Kawasaki Process

Kawasaki Steel Corporation has produced water-atomized iron powder (KIP 304AS grade) with high compressibility (7.05 g/cm3 green density at 1% zinc stearate pressed at 490 MPa), higher purity, improved compactibility, lower inclusion content, high green strength, and low ejection force. These property improvements are achieved by the stricter selection of scrap, more rigorous refining in the electric arc furnace, extended high-temperature first annealing, and the incorporation of a second annealing.

Figure 17(a) shows the new Kawasaki process in comparison to standard water-atomization processing (Fig. 17b). Scrap is selected more carefully than for the standard operation, and the melt is atomized by water applied at a 30% lower pressure and 25% lower volume than in the standard operation. The reduced volume of atomizing water facilitates a higher degree of particle spheroidizing, which thus increases the apparent density of the primary powder. The lower water pressure results in an increase in average particle size to about 120 /,!m. which is almost twice the size of primary powders from the standard process.

Scrap 1

Melting and refining 1

Atomizing 1

Drying 1

Primary powder

Fir&t annealing 1

First crushing and classifying

Second annealing 1

Second crushing and classifying

Secondary powder (product)

Scrap

Melting and refining

Atomizing

Drying \

Primary powder

Annealing \

Crushing and classifying

Secondary powder (product)

(a) Optimized process (b) Standard process

Fig. 17 Kawasaki manufacturing processes of atomized iron powders. (a) Optimized process of powders with high compressiblity and green strength. (b) Standard process

The primary powder is annealed in hydrogen at a slightly higher temperature and a longer duration than the anneal in the standard process. After annealing, crushing and classification are executed. The first crushing in the new process is harder than that under standard conditions. In addition, a second anneal crush and classification sequence is carried out. The second crushing was softer, and the sieve size of the second classification was 44% larger than under standard conditions.

The purpose of this modified production process is to obtain powders that allow high compressibility, low ejection force, and high green strength during high-pressure pressing. High compressibility improves die life, while lower forces for ejection reduce the potential for cracking of more complex shapes during part compaction and transfer.

However, compressibility and high green strength require careful optimization of particle shape. Compressibility (or fill density) increases for spheroidized particles, while compact green strength depends on particle "entanglements" that are enhanced by more irregular shaped particles. Therefore, the compatibility of high compressibility and improved compactability (green strength) requires optimization of factors besides just particle shape. Green strength is enhanced by preventing excessive spheroidization, while compressibility is improved by other factors such as reductions in impurities and inclusions, reduced dislocation density, and greater coarseness in the particle size distribution. Accordingly, various manufacturing conditions are optimized in the Kawasaki process to obtain iron powder with a good combination of green strength and compressibility (Fig. 18) and low ejection force requirements (Fig. 19). The characteristics of powders A, B, C, and D are summarized in Table 11.

Table 11 Characteristics of secondary iron powder compared in Fig. 18 and 19

Process conditions

Flow rate

Particle size

distribution, %

(s/50 g)

+180 .'m

+150 m

+106 m

+75 .'' ■' m

+63 "m

+45 m

-45 ■m

Powder A

Optimized (Fig. 17a)

23.5

5.2

14.2

24.7

24.0

8.2

13.5

10.2

Powder B

Standard (Fig. 17b)

25.4

0.1

7.3

19.3

26.7

10.0

12.5

24.1

Powder C

Commercial iron powder

21.6

1.5

8.4

19.4

26.2

9.6

16.4

18.5

Powder D

Commercial iron powder

25.1

0.1

4.0

15.3

22.9

9.5

14.0

34.2

Fe + 1%Z

tiSt Powder A

-P

owder C

i

'/y

Powder E

7 Pow

der D

400 500 600 700 Compacting pressure, M Pa

400 500 600 700 Compacting pressure, M Pa soo

Fig. 18 Relationship between green density and compacting pressure for four iron powders in Table 11. Source: Ref 25

26 25

« F

24

B

hi

£

22

o

(d

21

lu

26 25

20 19

Fe + 1

%ZnSt

Pov

vder B

Powd

C

/

/

/

Powell

Br D /

'/

/>0

*der A

6.80 6.90 7.00 7.10 7.20 7.30 7.40 Compacting pressure, M Pa

6.80 6.90 7.00 7.10 7.20 7.30 7.40 Compacting pressure, M Pa

Fig. 19 Relationship between ejection force and green density for four iron powders in Table 11. Source Ref 25

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