US 3771997 A
The use in the electroslag remelting or welding process of a starter tablet which includes oxygen deficient titania.
Description (OCR text may contain errors)
United States Patent [191 Clark TITANIUM OXIDES IN ELECTROSLAG PROCESSES  Inventor: Peter Stanley Clark, Nechells,
Birmingham, England  Assignee: Foseco International Limited,
Birmingham, England  Filed: July 24, 1968  Appl. No.: 747,073
 Foreign Application Priority Data Aug. 2, 1967 Great Britain 35,451/67  U.S. Cl. 75/10 R, 75/11, 75/94, 148/26  Int. Cl. C22d 7/00, C22b 9/10, 823k 35/24  Field of Search 75/10, 11, 12, 94, 75/129, 93; 148/23, 26; 106/55; 23/202  References Cited UNITED STATES PATENTS 2,792,287 5/1957 Moore 23/202 3,049,432 8/1962 Weber 106/57 3,189,473 6/1965 Weber 106/57 3,211,591 10/1965 Miltschitzky 148/26 2,490,179 12/1949 Willigen 148/23 1 Nov. 13, 1973 OTHER PUBLICATIONS Storms, Refractory Materials, Vol. 2 pp. 1-2 (Academic Press, 1967). Leybenzon & Tregubenko, Steel Production by Electroslag Remelting Method, pp. 27--31 (U.S. Dept. Comm., 1963).
Primary ExaminerL. Dewayne Rutledge Assistant Examiner-Peter D. Rosenberg AttorneyWolfe, Hubbard, Leydig, Voit and Osann Ltd.
 ABSTRACT The use in the electroslag remelting or welding process of a starter tablet which includes oxygen deficient titania.
15 Claims, No Drawings TITANIUM OXIDES IN ELECTROSLAG PROCESSES The present invention relates to oxides of titanium, and to their use in starter tablets used in the electroslag remelting or welding process.
If TiO is heated, either in vacuo or in a hydrogen or other non-oxidising atmosphere, at 950 1400C, some reduction of the material takes place and the previously white material changes colour to black or blueblack. By the treatment it is possible to produce a composition of empirical formula lying between TiO and TiO These oxides are, for simplicity, hereinafter referred to as oxygen deficient titania.
The normal resistivity of titania is of the order of ohm cm; the resistivity of oxygen deficient titania is generally in the region of 0.05 10 ohm cm.
In the electroslag remelting or welding process, the metal to be refined is used as an electrode, the electrode is immersed at its end in a bath of molten refining slag contained in a water-cooled mould, and electrical current is caused to pass through the system. The mould usually consists of a hollow steel cylinder with a copper base-plaste, the two components being separated by an insulating refractory gasket (e.g., asbestos). By reason of the electrical resistance of the system, due essentially to the slag component, heat is generated in amount sufficient to cause the tip of the electrode to melt. The molten metal falls through the molten slag layer in the form of droplets and thereby undergoes a cleansing action which reduces inter alia the sulphur and inclusion contents of the metal. The refined metal collects in the water cooled mould and is progressively solidified with a superior crystalline micro-structure to produce an ingot which possesses improved mechanical properties and is eminently suitable for subsequent rolling, drawing, forging and other working.
The slag used for the purpose may be of any composition which is suitable for the refining effect required, the principal restricting factor being that it must be electrically conductive to the correct degree so that its resistance to the electric current is responsible for producing the required heat. Usually, the composition of the slags has been based upon fluorspar, fluorsparlalumina, fluorspar/lime, fluorspar/magnesia, fluorspar/lime/alumina or fluorspar/magnesia/alumina systems with, possibly, minor additions of substances intended to effect specific alloying or refining modifications of the metal. As the remelted metal collects in the mould and its level rises, a thin layer of the slag remains between the metal and the mould wall, conferring a very smooth surface to the ingot produced.
In the electroslag welding process, the weld metal is used as the electrode and instead of providing a mould as in the electroslag remelting it is arranged that a cavity is formed between two opposite water-cooled shoes and two interspaced opposite metal elements which are to be welded together. The same technique of providing the molten metal from the electrode is used.
In terms of the type and quality of steel produced, the electroslag remelting process may be compared with the consumable are or vacuum arc steelmaking method. It does, however, offer considerable advantages over the latter process, both economically by requiring lower capital and running costs and technically in that the ingots produced have superior surface quality, a lower sulphur content, a more uniform and finer grained structure and much improved hot-working ductility.
