Introduction

SINTERING is a complex high-temperature process that consolidates and strengthens loose or compacted particles into a more dense coherent body. During sintering powder particles form coherent bonds and densify by pore shrinkage. There is no universally applicable theory of sintering, and much is not understood about the process; but sintering generally consists of six distinct, but not necessarily sequential, stages:

• Initial particle bonding

• Pore channel closure

• Pore rounding

• Densification or pore shrinkage

• Pore coarsening

A comprehensive and systematic theory of sintering has not been established due to the multitude of variables that influence sintering. Nonetheless the major factors that influence sintering response can be classified in terms of transport mechanisms, reaction types, and sintering parameters. These major factors include the following variables:

Mechanism of material transport

• Surface diffusion

• Lattice or volume diffusion

• Grain boundary diffusion

• Evaporation and condensation

Types of sinter reactions

• Single-component sintering of metals, oxides, or hydrides

• Multiple component sintering of solid solutions, intermetallic, or mechanically bonded systems

• Activated sintering with additives

Sinter Conditions

• Rate of heating and cooling

• Sintering time

• Sintering atmosphere

• Sintering temperature

Sintering time and temperature are the most significant factors from a practical perspective, with temperature being the most important variable. Particle size, compact porosity, and powder type (mixed, prealloyed, and diffusion-alloyed) also influence sintering practices.

Table 1 lists typical sintering temperatures for various P/M materials and ceramics. Sintering is generally performed at 2 4

temperatures around 3 to 5 of the absolute melting point or solidus of the material for a single-component system.

Multicomponent powder mixtures are generally sintered near the melting point of the constituent with the lowest melt temperature.

Table 1 Sintering temperatures for powder metal alloys and special ceramics

Sintered material

Sintering temperatures, °C

Aluminum alloys

590-620

Bronze

740-780

Brass, 890-910

Iron, carbon steels, low-alloyed steels (Cu, Ni)

1120-1150

Low-alloyed steels (Cu, Ni, Mo; Distaloy)

1120-1200

High-alloyed ferritic and austenitic steels (Cr, Cr-Ni)

1200-1280

Hard magnets (Alnico)

1200-1350

Hard metals (cemented carbides)(a)

1350-1450

Molybdenum and molybdenum-alloys

1600-1700

Tungsten(b)

200-2300

Heavy metal (W alloy)

-■ 1400

Ferrites (soft and hard)

1100-1300

Silicon nitride (with different additives)(c)

1750-2000

Silicon carbide (with different additives)(d)(e)

1750-2100

Alumina(e)

1400-1800

Zirconia (with different additives)(e)

1400-1750

Source: Ref 1

TiC-based hard metals (cermets) up to 1600 °C. ■■ 3000 °C, when direct sintering is used.

Highest temperature under pressured N2 atmosphere or in powder bed. Low temperatures for liquid phase sintering. Low temperatures for highly active powders.

Sintering times are typically 20 to 60 min under a protective atmosphere. Widely used furnace atmospheres include endothermic gas, exothermic gas, dissociated ammonia (DA), hydrogen, hydrogen-nitrogen mixtures, and vacuum. The main function of the atmosphere is to protect a part from oxidation or nitridation, as might occur when heating in air. Frequently, however, critical aspects of a sintering atmosphere also include reducing and carburizing power and capability for efficient removal of the lubricant. Some sintering furnaces contain so-called rapid burn-off zones for rapid and efficient removal of the lubricant. This usually entails an atmosphere with a higher oxidation potential, that is, a higher concentration of gases such as steam and carbon dioxide.

Inadequate sinter generally is indicated by low strength, low hardness, and improper dimensions. Causes of inadequate sintering are often related to the atmosphere, but several factors may be involved, such as:

• Sintering temperature too low

• Insufficient reducing agent

• Dew point too high in the hot zone

• High O2 content in hot zone

• Incorrect green density

• Incorrect belt speed or time at temperature

Corrective action includes:

Source: Ref 1

Measure and control dew point and O2 content in the hot zone Measure and increase H2 content

Check and correct belt speed and/or powder compositions

Further information on atmosphere control for sintered steel is discussed in the preceding article "Sintering Furnaces and Atmospheres" in this Volume.

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