Mechanical Properties

PM components are designed to meet structural criteria in many applications. It is possible to produce sintered structural or mechanical parts with properties equal to and even superior to those of parts made by more traditional routes.

The structural performance of a PM component usually depends on several mechanical properties either alone or in combination.

Unnotched Charpy impact strength figures for many PM materials range from 5 to around 35J.

  • Higher values are found in the case of high relative densities (>93-95%).
  • The ability of a material to resist crack initiation and propagation can be quantified only by adopting fracture mechanics concepts.

Fracture toughness values of PM materials are lower than that of wrought materials; in the case of PM steels with densities in the range 6.5 to 7.0 g/cm³, fracture toughness increases and crack propagation decreases.

  • However, the most important and experimentally verified aspect shows that in cyclic loading near the fatigue limit PM steels with a density of about 7.1 - 7.5 g/cm³ cracks propagate more slowly in comparison with nodular cast iron and wrought heat treatable steels.

This effect is due to the presence of pores which compensate for the weakening of the structure.

When components are designed with low tensile strength in critical features, such as gear teeth, the designer can use conventional design formulas.

  • Where strength of a feature is critical, it is essential to work with the PM manufacturer to optimise the design for manufacturing, determine the strength that can be reasonably specified, and establish the test and inspection procedures for maintaining performance.
  • Improvements in the strength could be achieved by filling the surface connected pores with a liquid metal that has a lower melting point (infiltration)
  • Elimination of flaws- using hot isostatic pressing.

Because tensile strength (Rm) is the oldest property used to characterise the mechanical behaviour of PM materials, and since there is a lot of experience behind the interpretation of its figures, designers have had good reasons for specifying it on their drawings.

  • They are, of course, right if the application actually requires Rm as a mandatory design value, to be directly used in calculations;
  • however, strength and fatigue concepts should not be mixed up: if parts are subject to dynamic loading, the conclusion about the performance of the component in service may only be approximate or even wrong when Rm (alone or with other properties, like elongation and impact strength) is used to establish the suitability of a material for an application where fatigue is present.

In mechanical design yield strength usually is more important than tensile strength: designers must take into account that dimensional stability is guaranteed only when stresses due to service loading are below this threshold value.

Material

Sintering Cycle

r

g/cm3

E GPa

Rm MPa

Rm MPa

Hardness

A5 %

J*

Static

Cyclic

HB

2.5/62.5

HV10

(HV20)

