Metal Science and Heat Treatment gol. 38, Nos. 9 - 10, 1996

THERMAL-SHOP EQUIPMENT

UDC 621.783.246:66.096.5

ELECTRIC FURNACES WITH A FLUIDIZED BED FOR IMPLEMENTING ENVIRONMENTALLY SAFE HEAT TREATMENT PROCESSES IN MACHINE BUILDING

A. P. Baskakov and E. M. Fainshmidt

Translated fi'om Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 19 - 23, September, 1996.

Current designs of electric tank furnaces with a fluidized bed that provide environmentally safe heat treatment of steels as tested under the conditions of machine-building plants are considered.

Some foreign finns are known to specialize in the pro- duction of furnaces with a fluidized bed either separately or along with other equipment. Russian industry has not yet started such production.

Several industrial and pilot furnaces (mainly induction electric furnaces) have been manufactured in shops for non- standard equipment for machine-building plants that use a fluidized bed as a heating agent in heat treatment processes. For example, five modifications of such furnaces have been produced by the Nizhnii Tagii plant, and ! - 2 furnaces each have been produced by the Nev'yansk, Chelyabinsk, and Magnitogorsk plants. In all cases the heating agent used in- stead of molten metals, salts, caustic alkalis, saltpeters, and mineral oils used in heat treatment practice in machine-build- ing plants was a fluidized bed of a fine-grained material (pre- dominantly white electrocomndum, which is an indifferent environmentally safe medium).

In some nonstandard processes (in special technologies or in the absence of natural gas) producers use molten lead and KCI, NaCI, BaCI 2 salts as media for oxidation-free heat- ing for hardening of structural and tool steels. Melts of KOH, NaOH alkalis are used for isothermal cooling of steels; melts of KNO 3 , NaNO 2 , NaNO 3 saltpeters are used for oxidation- free recrystallization annealing of copper alloys, heating for hardening of Duralumin, high-temperarure tempering of high-speed steels, and intermediate-temperature tempering of spring materials; Vapor mineral oil with t = 150 - 180°C (the flash point exceeds 250°C) is used for low-temperarure tem- pering of structural and tool steels.

All these chemicals contaminate the atmosphere of the shop and the waste waters, because the shops performing the mentioned processes (thermal, mechanical, machining) are often separated from the plant's purification system, which is

386

overloaded with waste from the chemical, galvanizing, and other shops. Thus, after washing parts heat treated in the melts and oils mentioned above, waste waters containing K ÷,

Na ÷, Ba ++, CI-, NO~, NO], OH- ions, metallic lead, alumi-

num, and zinc, and mineral oils pass directly into the sewage system and then into the soil and the water basins. This situ- ation is typical for machine-building plants in the Urals (see Table 1).

The use of a neutral environmentally safe heating agent prevents the appearance of the mentioned substances in the waste and makes the atmosphere of the shops healthier.

The first attempts to use a fluidized bed as a universal heating agent in heat treatment processes were made in the 1960s. At the present time some experience has been accu- mulated in the field of heat and mass exchange in fluidized beds and fluidizing technology [1, 2, etc.]. A characteristic feature o fa fluidized bed is its high effective thermal conduc- tivity. Intense circulation of particles whose volume heat con- duction exceeds that of gases by about three orders of magni- rude provides almost equal temperatures (accurate to 5 - 10°C) at all points of the bed even in large furnaces. If neces-

sary, the accuracy can be increased to + 3°C. Heating in a fluidized bed is similar to that in molten salts and metals and can be used at virtually any process temperature from room to 900°C.

The coefficient of heat transfer ct between a fluidized bed and the surface of a part immersed in it can exceed

1000 W/(m 2. K). Such high values ofct at gas velocities con- stiruting a tenth of a meter per second are caused first of all by the heat transferred by the solid particles of the fine- grained material used as the fluidization material. Heating in a fluidized bed is rapid; the heating rates in such a bed lie be-

0026-0673/96/0910-0386515.00 0 1997 Plenum Publishing Corporation

Electr ic F u r n a c e s w i t h a F l u l d i z e d B e d 3 8 7

tween those o f chloride melts and molten lead, and the me- dium is environmentally safe.

