US2693414A - Methods of casting titanium stabilized steel - Google Patents

Description

United States Patent O METHODS OF CASTING TITANIUM STABILIZED STEEL Walter H. Dunn, San Diego, Calif., and John C. Southard, deceased, late of La Mesa, Calif., by Margaret C. Southard, executrix, La Mesa, Califi, assignors to Solar Aircraft Company, San Diego, Calif., a corporation of California No Drawing. Application January 18, 1951,

Serial No.'206,708

8 Claims. (Cl. 75-429) of columbium and necessity for finding suitable alloys capable of being substituted for columbium stabilized alloys'is discussed in an article Conservation of Columbium by J. 'F. Tyrrell, Research Metallurgist, Solar Aircraft Company, ppearing in Metal Progress forluly 1950, pp. 6368, inclusive.

While AISI Type 321 steel has been available for many years in sheet and bar form and recognized as a satisfactory substitute for corresponding forms of A181 Type 347, castings of titanium stabilized steel have never beenentirely acceptable. Consequently, except for small amounts reported in 1946 and 1947 there has been no production commercially of titanium stabilized cast alloys of the CF type.

The difficulties which have beset the casters of titanium stabilized steels and have previously prevented the economic manufacture of such titanium castings may be briefly outlined as follows: I

-1. Fluidity.-Flow into small complex and even comparatively simple molds has been impeded by lack of fluidity in the melt. Higher temperatures than desirable have had to be used in order to assure complete filling of the mold, and solidification of certain areas far in advance of others has led to castings'of non-uniform internal structure.

2. Fading-The higher temperatures required have caused deleterious fading loss from the metal into the slag of a substantial amount of the titanium added to stabilize the material resulting in the rejection of castings for titanium content below the specified minimum.

3. Deleterious surface and internal inclusions.-Where in the formation of ingots for the rolling of A181 321, slag and surface defects of foreign matter may be chipped ofi? prior to rolling. In casting of stainless steel containing titanium previous efforts have been unsuccessful in keeping deleterious slag forming materials from passing from the ladle into the mold and remaining there. Some ofthese materials may rise to the surface in such a way as to mar its smoothness and detail and, particularly in intricate or small castings, others may remain entrapped in those areas where the cooling rate is slowest. There may produce apparently sound but actually dangerous castings.

This situation is further aggravated by present needs particularly in the small and more intricate shapes to raise the pouring temperature from approximately 2800 F. to 3100 F. and higher. Other things being equal, this increase of temperature greatly increases the production of slag.

A very considerable effort has been made by the producers of titanium alloys and others to produce sound, clean titanium stabilized stainless steel castings. The reasons for this continued effort are that titanium is far less critical than columbium and that its cost is less, and with greater use and production its price has been drop- P 8-, I

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Titanium is the ninth most abundant element in the earths crust and is available in large quantities in the deep sands of both coasts'of the United States. On the other hand, columbium is a relatively rare element and the present sources of supplyin North Africa and South America are not only distant but so limited that it is difficult to supply even the peace-time demands for this element. Furthermore, the ductility of castings stabilized with titanium has been shown to be superior to those stabilized with columbium. a

The foregoing problems are so pressing that the Alloy Casting Institute has recently instituted a research project with Batelle Memorial Institute in an effort to find a solution.

In view of the foregoing difl'iculties, disadvantages, and prior and current intensive efforts and failures by others to solve these problems, the aforementioned Tyrrell article, particularly the sub-head Sand Castings, page 66, .wherein it was mentioned that successful Type 321 castings have beenmade at Solar Aircraft Company, has raised considerable interest and comment by those skilled in the art. This interest has best been indicated by the many inquiries for further information and comments that such information would offer a distinct contribution to industry.

Heretofore, in the preparation of Type 321 ingots it has been a standard procedure to precede the addition of the ferro-titam'um alloy (usually 25 to, 40% Ti, low carbon and having an appreciable percentage of aluminum in the ferro-titanium) by the thorough deoxidation of the heat with aluminum.