Fe-1.5Cu

SL

SL

PS+R+SL

6.8

7.1

7.4

145

152

190

244

286

340

195

228

235

202

235

270

85

100

122

-

9.1

8.8

13.3

14.7

29.7

66.0

Fe-1.5 Cu-0.6C

SL

SL

PS+R+SL

6.9

7.2

7.4

135

153

182

433

483

500

379

418

470

350

398

460

122

142

160

-

3.6

4.4

6.8

9.5

12.1

35.3

Fe-0.45P

SL

SL

PL+R+SL

6.8

7.1

7.4

142

169

202

253

332

450

238

280

355

238

285

405

93

113

138

-

3.1

5.6

14.9

11.0

27.0

38.0

Fe-2Cu-2.5Ni

SH

SH

PS+R+SH

6.8

7.1

7.4

136

160

192

289

336

437

272

308

371

317

351

415

93

116

130

-

3.9

6.9

11.4

15.1

32.7

73.3

Fe-1.75Ni-1.5Cu

-0.5Mo-0.6C

SL 7.12 - 610 435 - - 210 3.0 24.0

Fe-4Ni-1.5Cu-

-0.5Mo-0.5C

SL

SH

7.25

7.3

-

700

850

370

480

- -

210

290

2.0

2.2

27.3

27.3

Fe-4Ni-2Cu-1.5Mo

-0.5C

SL

SH

7.1

7.1

-

1050

1180

550

700

- -

275

335

3.1

2.0

25.0

22.0

Fe-8Ni-1Mo-0.5C

SL 7.35 160 1020 570 725 - 340 4.5 -

Fe-0.8Mn-0.95Cr-

0.24Mo-0.6C

SL 7.03 168 713 615 - - 218 3.9 27.3

HEAT TREATED PM ALLOY STEELS

Fe-1.5Cu

SH+CN

SH+CN

6.8

7.1

137

158

635

770

- - -

537

582

<1

<1

6.3

7.8

Fe-1.5Cu-0.6C

SH+H1

SH+H1

6.8

7.1

137

145

750

810

737

773

730

770

-

252

312

1.3

1.6

5.7

8.0

Fe-1.75Ni-1.5Cu-

-0.5Mo-0.6C

SH+H2 7.12 - 1100 1050 - - 400 1.8 -

Fe-4Ni-1.5Cu-

0.5Mo-0.5C

SH+H3 7.25 - 1050 970 - - 420 0.9 15

Fe-0.8Mn-0.95Cr-

0.24Mo-0.6C

SH+H4 7.14 168 1177 1125 - - 329 1.4 15.3

Fe-2Cu-1.5Mo-0.4C

PS+R+SL+H3 7.52 - 1510 - - - 430 2.0 -

* - Unnotched Charpy test
SL - Sintered at 1120-1150°C x 20-30 min (standard sintering conditions for PM steels), in N2+H2 atmosphere
SH - Sintered at 1250-1280°C x 30-60 min (high sintering temperature), in N2 + H2
PS+R - Pre-sintered at 750-800°C x 30 min and re-pressed
CN - Carbonitrided at 920°C x 3.5h, quenched at 60°C in oil
H1 - Austenitized at 840°C x 1h, quenched at 60°C in oil, tempered at 430°C x 1h
H2 - Austenitized at 850°C x 30 min, quenched at 50°C in oil, tempered at 175°C x 1h
H3 - Austenitized at 860°C x 30 min, quenched at 60°C in oil, tempered at 175°C x 1h
H4 - Austenitized at 900°C x 30 min, quenched at 60°C in oil, tempered at 300°C x 1h

Data sources: Publications of C M Sonsino and Höganäs pamphlets on partially pre-alloyed iron powders,
published in 'Guide to Design of Sintered Parts' and reprinted with permission of Assinter, Italy

Material

Sintering Cycle

r

g/gm3

E

GPa

Rm

MPa

Rr0.2

MPa

(static)

HRB

A5

%

AISI 304L SL DA
SH DA
SH V
6.4
6.5
6.6
105
115
120
260
360
260
200
225
90
55
55
<0.5
5
12
AISI 316L SL DA
SH DA
SH V
6.4
6.5
6.6
105
115
120
240
380
240
170
230
100
55
55
20
<0.5
5
12
AISI 410 SL DA + HT 6.5 125 >620 - 23
(HRC)
<0.5
AISI 430 SL V
SL V
6.4
6.6
- 350
430
180
260
82
91
-

SL DA - Sintered at 1150°C, (in dissociated ammonia: nitrogen pick up!)
SH DA - Same as SL DA, but sintered at 1290°C SL V - Sintered at 1150°C in pure hydrogen or vacuum
SH V - Same as SL V, but sintered at 1290°C
HT - Hardening and stress relieving

Data source: National and International Standards,
published in 'Guide to Design of Sintered Parts' and reprinted with permission of Assinter, Italy

Material

r

g/cm3

E

GPa

Rm

MPa

Rr0.2

MPa

(static)

 

Hardness

HV5HRH

A5

%

Copper (99.5%)

8.3

8.5

-

180

200

60

160

40

65

66

81

25

15

Brass (8-12Zn)

7.8

8.2

51.7

54.5

115

140

65

80

30

35

61

63

9

11

Brass (20Zn-1-2Pb)

7.8

8.2

69

82.7

150

190

90

110

35

40

63

66

24

28

Bronze (9-11 Sn)