In principle, a fluidized bed can be heated by electricity (at least to 700 - 900°C) through the walls of a steel heat-re- sistant converter from resistance heaters placed along the walls o f the refractory lining of the shaft or by burning a gas, for example, in immersed burners [1]. World experience shows that gas furnaces have not found wide application in machine building.

In a machine-building plant natural gas is supplied to a few, usually specialized, shops (forge, heat-treatment, foun- dry). Choosing the concept o f electric heating o f a fluidized bed we managed to design a number o f mobile tank electric furnaces that can operate directly in flow lines o f mechanical shops, which makes it possible to eliminate transportation of parts into the heat-treatment department and back. The lower volume o f intershop transportation makes the process less ex- pensive and shortens the production cycle, making it more rhythmic.

It has been difficult to develop a process and equipment with a fluidized bed for low-oxidation recrystallization an- nealing o f parts maade o f copper and its alloys. In the range o f annealing temperatures (400 - 550°C) copper is intensely oxidized in air, forming a dense oxide film that is hard to re- move. Many parts have to be heated without oxidation, espe- cially if their sizes are finished or in order to provide for a high quality o f a deposited coating (for example, in tinning).

Previously, saltpeter melts were used for oxidation-free recrystallization annealing o f copper and its alloys and for heating parts made of aluminum alloys for hardening (500 - 525°C).

An alternative for oxidation-free heating in saltpeter was indirect (through the walls o f a closed muffle) heating o f a charge o f parts, but it did not provide uniform heating o f the

whole o f the charge; parts from different zones had different levels o f ductility, microstructures, and other properties (at the walls o f the muffle the parts were superheated, whereas in the center, underheating caused a deformation texture and the parts were rejected). In addition, the air left in the muffle oxi- dized the surface of the parts (to a dark-brown oxide film).

Direct heating o f a charge in a fluidized bed provides the requisite uniformity o f the distribution o f the temperature, properties, and microstructure. In order to eliminate the for- mation o f scale the fluidization was conducted using steam for the first time in the practice o f a machine-building plant. Steam played the role o f a fluidizing agent and, simultane- ously, o f a medium protecting copper and its alloys from oxi- dation [3, 4]. Steam from the plant's system was fed into the furnace at a temperature o f 100 - 110°C and a pressure o f up to 20 MPa. This method of heating was low-oxidation. A thin oxide film (colored from yellow to light-brown) was easily removable by pickling before tinning with a loss o f mass up to 0.01 g/cm 2 (in heating in a muffle the loss o f mass was up to 0.03 g/cm2; in the absence o f a protective atmosphere it was 0.1 - 0.12 g/era 2 for articles made of M 1 copper sheets after cold spinning).

The shaft electric furnace designed in order to implement the process described has the following characterizes: a power o f 80 kW, tm~ = 550°C, an output o f 250 kg/h, a steam flow rate o f 86m3/h, a length/width/height proportion o f 1730/1730/2700 mm, a functional space of 600/600/1500 ram, a mass o f conmdum of 320 kg, and a total weight o f 7.7 tons.

The shaft tank electric furnace (Fig. 1) consists o f a heat- resistant muffle 1 (with a square cross section) placed in a shaft 2 with a heat-insulating refractory lining 3 and regis- tahoe heaters 4. The square cross section provides maximum charging. A steam-distributing nozzle 5 is mounted on the bottom of the muffle and its upper plate is equipped with caps

TABLE 1

No. of Technology Treated material process

Heating agent

composition l, °C

Presence of harmful substances in waste waters

and the atmosphere

Oxidation-free heating for hardening The same The same

Carbon and alloyed steels High-speed steels Alloy edateels

Molten NaCI + KCI Molten NaCI + BaC h Molten lead

1 2 3*

4* Rapid heating for high-temperature The .same tempering

5 High-temperature tempering of cutting High-speed steels tools

6 Isothermal cooling (bainite quenching) 7 Tempering of spring items 8 ° Low-temperature tempering of items and

tools 9 Oxidation-free recrystallization annealing Copper-base alloys

I0 Chemical oxidation of items and tools Structural and tool steels 11 Hydrophobization of parts in powder Powder steels

metallurgy

The same

Molten KNO 3 + NaNO 3

Carbon and alloyed steels Molten KON + NaOH Spring steels Molten NaNO- 2 + NaNO 3 + KNO 3 Carbon and alloyed steels Vapor oil (ttp= 260°C)

