lean can mean sinter-hard and cost-effective

40 MPR July/August 2008 0026-0657/08 ©2008 Elsevier Ltd. All rights reserved.

technical trends

Lean can meansinter-hard andcost-effectiveWhen sinter-hardenable steels were first brought to market the cost of alloying elements was not a major issue. But times change, and economics with them, as any production engineer can attest. However, some aspects of the challenges raised by soaring commodity prices can be tackled by technology. The wider inclusion of improved cooling zones by furnace manufacturers in their products means that lower-cost, low-alloy steels can be successfully sinter-hardened…

The first sinter-hardenable alloys

introduced into the marketplace

relied on high concentrations of

alloying elements in the steel. As

sinter hardening has become more common,

sinter furnace producers have improved the

cooling zones in the furnace. The increase in

obtainable cooling rates allows lower alloyed

steels to be used to obtain sinter-hardened

parts. This is especially important in relation

to small parts, where higher cooling rates can

be achieved. This indicates the need for an

alloy with a composition that optimises the

levels of price sensitive elements while main-

taining the ability to sinter-harden in current

belt furnaces.

Traditional sinter-hardening PM steel

compositions utilise high levels of molyb-

denum (Mo), nickel (Ni) and copper (Cu)

along with high carbon (C) contents to

achieve martensitic microstructures in the

as-sintered condition. Historically, high

cooling rates could not be achieved in

the sintering furnace, leading to only the

most highly alloyed materials being used

for sinter hardening. With the advent of

accelerated cooling zones and the adop-

tion of these technologies, lower alloy

contents can be used. When alloy prices

are low, it is easier to use heavily alloyed

materials to ensure a martensitic micro-

structure forms regardless of section size

and cooling rate in the part. As price

pressures force a re-assessment of alloy

selection, it may be more cost effective to

invest in additional processing to reduce

the content of high priced alloying ele-

ments. The added processing may include

longer sintering times to fully utilise

admixed ingredients, accelerated cooling,

and an analysis of the actual cooling rate

in each part to determine the lowest alloy

content necessary for sinter hardening.

There are several approaches that can

be taken to address raw material costs.

The first is to introduce lower cost alloying

elements, such as chromium (Cr) and/or

manganese (Mn). These oxygen-sensi-

tive elements provide excellent harden-

ability, but may lead to higher processing

costs associated with powder production

and sintering. One benefit of these more

effective alloying elements is that lower

carbon levels can be used while maintain-

ing a martensitic microstructure. These

lower carbon martensitic alloys provide

lower dimensional variation and enhanced

mechanical properties [1]. The chro-

mium-containing Ancorsteel® 4300 and

Ancorsteel 4300L (0.3% Mo) are examples

of such alloys, where as-sintered marten-

sitic microstructures are common with

sintered carbon contents less than or equal

to 0.6 % (wt%) [2].

Another approach to lower cost sinter

hardening is the development of alloys

that have intermediate levels of alloying

elements. Earlier alloys used high levels of

molybdenum and nickel as powder costs

were relatively low compared with secondary

heat-treating steps. Ancorsteel 737 SH (MPIF

FL-4800 [3]) has the combination of good

Table I. Nominal composition (in wt%) of the base alloys studied.

Alloy Fe Mo Ni Mn

Ancorsteel 721 SH Bal. 0.9 0.5 0.4

Ancorsteel 737 SH (FL-4800) Bal. 1.2 1.4 0.4

mpr637p39_48.indd 40mpr637p39_48.indd 40 12/08/2008 15:48:3312/08/2008 15:48:33

metal-powder.net July/August 2008 MPR 41

compressibility and excellent hardenability,

but at current price levels, the 1.25% Mo and

1.4% Ni prealloyed in the powder make it

somewhat less attractive. Nevertheless, when

processing larger parts or where accelerated

cooling is not an option, slow cooling rates

within the part require these high levels of

alloying for sinter hardening. The diffusion

alloyed materials containing 4% nickel and

either 0.5% or 1.5% molybdenum also have

relatively high cost, and given that the nickel

is not prealloyed, do not take full advantage

of the alloying elements present. Those parts

producers that have the ability to

cool components at higher rates than

Table II. Comparison of the new alloy Ancorsteel 721 SH and FL-4800 at cooling rates of 0.7-2.2°C/s with 1 wt% Copper and

0.9 % Graphite.

