Ferrous Materials

Although lubricants are a necessary addition to compacting metal powders, they can have some critical, and often deleterious, effects on the ultimate premix. Hoeganaes Corporation (Ref 1) has made a comprehensive study of lubricants and their influence on the properties of iron premixes. Lubricant properties, such as bulk density and particle size, are very important; for example, high-bulk density and large particle size provide good flow and low stripping pressure, as illustrated in Fig. 5 and 6. To minimize premix segregation, the particle size of the lubricant must be smaller than the size of the largest particle of iron powder; for example, a 150- to 200-mesh (104- to 75-/'m) lubricant should be used with 100-mesh (150-/^m) iron powder.

Fig. 5 Relationship of mix flow and bulk density of Ancor MH-100 iron powder with 1.0% zinc stearate lubricant. Source: Ref 1

Bulk density, lb/ft3

15 20

200 300

Bulk dengiiy, kg/rn3

Bulk density, lb/ft3

15 20

a Jt o X

<1P

1

[

' 1 1 Carbide did g 48 tsi compaction

l

q a 3

V«; ° 0

o

- 30 ts

>i compa

rtion

-

fr^

-Cji

-

Bulk dengiiy, kg/rn3

tn a

1000 g-55

Fig. 6 Relationship of stripping pressure and bulk density of iron powders with 1.0% zinc stearate lubricant.

The melting point of the lubricant must be high enough to prevent melting or softening from heat developed during mixing. If this occurs, flow deteriorates, apparent density decreases, and the mix tends to agglomerate.

Higher lubricant content yields poorer flow, lower apparent density, and lower stripping pressure. Figure 7(a) shows the effect of lubricant content on apparent density of atomized stainless steel powder. The effect of increasing amounts of three lubricants on the flow of iron powder is shown in Fig. 7(b) and 8.

Fig. 7 Effect of lubricant additions and mixing time on (a) apparent density and (b) flow time of water-atomized stainless steel powder

Fig. 8 Effect of increasing amounts of three lubricants on the flow rate of Ancor MH-100 iron powders. Lubricant A-1: 82% stearic acid, 15% palmitic acid, 1.0% oleic acid; lubricant A-2: 49% stearic acid, 50% palmitic acid, 0% oleic acid, lubricant A-3: 41% stearic acid, 51% palmitic acid, 6% oleic acid. Mixing time of 30 min. Source: Ref 1

Stripping pressure is the initial amount of pressure required to start the ejection process of a green compact from the die after compaction. The sliding pressure is lower than the stripping pressure and can be defined as the pressure required to complete the compact ejection cycle. Both stripping and sliding pressures are dependent on the compaction pressure and type of lubricant, as shown in Fig. 9 and 10. Thus, different lubricants may be required for steel and carbide tooling materials.

4500

4000

Compaction pressure, MPa &00

3500

I 3000

c "CL

2000

1500

4000

3500

I 3000

c "CL

2000

1500

j

o Litlhiu'm stearate * Zinc stearate

A CMnarir a^Frl

A

iEBSi

Acrawax)

i

i

25 0

ZJ trt

30 40 50

Compaction pressure, tsi

25 0

ZJ trt

30 40 50

Compaction pressure, tsi

Fig. 9 Effect of compaction pressure and 0.75% lubricant on stripping pressure of Ancorsteel 45P (Ancorsteel

1000B + 2.90 wt% ferro-phosphorus)

Compaction pressure, MPa soo

1000

1600

11400

1000

1000

1600

11400

1000

i i o Lilhium stearate • Zinc stearate

k

>-------

J> EBS £/

era was)

:

40 50

Compaction pressure, (st

40 50

Compaction pressure, (st

Fig. 10 Effect of compaction pressure and 0.75 wt% lubricant on the sliding pressure of Anchorsteel 45P (Anchorsteel 1000B + 2.90 wt% ferro-phosphorus)

Stripping strength can vary considerably with the type of lubricant used. When compared to parts pressed without lubricant, zinc stearate and stearic acid decrease sintered strength only slightly, as illustrated by Fig. 11 and 12. However, several lubricants, such as calcium and barium stearates, added to pure iron or to an iron and 1% graphite mixture cause significantly large decreases in sintered strength, as shown in Fig. 13. The appearance of P/M parts after sintering also can be affected by lubricants. Although the effect on iron parts is not as significant as with brass and nickel-silver powders, spotty surfaces and discoloration can be a major concern. Lubricants that vaporize completely during sintering generally leave a clean surface. Dark mottled surfaces occasionally result from zinc and calcium stearate lubricants, due to condensed zinc metal or calcium oxide deposits.

