Removal of Carbon from 25%Cr-Fe Alloy During Vacuum Arc Remelting*

UDC 669.1 87.3-982: 6 6 9 .0 46.564 : 669.15'26-1 92

Removal of Carbon from 25%Cr-Fe Alloy during Vacuum Arc Remelting*

By Yasushi NAKAMURA,** Hidetake ISHIKAWA,*** Takamasa ONO**** and Masatoshi KUWABARA**

Synopsis have been performed on this rea cti on from the vIew A kinetic study was made on the reaction, g + Q = CO, taking place point of carbon removal. H ence, the present exp eri­ during vacuum arc remelting of 25%Cr- Fe alloy to know availability m ent of V AR in a small unit is m ade to elucidate fac­ of this remelting process for Ihe jJroduclion of high-chromium .ferritic stain­ to rs influencing carbon removal during V AR of less steel with extremely lo w carbon content. Electrode steel to be re­ 25%Cr- Fe alloy. melted contained about 0.015- 0.05% carboll. Oxygen concentration was higher than that of carbon. T he residual carbon concentration in the II. Experimental resulting ingot, G 1" was found to be determined by the three factors : the remelting rate, m, the to tal surface area of the metal/)ool at the ingot An equipment designed by J a pan Vacuum E ng. to/) and the metal fluid layer at the electrode tij), S, and the carbon con­ Co., Ltd. and remelting procedure were essentially 4 eentra tion in the electrode, Ce. The observed results cOlyorm fairly 10 simila r to usua l ones described in the litera ture. , 5 ) Ihe following relation. The experimenta l conditions a re shown in T a ble I. (G,/Cp)'I' = I +(k/2)(pS/m) The 25%Cr- Fe a lloy with carbon content of 0.05 - where, p is the dellsi/.y alld k is the mass transfer coefficient. T his 0.0 15% a nd oxygen content higher than that of car­ equation was derived all Ihe assllln/)lioll that the transfer q{ carbon across bon was melted a nd cast in a vacuum induction the boundary lo;yer ill Ihe mells is the rale-conlrolling stell. T he value furnace. The ingots thus mad e were forged and of k was obtained to be 0.025 cm /sec. machine-cut to form desired shapes of the electrodes. The electrode was verti call y supported by a water­ I. Introduction cooled shaft in the furnance. During the remelting, High-chromium ferritic sta inless steels with ex­ the descending length of the scha ft was measured at tremely low carbon (less than 0.005%) a nd nitrogen regula r intervals a nd thereby the rates of the elec­ contents a re known to exhibit excellent corrosion­ trode consumption, v., and the ingot growth, Vp , were and impact-resistances .1- 3) It is, however, diffic ult calculated in cm /min unit. Opera ting pressure a t to remove carbon to such a low level from liquid the furnace head was a lso measured with a vacuum high-chromium steels with conventiona l degassing gauge. After remelting, the ingot was longitudinally furnaces, su ch as DH, RH, and AOD, without a large sectioned, and samples for chemica l analyses of car­ a mount of chromium loss . Thus these a lloys have bon, oxygen, and chromium were taken from the heen commerciall y produced by the electron-beam center of the ingot. T otal uncertainty in the m easure­ continuous melting and the va cuum inducti on melting m ent of carbon content was below 10% of the con­ processes. 2,3) H owever, considerable losses of chro­ centration. Distribution of carbon along the ingot mium a nd manganese due to the evapora tion a nd axis was prelimina rily examjned , but no significant rela tively high cost for the insta llation seem to limit variation beyond experimental limits was observed . their application. It is, therefore, of metallurgical interest to study the degree of carbon removal in con­ III. R esults ventiona l refining p rocesses other than electron-beam The observed contents of solute elements in the melting a nd vacuum induction m elting. resulting ingot after V AR (final solute content) are Consumable vacuum arc rem elting (V AR) process compared with those in the electrode (initial solute is expected to give su ffi cientl y high degree of carbon content). Fina l chromium content is found to be removal, because the apparent operating pressure of a bout 0.2% less tha n the initial one. Some of the V AR is as low as that in a vacuum induction furnace. observed values of initia l and fin a l contents of carbon A number of works conce rning phys ical and chemical or oxygen are shown in Table 2. The table also phenomena occuring in VAR have been reported . shows the ra tio (Ll) of removed carbon to removed The results are revi ewed by Child and Oldfield4 ) and Table I. Experimental conditions Wood and Beall. 5 ) M ost steels to be remelted are Melting ra te deoxidized before the electrode preparati on . H ow­ M old Electrode Current Voltage Pressure Dia. m ID x L (kA) ever, residual oxygen in the electrod es is further re­ (mm) (mm) (V) (fL Hg) (g/min) duced during V AR due to the reaction between carbon and oxygen in the electrodes; Q+Q=CO.6,7) Un­ 103 X 500 48/50/57 1. 2-2 .5 24-26 0 .2-10 420-1340 fortunately, few systematic a nd qua ntitative studies 145 X 500 50/85/90 1.5-3. 0 25-27 0 .5-10 720-1400