Base

Alloy

Copper

(wt%)

Graphite

(wt%)

Cooling

Rate (°C/s)

Compaction

(MPa)

Density

(g/cm3)

DC

(%)

Hardness

(HRA)

YS

(MPa)

UTS

(MPa)

Elong.

(%)

721 SH 1 0.7 0.7 550 6.96 0.21 52 449 559 1.2

721 SH 1 0.7 0.7 690 7.11 0.23 54 540 644 1.3

FL-4800 1 0.7 0.7 550 6.83 0.31 65 726 783 1.0

FL-4800 1 0.7 0.7 690 6.98 0.33 67 777 875 1.0

721 SH 1 0.7 1.6 550 6.94 0.29 66 770 884 1.1

721 SH 1 0.7 1.6 690 7.10 0.31 68 858 955 1.1

FL-4800 1 0.7 1.6 550 6.83 0.33 67 767 785 0.8

FL-4800 1 0.7 1.6 690 6.99 0.35 69 849 906 0.9

721 SH 1 0.7 2.2 550 6.87 0.30 71 804 903 1.0

721 SH 1 0.7 2.2 690 7.03 0.33 71 873 963 0.9

FL-4800 1 0.7 2.2 550 6.83 0.34 67 732 823 0.8

FL-4800 1 0.7 2.2 690 6.99 0.37 69 891 945 0.8

Table III. Comparison of the new alloy Ancorsteel 721 SH and FL-4800 at cooling rates of 0.7-2.2°C/s

with 2 wt% Copper and 0.9 wt% Graphite.

Base

Alloy

Copper

(wt%)

Graphite

(wt%)

Cooling

Rate (°C/s)

Compaction

(MPa)

Density

(g/cm3)

DC

(%)

Hardness

(HRA)

YS

(MPa)

UTS

(MPa)

Elong.

(%)

721 SH 2 0.9 0.7 550 6.95 0.23 63 679 840 1.2

721 SH 2 0.9 0.7 690 7.09 0.27 66 733 912 1.2

FL-4800 2 0.9 0.7 550 6.85 0.15 68 654 818 1.2

FL-4800 2 0.9 0.7 690 7.00 0.22 69 693 996 1.4

721 SH 2 0.9 1.6 550 6.93 0.27 68 637 849 1.1

721 SH 2 0.9 1.6 690 7.08 0.31 71 701 954 1.2

FL-4800 2 0.9 1.6 550 6.85 0.16 69 634 829 1.2

FL-4800 2 0.9 1.6 690 7.00 0.22 70 685 983 1.4

721 SH 2 0.9 2.2 550 6.87 0.29 73 706 902 1.1

721 SH 2 0.9 2.2 690 7.00 0.32 74 759 919 1.0

FL-4800 2 0.9 2.2 550 6.85 0.18 69 618 944 1.3

FL-4800 2 0.9 2.2 690 6.99 0.23 70 649 914 1.1


42 MPR July/August 2008 metal-powder.net

conventional cooling need not pay for extra

alloying when a leaner alloy would suffice.

With that in mind, a new alloy, Ancorsteel

721 SH, has been developed by Hoeganaes

Corporation for lower cost sinter hardening.

The nominal compositions of

Ancorsteel 721 SH and Ancorsteel 737

SH (FL-4800) are given in Table I. The

new alloy, Ancorsteel 721 SH, contains the

same prealloyed constituents as FL-4800,

however with 0.3% less Mo and 0.9% less

Ni. All mixes for this study were prepared

with 0.75% EBS wax (Acrawax® C) as the

lubricant and varying amounts of Asbury

type 3203H graphite. Admixed copper was

used to produce alloys with 1% and 2%

Cu. Transverse rupture strength, dogbone

tensile, and impact bars were pressed at 415

MPa (30 tsi), 550 MPa (40 tsi) and 690 MPa

(50 tsi) and sintered in 90% N2- 10% H2

(vol%) atmosphere at 1120°C (2050°F) for

15 minutes in an Abbott continuous-belt fur-

nace with an accelerated cooling system.

Jominy evaluation

Three average cooling rates were

chosen for study: 0.7°C/s (1.3°F/s),

1.6°C/s (2.8°F/s) and 2.2°C/s (4.0°F/s).

