Powder Quality

Contamination-Free Stainless Steel Powder. Widely used commercial grades of stainless steel powders are summarized in Table 12. Those grades containing tin have superior corrosion resistance over their tin-free equivalents on account of an enrichment of tin on the surfaces of the sintered parts.

Table 12 Compositions of stainless steel powders

Composition1"3"1, %

Cr

Ni

Si

Mo

Cu

Sn

Mn

C

S

P

Austenitic grades

303

17-18

12-13

0.6-0.8

0.3(b)

0.03(b)

0.1-0.3

0.03(b)

303LSC

17-18

12-13

0.6-0.8

2"c"

1(c)

0.3®

0.03(b)

0.1-0.3

0.03(b)

304L

18-19

10-12

0.7-0.9

0.3®

0.03(b)

0.03(b)

0.03(b)

Ultra 304L

19(c)

11"c"

0.8(c)

0.8(c)

0.2(c)

0.02(c)

0.01(c)

0.01(c)

316L

16.5-17.5

13-14

0.7-0.9

2-2.5

0.3®

0.03(b)

0.03(b)

0.03(b)

316LSC

16.5-17.5

13-14

0.7-0.9

2-2.5

2"c"

1(c)

0.3®

0.03(b)

0.03(b)

0.03(b)

317L

19(c)

14(c)

0.8(c)

3W

0.3"b"

0.03(b)

0.03(b)

0.03(b)

SS100

20(c)

17(c)

0.8(c)

5(c)

0.2(c)

0.02(c)

0.03(b)

0.02(c)

Martensitic grade(d)

410L

12-13

0.7-0.9

0.3(b)

0.03(b)

0.03(b)

0.03(b)

Ferritic grades

430L

16-17

0.7-0.9

0.3(b)

0.03(b)

0.03(b)

0.03(b)

(a)

Oxygen content, 1000-2500 ppm; nitrogen content, 200-500 for all listed

grades.

(b)

Maximum.

(c)

Typical.

(d)

With C < 0.03%, this grade is ferritic. Graphite addition to the low carbon

grade powder renders it martensitic upon sintering.

When powder producers manufacture stainless steel powders in a facility or with equipment where cross contamination with iron or low-alloy steel powder is possible, the sintered parts are likely to have low-corrosion resistance because the electrochemically less noble contaminants will form so-called galvanic couples with the stainless steel particles in which the less-noble particles oxidize (corrode) (Ref 11). Austenitic stainless steel powders contaminated with ferritic stainless steel powders also form galvanic couples, that is, the corrosion resistance of a sintered austenitic stainless steel is significantly reduced if it is contaminated with ferritic stainless steel particles. The only exception to the above statement is when the contaminant particles dissolve during sintering, which is possible for fine particles (<38 /m) and high sintering temperatures (>1260 °C, or 2300 °F).

Contamination can also occur at the part producer's facility if stainless steel and carbon steel processing are done under one roof.

It is important to assure that a stainless steel powder is contamination free prior to pressing and sintering because after sintering, the identification of contamination as the cause for inadequate corrosion resistance can be rather difficult and time consuming. The authors have found that as little as 100 ppm of iron contamination in 316L can reduce corrosion resistance in aqueous NaCl by 50%. It is for this reason that stainless steel powders as well as green parts should be subjected to the so-called copper sulfate or Ferroxyl test prior to compacting and, additionally, prior to sintering if contamination in the part producer's facility is possible.

Copper Sulfate Ferroxyl Tests. For the detection of contamination of a stainless steel powder with a less-noble metal powder, two tests are available.

The copper sulfate test (ASTM 380) is based on the plating out of elemental copper from a concentrated copper sulfate solution onto particles that are less noble than copper, in accordance with:

The plated-out copper can be seen within minutes with the naked eye or preferably with a low-power microscope. A serious shortcoming of the copper sulfate test is that lubricants prevent the copper sulfate solution from properly wetting the powder or green compact.

The Ferroxyl test is based on the development usually within minutes, of a blue precipitate, Turnbull's blue, when elemental iron or ferrous ions react with hexacyanoferrate (III) in accordance with:

K+(aq) + Fe2+(aq) + [Fe(CN)6f(aq) ■ >KFeinFen(CN)6(s) (Turnbull's blue)

Ref 12 summarizes optimum solution strengths for performing this test. This test is also mentioned in ASTM Standard for Cleaning and Descaling Stainless Steel Parts (Ref 13).

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