* Presented a t the 87th lSI] Meeting, April, 1974, in N ippon University, Narashino 275. M anuscript recieved August 27, 1974. ** Fundamental R esearch La bs., Nippon Steel Corp. (NSC), Ida, K awasaki 2 11. * * * Formerly Fundamental R esearch Labs., presently Yawata Works, N SC, Edamitsu-cho, Yawata-ku, Kita kyushu 805. * * * * Formerl y Fundamental R esearch Labs., Presently H ika ri Works, N SC, Hikari 743.

( 286 ) Re searc h No te Transactions ISIJ, Vol. 15, 1975 ( 2 87 )

Table 2. R esu lts of chemical analyses

Exp. No. V-9 V-14 V -29 V-33 V-50 T -1 4

Initia l 0.024 0.025 0 .020 0.038 0 .019 0 .013 Carbon content (%) Final 0 .0058 0 .0066 0 .0047 0.011 0.0044 0.0026

Initial 0.031 0 .046 0.054 0.055 0.026 0 . 01 7 Oxygen content (%) Final 0.0071 0 . 02 3 0.034 0 .020 0.0065 0 .0070 ---- j 1.01 1.07 1.03 1.03 1.00 1. 25 ------j : ratio of removed carbon to removed oxygen in mole oxygen in mole. The observed values of L1 for most Mold ID Ce m. o f the present experiments are elose to unity. This (mm) (%) (kg/min) result indicates that carbon-monoxide is the main product of the reaction between carbon and oxygen o 145 0 .019 0 .91 in the alloy, Q + Q = CO. • 145 0.021 1.40 The relation between final carbon and oxygen con­ 103 0 .012 0.56 tents is examined for the cases where electrodes with •o 103 0.025 0.60 different oxygen contents but with the same carbon .6. 103 0.018 1.40 content are rem elted at a specified remelting rate. The observed values of the final carbon content are O.O I O~----~---~,-,-,,~ found to be independent of the final oxygen content : " I Torr I. ~ \ 2Torr' as shown in Fig. Dotted curves represent the rela­ I ~', I , tion between carbon a nd oxygen contents in liquid I " 0 25 % Cr- Fc alloy in eq uilibrium with carbon-monoxide ~ -a~---'-D>-----".'~-- ~ 0.005 - I "'::" at I 600°C, which is calculated from thermodynamic '- .{)>------o--<~> 8 I 01 \ data. - The o bse rved values are far from the , , . . equilibrium expected from the operating press ures \ ...... _ 0.1 Torr m easured at the furnance head. The pressure over '-__ Q.OI T';-;'~------0.01 0.02 the liquid metal pool is reported to range from 1O - 2~ (%0 ) 20 Torr above the operating pressure. 5, 11 , 121 How­ ever, the equilibrium values at such relatively higher Fig. I. Plot of the observed carbon- vs . oxygen-content after VAR. Dotted li nes show the theoreti cal re­ pressures seem unlikely to give any rea onable ex­ lation between oxygen and carbon contents in liquid p lanation for the observed values. 25% Cr- Fe a ll oy in equilibrium with CO a tmos­ As shown in Fig. 2, the observed values of the degree phere. Pressures of CO a rc shown in the fi gure of carbon removal, G1,/G., where Gp a nd Ge are the final and initial carbon co ntents, respecti vely, arc r-T-"""""~~------~ O . l approximately expressed by a function of single vari­ 3 able, I /vp+ I /ve, where Vp and Ve are the rates of the 0 • ingot growth a nd the elec trode consumption in o. o · cm /min unit, respectively. This vari able is equiv­ lj 0 0, 0.2 :..5 2 00 0 alent to pS/m, where m is the remelting rate in g /min '> <) G 0.3 unit, S is the total fr ee-surface area of the metal pool 8 <6- 0 0.4 at the top of the ingot and of the m etal fluid layer a t 0.5 the tip of the electrode, and p is the density. 1.0 10 2