Nevertheless, the process suffers certain limitations which have proved difficult to overcome. The principal disadvantages arise from the difficulties in the starting procedure. It will be appreciated that it is not sufficient merely to place a quantity of powdered slag in the mould, lower the electrode and expect a current to flow through it; considerable arcing occurs, possibly damaging the copper base-plate of the mould and certainly causing severe power surging.
One technique for meeting this difficulty is to place in the mould a small quantity (e.g., an ounce or two) of a sodium or potassium nitrate/aluminium/iron oxide or nitrate/aluminium/magnesium mixture and lower the electrode on to it. Powdered slag is then poured into the space between electrode and mould wall, the electric power is switched on to trigger the reaction between the aluminium/magnesium and the nitrate and- /or iron oxide. The exothermic reaction assists in melting a small quantity of the slag which then becomes conductive and the remainder is gradually melted. However, even this method needs a high starting current (much higher than the current for normal running of the process) and the electrical system has to be constructed in a way that it will withstand current surges without being damaged. As only a small quantity of slag is initially melted, it cannot rapidly attain the required running temperature whilst unmelted powdered material continues to chill the slag pool. Thus, initially the slag is rather viscous and can contain lumps of unmelted material which are likely to become entrapped in any molten metal which is produced. In fact, it is quite usual to crop and discard the bottom 5 percent of the ingot produced because of its poor quality and high slag inclusion content.
Furthermore, the position of the electrode in the slag is usually automatically controlled so that it produces the required current density, i.e., when the resistance of the system changes the extent to which the electrode is immersed in the slag is automatically varied to compensate, so that a steady current density is maintained. With the violent current fluctuations resulting from the uneven melting of the slag, the electrode tends to move up and down in rapid oscillation in an attempt to stabilise the current density. This may result in the electrode touching the mould bottom and becoming welded thereto. In addition, it is often found that the powdered slag remains suspended above the liquid level and bridges across the annulus between electrode and mould wall. As the remelting process proceeds, the level of metal and slag rises in the mould and eventually the bridges are partly consumed. This causes intermittent cooling of the slag bath with consequent rises in viscosity. The usual effect is the production of an ingot with a corrugated surface, due to the formation of an uneven thickness of solidified slag on the mould walls.
Various types of starter tablets have been proposed, for example as disclosed in our copending application, US. Pat. No. 634,737, and now abandoned.
These types function well, but necessitate a more complex manufacturing procedure than the starter composition of the present invention.
It has now been found that the starting difficulties referred to above may be largely eliminated or overcome by the use, in contact with the electrode of a composition containing oxygen deficient titania.
According to the present invention, therefore, there is provided in the electroslag remelting or welding process the use of a starter composition which comprises oxygen deficient titania. The starter composition preferably also includes other slag forming ingredients. The composition preferably includes 40-99 percent by weight oxygen-deficient titania.
On initiating the electroslag remelting or welding process by passing the electric current, the composition heats up due to the oxygen deficient titania therein acting as a resistance heating element, and which. melts to form a shallow pool of fused titania in which the remaining ingredients of the composition dissolve to form the required refining slag.
in certain circumstances, oxygen deficient titania may be used by itself in powder or compacted form as the starter material, but it is preferable to include other slag forming ingredients in the composition in order to adjust the composition of the slag produced and its melting point. Alkaline earth metal fluorides may be included in the composition, preferably in the proportion of up to 20 percent of the composition.
The composition may be matched to that of the metal of the refining or welding electrode. in a preferred form of the invention, bonded tablets comprising 20-60 percent by weight iron powder and 8040 percent by weight oxygen deficient titania may be used quickly and easily to initiate the electroslag remelting or welding process.
The binder, if used, may be any binder which will hold the tablet in coherent form and have no adverse effect on the properties thereof. Inorganic binders such as modified clays are preferred for economic reasons, the binder most preferably used being bentonite. Thus, the present invention provides, an electroslag starter tablet which comprises oxygen deficient titania, a slagforming agent and a binding agent, bonding the ingredients to a coherent tablet.
However, it is still more preferable to employ compositions which have been sintered into tablet form. Thus, tabletted compositions according to the present invention may be prepared by incorporating in a composition comprising normal titania, TiO,, a suitable binder, e.g., l-4 percent by weight gum arabic and tabletting the mixture, e.g., by moistening and pressing into the required shape. The tablet is then sintered in air at about 800C during which process the gum arabic burns away. The coherent tablet thus formed is heated in a suitable reducing atmosphere until the titania has the required oxygen deficiency and electrical conductivity.
