Lect 3-Furnaces & Equipment

8/12/2019 Lect 3-Furnaces & Equipment

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H. Kandil MPE 355 Furnaces & Equipment 2/14

1.2 Modes of Heating Three basic modes of heat transmission; conduction, convection, and radiation

(a) Conduction The transfer of heat from one part to another due to temperature gradient without

displacement of particles Conduction involves the transfer of kinetic energy from one molecule to another Heat flow continues until equilibrium occurs Conductivity of metals are relatively high

(b) Convection The transfer of heat takes place by the motion and mixing of fluids; liquid or gas Fans are used to increase the heat transfer coefficient of the system Fig. 2 shows a typical convection heating furnace The furnace is heated by recirculating, forced air convection

(b) Radiation A body emits radiant energy in all directions by means of electromagnetic waves When this energy (4-7 m) strikes another body, it raises it temperature The amount of heat absorbed depends on the absorption coefficient of the receiver

2. CLASSIFICATION OF FURNACES Furnaces are classified here by the type of heat transfer medium used There two heat transfer media; gas (air or vacuum) and liquid (molten metal or molten salt

bath)

2.1 Batch and Continuous Furnaces In some plants, heat treating furnaces are classified as batch or continuous types

Figure 10.2 Low-temp. heat processing furnace heated by convection.


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2.1. A Batch Furnaces

A batch-type furnace refers to one that is loaded with a charge and then closed for the pre-heating cycle

After completing the heating cycle, the work load may be cooled in the furnace (such as for

annealing), removed, and cooled in still air (as in normalizing), or quickly cooled(quenching (as by immersion in oil or water

Figs. 1, 2, 3, and 4 illustrate some batch-type furnace. Fig. 1 shows a large car-bottom type Fig. 2 is a batch furnace used for tempering, stress relieve, or process (subcritical)

annealing Fig. 3 shows another batch-type furnace heated by radiation, a real work horse This furnace is heated by metallic electrical

heating elements up to 980 oC With SiC heating elements temp. goes up to

1260 oC

Fig. 4 is a pit-type vertical loader, where the work is surrounded by gas-heated radianttubes (indirectly fired)

A convection assist is supplied by gas-heatedradiant

Figure 10.3 Small batch-type furnace. Notesteel frame, insulating brick, electricalresistors and roll-top table in front of furnace for handling work.

Figure 10.4 Batch-type multiple- purpose furnace that features anintegral quenching system.


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B.4 Rotary-Hearth Continuous Furnaces

Fig. 6 shows a straight thru type of continuous furnace The furnace is heated with gas-fired radiant

tubes

2.2 Liquid Bath Furnaces

Heating by immersing the workpiece in a liquid

2.2. A) Molten metal baths

Heating parts in molten metal ( usually molten lead) is a very old practice Disadvantages:

Molten lead is heavy, thus steel parts float if not anchored Lead adheres to steel parts

2.2. B) Molten salt baths

Salt bath offers several advantages: Salts are available for operating temp. range 175 oC - 1260 oC Parts do not scale while they are in molten bath Thin film of salt remains on work after quenching so clean hardening is easy Varieties of salts are available

The principal disadvantage is: Necessary cleaning of salts after heat treatment, which is difficult for complex parts

Types of Salt Baths: The simplest form involves heating a metal pot filled with a low-melting-temp. salt with

electric immersion heaters This type is applicable to temp. range of 175 oC - 345 oC Fig. 7 (a) shows a common type of fuel-fired pot furnace used for molten metal or salt

Figure 10.6 Rotary hearth forging or heat treating furnace.


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This type of furnace can be used for heating metals up to 900 oC

Fig. 7 (b) shows a similar furnace to Fig. 7(a), except it is heated by electrical resistance Furnaces of liquid bath operating in the temp. range of 760 oC - 1260 oC are shown in Figs.

7(c) and (d) of immersed and submerged electrode type, respectively

In these types, heat is generated by resistance to current flow thru the molten salt from oneelectrode to the other

This creates a stirring action at the electrodes, providing uniform temp. within the bath A principal disadvantage of Fig. 7(d) furnace is the deterioration of immersed electrodes

just above the salt line where heat is intense

Figure 10.7 Principal types of salt bath furnaces. Types (a) and(b) also can be used for lead baths.


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2.4 Vacuum Furnaces A heated chamber evacuated to a vacuum pressure for thermal operations Vacuum heat treating can be used to:

Prevent reactions at the work surface, such as oxidation and decarburization

Remove surface contaminants such as oxide films Add a substance to the work surface, such as carburization Remove dissolved contaminants, using the degassing effect of the vacuum Join metals by brazing or diffusion bonding

The degree of vacuum is 1% Highly alloyed materials require a harder vacuum, less than 1%, which requires a strongconstruction of hot-wall furnace to avoid implosion under vacuum

Fig. 10 shows a three-chamber furnace Vacuum furnaces are economical, howeverwith high initial cost

Figure 10.9 Externally gas-firedluidized-bed urnace.

Figure 10.10 Three-chambercold-wall vacuum oil-quench furnace.


