TECHNICAL ACHIEVEMENTS OF M. V. FRUNZE ALL-UNION

SCIENTIFIC-INDUSTRIAL ASSOCIATION

TECHNOLOGY OF FABRICATION OF WELDED-CAST IRON HEAT EXCHANGERS N. M. Sytnik and V. N. Likhanosov

UDC 621.791.042:66.045.1-47-462

Replacement of high-alloy steel by cast iron, which is highly resistant to corrosion in concentrated sulfuric acid, for fabrication of heat exchangers for sulfuric acid manufacture allows saving of scarce metals and reduction of equipment cost. The problem of fabrication of welded cast iron structures which are poorly ductile and tend to undergo metastable crystallization when cooled rapidly has not been solved in practice. Therefore, development of a technology for fabrication of welded-cast heat exchangers calls for resolving the problems associated with both the metallurgical and the technological peculiarities of welding of structural cast irons. Welding of cast iron using nickel-containing electrode wires and piece electrodes does not require preheating of the welded joints to a high temperature. Studies have shown that of the known electrode materials of this type the PANCh-II thin wire containing nickel mixed with small amounts of copper, manganese, and rare-earth metals as its matrix ensures maximum corrosion resistance of welded cast iron joints in sulfuric acid. One of the defects of the PANCH-II wire is the tendency of the metal of the weld made with a wire of this composition to form hot cracks, for the prevention of which techniques like slit grooving, inappreciable penetration of the matrix metal, etc., are to be applied, which greatly complicates preparation of the article for welding and joining. Moreover, automatic welding with thin wire is troublesome because of the difficulty in delivering the wire into the slit groove if the wire deviates a little from the groove axis. High-quality joints were obtained by welding a seven-tube heat exchanger using the PANCHii wire with preheating to 250~ Welding of a larger number of tubes appeared impossible: increased welding strains produced cracks along the joiners of the tube grid. Interspace annealing to remove the strains is not possible because of formation of a skin which is difficult to remove in the slit-like gaps and which facilitates formation of welding defects. From the point of both metallurgical weldability and corrosion resistance most propitious is to use a filler metal that is identical in composition with the metal being welded. However, the cast iron joints studied could not be welded by such a technique using tradi, tional electrode wires because these wires were developed for bath welding rather than beading, the application of which is dictated by the construction of the welded joints. Welding of cast iron by an identical electrode metal is difficult because of formation of cracks and metastable structures in the welded joint. The resistance of cast iron to crack formation and the cooling rate at which cast iron crystallizes in a stable system can be increased by optimum modification and alloying [1-3]. An analysis of [4-7] shows that for arc welding modification of cast iron by calcium helps to get most suitable structures. Furthermore, the optimum content of this modifier in the electrode wire was determined in [8]. Therefore, in this work we studied only the effect of alloying of optimally modified cast iron on the formation of cracks and metastable structures. The tendency of the deposited cast iron to form cracks (the number of cracks was counted on a 200 mm long segment of the weld bead) was ascertained from the minimum welding heat input q/v at which no crack is formed in the weld bed. The beads were deposited on 400 • 150 • 30 mm plates made of SCh20 cast iron without preheating under the following conditions: welding current Iweld = 80-500 A, arc voltage U a = 18-44 V, wire delivery rate v w = 0.0190.066 m/sec, welding rate Vweld = 0.0028 m/sec, electrode extension s = 50 mm. In the case of 2.8-mm diameter powder wire containing (%) 5-6.1 C, 2.7-3.4 Si, and 0.67-0.86 Mn and the filling factor Kf = (26 ~ 3)%, the deposited cast iron contains (%) 3.2-3.4 C, 2.5-3 Si, and Translated from Khimicheskoe

i Neftyanoe Mashinostroenie,

No. 12, pp. 21-22, December,

1990. 620

0009-2355/90/ii12-0620512.50

9 1991 Plenum Publishing Corporation

Fig. I. Microstructure of modified and alloyed cast iron joint welded with preheating at 573 K (lweld = 360 A, U a = 38 V). x200: a) weld metal and b) fusion area. 0.5-0.7 Mn. The theoretical content of the alloying elements in the weld metal is (%) 4.4 Ni, 1.15 Cr, 1.2 Mo, and 0.03 B. It was proved experimentally that crack formation in deposited unmodified cast iron of the base composition can be prevented by welding at q/v ~ 74.7"i0 s J/m. Optimum modification of cast iron of the same composition helps to reduce q/v down to 68.1.i0 s J/m. Alloying of modified cast iron separately with nickel (1.46%), boron (0.02%), chromium (0.4%), andmolybdenum (0.7%) lowers q/v respectively to 64"105 , 62"105 , 60.105 , and 58"105 J/m. In order to optimize the cast iron composition in terms of the al~oying elements, a multifactorial experiment was performed by a method described in [9]. The minimum welding heat input at which no crack is formed in the weld bead was taken as the optimization parameter Y. The following regression equation was obtained by mathematical analysis of the experimental data: Y = 7.4 - 1.26xi - 0.27x 2 - 2.48x 3 + 55x 4 MJ/m, where xl, x2, x3, x 4 are the contents boron.

