E F F E C T OF T H E C O M P O S I T I O N OF D E O X I D I Z I N G A D D I T I O N S ON T H E S T R E N G T H OF C A S T S T E E L IN C O R R O S I V E M E D I A V. P. Rudenko Fiziko-Khimicheskaya Mekahnika Materialov, Vol. 2, No. 5, pp 512-514, 1966 Owing to the simplicity of casting equipment and low cost of casting as a metal fabrication method, cast machine and instrument parts are widely used in mechanical engineering. It is therefore natural that a great deal of attention is paid to the problem of improving the quality of steel castings. The easiest way of achieving this objective is by improving the deoxidizing treatment of steel. Without introducing any substantial change in the casting process, a considerable improvement in the physical and m e c h a n i c a l properties of steel can be achieved simply by changing the composition of the deoxidizing additions [1]. At present, the most widely used deoxidant is aluminum; a recent development is the use of calcium silicide and ferro-cerium (added singly or in combination) which may raise the m e c h a n i c a l properties of cast steel to a level usually associated with rolled steel [2]. However, insufficient attention has so far been paid to the effect of the composition of deoxidants on the strength of steel in corrosive media. It was with a view to elucidating this problem that, in collaboration with the Foundry Technology Department of the Zhdanov Metallurgical Institute, we carried out a series of trial melts of steel 35, deoxidized with the following additions: 1) 0.10~ A1; 2) 0.10% A1 + 0.15~ SiCa; 3) 0.10% A1 + 0.15% FeCe; 4) 0.10% A1 + 0.15% SiCa + 0.15~ FeCe. The specimen blanks were cast in sand molds by a method described in [3], subjected to a heat treatment which is most widely applied to steel castings (normalizing at 900 ~ C followed by a 2 hr tempering treatment at 680 ~ C), and ground to Class 9 surface finish. Fatigue tests in a corrosive medium (8% NaC1 solution) were carried out on type MUI machines by a method developed at the Institute of Physics and Mechanics [4]; a basis of 5 x 10 ~ cycles was used. The corrosion resistance of steel was determined by a volumetric method in a 58% HzSO4 solution [5]. By using a method described in [6], we were able to measure the electrode potentials of the steels tested in both corrosive media. In order to determine the effect of the deoxidizing treatment on the strength of steel in hydrogen-charging media, fatigue tests were carried out in a 26% H2SO4 solution on notched specimens, which were cathodically polarized at a current density of 5 a / d i n 2 ; a technique described in [7] was used in these tests. The results (see table) showed that deoxidation with (A1 + CaSi) and (A1 + FeCe) has a beneficial effect on the corrosion-fatigue strength of cast steel, the improvement being even more pronounced after deoxidation with (A1 + CaSi + FeCe): In the latter case, the conventional fatigue l i m i t is 50% higher than that of steel deoxidized with a l u m i n u m alone and approaches the level recorded for rolled steel. Physical, Mechanical and Electrochemical Properties of Cast Steel 35 After Various Deoxidizing Treatments Deoxidants a ~plied AI
+ + + + +
SiCa
re-t Electrode Electrode Corrosion sistance - . , ~ in -Ipotentlat potential o=.~ 070 1-120tJ 4 . ~,.a / m m z) in ~e = --)-.lOO% in 3% solution 4 " i'ln ~3~1o NaC1 so- KH~ (cm / H2SO4so" FeCe lution, mV /era 2) ]Jlution, m y
+ (rolled)
4.0 5.5 5,0 6.0 7.0
18,2 23.5 21,7 25.0 25.9
640 639 529 628 617
2.80 2.47 2.85 2.78 2.51
903 890 900 894 891
ghydrogen' % 63.7 68.1 67.5 75,8 69.(
It is known that the conventional corrosion-fatigue l i m i t depends, in the main, on two factors: the m e c h a n i c a l factor (which generally covers the resistance of a given m e t a l to cyclic loads), and the e l e c t r o c h e m i c a l factor (i. e . , the resistance of a given m e t a l to corrosion in a given medium). The lowest conventional fatigue l i m i t was recorded for cast steel deoxidized with a l u m i n u m alone; this is because this steel has a low fatigue strength in air and the highest electrode potential, i . e . , the lowest degree of electrochemical homogeneity. The electrode potential of steel deoxidized with (A1 + CaSi) is practically the same; owing, however, to its higher fatigue strength in air, this steel has also a higher corrosion-fatigue l i m i t . Electrode potential measurements showed