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Fig. 11 shows a single-chamber vacuum furnace

Another vacuum furnace used for carburizing is shown in Fig. 12

3. FURNACE PARTS AND FIXTURES Interior components are made of refractory materials Metal parts may vary from simple low-carbon steel (0.15-0.2% C) up to hi-alloy steel C-steels scale at temp. higher than 425 oC, in addition to loosing strength 310 St. St. is used for furnace parts used at temp. below 650 oC

Fixtures & Baskets Fixtures are used for holding the workpieces More energy is needed to heat and cool the fixtures or baskets Fixturing is essential to minimize distortion of workpiece and to allow uniform cooling Fig. 13 shows typical trays for heat treating in carburizing furnaces

Figure 10.11 Single-chamber batch-type pressure-quench vacuum furnace.

Figure 10.12 A batch-graphite integral oil-quenchvacuum furnace with vacuum carburizing capability.


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A typical multipurpose tray, shown in Fig. 14, is fabricated from welded components of a35% Ni-18% Cr steel

These trays can be used in either batch-type or continuous furnaces

For processing salt baths or fluidized beds, such trays are designed higher and narrower For heat treating in a pit-type furnaces, a round basket or tray (Fig. 14) is used Automotive parts such as gears, pinions, axles, and shafts typically require fixturing Fig. 15 shows a tray for carburizing ring gears

Figure 10.13 Typical heat treat alloy carburizing furnace trays.Dimensions given in inches.

Figure 10.14 Bar frame type basket.

Figure 10.15 Tray/fixture for carburizing ring gears.


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Design Criteria Designing trays, fixtures, and baskets, the following points should be considered:

Eliminate their use when possible Reduce weight to min. that provide the required strength to handle the charge In quenching applications, design fixtures with openings to allow circulation of quenching

medium

4. QUENCHING MEDIUMS AND SYSTEMS Quenching is the rapid cooling of a metal from elevated temperature Accomplished by immersing in water, oil, or a polymer Forced or still air is sometimes used There two major applications in quenching:

Cooling of quench-hardenable steel s to develop desired microstructure and mechanicalproperties

Rapid cooling to attain solid solution to permit precipitation hardening by aging

Metallurgical Aspects Most steels are quenched to control the transformation of austenite to desired phases Microstructure that may be obtained are indicated on TTT diagrams Martensite is the as-quenched microstructure that is usually desired as indicated by curve Ain Fig. 16

Cooling Rates When cooling C-steel, a cooling rate 55 oC/s is necessary to avoid the nose Under ideal conditions, water provides a cooling rate of about 278 oC/s

Figure 10.16 Transformation diagrams and cooling curves for AISI 8630 steel, indicatingthe transformation of austenite to other constituents as a function of cooling rate.


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A simple tank system for water quenching is shown in Fig. 17 A supply of fresh water is continuously fed into the bath and allowed to overflow to the

drain

For continuous high production rates, specific quenching arrangement is shown in Fig. 18

Figure 10.17 Temperature-controlledoverflow tank for water quenching.

Figure 10.18 Schematic of a typical installation for high-volume batch quenching ofcarburized or hardened parts on trays. Directional vanes in the oil stream distributethe oil flow uniformly. Unit contains combined heating and cooling elements and provision for blanketing the surface of the oil with an inert gas atmosphere. Radianttube is used for heating.


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5. FURNACE ATMOSPHERES Austenitizing must be performed under surface protection of workpiece. Otherwise,

workpiece may be severely oxidized (coated with rust) and decarburized, or both Severe oxidation of C-steels begins at 425 oC At higher temperatures, the rate of oxidation becomes exponential Under high temperature conditions, decarburization also occurs (amount of C deceases) Furnace atmospheres provide neutral heating, i.e., keeping workpiece surface clean and

unchanged Principal types of gaseous atmosphere are listed below in an increasing order of cost:

Natural (conventional air), Atmosphere derived from productions of combustion in a directfuel-fired furnace, Exothermic (generated), Endothermic (generated), Nitrogen base,Vacuum, Dissociated ammonia, Dry hydrogen (dried bottled gas), Argon (from bottles)

6. TEMPERATURE CONTROL SYSTEMS

Temperature should be controlled to within 2.5 oC The temperature control system consist of three stages; temperature sensors, temperature

measurement, and temperature control

6.1 Temperature Sensors One variable is measured then translated, or converted, to another Environmental temperatures are measured by expansion and contraction of a column of

fluid or of a metal By means of calibration, these variable are converted to numerical temperature readings

Thermocouples are the most widely used sensors for measuring Temp. in furnaces Thermocouples consist of two dissimilar homogenous metal wires Thermocouples have two ends; measuring hot end and the reference or cold end The electrical output in EMF is proportional to the difference in temperature between the

measuring junction (hot) and the reference junction (cold)

6.2 Temperature Measurements Measurement instruments measure the output signal of the temperature sensor and

convert it to a temperature indication

6.2 Temperature control A temperature controller must provide sufficient energy to satisfy process requirement Variations include changes in process load, fuel characteristics, and ambient temperature In operation, the controller set point that represents the desired temperature is compared

with the process or actual temperature Based on the comparison between desired and actual temperature, the controller regulates

the energy flow to the process

Lect 3-Furnaces & Equipment

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