IV. Discussion pS 11/ 1 f c+ 1 r" ( min lcm ) 6 The work of Belyanchikov et at. , 71 shows that gas Fig. 2. Relation between the degree of the carbon removal, evolution in the interior of the metal pool increases (Cp /C,), and (pS /m.) with increasing the remelting rate. In the present work this increase, however, is not discernible. Ac­ mining step, because carbon transport-control in other cordingly, the gas/melt interfaces considered are ex­ steps is inconsistent with the present results that the pected to be main reaction sites for carbon removal observed degree of carbon removal is nearly indepen­ during VAR, as was pointed out by Child and dent of oxygen content, initial carbon content and O ldfield. 41 operating pressure. A simple interpretation of the Carbon transport from both of the metal pool and experimental results is made on the following ass ump­ the fluid layer to the furnace head consi t of a seri es tions: (a) the carbon concentration at the fr ee surface of consecutive steps. A theo retical consideration for is negligible, (b) the carbon concentration is homoge­ each step reveals that the simultaneou transfer of neous throughout the bulk of the melt, and (e) the carbon and oxygen across a boundary layer in the melt values of the mass transfer coeffi cient of carbon in the adjacent to the gas/melt interface is the rate-deter- metal pool and the fluid layer are almost similar to

Research Note ( 288 ) Transactions ISIJ, Vol. 15, 1975

each other. T he considera tion of material balance for 0.05% in order to achieve the fin al carbon level of continuous remelting system in a steady state, thus, less tha n 50 ppm in VAR processing. Such level of yields the folIowing equation. carbon content in the electrodes is easil y obtained by using conventional degassing furnaces. In addition, m(C,-C'l ) pkS.C (12 /16)/ll ...... (1 ) = a = it is noted tha t the loss of chromium during V AR is J m(Ca-Cp) = pkSpCp = (12 /16)t1 p ...... (2) suffic iently small in comparison with an electron­ 2 where, k is the mass tra nsfer coeffici ent for carbon, beam continuous refining process. , 17) As the result Cd, is the carbon concentra tion in metal d roplets de­ of the present considera tion, V AR would be practical­ tached from the electrode tip, Sp and S. a re the free ly used as a n effective method for producing high­ surface areas of the metal pool a nd the Huid layer, chromium ferritic steels with extremcly low carbon content. resp ectively, and /lI. and (/J p a re the ra tes of mass transfer for oxygen in the fluid layer a nd the m etal pool, resp ectively. Elimination of C'l from these R EFER ENCES equations a nd substitution* of (Se+ Sp)/2 for (S.Sp)1/2 I) W . O. Binder and H . R. Spendelow, Jr.: T ra ns. ASM , 43 may yield: ( 195 1),759. 