Sintering of Alloy Steels

Sintered alloy steels are made from various powder types that may be elemental, prealloyed, diffusion alloyed, or a hybrid mix of these types. The most common alloying method is mixing and compaction of elemental powders, which then alloy during sintering by diffusion. Mixing of elemental powders is advantageous because elemental powders have high compressibility due to the absence of solid solution strengthening. However, in order to achieve adequate homogenization, uniform composition and good diffusion rates must be maintained during sintering. Ferrous master alloys are sometimes used for base powders in admixed systems. Powders manufactured from ferrous master alloys can also be advantageous when the alloy elements are strong oxidizers (like chromium or manganese).

In contrast, prealloyed steel powders are alloyed during atomization and do not require homogenization during sintering. This allows a higher level of chemical and structural homogenity in sintered parts. Prealloyed powders are more expensive and have lower compressibility than elemental mixes, but they are used in high-performance parts that require structural integrity in demanding applications. Diffusion alloyed powders are an intermediate powder form where alloy elements are partially bonded onto base iron particles. Because the elements are not fully alloyed, the powders still have good compressibility, while the bonding improves the level of chemical homogeneity. Diffusion bonded powders typically involve iron alloying with nickel, molybdenum, and/or copper and can be sintered like elemental mixes. Prealloyed powders include a wider variety of possible compositions.

Sintering of Ferrous Alloys with Copper, Nickel, Molybdenum, and/or Phosphorus. A major constraint in conventional P/M alloy development is that typical production furnaces and atmospheres require elemental mixes and diffusion-alloyed powders based on elements with a low affinity for oxygen (copper, nickel, molybdenum, and phosphorus). Nickel, molybdenum, phosphorus, and copper have low affinity with oxygen (Table 5) which thus allows reduction of their oxides in typical mesh-belt furnaces (which cannot exceed 1150 °C and have typical atmosphere dew points around -10 to -40 °C). Under these typical production conditions, elements with high-oxygen affinity (chromium, manganese, and silicon) may not be adequately reduced during sintering. This limitation on alloying can be overcome to some extent by vacuum sintering, high-temperature sintering, or by using prealloyed powders.

Table 5 Alloying properties of selected elements in iron

Alloying element

Hardening

Diffusion

Enthalpy of

Grossman

coefficient

oxide formation,

factor(a)

(Dx/DFe)(b)

kJ/g ■ atom

of oxygen

Copper

1.7

1

-150

Nickel

1.4

0.5

-250

Cobalt

2

0.5

-270

Tungsten

4.5

-270

Molybdenum

3.7

5

-310

Iron

-350

Chromium

3.1

5

-540

Manganese

4.5

2.5

-500

Vanadium

5

-620

Boron

-650

Silicon

1.7

10

-680

(a) Grossman factor for 1 wt% alloy.

(b) Diffusion coefficient of alloying element in ! -iron (Dx), self-diffusion coefficient of ) -iron (D¥e)

(a) Grossman factor for 1 wt% alloy.

(b) Diffusion coefficient of alloying element in ! -iron (Dx), self-diffusion coefficient of ) -iron (D¥e)

Nickel-Steel. Sintering of mixed iron-nickel powders is subject to inhomogeneous alloying due to the low diffusion rate of nickel into iron (Table 5). Fine (carbonyl) nickel powder should be used when mixing with iron powder, and the sintering temperature should be at least 1100 °C (2010 °F) or preferably up to 1200 °C (2190 °F).

Cu-Ni-Mo Steels. Copper-nickel steels are sintered at temperatures up to 1150 °C, and the temperature must be increased to at least 1200 °C when molybdenum is added (Ref 3).

Phosphorus Steel. Unlike wrought forms (where phosphorus causes grain boundary embrittlement), phosphorus is a common alloying element in P/M Steels. phosphorus P/M steels provide a unique combination of strength and ductility, which is attributed to the formation of a eutectic liquid phase in the initial stages of sintering (starting at 1050 °C) and the stabilization of ferrite even at normal sintering temperatures of 1120 °C (2050 °F).

Sintering of Ferrous Alloys with Chromium, Manganese, and Silicon. Chromium, manganese, and silicon have a higher affinity with oxygen (Table 5), which thus requires cleaner atmospheres and high enough temperatures for adequate oxide reduction (Fig. 2b) during sintering. A nitrogen atmosphere with a small amount of hydrogen and hydrocarbons can ensure oxide reduction (Ref 3). Another general approach is to tie up the reactive alloying elements in compound form by adding alloy powders from either ferroalloys (ferrochromium, ferromanganese, and ferrosilicon) or as carbides. Addition of carbides (or carbon ferroalloys) has a low-alloying cost, but carbides increase tool wear and require sintering temperatures above 1200 °C (2190 °F) in typical production atmospheres (Ref 3). This practice is not prevalent.

Chromium Steel Sintering (Ref 3). Chromium steel parts are sintered from powder mixes, prealloyed powders, or from special master alloys based on carbides, sigma-phase compositions, and other complex alloying with carbides. To obtain a homogenous structure from sintering of powder mixes, the sintering temperature should be above 1250 °C (2280 °F) with the formation of a liquid phase in the presence of carbon. Effective carbon control is a decisive factor, but high strength chromium alloy steel can be achieved (Table 6).

