Prealloy Diffusion Alloyed Irons

Prealloyed irons are completely alloyed with hardenability agents such as manganese, nickel, molybdenum, and chromium to provide improved dynamic properties upon densification and heat treatment.

Diffusion alloyed irons are produced by coreducing the iron oxide with the oxides of these elements to provide partial alloying. The advantage of partial alloying compared to prealloying is improved compressibility, allowing for higher green density at a given compaction pressure.

In rotating bending fatigue (RBF) testing, as well as in component testing, it has been found that these alloys provide improved fatigue strength at higher densities than the preblended steel compositions. These alloys are frequently specified for highly stressed components where fatigue strength and impact resistance are required.

Figure 7 shows the improvement in fatigue strength compared to tensile strength at two density levels after hardening. As shown, the fatigue strength improvement is most pronounced with increased density and combination of a prealloy with elemental nickel to form a composite microstructure of tempered martensite and nickel-rich austenite.

m CB

Cn c

Ultimate tensile strength, M Pa 1035 1105 1170 1240

1310

1310

FN-0405

FL-4405

F i .¿an*;

B

B

FLN-42C

5

b y

A /

^ A 4

j B/

A (T

A '

A B

7.0 g/cm 7.3 g/crn

1380 415

130 140 150 150 170 180 Ultimate tensile strength, ksi

flj CL

OJ D

Fig. 7 Influence of alloy content on fatigue strength of heat treated low-alloy steels

The process route taken to make a component also is a significant factor in developing optimal properties. Table 1 shows the comparison of properties attained from an FL-4405 alloy by a single press sinter (SPS), a double press double sinter (DPDS), and a DPDS coupled with a high temperature sinter (HTS). As shown, HTS combined with high density provides a substantial increase in static and dynamic properties after heat treatment.

Table 1 Influence of density and sinter cycle on heat treat properties for 0.85 Mo-0, 13 Mn-0.40C alloy

Sinter

Density

Sinter

Atmosphere

Sintered

Heat

Charpy

Apparent

Microhardness,

Carbon content,

cycle

g/cm3

temperature

ultimate

treated00

impact

hardness,

%

tensile

ultimate

HRC

strength

tensile strength

°C

°F

MPa ksi

MPa ksi

J

ft ■ lbf

Surface

Core

Surface

Core

Press

7.02

1120

2050

n2/h2

515 75

1205 175

16

12

47

65

60

0.83%

0.57%

and

sinter

DPDS

7.42

1120

2050

n2/h2

635 92

1380 200

27

20

53

63

54

0.77%

0.44%

DPDS

7.46

1285

2350

n2/h2

690 100

1535 223

47

35

56

63

51

0.73%

0.40%

and

(surface)

HTS

(a) Heat treat: carburize at 900 °C (1650 °F) for 2 h, oil quench and temper 175 °C (350 °F) for 2 h

(a) Heat treat: carburize at 900 °C (1650 °F) for 2 h, oil quench and temper 175 °C (350 °F) for 2 h

Preblended, prealloyed, and diffusion alloyed powders are being used in high-density components that are highly stressed during service. The component shape and process requirements usually dictate which type of powder is specified. If secondary machining is required, a less hardenable alloy may be necessary for improved machinability. All of these process requirements need to be considered to make the part cost effectively. The heat treatment must also be included in this analysis. These considerations need to be determined prior to design and manufacture of hard tooling. Dimensional change varies depending on the base powder type and composition as well as density and process conditions.

Figure 8 shows the dimensional changes occurring in three powder types at a given density with different graphite additions. Here, the transverse rupture bars of each material were sintered together at 1130 °C (2070 °F) for 30 min in N210%H2 atmosphere then quenched from 870 °C (1600 °F) in a fast oil and temperature at 180 °C (350 °F) for 1 h. As shown, significant variation can occur, depending on type of powder and the extent of additives. It is very difficult to change powder compositions once tooling has been developed and maintain part tolerances.

Fig. 8 Effect of carbon and heat treatment on size change. Density: 7.0 g/cm3. TR, transverse rupture

Heat Treatment of Ferrous Powder Metallurgy Parts

Howard Ferguson, Metal Powder Products Company

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