Fig. 11 Effect of zinc stearate additions on sintered strength of Ancor MH-100 compacted powders sintered 40 min at 1120 °C (2050 °F) in purified exothermic gas. Preheat of 650 °C (1200 °F). Bulk density: (a) 141 kg/m3

(8.8 lb/ft3); (b) 256 kg/m3 (16.0 lb/ft3) (c) 91 kg/m3 (5.7 lb/ft3). Particle size (a) 1.9 Jt'm; (b) 4.5 i'm; (c) 1.0 i'm. Graphite addition of 0%, open circle data point; graphite addition of 1.0% closed circle data point. Without lubricant, solid line; with lubricant, dashed line. Source: Ref 1

Fig. 12 Effect of zinc stearate additions on sintered strength of Ancor MH-100 compacted powders sintered 40 min at 1120 °C (2050 °F) in purified exothermic gas. Preheat of 650 °C (1200 °F). Bulk density: (a) 354 kg/m3 (22.1 lb/ft3); (b) 434 kg/m3 (27.1 lb/ft3); (c) 384 kg/m3 (24.0 lb/ft3). Softening temperature: (a) 64 °C (147 °F); (b) 55 °C (131 °F); (c) 54 °C (129 °F). Graphite addition of 0%, open circle data point; graphite addition of 1.0%, closed circle data point. Without lubricant, solid line; with lubricant, dashed line. Source: Ref 1

Fig. 12 Effect of zinc stearate additions on sintered strength of Ancor MH-100 compacted powders sintered 40 min at 1120 °C (2050 °F) in purified exothermic gas. Preheat of 650 °C (1200 °F). Bulk density: (a) 354 kg/m3 (22.1 lb/ft3); (b) 434 kg/m3 (27.1 lb/ft3); (c) 384 kg/m3 (24.0 lb/ft3). Softening temperature: (a) 64 °C (147 °F); (b) 55 °C (131 °F); (c) 54 °C (129 °F). Graphite addition of 0%, open circle data point; graphite addition of 1.0%, closed circle data point. Without lubricant, solid line; with lubricant, dashed line. Source: Ref 1

Fig. 13 Effect of calcium and barium stearate additions on sintered strength of Ancor MH-100 compacted powders sintered 40 min at 1120 °C (2050 °F) in purified exothermic gas. Preheat of 650 °C (1200 °F). (a) and

(b) Calcium stearate. (c) Barium stearate. Bulk density: (a) 104 kg/m3 (6.5 lb/ft3); (b) 431 kg/m3 (26.9 lb/ft3)

(c) 178 kg/m3 (11.1 lb/ft3). Particle size (a) 11 /'m; (b) 18.8 /'m; (c) 1.2 /^m. Graphite addition of 0%, open circle data point; graphite addition of 1.0%

Fig. 13 Effect of calcium and barium stearate additions on sintered strength of Ancor MH-100 compacted powders sintered 40 min at 1120 °C (2050 °F) in purified exothermic gas. Preheat of 650 °C (1200 °F). (a) and

(b) Calcium stearate. (c) Barium stearate. Bulk density: (a) 104 kg/m3 (6.5 lb/ft3); (b) 431 kg/m3 (26.9 lb/ft3)

(c) 178 kg/m3 (11.1 lb/ft3). Particle size (a) 11 /'m; (b) 18.8 /'m; (c) 1.2 /^m. Graphite addition of 0%, open circle data point; graphite addition of 1.0%

Lubricant removal, prior to the actual sinter operation, is vitally important for several reasons. Generally, this removal is accomplished during the sintering preheat operation and is referred to as "burn-off." Because of highly adverse effects of residual lubricant carbon deposits when sintering stainless steel, resulting in lowered corrosion resistance, Moyer (Ref 2) conducted an extensive study of the burn-off characteristics of common lubricants (lithium stearate, zinc stearate, Acrawax C, and Nopco Wax) in 316L powder compacts. Burn-off temperatures of 370, 425, and 480 °C (700, 800, and 900 °F) were used in both air and dissociated ammonia atmospheres. This study revealed that the waxes leave no residue at 425 °C (800 °F) or higher; the stearates, however, leave approximately 15% residue even when burned at temperatures up to 540 °C (1000 °F). Maximum lubricant burn-off is achieved at about 425 °C (800 °F). Burn-off is less controllable in dissociated ammonia, and the amount of lubricant removed decreases as the compacting pressure is increased. Carbon burn-off is incomplete when compacts are burned off in a dissociated ammonia atmosphere.

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