Molten NaNO 3 + KNO 3 Solution of NaOH, NaNO2, NaNO 3 GKZh-94,** gasoline

780 - 900 Na +, K +, CV, HCI vapor 1260 - 1280 Na ÷, Ba ''+, HCI vapor 800 - 850 Pb vapor, Pb in waste

waters

650 - 680 The same

560 Na +, K +, NO~

320-400 Na +,K +,OH- 230 - 350 Na +, K +, NO~, NO~ 160 - 180 Volatile hydrocarbons, oil.

tars

400-550 Na ÷,K ÷,NO~ 130-135 Na +,OH-,NOT,NO~

20 Gasoline vapor, GKZh

*Special technology. **Hydrophcbizing silioon-conmining liquid.

388 A . P . B a s k a k o v a n d E. M. F a i n s h m i d t

6. A dense bed of electrocorundum 7 (GOST 3647-71, parti- cle size 120 I.tm) 600 mm high is poured onto the steam-dis- tributing mesh. Steam is fed through nozzle 8 into the steam- distributing nozzle virtually under atmospheric pressure and is filtered uniformly over a horizontal cross section of the conveiter through the bed of corundum particles, fluidizing the latter. The steam also serves as a protective atmosphere that provides virtually oxidation-free heating of copper parts. The fluidized bed is heated through the walls of the con- verter; the latter is heated, in turn, by radiation of the resis- tance electric heaters.

Due to intense mixing in the functional volume of the fluidized bed the temperature becomes uniform (the tempera- ture difference is 2 - 3°C). The functional volume is situated 60 - 70 mm above the openings in the gas-distributing caps 6.

In the start-up period dried compressed air is fed from the shop system in order to avoid condensation of steam on the cold installation and moistening of the corundum, which hampers normal fluidizing.

The furnace is equipped with a water tank 9 connected with a tambour 10. The furnace, the tambour, and the tank have a common cover 11 with a longitudinal groove for pull- ing a container with parts. Steam passes through the settling zone (the space above the fluidized bed), is separated from the eleetroeorundum particles, and is then sucked into a ven- tillation pump 13 through a longitudinal groove in the cover 11. An excess steam pressure is retained under the cover, which creates a steam jacket in the tambour and above the water tank. For this reason, parts moved into the water tank hardly oxidize. The container with the parts is moved into the furnace by a telpher, a vertical pull rod moves along the lon- gitudinal groove in the cover, and the charge is immersed in the fluidized bed when it reaches the end of the groove. The container is mounted on a stationary support 14.

When the heating is finished, the charge is transported with the help of the telpher into the water tank for 10 - 20 sec. The cooling time of the charge is chosen so that the parts withdrawn from the water have a temperature of 100- 150°C, at which excess moisture evaporates from their surface.

The mass of the charge is 100 - 120 kg, the heating to the annealing temperature for each stack in the charge lasts 5 - 7 min, and the annealing lasts 40 min (with allowance for the 30-min hold at 380-450°C required by the specifica- tions).

By increasing the heating temperature to 500 - 550°C the annealing time can be reduced to 15 - 20 min, but this can be accompanied by growth of the grains to size No. 5 (at t < 450°C the grains correspond to size No. 7).

We used the same fluidizing technique in another techno- logical process, namely, rapid steam oxidation of sintered steels (instead of hydrophobization impregnation in GKZh- 94 and convective steam oxidation) [5]. In this case the sec- ond (technological) function of steam consists in oxidizing iron and the impurities and filling the branched pore (capil- lary) system with this oxide phase. This increases substan- tially the corr_,~sion resistance of the sintered steels. For exam-

8 6 7 : ~ 1

Fig. 1. Shaft tank elecUic furnace with a fluidized bed for oxidation-free r~ crystallization annealing of parts made of copper and it3 alloys.

pie, corrosion spots appear on parts made of steel SP 50D2 after sintering (without closing the pores) in 1 - 2 h, after hy- drophobization they appear in 12 h, after convective steam oxidation they appear in 12 h, and after rapid steam oxidation in a fluidized bed they appear in 48 h (in cyclic tests in a 3% solution of NaCl at 18 - 25°C).

