Sintering of Ferrous Materials

Pressed and sintered iron-base materials represent the largest segment of the P/M parts industry. Ferrous P/M parts are made from iron powders, alloy steel powders, or mixtures of (a) iron and graphite powders, (b) iron (or steel) and copper powders, or (c) iron, copper, and graphite powders (with or without powders of other metals). The overall sintering process consists of several stages and temperature regimes (Table 2). Excluding lubricant burnoff, the following reactions occur during sintering of ferrous powders:

• Metallurgical bonds develop from the mechanical interlocks between powders in the compact.

• Metal oxides are reduced by reaction with the carbon from the blended graphite powders.

• Carbon alloying of the iron powder is obtained by diffusion of carbon from graphite powder.

• Densification of P/M compacts is achieved by shrinkage and the reduction of porosity.

Table 2 General temperature regimes during steel sintering

Process stage(a)

Temperature range, °C

Lubricant removal, in the liquid state

150-200

Lubricant removal, by gaseous decomposition

300-600

Refining (reduction of iron oxide)

750-1000(b)

Carbon diffusion

900-1050

Diffusion of alloy additions

1050-1300

Carbon pick-up by the atmosphere (in some cases)

1000-1300

Carbon restoration

1050-800

Microstructure formation

950-600

Final cooling, under reducing or "inert" conditions

Cool from 600 to 50

Not all stages apply and some stages may overlap.

Reduction of metal oxides other than iron oxide may require higher temperatures depending on the element and furnace atmosphere.

Conventional sintering generally is conducted at 1120 °C (2050 °F) in mesh belt conveyor furnaces with conveyor mesh belts made from nickel-chromium alloy wire. Mesh belt furnaces can be used at temperatures up to 1150 °C (2100 °F) only, due to the temperature limitation of the belt material. For high-temperature sintering applications, walking beam furnaces have been successfully used for continuous operation at temperatures up to 1315 °C (2400 °F). Furnace temperatures rarely exceed 1350 °C (2460 °F) in ferrous P/M operations (Ref 3).

Sintering temperatures depend on the material and desired properties. Alloy steels typically require higher sintering temperatures (Table 3) and times to promote homogenization. Typical sintering practices for several ferrous materials, with an emphasis on the types of atmospheres used and the resulting as-sintered microstructures and properties, are described in this section. In practice, sintering parameters are usually determined empirically from a combination of technical requirements and economic factors (and furnace type).

Source: Ref 2

Table 3 Sintering temperatures for various iron-base alloys

Material

Sintering temperature^"1, °C

Fe, Fe-Cu, Fe-Cu-Ni

1120-1280

Fe-C, Fe-Cu-C, Fe-Cu-Ni-C

1120-1150

Fe-Cu-Ni-Mo (Distaloy)-C

1120-1200

Fe-Mn, Fe-Mn-C

1120-1280

Fe-Cr, Fe-Cr-C

1150-1280

Fe-Mn-Cu

1120

Fe-Cr-Cu

1200-1280

Fe-Mo, Fe-Mo-C

1120-1280

Fe-Cr-carbide

>1280

Fe-V carbide

>1280

Fe-Ti-carbide

1200-1280

Fe-W-carbide

1200-1280

Fe-Mn-Cr-Mo-C, Fe-Mn-V-Mo-C

>1280

Fe-Cr-Ni

(a) Temperatures represent the range of typical or possible sintering temperatures.

Source: Ref 3

(a) Temperatures represent the range of typical or possible sintering temperatures.

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