Furnace Zones Concept Ref 23 24 25 26 27

Typical sintering furnaces can be thought of as having three or more interconnected zones (depending on the material being sintered), each with a separate function. The sintering process consists of several sequential phases, each requiring a unique combination of temperature, time, and atmosphere composition; flow; direction; and circulation. Each phase of the sintering process occurs in a specific zone of the furnace. Separating these zones and phases conceptually enhances design flexibility. A close match between the temperature and atmosphere of each zone and the function of each phase results in an optimum overall sintering process. In a single system, the base nitrogen can be modified with other gases or active ingredients to produce an appropriate and optimum atmosphere composition for each sintering phase before introduction into the proper furnace zone.

To understand the optimum combination of temperature, time, and atmosphere for each sintering phase, the processing sequence of a P/M part through the sintering furnace must be visualized. If the first sintering phase is completed effectively and efficiently in the first furnace zone, then the second phase in the second furnace zone, and each successive phase, is likely to be successful. The net result is improved productivity (production rate, quality, consistency, safety, versatility, and cost). Figure 7 shows a schematic of the processing sequence for sintering steel.

For example, if a P/M part composed of iron, 1% graphite, 2% copper, and 1% lubricant was processed in this furnace, at least five phases would be required to complete the sintering process. These would occur in five different zones of the furnace. The requirements for each sintering phase and furnace zone are outlined in Fig. 7 and are summarized in Table 6.

Table 6 Furnace zones, sintering phases, and atmosphere requirements to sinter an iron, 2% copper, 1% graphite, and 1% lubricant part

Furnace zone

Temperature

Time, min

Sintering phase

Sintering subphases

Atmosphere requirement

Atmosphere composition(a),

%

Flow

Circulation

°C

°F

Hydrogen(b)

Methane

Water(c)

1 delube

425650

8001200

10-30

Delubing

Lubricant vaporizing, vapor burning, flushing vapors to entrance

Fast, uniform heat transfer, slightly oxidizing

2-7

0-0.1

0.5-1.5

High toward entrance

Highly desirable

2 preheat

6501070

12001960

5-15

Elimination of particle surface oxides

Oxide reducing, graphite diffusing, copper melting, and coating particle surfaces

Highly reducing to surface oxides, neutral to carbon fast, uniform heat transfer

5-15

0.1-0.3

0.010.03

High toward entrance

Desirable

3 high

1070-

1960-

10-30

Particle bonding

Copper diffusing, neck

Neutral to carbon

3-8

0.2-0.5

0.01-

Medium

Desirable

temperature

1150

2100

(bond) growing, pore rounding

reducing

0.02

toward entrance

4 slow cool

1150-

2100-

5-15

Carbon restoring

Carbon transferring,

Uniform slow cooling,

2-7

0.3-1.0

0.01-

Medium

Highly

815

1500

homogenizing

slightly carburizing

0.02

toward entrance

desirable

5 cool down

81550

1500120

30-90

Cooling down, optionally oxidizing

Pearlite forming, part cooling down, preventing oxidation or controlling light

Slightly reducing or neutral (optionally oxidizing) to iron,

0-2

0-0.1

0.010.02

Low to medium partially

Highly desirable

(a) Balance is essentially nitrogen with or without small amounts of carbon monoxide and carbon dioxide.

(b) Hydrogen can be derived from hydrogen storage vessel, dissociated ammonia, endothermic gas, or dissociated methanol.

(c) If slightly oxidizing atmosphere is required, water is relatively higher or a small amount of air is introduced along with nitrogen in water-cooled section at a selected location.

Prime objectives in sintering this green part include the absence of sooting (internal or external). Admixed graphite should be effectively and uniformly diffused into iron particles as combined carbon. This results in an optimum pearlitic microstructure during cooling with minimum carbon loss. Admixed copper, after melting, should be uniformly distributed over all particle surfaces in the P/M part.

High structural integrity (maximum possible number of bonds or necks between particles, high average bond or neck area, and relatively rounded pores) is desirable. Surface carbon should be approximately equal to the carbon content in the core or interior. Emerging parts should be clean and bright or should have a controlled thin surface oxide to improve corrosion resistance. To meet these technical objectives economically, the total flow of atmosphere should be held to a minimum, with the least amount of added active gases.

For the purposes of the example, the atmosphere introduced into the furnace is nitrogen-base system 2 (carbon monoxide free), with 5% hydrogen and 0.5% methane. If the furnace is conventional, all of the atmosphere is introduced at one inlet--generally in the slow-cooling section between the hot zone and the water-cooled section. As a consequence, atmosphere composition (water, hydrogen, and methane content) is generally uniform throughout the different zones or sections as the atmosphere flows from the inlet toward both ends. This means that each active ingredient is at the same level whether or not the atmosphere is in a zone where a particular ingredient is needed. For the furnace to function, it is necessary to compromise between the atmosphere needed in one part of the furnace and that needed in another.

In the atmosphere system used in a zoned sintering furnace, however, different atmospheres (composition, flow, direction, and circulation) are introduced to different zones of the furnace (Fig. 14) to obtain optimum results. Wet nitrogen is introduced near the loading end of the furnace, hydrogen in the center, and dry nitrogen near the exit. Custom-designed injectors provide proper flow, circulation, and direction to these atmospheres. Injectors are also designed to act as gas barriers and reduce overall atmosphere flow. As a result of this zone, the constituents peak where they are needed (Fig. 14). In this example, water (dew point) is at the highest level near the entrance in the preheat zone, where delubing takes place. Hydrogen is introduced at a peak level at the mouth of the hot zone and in the hot zone, where it efficiently reduces surface oxides. Methane is added in sufficient amounts to reach maximum levels at the end of the hot zone and at the beginning of the slow-cooling zone, where it is needed for carbon control. Only nitrogen is added in the cooling zone, constituting nearly 100% of the atmosphere near the exit end of the furnace; furthermore, there is no need for a flame at the exit end. Therefore, exiting parts are cooler, and operators are safer and more comfortable.

In contrast to the conventional system, no compromise is needed to balance atmosphere makeup and furnace function; the zoned sintering atmosphere facilitates optimum sintering regardless of makeup or other part specifications. In zoned systems, atmosphere is introduced as needed with respect to composition, quantity, and location. Atmosphere zoning, combined with suitable temperature and time, allows each phase of the sintering process to be completed effectively, efficiently, and economically at the lowest possible atmosphere flow rates.

Nitrogen-base zoned atmosphere systems are compatible with existing furnaces, and the constituents can be varied to permit sintering of a wide variety of materials. A well-designed nitrogen-base system can meet the needs of a plant that sinters a wide range of ferrous and nonferrous parts. Technically, zoned atmospheres can achieve the same results as endo, dissociated ammonia, and exo atmospheres, while being more energy efficient, easier to operate, safer, and more flexible.

Nitrogen-base zoned atmosphere systems are capable of meeting the varying requirements imposed by furnace components. Atmospheres must be mild to furnace components. Conveyor belt alloys, for example, oxidize easily at sintering temperatures. However, under very dry and reducing atmospheres, the protective oxide layers are removed, and belt links tend to weld together. The belt "crackles" and gradually loses strength. Furthermore, belts, heating elements, and muffles become brittle and have short service lives under carburizing atmospheres. In addition, insulating brick oxides are reduced if the atmosphere is too reducing, which shortens service life.

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