The tank furnace for rapid steam oxidation differs from the one described above by the absence of a water tank (the charge of oxidized parts is cooled in still air) and preliminary heating of steam to 3 0 0 - 350°C [6]. Furnaces of both types operate stably and reliably in several plants.

A universal tempering tank furnace with a bed fluidized by compressed air from the plant's system, which is dried in an oil and moisture separator, has been designed for all kinds of tempering (low-, intermediate-, and high-temperature). The air flow rate is 65 m3/h, and the rate of fluidizing is 0.2 m/sec. The functional space (a converter made of steel KhI8NIOT) is 600 mm in diameter and 1900 mm high, and the dense electrocorundum bed is 650 mm high. The furnace operates in the 150- 650°C temperature range, i.e., can be used for isothermal cooling or can serve as a cooling (harden- ing) tank for martensitic hardening of alloy steels (with switched-off heaters). The design of the furnace is described in detail in [3, 4].

In our opinion, the problem of low-temperature heating ofa fluidized bed (up to 700°C) has been solved successfully, which is confirmed by the reliable operation of the existi~lg

Electric Furnaces with a Fluldized Bed 389

Air

high-temperature tank furnace with induction heating (a cast thick-wailed heat-resistant converter with a gas-distributing mesh and a poured corundum bed placed in an inductor and heated by current of industrial frequency) presented in Fig. 2 has shown good prospects for the method [3, 4].

We also believe that the experience of rapid nitriding in a furnace with a vibrofluidized bed [7] is quite promising and environmentally safe.

C O N C L U S I O N S

The developed industrial electric furnaces with a fluidized bed (of three modifications) and semi-industrial fur- naces (of two modifications) have been tested successfully under industrial conditions and provide environmentally safe heat treatment processes.

Fig. 2. Shall tank electric furnace with induction heating of the fluidized bed: / ) inductor;, 2 ) monolithic lining (heat-resistant concrete);3 ) converter;, 4 ) air-di~atbuting nozzle; 5 ) caps; 6 ) fluidized bed.

industrial furnaces designed as described above and provid- ing environmentally safe heat treatment processes.

Laboratory and engineering developments exist for high- temperature (up to 900°C or more) heating of the fluidized bed. Some experimental designs have been tested (oxidation- free heating for hardening). We do not consider these designs in the present paper, because the creation of production proto- types (if needed) will be accompanied by inevitable changes. However, it should be noted that the experimental model o f a

REFERENCES

1. A. E Baskakov, Heating and Cooling Metals in a Fluidized Bed [in Russian], Metallurgiya, Moscow (1974).

2. A.P. Baskakov, B. V. Berg, and A. F. Ryzhkov, Processes o f Heat and Mass Transfer in a Fluidized Bed [in Russian], Metallurgiya, Moscow (1978).

3. E. M. Fainshmidt, A. S. Zavarov, and Yu. B. Pirogov, Heat Treat- ment o f Machine Parts in a Fluidized Bed [in Russian], TsNI- INTI, Moscow (1984).

4. A. S. Zavarov. A. P. Baskakov, S. V. Graehev, and E. M. Faiush- midt, "Use of a fluidized bed for heat and chemical heat treat- ment," Metalloved. Term. Obrab. Met., No. 10, 35 - 40 (1984).

5. USSR Inventor's Certificate 1321523, "A method for steam oxi- clarion of sintered articles of iron powders," Byull. Otkryt. Izo- bret., No. 25 (1987).

6. E. M. Fainshmidt, T. A. Pumpyanskaya, and A. A. Shalamov, Fabrication o f Powder Parts [in Russian], TsNIINTI (1986).

7. Patent 2007497 RF, "An installation for nitriding articles in a vi- brofluidized bed," Byull. Otkryt. Izobret., No. 3 (1994).