7.4

7.8

37.9

40

140

190

110

130

35

40

63

66

4

5

Nickel Silver (18Ni-18Zn)

8.0 76.8 220 140 45 69 11

Notes on sintering cycles:
Sintering temperatures are:
- 930-1000°C for copper; densities >8.3 g/cm3 require re-pressing
- 815-930°C for brass and nickel silver
- 800-875°C for bronze

Data source: National and international standards, published in 'Guide to Design of Sintered Parts' and reprinted with permission of Assinter, Italy

Material

r

g/cm3

E

GPa

Rm

MPa

Rr0.2

MPa

(static)

Hardness

HV5 HRH

A5

%

2014

2.50
2.64
49
59
150
180
115
150
- 70
75
3
3

2014-T6

2.50
2.64
49
59
250
300
235
280
- 85
90
1
2

6061

2.42
2.55
47
56
100
125
65
80
62
65
- 4
6

6061-T6

2.42
2.55
47
56
140
210
130
195
72
-
-
80
0.5
2

7075

2.51 49 205 150 90 - 3

7075-T6

2.51 49 310 275 - 80 2

Notes:
All properties measured after sintering and sizing
T6 = solution hardening and artificial ageing

Date Source: E Mosca, Powder Metallurgy - Criteria for Design and Inspection, published in 'Guide to Design of Sintered Parts' and reprinted with permission of Assinter, Italy

Creep is very slow plastic deformation occurring at stress levels below the yield point.

  • PM components may offer economical alternatives to injection-moulded plastics, composites and die-castings designed for temperatures to 400° F (205° C).

For example, a redesign for PM may require fewer or smaller fasteners, require less thread depth in tapped holes, allow threaded fasteners in tapped holes rather than through bolts and nuts, and eliminate the need for steel inserts to distribute concentrated loads.

When hardness is measured on a porous material, the size of the identation is the combination of two factors:

  1. the resistance of the base material to the penetration of the indenter
  2. and the resistance to plastic deformation and fracture of the welds which characterise the sintered structure, stressed beyond its yield point.

Thus compared with dense materials, hardness values of PM materials are always more scattered and scatter increases with decreasing test load.

  • For example, in the case of PM steels because there is a decrease in the number of pores whose effect is averaged during the penetration of the indenter;
  • ISO standards prescribe that the average value be indicated on drawings.

Hardness can also for an indirect and rough evaluation of Rm of PM parts, provided they are as-sintered and homogeneous as regards structure and porosity distribution; it cannot be used in the case of heat treated PM parts.

Elongation values give a good indication of the material's ductility, ie. its ability to absorb substantial amounts of energy in case of breakage.

  • For PM designers are limited to elongation values ranging between 1 and about 10%.
  • With the same alloy composition a decrease of density also decreases elongation values; the decrease is steeper at a relative density higher than about 90% (transition from closed to open porosity).

MOE is directly affected by material density.

  • Tabulated values foe MOE indicate lower values for PM alloys than for equivalent wrought and cast alloys, higher values than for die cast alloys, and much higher values than plastics and most composites.
  • Higher MOE may allow opportunities to reduce wall thickness and eliminate reinforcing features, such as gussets and ribs, when redesigning plastics, composites and die-castings for PM.

The relation between Young's modulus (E) and density is approximately linear for relative densities higher than 80%:

  • E value is halved (E ~ 210 Gpa in case of iron) by decreasing the density in the indicated range;
  • the decrease becomes steeper with porosity greater than 20%.

For Poisson's ration (v) an increase of porosity up to 15-20% for ferrous PM materials involves a gradual decrease of the value of v from ~ 0.29 to ~ 0.26.