The cooling rate was measured in the

sample between 650°C (1200°F) and

315°C (600°F). All samples were tem-

pered at 205°C (400°F) for one hour

in a nitrogen (N2) atmosphere prior to

testing. Measurements were performed

in accordance with the relative MPIF

standards [4].

Hardenability of the new alloy was

evaluated using the Jominy end-quench

method. Cylindrical test specimens were

machined to a length of 100 mm (4 in.)

and 25 mm (1 in.) diameter from blocks

compacted to a green density of approxi-

mately 7.0 g/cm3. All bars were initially

sintered in 90% N2 – 10% H2 (vol%)

atmosphere at 1120°C (2050°F) for 15

minutes in a continuous-belt furnace.

These samples were austenitised at 900°C

(1650°F) for 30 minutes in 90% N2 – 10%

H2 (vol%) atmosphere prior to end-

quenching. The Jominy end-quench test

method and hardness measurements were

carried out according to ASTM standard

A 255 [5] and MPIF standard 65 [4].

Figure 1 demonstrates the compress-

ibility of Ancorsteel 721 SH in compari-

son to the more heavily alloyed version,

FL-4800. It is apparent that the new alloy

has an improved compressibility with the

capability of achieving a green density of

7.1 g/cm3 or better at a compaction pres-

sure of 690 MPa. The increase in density

at similar compaction pressures, in part,

plays a significant role in the new alloy’s

capacity to achieve and even surpass

the mechanical properties of FL-4800.

Processing considerations (ie accelerated

cooling zones) strongly dictate the effec-

tiveness of the new alloy in providing the

necessary mechanical properties attrib-

uted to a viable sinter-hardening alloy.

Mixes of Ancorsteel 721 SH and FL-4800

were prepared with additions of 1% Cu

with 0.7% graphite (Table II) and 2% Cu

with 0.9% graphite (Table III). Compaction

pressures of 550 MPa and 690 MPa are pre-

sented which allow for an easy comparison

of mechanical properties as a function of

sintered density. The hardness and tensile

data are arranged according to conventional

(0.7°C/s) and accelerated cooling (1.6-

2.2°C/s) conditions within a continuous-belt

furnace. FL-4800 clearly displays superior

mechanical properties over Ancorsteel 721

SH at conventional cooling rates when only

1% Cu and 0.7% graphite is added. This

demonstrates the capability for a fully



hardening FL-4800 under even the most dif-

ficult processing conditions. However, if the

cooling rate can be increased beyond 0.7°C/s

during processing, Ancorsteel 721 SH is

capable of exhibiting comparable values to

FL-4800 at a lower alloy cost. Though not

presented in this study, the new alloy even

offers a sufficient hardenability with no

admixed copper, similar to that of FL-4800,

given an increased cooling rate [6].

Section size and belt speed are increas-

ingly more important factors when con-

sidering the usefulness of accelerated

cooling with a lean sinter-hardening alloy.

Therefore, careful consideration as to the

amount of admixed constituents should be

explored to fully utilise the alloy. With the

addition of 2% copper and 0.9% graph-

ite to Ancorsteel 721 SH, the mechanical

properties appear to be less sensitive to

cooling rate, although reduced levels are still

Figure 2. Apparent hardness of Ancorsteel 721 SH and FL-4800.Figure 1. Compressibility of Ancorsteel 721 SH and FL-4800.

Figure 3. Dimensional change of Ancorsteel 721 SH and FL-4800. Figure 4. Elongation of Ancorsteel 721 SH and FL-4800.

Figure 5. Ultimate Tensile Strength of Ancorsteel 721 SH and FL-4800. Figure 6. Yield Strength of Ancorsteel 721 SH and FL-4800.


metal-powder.net July/August 2008 MPR 45

apparent at conventional cooling rates when

compared with FL-4800. Beyond conven-

tional cooling, Ancorsteel 721 SH with 2%

Cu notably achieves the same, if not greater

properties over FL-4800.

Figures 2-6 highlight the mechanical

properties of the alloys studied at a com-

paction pressure of 690 MPa over the range

of cooling rates tested. With the addition

of 2% copper and 0.9% graphite, the two

alloys perform similarly. Both alloys have

sufficient hardenability, over the three

cooling rates, to achieve 30 HRC and above,

as shown in Figure 2. A major difference

between Ancorsteel 721 SH and FL-4800

is the dimensional change after sintering.

