Stainless Steel Powders

Water-Atomized Stainless Steel Powders. Early experiments concentrated on producing stainless steel powders from elemental components and from alloy powders by sensitization embrittlement and grinding of stainless steel sheet. Water atomization became the established process in the 1950s for producing stainless steel powders for conventional die compaction and sintering. Over the years, slight modifications to the existing wrought compositions led to improved compacting properties. Recently, further modifications in composition, combined with processing precautions, have led to improvements in corrosion resistance.

Melting of virgin raw materials is performed in open air or vacuum induction furnaces. Low manganese concentrations (<0.3%) and deoxidation with ferrosilicon to achieve 0.7 to 1.0% Si in the alloy prevent excessive oxidation of the powder during water atomization in an inert gas (nitrogen)-purged atomization chamber. Typical water pressures for producing predominantly - 80 mesh powder are about 14 MPa (2 ksi).

Compositions and powder properties of commercial grades of stainless steel powders are given in Table 12. These powders normally are used in the as-atomized condition, although martensitic and ferritic grades may be annealed to improve green strength and compressibility. Particle shape (Fig. 6) and other powder characteristics are controlled to produce powders with apparent densities ranging from 2.5 to 3.2 g/cm3, adequate green strength (Fig. 7a), and good compressibility (Fig. 7b). In Fig. 7(a) and 7(b), the higher green densities are typically obtained for the lower alloyed grades. Also, low contents of interstitials (carbon, oxygen, nitrogen) improve green strength, compressibility, and corrosion resistance. For applications requiring high green strength, Acrawax or stearic acid, instead of lithium stearate, may be used as lubricant. However, the improvement of green strength comes at the expense of reduced compressibility.

Table 12 Commercial P/M grades of water-atomized stainless steel powder

Composition, wt%

Apparent

Flow

Sieve analy

sis, %

Cr

Ni

Si

Mo

Cu

Sn

Mn

C

S

P

Fe

O (ppm)

N (ppm)

density(a), g/cm3

rate(a), s/50 g

+100 mesh

-325 mesh

Austenitic grades

303

17-18

12-13

0.6-0.8

0.3(b)

0.03(b)

0.1-0.3

0.03(b)

bal

1000-2500

200-500

3.0-3.2

24-28

3(b)

40-60

303LSC

17-18

12-13

0.6-0.8

2(c)

1(c)

0.3(b)

0.03(b)

0.1-0.3

0.03(b)

bal

1000-2500

200-500

2.8-3.0

26-30

1-3

30-40

304L

18-19

10-12

0.7-0.9

0.3(b)

0.03(b)

0.03(b)

0.03(b)

bal

1000-2500

200-500

2.5-2.8

28-32

1-4

35-45

ULTRA 304L

19(b)

11(c)

0.8(c)

0.8(c)

2(c)

0.2(c)

0.02(c)

0.01(c)

0.01(c)

bal

2.7(c)

30(c)

3(c)

40(c)

316L

16.5-17.5

13-14

0.7-0.9

2-2.5

0.3(b)

0.03(b)

0.03(b)

0.03(b)

bal

1000-2500

200-500

2.6-3.0

24-32

1-4

35-45

316LSC

16.5-17.5

13-14

0.7-0.9

2-2.5

2(c)

1(c)

0.3(b)

0.03(b)

0.03(b)

0.03(b)

bal

1000-2500

200-500

2.9(c)

25(c)

1(c)

42(c)

317L

19(c)

14(c)

0.8(c)

3(c)

0.3(b)

0.3(b)

0.3(b)

0.3(b)

bal

1000-2500

200-500

2.9(c)

27(c)

2(c)

44(c)

SS100

20(c)

17(c)

0.8(c)

5(c)

0.2(c)

0.2(c)

0.3(b)

0.2(c)

bal

1000-2500

200-500

2.9(c)

29(c)

2(c)

45(c)

Martensitic grades(d)

410L

12-13

0.7-0.9

0.3(b)

0.3(b)

0.3(b)

0.3(b)

bal

1500-2500

200-500

2.9(c)

27(c)

39(c)

Ferritic grades

430L

16-17

0.7-0.9

0.3(b)

0.3(b)

0.3(b)

0.3(b)

bal

1500-2500

200-500

2.8-3.0

27-30

1-3

40-50

434L

16-18

0.7-0.9

0.5-1.5

0.3(b)

0.3(b)

0.3(b)

0.3(b)

bal

1500-2500

200-500

2.8-3.0

27-30

1-3

40-50

(a) Determined on unlubricated powder.

(d) With C <0.03%, this grade is ferritic; graphite addition to the low-carbon grade powder renders it martensitic upon sintering.

Fig. 6 Scanning electron micrograph of water-atomized 304L stainless steels. -100 mesh. 150x

Fig. 7 Typical compressibility and green strength ranges of stainless steel powders. Die lubricant, 1% lithium stearate. (a) Austenitic grades. (b) Ferritic and martensitic grades

To ensure that a stainless steel powder is free from contamination with iron, low-alloy steel, and other powders capable of producing galvanic corrosion, the copper sulfate or Ferroxyl test may be applied. Details of these tests are described in the section "Sintering of Stainless Steel" in the article "Production Sintering Practices" in this Volume.

Gas-Atomized Stainless Steel Powders. Gas (nitrogen and argon)-atomized stainless steel powders have particles that are spherical in shape (Fig. 8). These powders have high apparent densities of about 5 g/cm3 and excellent flow rates. They require special methods of consolidation such as hot isostatic pressing, cold isostatic pressing, or extrusion. Oxygen contents are similar to those of wrought stainless steels. Much emphasis is being placed on avoiding cross-contamination and contamination with ceramic (slag) particles. The latter are controlled to very small levels of concentration as well as to very small sizes, typically less than 25 /'m and preferably less than 10 t-' m. Such control is essential if fatigue life, toughness, and other properties are to be maximized and exceed those of I/M materials.

Fig. 8 Scanning electron micrograph of nitrogen-atomized 316L stainless steel. 65x

Commercially available grades include AISI grades 410, 440C, 446, 304L, 304, 347, 316L, 316, and 317. Chemical compositions are similar or identical to conventional wrought grades. Special compositional adjustments are unnecessary due to the absence of water during atomization and due to the special consolidation methods.

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