Table 6 Properties of chromium alloyed steels prepared from elemental powders

parameters

Density, g/cm3

Dimensional change, %, linear

Ultimate tensile strength

MPa

Yield strength (0.2 offset), MPa

J

HV30

/'m

Fe-2% Cr-0.7% C

<25 'm

1 h/H2

7.23

0.83

937

740

2.9

38

274

2 h/vac

7.36

0.26

863

546

10.9

58

230

45-63 m

1 h/H2

7.26

0.65

643

544

3.5

10

273

2 h/vac

7.34

0.3

755

490

9.2

38

210

Fe-1.0% Cr-0.5% Mo-1.0% C

<25 ■ m

1 h/H2

7.16

0.92

927

776

3.4

38

278

2 h/vac

7.35

0.18

923

635

8.0

29

266

Bondaloy

1 h/H2

7.29

0.38

948

800

4.2

29

285

2 h/vac

7.45

-0.33

961

648

5.1

29

275

Fe-1.0% Cr-0.5% Mo-1.0% C

<25 'm

1 h/H2

7.17

0.89

855

718

1.0

1.9

278

Compacted at 1200 MPa, sintered at 1 h at 1270 °C/H2 or 2 h at 1270 °C/vacuum as listed. vac, vacuum

Compacted at 1200 MPa, sintered at 1 h at 1270 °C/H2 or 2 h at 1270 °C/vacuum as listed. vac, vacuum

The problem of sintering chromium steels at high temperatures can also be solved by adding chromium to the iron powder in the form of low-melting master alloy. Higher hardness was achieved in a Fe-1.5Cr-0.25C sintered steel based on a low-melting master alloy (33%Cr5%C-8%MN+Ni+Mo+P) as compared to chromium carbide or ferrochromium additions. With the low-melting master alloy, the material can be effectively sintered at 1120 °C (Ref 3).

Manganese Steels. Manganese is one of the least expensive alloying elements for iron, but is not widely used in P/M steels due to high reactivity with oxygen. It can be added to iron in elemental form or as ferromanganese alloys. It is also added with other elements as low-melting master alloys (Table 7).

Table 7 Chemical composition of master alloys for alloying powder steels with manganese for liquid-phase sintering

Chemical composition, wt%

T1,

s

\T,

Mn

Ni

Cr

Mo

Fe

Cu

Si

OC

OC

OC

40

30

15

5

10

1171

999

172

44

25

11

19

1204

1166

38

55

18

3

8

14

2

1129

943

186

56

24

3

6

11

1157

1032

125

47

20

13

6

14

2.5

1210

1166

45

75

25

1054

982

72

74

12.5

12.5

1

1060

927

133

36

30

18

6

10

1207

1096

111

41

25

18

6

10

1216

1077

139

38

23

18

6

15

1229

1093

136

64

16

10

10

1188

1132

56

56

14

15

15

1260

1093

167

56

14

15

5

10

1227

1102

125

59

11

15

5

10 . . .

1249

1116

133

53

17

15

5

10 . . .

1216

1093

123

56

14

22

8

1335

1049

286

50

20

15

5

10 . . .

1204

1143

61

46

24

15

5

10 . . .

1204

1088

116

72

14

. . . 14

2

1043

966

77

T1, liquidus temperature; Ts, solidus temperature. Source: Ref 3

T1, liquidus temperature; Ts, solidus temperature. Source: Ref 3

Manganese steels can be sintered efficiently at 1120 °C in typical production atmospheres due to self gettering of the atmosphere. Manganese sublimation occurs at sintering temperatures and manganese vapor reacts with oxygen in the furnace atmosphere, which in effect cleans oxygen from the furnace atmosphere (Ref 3). Table 8 presents mechanical properties of manganese steels prepared from atomized iron powder and electrolytic manganese sintered at different temperatures. The properties change only slightly if ferromanganese is used instead of electrolytic manganese. Dimensional stability was obtained at 2 to 3% Mn.

Table 8 Effect of sintering temperature and time on mechanical properties of Fe-Mn steels

Alloy

Sintering

Density,

Yield strength,

Ultimate tensile

Elongation,

°C

Time, min

g/cm3

%

strength, %

%

Fe-2Mn

1200

50

7.03

170

300

11.0

1200

100

7.05

220

340

10.0

1250

50

7.06

200

320

10.0

1280

100

7.06

200

330

15.0

Fe-4Mn

1200

50

6.9

340

490

3.5

1200

100

6.91

320

530

4.0

1280

50

6.92

330

530

4.0

1280

100

6.92

330

550

4.0

Fe-6Mn

1200

50

6.79

370

600

3.2

1200

100

6.81

420

650

4.0

1280

50

6.8

450

610

2.5

1280

100

6.87

440

620

4.0

Manganese Silicon Steels. Iron silicon materials are not used in the fabrication of P/M parts due the excessive shrinkage of parts during sintering. However, the shrinkage is reduced by the addition of manganese in elemental form or by master alloys. To increase activity, additions for liquid phase sintering are also considered (Ref 3).

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