2) T. H. H a rrington and R . B. Pa lmer : Elect. FUTIl. Proc., (C./Cp)I/2 ~ 1 +(k/2)(pS /m) ...... (3) 28 ( 1970),34. 3) R. A. Lula : 6th Ann. Offshore T ech. Conf. , TMS-AIME, where, S= S. +Sp. ( 1974), O TC-1 96 1. A plot of the observed values of (C./Cp )1I2 against 4) H . C . Child and G. E. Oldfield: Vacuum M et., O. Winkl er thosc of (pS/m) is shown in Fig. 2. The observed and R . Ba kish, ed ., Chap. 5, E lsevier Pub. Co., ( 197 1). valucs may b e fairly fitted by a straight line. The 5) F. W. Wood and R. A. Beall : Bureau of Mines, B uiletin, va luc of the gradi ent of thc line is 0.74 cm /min, from ( 1965), No. 625. which the valu e of k can be calculated to be 0.025 6) L. N. Belyanchi kov, R. N. G rigorash and A. V . Panov: cm /sec. The values of the ma s transfer coefficient Doklady. Vysslz. Shkoly . Metall. , ( 1959), No.2, 48. 7) L. N . Bclyanchikov, R . N. C rigorash and A. V . Panov : for carbon have been reported to be 0.035 13) and h.uest. VUZ-Clzem. Met., ( 196 1), No.9, 79. 0.03214) for liquid p ure iron, a nd 0.01 515 ) for liquid 8) T . Fuwa and ] . Chi pman: Trans. AIME, 218 ( 1960),887. stainless steel from the experiments on gas/melt reac­ 9) T. Fuwa a nd J . Chipman : T ra ns. A/ME, 215 ( 1959), 708. tions. The p resent result is fully close to those re­ 10) Y. Nakamura, T. Ono, a nd K . Segawa: Proc. I CST IS, ported. It see ms, therefore, likely tha t the semi­ Supp!. Trans. l SI}, 11 ( 197 1), 456. quantitative equation derived a bove is reaso nable. II ) E. W . J ohnson, R. Itoh, R . L . R eadal, a nd M . L . Hill: Estimation of the degree of carbon removal in a Tra ns. 8 th Vacuum Symp. a nd 2nd I ntern. Congress, 2 large V AR unit will be made with the aid of Eq. (3). ( 196 1), 749 (Pergamon Press, 1962). Barraclough l 6) reported that the values of the remelting 12) S.J. Noesen : Vacuum Syrnp. T rans., ( 1957), 150 (Per­ rate and the ingot diameter are 5.9 -1 3 kg /min and gam on Press, 1958). 41-69 cm, respectively, when the electric current is 13) H . Nomura and K . M ori: Tetsu-to -Haganc, 58 ( 1972), 29. 10-20 kA. Thus, the value of (pS/m) is evaluated to 14) P. A. Distin, G. D. H all et, a nd F. S. Richarclson: }ISI, 206 ( 1968), 821 . be 2.2-2.8 min/cm, assuming tha t S. is equal to Sp/3. 15) T . W a tanabe and T. Tohge: Tetsu-to- Haganc, 59 ( 1973), In this case, the relation given by Eq. (3) shows tha t 1224. carbon can be reduced by 85 - 90% of the initial con­ 16) K. C . Barraclough: Steel Times, 188, Ma rch 20 ( 1964), tent from the 25 % Cr- Fe alloy during V AR. Ac­ 376. cordingly, carbon content in the electrodes to be 17) Y. Nakamura ancl M. Kuwaba ra: Trans. l SI}, 15 ( 1975), remelted should be controll ed in the range of 0.03- 103.

* T hi s substitution will give less than 5% u ncerta inty for (CpIC. ) under the present experimental cond itions.

Research Note