Counter to general belief, we have discovered that the aluminum rather than the titanium was primarily responsible for the lack of fluidity in melts of stainless steel using ferro-titanium'as a stabilizing agent. We have found that clean, sound castings of all sizes and shapes can be effectively produced by using a deoxidizer containing no aluminum and a titanium containing stabilizing agent having .little or no aluminum.

An approach to the desired stabilizing agent has been found in an alloy available to industry for a considerable time and described in Comstock Patent-2,064,150. This alloy contains approximately 40% Ti, 11% Si and 3% Al. It was developed primarily as a deoxidizing agent for common carbon steels, and has not to our knowledge been previously considered for use in the production of stabilized stainless steel castings. The prime difiiculty in using the alloy in this manner is that its high silicon content added to that already present in the charge tend to bring the total silicon content above the usual composition range. Further, the well known oxidizing properties of titanium tend to exert themselves and the titanium fades into the slag rather than remaining full strength in the melt as a stabilizer.

We have discovered methods of overcoming these difficulties and to recover, and retain in the metal as cast, a large proportion of the total titanium added; likewise, we have discovered other ways of deoxidizing without the aluminum titanium combination which leave a substantial'portion of the titanium in the final alloy and lincrease the fluidity of the titanium stabilized materia Accordingly, it is a prime object of our invention to alleviate the demand for the critical material columbium by the provision of columbium-free castings having physical properties equivalent to the columbium stabilized stainless steel AISI Type 347.

It is a further object of our invention to provide a method of melting and casting stainless steels such that the composition of the metal as cast retains sufiicient titanium to effect stabilization.

A further object of our invention'is toprovide titanium stabilized stainless steel castings of. all sizes and shapes substantially freeof inclusions and of deleterious admixtures tending to rise to the surface of said castings in such a way as to mar their smoothness and detail.

A still further object of the invention is to provide titanium stabilized castings substantially free of aluminum and/ or aluminum compounds.

Another object of the invention is to provide a thorough. deoxidation of the melt, withoutthe use of aluminum,

prior to the introduction of titanium, so that little of the titanium is lost in the deoxidation of the metal.

A further object of the invention is to accomplish the above objects with the loss of only a small fraction of the entire titanium added.

Other objects andadvantages will become apparent f11"o m the following description and from the appended. c aims.

For clarity certain specific heats. will be described using a basic furnace charge for the production of titanium stabilized; stainless steel castings but it should be understood that the methods described herein are by no means limited to this specificationand are governed rather by the appended claims. The composition of the basic furp harge. isv lis e n. Table I.

Table I FURNACE CHARGE FOR 18-8 Ti Percent by weight 19.70

Just prior to tapping'the furnace substantial deoxidation of the melt is accomplished with aluminum-free alloys which leave no deleterious metal residue in the melted steel and prevent substantial titanium losses due to fading. These may take the form of calcium silicon or Zirconium alloy (Zr- 35-40%, Si 47-52%, Fe 23-12%, C 0.5% max.). Tenpoints (0.10%) of each of these alloys has been; found tobe sufiicient and to substantially deoxidize the melt. It is believed that calcium silicon manganese or other substantially aluminum-freedeoxidizers might be used alternately.

Also just prior to. tapping the furnace 80 points (0.80%) of the FeTialloy'substantially as described in the Comstock patent, supra, is added to the ladle cold. Table 11 following indicates the relation of time and per cent. of titanium in samples taken from a 1,000lb. ladle. after -it hasbeen filled with the melt.

Table II (Heat No. 2775) Percent Percent T1 Added Ladle Sample Temp. T1me, Minutes o T1 in lagzioe) Tapped Poured F. sample sible after the addition oftitaniurnin order. to prevent fading.

The chemical composition, of the metal ascast and likewise the physical properties of certairnsamples are given in Table III below:

This experience is 4 Table III CHEMICAL COMPOSITION AND MECHANICAL PROPERTIES Cr percent by weight 20.00 Ni do 9.01 Mn do 1.25 Si do.. 1.04 C d0 0.074 P do 0.017 S do 0011 T1 See heat 2775 above Fe Balance TS, YP; R. A., El. Test Bar 10m Sample p. s. i. p. s. 1. percent: percent It will; be seenfrom the above composition, and mechanical properties that the castings produced not. only are completely satisfactory but also have mechanical. properties as good orbetter than those of Type 347 castings.