Current die dimensions would require

modification to account for the increase in

part expansion beyond that which is cur-

rently seen with FL-4800. However, one

benefit of the lower alloy content (especially

nickel) in Ancorsteel 721 SH is less retained

austenite in the as-sintered microstructure.

The lower retained austenite content will

lead to better dimensional consistency in

parts. Additionally, elongation, ultimate

tensile strength, and yield strength are com-

parable in level and observed trend for parts

admixed with 2% copper and 0.9% graph-

ite for both alloys. These results suggest

that Ancorsteel 721 SH is a feasible sinter-

hardening alternative to FL-4800.

At 1% copper and 0.7% graphite, the

new alloy exhibits a reduced level of hard-

ness than that of FL-4800 at the lower

cooling rates as a result of diminished

hardenability. Nevertheless, Ancorsteel

721 SH is capable of demonstrating an

improved hardness at the highest cooling

rate, 41 HRC, exceeding FL-4800. Under

conventional cooling rates, Ancorsteel

721 SH is clearly inferior to FL-4800 in

hardenability and therefore mechanical

properties. Accelerated cooling in con-

junction with enhanced compressibility of

the alloy does, however, provide the capa-

bility to greatly enhance these properties

to comparable values.

Microstructure revealed

The microstructure of Ancorsteel 721 SH

admixed with 1% Cu and 0.7% graphite

is increasingly martensitic when proc-

essed at higher cooling rates, Figure 7.

When slow cooled at 0.7°C/s, this new

alloy largely transforms into a pearlitic

(with some bainite) microstructure, which

agrees well with the lower mechanical

properties already presented. At increased

cooling rates however, the microstructure

almost fully transforms into martensite;

increasing from approximately (a) 15%

martensite, 85% bainite/pearlite, to (b)

72% martensite, 28% bainite/pearlite,

and (c) 87%martensite, 13% bainite/

pearlite at the highest cooling rate; based

on a point-count method [7]. This clearly

displays the significance and necessity of

using accelerated cooling rates in order

to take full advantage of this lean sinter-

hardening alloy.

When admixed with 2% copper and

0.9% graphite, the microstructural develop-

ment of Ancorsteel 721 SH follows a simi-

lar trend when processed with increasing

cooling rates (Figure 8). With the increased

amount of admixed constituents, the

slow-cooled microstructure transforms to

a greater fraction of martensite compared

with the 1% copper version; starting at

approximately (a) 45% martensite, 55%

bainite/pearlite for a cooling rate of 0.7°C/s,

to (b) and (c) 97% martensite, 3% bainite/

pearlite for cooling rates of 1.6 to 2.2°C/s,

Figure 7. Microstructures of Ancorsteel 721 SH with 1% Cu and 0.7% graphite compacted at 690 MPa, sintered at 1120 °C for 15 minutes, and cooled at (a) 0.7 °C/s (b) 1.6 °C/s and (c) 2.2 °C/s.

Figure 8. Microstructures of Ancorsteel 721 SH with 2% Cu and 0.9% graphite compacted at 690 MPa, sintered at 1120 °C for 15 minutes, and cooled at (a) 0.7 °C/s (b) 1.6 °C/s and (c) 2.2 °C/s.

Figure 9. Jominy end-quench hardenability of Ancorsteel 721 SH and Ancorsteel 4600V.


respectively. A small amount of retained

austenite was observed in the martensitic

microstructure. As mentioned previously,

the increase in copper and graphite content

generally leads to the formation of retained

austenite, typically around the prior particle

boundaries [7], potentially generating a

negative impact on properties. The amount

of retained austenite appeared to be lower

in the Ancorsteel 721 SH alloy compared

with FL-4800. This alloy is most effective

when sintering furnaces are equipped with

accelerated cooling zones.

References1. B Lindsley and T F Murphy,

“Effect of post sintering thermal

treatments on dimensional preci-

sion and mechanical properties

in sinter-hardening PM steels”,

Advances in Powder Metallurgy

& Particulate Materials, MPIF,

Princeton, NJ, 2007.