(%) respectively of nickel, chromium, molybdenum,

and

The optimum composition of the weld metal determined by the regression equation for the cast iron welding conditions without heating was corrected with reference to the conditions of welding with preheating of the welded joint to 373 K. Thereupon powder wire of the following composition was obtained (%): 5.1-6.43 C, 2.9-3.64 Si, 0.5-0.65 Mn, 0.49-0.62 Cr, 1.181.36 Ni, 0.83-1.05 Mo, and 0.65-0.82 B [i0]. Our studies showed that cast iron welding by powder wire of the above composition with preheating to 573 K allows one to obtain weld metal without cementite formation (Fig. i). The ultimate strength of the SCh20 cast iron welded joint is 200-220 MPa. The rate of

Fig. 2. Nature of breakdown of tube grid due to thermal stress and expansion of cast iron.

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Fig. 3.

Welded-cast heat exchanger.

corrosion of the base metal and the weld metal in 93% sulfuric acid at 60~ and 0.84 g/m2.h respectively.

in i00 h is 0.7

Thus, a powder wire has been developed that allows stable welding of cast iron by the bead technique and gives a metal of the welded joint at a preheating temperature of 473-573 K which is identical in properties (chemical composition, corrosion resistance, structure, and strength) with the base metal. High-quality welding of individual tubes with a tube grid using this wire is ensured by preheating the welded joint to 423-473 K, the welding conditions being Iweld = 280-300 A, U a = 26-28 V, v w = 0.028 m/sec, Vweld = 0.003 m/sec, Z e = 50 mm. For welding a seven-tube heat exchanger the temperature needed to be raised to 673 K. However, preheating to this temperature is ineffective if the number of tubes in the heat exchanger increases. For instance, to prevent crack formation in the welded joint of a heat exchanger having 31 tubes, it has to be preheated to 873 K. The joint obtained thereby meets all demands made on welded vessels used under pressure. Preheating of a cast iron heat exchanger having a large number of tubes 2 m and more in length to 873 K leads to irreversible increase in its size (a 2-m long heat exchanger "grows" by 4 mm and more) and uneven heating of the tubes along the cross section of the bundle gives rise to considerable thermal stresses. The combined effect of these factors is the cause of detachment of the tubes from the tube grid and breakdown of the metal of the tube grid in the connector area (Fig. 2). Therefore, in welding tubes with the tube grid it is necessary to take account of the processes associated with graphitization of cast iron and to adopt measures to ensure uniform heating across the tube bundle. Thermal stresses can be reduced by setting appropriate rates of cooling of the heat exchanger. For instance, the permissible heat exchanger heating rate for welding tubes with the first tube grid is 150~ and for welding with the second grid it must be reduced down to 75~ the heat exchanger cooling rate in both cases must not be higher than 25~ The technological process developed on the basis of the experimental data obtained was tested in the fabrication of a 2-m long pilot heat exchanger having tube grids 425 mm in diameter and 31 tubes with a diameter of 45 mm and a wall thickness of 4.5 mm (Fig. 3). The heat exchanger passes the tests successfully. LITERATURE CITED i~

2. 3. 4. 5.

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Yu. A. Sterenbogen, V. F. Khorunov, and Yu. Ya. Gretskii, Welding and Beading of Cast Iron [in Russian], Naukova Dumka, Kiev (1966). N. G. Girshovich, Crystallization and Properties of Cast Iron in Castings [in Russian], Mashinostroenie, Moscow-Leningrad (1966). A. G. Korotkov, I. S. Koifman, T. V. Egorshina, et al., "High-phosphorus cast irons alloyed with boron," Lit. Proizvod., No. 7, 14-15 (1968). Yu. Ya. Gretskii and V. A. Metlitskii, "Welding of high-strength cast iron castings with powder wire containing yttrium," Lit. Proizvod., No. i, 6-7 (1975). V. A. Metlitskii, Yu. Ya. Gretskii, and G. M. Kroshina, "Effect of scandium on the structure of deposited cast iron," Avtomat. Svarka, No. ii, 74-75 (1980).

6. 7.

8. 9. I0.

N. M. Sytnik, "Modification of deposited cast iron with calcium," Svaroch. Proizvod., No. 3, 18-19 (1982). N. M. Sytnik, Yu. F. Gartsunov, and A. I. Lyubich, "Welding of high-strength cast iron castings with powder wire containing rare-earth oxides," Lit. Proizvod., No. 8, 20-21 (1980). N. M. Sytnik, P. M. Nesvit, N. I. Kopersak, and Yu. F. Gartsunov, "Production of cast iron with spheroidal graphite in weld metal," Lit. Proizvod., No. 7, 20-21 (1973). Yu. P. Adler, E. V. Markova, and Yu. V. Granovskii, Experiment Design for Searching Optimal Conditions [in Russian], Nauka, Moscow (1971). USSR Inventor's Certificate No. 107,688, IPC B23 K 35/36, Composition of Powder Wire for Cast Iron Welding [in Russian].

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