Comparison of parts manufacturing processes in terms of shaping capabilities
Property Investment casting MIM
Min. bore diameter 2mm 0.4mm
Max. depth of a 2mm dia blind hole 2mm 20mm
Min. wall thickness 2mm <1mm
Max. wall thickness unlimited 5mm
Tolerance at 14mm dimension +/- 0.2mm +-0.06mm
Surface roughness Ra 5µm 4µm

(Schlieper, EPMA MIM Short Course, Delft, September 1992)

Dent resistance is the ability of a component to withstand impact without permanent deformation.PM materials with dent resistance are not readily handled by standards methods.

This specific property can be increase leading to the reduction in porosity of the surface layer, increasing the surface hardness and the wear resistance.

Could be achieved by the filling of some of the porosity.

PM parts working in mechanical devices usually require no protection for corrosion because the presence of lubricants guarantees safe operation.

However, where PM parts are exposed to high humidity or corrosive media they are more prone to corrosion than their wrought counterparts.

Surface pores increase the surface area exposed to the environment and they can also act as pockets where corrosive media have an enhanced effect.

  • Corrosion resistance can be assured by the use of stainless steels, or by zinc plating followed by chromating of PM steel components.
  • Steam treatment and resin impregnation also make PM parts less prone to corrosion.

Material

r

g/cm3

Bmax

Br

µ max

Hc

µWcm

T

kG

T

kG

A/m

Oe

Pure iron

6.9
7.4
1.3
1.5
13
15
0.85
1.2
8.5
12
2000
4000
175
127
2.2
1.6
12.5
11

Fe-0.45P

7.0
7.4
1.4
1.55
14
15.5
1.0
1.2
10
12
3200
4500
127
80
1.6
1.0
19
18

Fe-3Si

7.1 1.3 13 1.1 11 5000 70 0.9 50

Fe-50Ni

7.1
7.5
1.15
1.3
11.5
13
0.8
0.9
8
9
11000
15000
24
20
0.3
0.25
70
60

Fe-50Co

7.25 1.7 17 0.56 5.6 2000 159 2.0 60

Data sources:
Published in 'Guide to Design of Sintered Parts' and reprinted with permission of Assinter, Italy

The temperature ranges in which most PM components operate have little effect on copper, iron, steel and stainless steel alloys. In those applications where anticipated temperatures are high or low enough,alloy formulation can be modified to improve the properties.

The stability of mechanical properties at elevated temperatures often makes PM an economical alternative to injection moulded plastics, composites and die castings, which can experience loss of strength and reduced MOE at approximately the same temperatures that induce creep.
When redesigning for PM, design economies discussed previously for MOE, strength and creep may be applicable.

PM alloys exhibit an endurance limit, as do cast and wrought alloys of similar composition. The endurance limit increases with increasing component density.

  • Fatigue performance of a PM component relative to tensile strength is reasonably consistent.
  • An increase in mechanical properties is achieved when pores are filled with an organic rather than metallic material.
  • The operation prevents the entry of potentially corrosive electrolyte during subsequent plating operations.

Considering what has been said before on fracture toughness, fatigue endurance limits increase with increasing density.

  • Fatigue strength of unnotched PM steels is lower than that of wrought steels;
  • however, in the presence of notches (Kt > 2.0) the difference practically disappears, provided that density is 7.1 g/cm³ or more.
  • Moreover, porous materials are less sensitive to an increase of Kt than their dense counterparts.
  • Of course there is an increase of notch sensitivity with increase of hardness.

Fatigue strength is greater under bending than axial loading, due to the difference in the influence of pores, which is less severe in the case of bending.

An improvement in strength can be obtained by local densification of the already sintered part, but the highest improvements are obtainable by means of post sintering heat treatments.

Carburizing or carbonitriding provide the maximum gains as residual compressive stresses can be introduced.

