.

The mechanism by which silicon suppresses carbide formation is related its diffusion. As

the carbides form the silicon is rejected from the carbide into the ferrite. This raises the

silicon level outside the carbides and slows the rate of silicon diffusion which, in turn

slows, the growth rate of the carbides. Figure 11 shows EDX analysis of the silicon

levels in the carbide (yellow) and ferrite (red) of the material from a Fe-1.1 w/o C + 0.60

w/o Si based alloy.

Figure 11: EDX analysis of carbide in Fe-1.1 w/o C + 0.60 w/o Si.

The red line indicates that the silicon level is higher in the ferrite than in the carbide that

forms at the grain boundaries. Semi-quantitative estimates indicate that the silicon level

in the ferrite is approximately 1.0 w/o while the carbide contains 0.20 w/o silicon. This

increase in the silicon level in the ferrite leads to a gradient slowing the growth of the

carbide and therefore leading to decreased carbide formation in the alloy containing

silicon. As a result the high carbon alloy containing silicon exhibits less carbide

formation then a similar alloy without the silicon.

Table III shows data for a Fe-C system with a range of carbon levels from 0.70 w/o to 1.3

w/o graphite. One composition has no silicon while the other has 0.60 w/o silicon. Not

only does the silicon addition lead to higher strength (both yield and ultimate tensile) and

hardness, but the impact energy is higher, particularly at graphite levels >1.1 w/o in the

alloy containing silicon.

Table III. Effect of Silicon and Carbon on Properties of PM Steels.

Sintered

Density

Apparent

HardnessElongation


3 psi) (MPa) (10

3 psi) (MPa) (%)

0.70 w/o CarbonNo Si 7.15 22 16 42 59.3 408 37.8 260 4.7

0.60 w/o Si 7.10 24 18 46 67.7 466 43.7 301 4.0

0.90 w/o CarbonNo Si 7.09 20 15 48 74.4 512 44.2 304 3.8

0.60 w/o Si 7.08 23 17 52 85.9 591 53.3 367 3.4

1.10 w/o CarbonNo Si 7.09 17 13 51 83.7 576 51.5 354 3.1

0.60 w/o Si 7.05 19 14 54 93.8 645 60.0 413 2.8

1.30 w/o CarbonNo Si 7.05 5 4 48 60.7 418 46.6 321 1.7

0.60 w/o Si 7.02 13 10 55 87.1 599 62.3 429 2.2

Impact

EnergyUTS

0.20% Offset

Yield

The use of silicon in combination with high carbon levels (> 1.0 w/o) has been shown to

be effective in a number of alloy systems including iron-carbon-molybdenum, iron-

carbon-nickel and iron-carbon-copper.

Economics of Alloying with Silicon

It is obvious from Table II that silicon additions increase the mechanical properties in a

number of different ferrous alloy systems. The cost savings are difficult to quantify.

However the use of silicon in combination with high graphite levels can lead to direct

replacement of certain alloying elements. In addition, if these elements are used in

combination with sinter-hardening, considerable savings in alloy cost can be realized.

These cost savings can be balanced against the use of high temperature sintering. Table

IV shows typical property data for sinterhardening grades contained in MPIF Standard

35. Most of these alloys use combinations of chromium, copper, molybdenum,

manganese and nickel in excess of 3 w/o. In Table IV the mechanical properties of an

alloy comprised of 0.50 w/o Mo with 0.75 w/o Si is compared with the sinter-hardened

steels at a density of 7.0 g/cm3. In addition to matching the strength level of most of the

sinter-hardening grades, with higher alloy content, the silicon containing alloy maintains

some ductility.

Table IV. Comparison of Mechanical of Experimental Alloy with MPIF Sinter-

hardening Grades (density = 7.0 g/cm3).

TRS TRSApparent

HardnessElongation

Material Designation Code (103 psi) (MPa) (J) (ft.lbf) (HRC) (10

3 psi) (MPa) (10

3 psi) (MPa) (%)

FLNC-4408-105HT 220 1514 16 12 25 115.0 791 D D <1

FLC-4608-95HT 180 1238 15 11 36 105.0 722 D D <1

FLC-4805-140HT 240 1651 13 10 34 150.0 1032 D D <1

FLC2-4808-110HT 230 1582 19 14 35 120.0 826 D D <1

FLC2-5208-110HT 255 1754 15 11 30 120.0 826 100.0 688 <1

FL-5305-135HT 265 1823 13 10 35 145.0 998 D D <1

Ancorsteel 50HP +0.75 w/o Si +

0.90 w/o Graphite278 1913 19 14 34 145.0 998 107.5 740 1.5

Impact Energy UTS 0.20% Offset Yield

CONCLUSIONS

Silicon solid solution strengthens the ferrite of pearlitic PM steels resulting in an

increase in strength.

Increasing the silicon content leads to a refined pearlitic microstructure resulting

in an increase in strength and hardness accompanied by an increase in impact

energy.

Silicon can be added to a number of common ferrous PM alloy systems leading to

improved mechanical properties.

Silicon increases the hardenability of iron-based PM alloys and has synergistic

effects with chromium, manganese and molybdenum.

Silicon suppresses the formation of carbides at carbon levels >0.80 w/o leading to

improved mechanical properties.

When used in molybdenum containing alloys, silicon can enhance the sinter-

hardening behavior of the alloys.

REFERENCES

1. B. Lindsley, “High Performance Manganese Containing PM steels”, Advances in Powder

Metallurgy& Particulate Materials, compiled by R. Lawcock, A. Lawley and P. J. McGeehan,

MPIF, Princeton, NJ, 2008, part 7, pp. 17-25. 2. J. Tengzelius, S. Grek, C. Blande, , “Limitations and Possibilities in the Utilization of Cr and

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Powder Metallurgy and Particulate Materials, 2008, compiled by A.Lawley. P. McGeehan

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