According to a further feature of the invention, there is provided a process for the production of an electroslag startei' tablet which comprises the steps of bonding in tablet form a mixture of titania and optionally at least one material selected from glass powder, iron powder and alkaline earth metal fluorides and subjecting the resulting tablet to reduction at a temperature of 950-l400C. The ingredients may be bonded with an organic gum or resin, e.g. gum arabic.
In a specific process normal titania may be mixed with gum arabic and glass powder, compacted into a tablet, dried at 140C and then heated in a suitable reducing atmosphere.
The following examples of oxygen deficient titania tablets serve to illustrate the invention:
EXAMPLE 1 TiO,(x 1.65 l.98) 98 percent Glass powder 2 percent resistivity of the above composition is approximately 0.5 ohm cm.
EXAMPLE 2 TiO (x L65 1.98) 49 percent Iron filings 49 percent Glass powder 2 percent resistivity of the above composition is 0.1 1.5 ohm cm.
EXAMPLE 3 TiO, (x 1.65 L98) 44 percent Iron filings 44 percent Fluorspar 10 percent Glass Powder 2 percent resistivity of the above composition is 0.25- 0.5 ohm EXAMPLE 4 TiO, (x 1.65 L98) percent Carbonyl Nickel powder 18 percent Glass powder 2 percent resistivity of the above composition is 0.1 0.5 ohm cm.
In each of the above examples, the recipes given are of the finished, sintered, tablet. These tablets were obtained by mixing 93 parts of the appropriate mixture with two parts of gum arabic and 5 parts of water. The composition was then compressed into tablet form using a pressure of 8-10 tons p.s.i. and ignited at 800C to burn the organic matter and to sinter the tablet. This was followed by heating at 1,000 to l,400C in a hydrogen atmosphere for two to three hours. This procedure provided tablets of density between about 4 gmslcc (for example 1) and about 6 gms/cc for those compositions containing a high proportion of iron. in example 3, for example, a tablet of two inches diameter and one inch height was manufactured from gms. of the mixture. This was suitable for use as a starter block in a six inch diameter mould.
The use of gum arabic ensures cohesion in the green state but it is not essential if reasonable care is taken in handling the tablets before igniting.
I claim as my invention:-
l. In a process involving passage of an electric current through a molten slag between a meltable and a non-meltable electrode, the improvement which comprises initiating the process in the presence of a composition comprising a substantial proportion of oxygen deficient titania wherein the ratio of oxygen to titanium is in the range of from about 1.65 to about 1.98.
2. A process according to claim 14 wherein the com position includes up to 20 percent by weight of at least one alkaline earth metal fluoride.
3. A process according to claim 14 wherein the composition includes iron powder.
4. A process according to claim 14 wherein the composition includes glass powder.
5. A process according to claim 14 wherein the composition is used in the form of a bonded tablet.
6. A process according to claim 5 wherein the binding agent of said tablet is a clay.
7. A process according to claim 14 wherein the composition is used in the form of a sintered tablet.
8. A process for the production of an electroslag starter tablet which comprises the steps of bonding in tablet form a mixture of titania and at least one material selected from the group consisting ofglass powder, iron powder and alkaline earth metal fluorides, and subjecting the resulting tablet to reduction at a temperature of 950-l400C so as to decrease the ratio of oxygen to titanium in the titania to from about 1.65 to 1.98.
9. A process according to claim 8 wherein the ingredients are bonded with a binding agent selected from the group consisting of organic gums and resins.
10. A process according to claim 9 wherein the ingredients are bonded with 14 percent by weight gum arabic.
11. An electroslag starter body consisting of a coherently bonded tablet which consists essentially of oxygen deficient titania wherein the oxygen to titanium ratio is from about 1.65 to 1.98, a slagforming agent and a binding agent, said oxygen deficient titania being capable upon passage of electric current therethrough of melting to form a pool of fused titania in which the other components of said starter body dissolve to form a refining slag.
12. An electroslag starter body which consists of a coherently bonded tablet which consists essentially of 20-60 percent by weight iron powder and 40 percent by weight oxygen deficient titania wherein the oxygen to titanium ratio is from about 1.65 to 1.98, said oxygen deficient titania being capable u'pon passage of electric current therethrough of melting to form a pool of fused titania in which other components may be dissolved to form a refining slag.
13. A process for starting electroslag remelting and welding processes which comprises resistance heating via a meltable and a non-meltable electrode a slagforrning body containing oxygen deficient titania to form a molten slag and raising the slag to a temperature such that the meltable electrode melts to form droplets which fall through the slag.
14. The process of claim 1 wherein the meltable electrode is metal to be refined which after melting passes through the molten slag and is collected.
15. The process of claim 1 wherein the meltable electrode is weld metal which after melting serves to join metal pieces being welded.