Material

r

g/cm3

 

Prod Cycle

Endurance Limits - N/mm2; Ps = 50%

Kt=1

Kt=2.8

R=0

R=-1

R=0

R=-1

SAE 1015

7.87   172 190 86 95

Nodular cast iron GH40-60

7.30   133 160 65 78

Fe-0.45P

6.8
7.1
SL
SL
77
108
91
126
45
55
56
68

Fe-1.5Cu

7.1 SL 81 100 48 58

Fe - 1.5Cu Carbonitrided

7.1 SL + CN 164 245 110 158

Fe - 1.5Cu - 0.6C

7.2
7.4
SL
PS+R+SL
130
145
165
172
64
80
84
100

Fe-1.5Cu-0.6C hardened and

tempered

7.1 SL+H1 155 222 70 102

Fe-2.0Cu-2.5Ni

7.1 SH1 119 145 64 79

Material

r

g/cm3

Prod Cycle

Endurance Limits - N/mm2; Ps = 50%

Kt=1

Kt=2.0

R=1

R=-1

R=0

R=-1

SAE 1015

7.87 - 190 210 140 155

Nodular cast iron GH40-60

7.30 - 151 182 112 135

Fe-0.45P

6.8

7.1

SL

SL

86

135

100

153

70

84

86

100

Fe-1.5Cu

7.1 SL 123 144 90 105

Fe - 1.5Cu Carbonitrided

7.1 SL+CN 292 477 193 300

Fe - 1.5Cu - 0.6C

7.2

7.4

SL

PS+R+SL

127

146

160

198

102

115

137

97

Fe-1.5Cu-0.6C hardened and

tempered

7.1 SL+H1 214 307 119 173

Fe-2.0Cu-2.5Ni

7.1 SH1 117 146 105 124

Fe-4.0Ni-1.5Cu-0.5Mo-0.6C

- SH1 - 164 - 153

Fe-0.8Mn-0.95Cr-0.24Mo-0.6C

7.2 SH2 - 298 - -

Fe-0.8Mn-0.95Cr-0.24Mo-0.6C

hardened and tempered

7.2 SH2+H2 - 395 - -

Ps - Probability of survival. Safety margins for the design: divide by 1.6-2 the values given in the tables and compare with the maximum local stress calculated for the part.
Kt = notch factor
R = õmin/õmax = ratio between minimum and maximum cyclic stresses
(R = 0, pulsating loading; r = -1, alternating loading)
SL = sintered at 1120°C for 30 min; SH1 = 1250°C for 30 min; SH2 = 1280°C for 40 min
PS + R = pre=sintered at 750-800°C for 30 min and re-pressed
CN = carbonitrided at 920°C for 3.5h, quenched in oil at 60°C
H1 - austenitised at 840°C for 1h, quenched in oil at 60°C, tempered at 430°C for 1h
H2 = austenitised at 900°C for 30 min, quenched in oil at 60°C, tempered at 300°C for 1h

Data source: C M Sonsino; private communication, published in 'Guide to Design of Sintered Parts' and reprinted with permission of Assinter, Italy

The mechanical properties of structural PM components are influenced mainly by the residual porosity.

  • Porosity reduces the amount of metal actually present in a given section of the part and when it is loaded the pores themselves can act as stress raisers.

In wrought materials tensile strength can be controlled by the composition of the alloy and the structural changes brought about through heat treatment. Similar strength levels can be achieved in PM components by raising the density level but not adding alloying elements, or by maintaining an acceptable level of porosity but adding alloying elements.

Yield strength follows the same pattern. However, these are extremely intricate inter-relations, and it is therefore important to discuss with the PM producer the choice of both material and production cycle.

Porosity affects the magnetic properties of PM materials.

  • It must therefore be minimised when high induction (Bmax) is needed;
  • remanence (Br) is affected in the same way.

Pores also limit the mobility of domain walls, i.e. an increase in density gives higher values of permeability (µ) and lowers coercive force (Hc).

  • Other aspects, like grain size, presence of work hardening (annealing may be required), presence of impurities in the lattice (C, N), have the same importance as in the case of dense materials.

Recently, new magnetic powders based on iron where the single particles are coated with an insulating resin have been developed for AC applications (low eddy current losses).

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