Under certain conditions it had been. found. imprac-- furnace rather. than-tothe ladle. When the alloy is added;

to the furnace in this manner. it has been found that less. recovery is. effected. It: isbelieved that-this is due in part to the fact that the ferrotitanium is added to the top and therefore does. not have the opportunity to. filter up through asin thecasewhere it is added to-the, bottomof the ladle. Also, convection within the furnace may tend; to bring the lighter weight titanium 'containing compound;

to the surface morerapidly and before itssolution in the base charge may be: fully completed. There may alsobe increased fadmg or titanium release due to the higher.

temperature ,within thefurnace.

However, ithasbeen foundthat where 60 or. 300 lb.

ladlesmustbe usedand the ferrotitanium. added directlyto the furnace charge, satisfactory castingsmay beprodu ced if the initial addition of points of titanium is supplementedby smaller, amounts added prior to, each: tapping of the furnaces. Tables IV and V following areillustrative of this method.

Table-IV (Heat #2800) FURNACE ADDITIONS OF Ti (40% FeTi) 300 :LB. LADLE Percent Ti Added Ladle Sample Temp. t Time, M1n.-Secs. T1 in to Tapped Poured F. Furnace Sample Table V (Heat #2801) FURNACE ADDITIONS OF Ti (40% Fe'li) 60 LB. HAND SHANK LADLE Percent Percent TiAdded Ladle Sample Temp. Time, Min-Secs. Y o Tim I Futgace Tapped Poured F. Sample It will be seen from the examples given above that time plays an essential part in the degree of recovery. which may be expected. The optimum time will, of course, vary depending upon the sizes of ladle used and the part to be cast. As a rule, approximately three to eight mmutes are allowed to elapse between the addition of sup plemental amounts of titanium to the furnace and the time the ladle is tapped. This again allows time for this titanium to go into solution but insufficient time for it to fade very materially.

Though ferrotitanium has been mentioned and used in our tests as the source of the titanium stabilizer, other titanium alloys of low aluminum content might be used or even titanium metal if such becomes economically expedient.

One additional factor should be stressed in the use of silicon compounds both in the original deoxidant and as a substantial portion of the titanium stabilizing compounds. As mentioned before, caution must be used in the selection of the metals forming the charge in order to keep the total silicon content of the casting below the specified maximum. The use of titanium or a titanium alloy of still lower aluminum and silicon content than that described in the Comstock patent will ease this situation should use thereof become commercially expedient. At present the solution to this problem resides in the selection of materials low in both aluminum and silicon content for the basic furnace charge. We have been careful in the selection of remelt and scrap to select that which is low in aluminum and silicon for our castings. The silicon content of the charge can be further reduced if necessary by the addition of Armco Ingot Iron which has only traces of silicon. It is believed also that in an arc furnace procedure some of the silicon in the charge may be reduced prior to the addition of the ferrotitanlum, making the silicon content of the initial charge less critical.

From the foregoing description it will be seen that we have provided successful means for producing columbium-free castings having physical properties equivalent to the columbium stabilized steel A181 347, whereby the critical shortage of columbium may be considerably alleviated. In so doing, we have solved in a simple and economical fashion the longstanding problem of producing satisfactory titanium stabilized stainless steel cast1ngs of all sizes and shapes the production of which has not heretofore been commercially successful or practical.

This invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The present embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims rather than by the foregoing description, and all changes which come within the States Letters Patent is:

1.,In a method of producing stainless steel casings capable of commercial use substantially as cast and containing titanium as a stabilizing agent, the steps of deoxidizing the furnace charge completely with strongly reducing aluminum free compounds just prior to tapping, adding ferro-titanium alloy containing less than 4% aluminum by weight to said charge while in a completely deoxidized condition, and pouring such charge into at least one sand mold immediately after such compounds have alloyed with the charge.

2. The method as defined in claim 1, wherein the deoxidizing agent is calcium silicon, calcium manganese or aluminum free compounds thereof.