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that additions of calcium silicide are unlikely to have any significant effect on the electrochemical properties of cast steel in 3% NaC1 solution. Ferro-cerium, on the other hand, reduces the electrode potential of steel 35 in 3~ NaC1 solution, which has a beneficial effect on its corrosion-fatigue strength. Quite a different picture is presented by the results of corrosion tests in 58% }-12804 solution, in which the steel oxidized with (A1 + CaSi) was found to have the highest corrosion resistance, even higher than roiled steel. Compared with the corrosion resistance of steel deoxidized with aluminum alone, the corrosion resistance of steel deoxidized with (A1 + FeCe) is slightly reduced and that of steel treated with (A1 + FeCe + CaSi) increased. Measurements of electrode potentials of the experimental steels in this medium showed that steel deoxidized with (A1 + CaSi) has a lower electrode potential than steels treated with A1 or (A ! + FeCe). Evidently, CaSi additions reduce the electrode potential of steel, i. e., m a r e it more noble, and increase its resistance to corrosion in sulfuric acid, which is practically unaffected by deoxidation of steel with FeCe. A good indication of the ductility of steel is given by the results of fatigue tests on hydrogen-charged notched specimens. The values of a coefficient measuring the effect of hydrogen (Shydrogen = (Ohydrogen-charged/Oin air) x x 100%), i . e . , the ratio of the fatigue limits of hydrogen-charged specimens to those of specimens tested in air, are given in the table. Here, again, the lowest fatigue limit was recorded for aluminum-deoxidized steel; the addition of FeCe increases the ductility of steel and, consequently, the value of ghydrogen; a slight increase is observed after deoxidizing with CaSi. Deoxidation with FeCe and CaSi increases the ductility of steel by changing the nature and shape of nonmetallic inclusions [8]. Best results, however, are obtained after deoxidation with (A1 + CaSi + FeCe) which combines the beneficial effects of calcium silicide and ferro-cerium. The fatigue limit of hydrogen-charged specimens of steel deoxidized in this way is increased by 1~70 (as a result of increased ductility) and is higher than that of roiled steel. Summary 1. The composition of deoxidizing additions has a substantial effect on the fatigue strength of cast steel in corrosive media, the best results being obtained with a three-component (A1 + CaSi + FeCe) deoxidant. Best properties for use in NaC1 solutions are conferred on steel by deoxidizing it with FeCe, deoxidation with CaSi ensuring the best properties for use in H2SO4 solutions. 2. Deoxidation with calcium silicide and ferro-cerinm produces a change in the nature and shape of nonmetallic inclusions and, as a result, increases the ductility of steel and reduces its tendency towards hydrogen emhritttement. 3. Deoxidation with (A1 + CaSi + FeCe) increases the fatigue strength of cast medium-carbon steel in corrosive and hydrogen-charging media to the level characteristic of rolled steel. REFERENCES 1. Yu. A. Shurte, Nonmetallic Inclusions in Electrically Smelted Steel [in Russian], Metallurgiya, 1964. 2. Yu. A. Shurte, I. P. Volchok, V. V. Lunev, and V. P. Rudenko, FKhMM [Soviet Materials Science], l, no. 5, 563-566, 1965. 3. Yu. A. Shul'te, G. V. Karpenko, P. A. Mikhailov, S. I. Gladkii, A. V. Kuslitskii, I. P. Volchok, V. P. Rudenko, and V. V. Lunev, Tekhnologiya i organizatskya proizvodstva, no. 4, 57-58, 1966. 4. G. V. Karpenko, Strength of Steel in Corrosive Media [in Russian], Mashgiz, 1963. 5. G. V. Akimov, Theory and Methods of Investigating Corrosion of Metals [in Russian], Izd. AN SSSR, 1945. 6. I. V. Karpenko, E. M. Gutman, and A. K. Mindyuk, FKhMM [Soviet Materials Science], 1, no. 2, 172-181, 1965. 7. G. V. Karpenko and R. I. Kripyakevich, Effect of Hydrogen on Properties of Steel [in Russian], Metallurgizdat, 1962. 8. V. P. P,udenko, I. P. Volchoki, et al., FKhMM [Soviet Materials Science], ~, no. 4, 1966. 14 March
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1965
Institute of Physics and Mechanics AS UkrSSR, L'vov