2. P King and B Lindsley,

“Performance Capabilities Of

High Strength Powder Metallurgy

Chromium Steels With Two

Different Molybdenum Contents”


& Particulate Materials, MPIF,

Princeton, NJ, 2006.

3. MPIF Standard 35, Materials

Standards for PM Structural Parts,

2007 Edition.

4. MPIF Standard Test Methods

for Metal Powders and Powder

Metallurgy Products, 2007 Edition.

5. ASTM A 255, Standard Method of

End-quench test for Hardenability

of Steel.

6. B Lindsley, “Alloy Development of

Sinter-Hardenable Compositions”,

EuroPM Proceedings, 2007.

7. T F Murphy, M Baran, “An

Investigation Into the Effect of

Copper and Graphite Additions

to Sinter Hardening Steels”,


& Particulate Materials – 2004,

compiled by W B James and R

A Chernenkoff, Metal Powder

Industries Federation, Princeton,

NJ, Part 10, pp. 266-274.

8. W B James, “What is Sinter-

Hardening?” Presented at PM2TEC

1998.

Turn to page 48



The hardenability of ferrous alloys has

been well documented using the Jominy

end-quench test. This method is used as

an indicator of the depth, or thickness to

which an alloy is capable of being trans-

formed to martensite. Figure 9 compares the

hardness profiles along the length of water

end-quenched bars of Ancorsteel 721 SH to

another commonly used sinter-hardenable

alloy, Ancorsteel 4600V (MPIF FL-4600 [3]),

which has 0.5% molybdenum, 1.8% nickel,

and 0.2% manganese. Notably, Ancorsteel

721 SH with 2% copper and 0.9% graphite

through-hardens in excess of 75 mm

(3 in.), maintaining a hardness above

45 HRC throughout the majority of the

bar length. The higher content of carbon in

solution is generally a significant contribut-

ing factor in maximum attainable harden-

ability [8].

Nevertheless, the difference between

the two alloys is clearly a result of other

alloy additions and their relative alloy-

ing behavior [2]. It is well known that the

hardenability factors for manganese and

molybdenum are greater than that of nickel,

and as Ancorsteel 721 SH contains more

manganese and molybdenum than FL-4600,

the Jominy results represent the expected

performance of the alloy constituents. This

material feature, in connection with acceler-

ated cooling, should allow for parts with

thick cross-sections to be constructed from

this new alloy. Even with a reduced level

of admixed copper and graphite, the alloy

retains an appreciable level of hardenability,

not dropping below the J Depth (65 HRA)

until approximately 57 mm (36/16 in.). In

comparison with FL-4600, Ancorsteel 721

SH is a superior sinter-hardening grade at

similar levels of admixed copper and graph-

ite. The amount of admixing can be tailored

to meet customer specific requirements

based on part dimension, sinter furnace

design, and required mechanical constraints.

To summarise: A new sinter-hardening

alloy, Ancorsteel 721 SH, has been devel-

oped by Hoeganaes Corporation as a viable

alternative to address the continuing trend

in metal price volatility. The reduced level of

alloying, in comparison with its predecessor

Ancorsteel 737 SH (FL-4800), can lead to

insufficient hardening under conventional

cooling parameters (~ 0.7°C/s). Nonetheless,

with the incorporation of advanced cool-

ing systems in modern sintering furnaces,

accelerated cooling rates of 1.6 to 2.2°C/s are

capable of yielding almost fully martensitic

microstructures within the new alloy. The rel-

ative amount of martensitic transformation

under accelerated cooling, of Ancorsteel 721

SH, provides suitable mechanical properties

rivalling those of FL-4800 at a reduced cost.

In the event part manufacturers are willing

to utilise elevated cooling systems along with

judicious tailoring of the premix, Ancorsteel

721 SH provides the appropriate level of

strength and mechanical response required

for selective components.

The AuthorsTHIS article was adapted from

Introduction of a new sinter-harden-

ing PM steel, a paper given at the

Washington World Congress, by Peter

Sokolowski, Bruce Lindsley and Francis

Hanejko, who work for Hoeganaes

Corporation. The authors thanked

Gerard Golin for his assistance in provid-

ing microstructures and analysis as well

as Andrew Chan and Paul Fallis for col-

lecting the data found within this paper.


lean can mean sinter-hard and cost-effective

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