3. In a method of producing titanium stabilized stainless steel castings capable of commercial use as cast using a furnace, the steps of melting a stainless steel charge in the furnace, substantially completely deoxidizing the charge with strongly reducing aluminum free compounds just prior to tapping said furnace, adding to the charge while in such deoxidized condition a composition formed predominantly of titanium and having less than 4% aluminum by weight so that relatively little of the titanium is lost in the further deoxidization of the charge, and pouring such charge in sand mold immediately after such composition has alloyed with the charge.

4. The method as defined in claim 3 wherein the deoxidizing agent is calcium silicon, calcium manganese or compounds thereof.

5. In a method of producing titanium stabilized stainless steel castings capable of commercial use substantially as cast, the steps of deoxidizing the charge completely with strongly reducing aluminum free compounds, adding a titanium alloy having less than 4% aluminum by weight to the ladle prior to filling the ladle with the charge, filling the ladle with the deoxidized charge whereby the titanium alloy is allowed to filter up through the charge, mix thoroughly, and alloy therewith, and pouring said charge from said ladle into sand molds.

6. In a method of producing titanium stabilized stainless steel castings capable of commercial use substantially as cast using a furnace and a ladle exceeding 300 pounds; the steps of melting a stainless steel charge in the furnace; completely deoxidizing the charge just prior to tapping the furnace with calcium silicon, calcium manganese, or aluminum free compounds thereof; adding to the ladle just prior to tapping the furnace a titanium containing compound having less than 4% aluminum by weight; filling the ladle with the deoxidized charge whereby the titanium stabilizer is allowed to filter up through the charge, mix thoroughly, and alloy therewith, and pouring said charge from said ladle into sand molds immediately after said titanium alloy has alloyed with said charge in said ladle.

7. A method of forming titanium stabilized stainless steel castings capable of commercial use substantially as cast comprising the steps of deoxidizing a stainless steel melt sufiiciently with a strong aluminum free deoxidizer sufiiciently so that upon the addition of titanium to such melt there will be practically no oxidization thereof, adding a stabilizing agent containing a high percentage of titanium and having less than 4% aluminum by weight to such melt while in a deoxidized condition, and subsequently pouring such melt into sand molds immediately after such stabilizing agent has alloyed with the stainless steel melt.

8. A method of forming titanium stabilized stainless steel castings which are substantially free of surface defects and to be used at least in part in the as cast form comprising the steps of deoxidizing a stainless steel melt sufliciently so with a strong aluminum free deoxidizer so that upon the addition of titanium to the melt there will be no oxidization thereof, adding a composition containing a high percentage of titanium and having less than 4% aluminum to such melt while in such deoxidized condition, and subsequently pouring such melt into sand molds immediately after such composition has alloyed with the stainless steel melt.

(References on following page) 1 References Cited-Lin -.the filezofithis patent .OTHER- REFERENCES UNITED"STATESPATENTS .Electric Furnace Proceedings, A. I. M. E., 1950 Proce'edings, page 121. Number Name Electric Furnace Proceedings, A. I. M. E., 1947, pages =.Date 2,095,400 -Scharschu l'.Oct- 12,1937 and 174 Titanium and Its Use in Steel,xpage 14, published in 1.94% by The Titanium Alloy Manufacturing Co., New -Yor

Claims (1)

1. IN A METHOD OF PRODUCING STAINLESS STEEL CASINGS CAPABLE OF COMMERCIAL USE SUBSTANTIALLY AS CAST AND CONTAINING TITANIUM AS A STABILIZING AGENT, THE STEPS OF DEOXIDIZING THE FURNACE CHARGE COMPLETELY WITH STRONGLY REDUCING ALUMINUM FREE COMPOUNDS JUST PRIOR TO TAPPING, ADDING FERRO-TITANIUM ALLOY CONTAINING LESS THAN 4% ALUMINUM BY WEIGHT TO SAID CHARGE WHILE IN A COMPLETELY DEOXIDIZED CONDITION, AND POURING SUCH CHARGE INTO AT LEAST ONE SAND MOLD IMMEDIATELY ATFER SUCH COMPOUNDS HAVE ALLOYED WITH THE CHARGE.