US2360797A - High tensile strength, noncorrosive alloy - Google Patents

Description

Patented Oct. 17, 1944 UNITED STATES PATENT OFFICE 2,360,797 HIGH TENSILE STRENGTH, NONCORROSIVE ALLOY Charles J. Schafer, Baltimore, Md.

No Drawing. Application May 13, 1941, Serial No. 393,261

3 Claims.

commerce but they are 'of markedly different composition and possess advantageous properties not displayed by such steels.

Furthermore, the alloys provided by the present invention are much cheaper and easier to produce than special steels and other alloys heretofore employed for the same purposes as those to which the present alloys are applied.

The qualities just mentioned, together with others which will hereinafter be-set forth, render my novel alloys susceptible of a wide variety of uses, among which may be cited the manufacture of ship plates both for commercial and naval use, military armor plate and construction material, cutlery, surgical and other instruments, building constructions and sheathing, and other uses too numerous to mention where high tensile and stainless steels are now employed.

The novel alloys involved in the present invention may contain varying percentages of zirconium, nickel, chromium, iron, yttrium, and aluminium. In order to attain certain variations in the properties of the alloys, the percentages of the various ingredients may be altered and in fact certain of the constituents may be omitted altogether. Certain of the alloys thus produced resemble somewhat the 18:8 stainless steel which has gone into very extensive use. This steel contains chromium and nickel in the ratio of 18 to 8. However, none of the present stainless steels employs zirconium as a principal alloying constituent, and few of them are carbonfree. Attempts have been made to aifect the carbon structure of these austenitic alloys by the addition of smaller quantities of zirconium, but the amount of zirconium added has seldom exceeded about 2.5%, and the resulting product could hardly be called a zirconium alloy.

Certain specimen analyses of alloys produced in accordance with the present invention are set forth below.

An alloy which is particularly useful in the production of ship plates especially in connection with the merchant marine has the following approximate analysis:

' Per cent Zirconium 10-15 Nickel About 12 Chromium ut 6 Carbon ut .040 Silicon out .05 Iron Balance Another alloy which may serve as armor plating for certain structures, ships, vehicles, or the like, and also for general utility where somewhat greater tensile strength and resistance to impact is desired, is as follows:

An alloy of similar content but adapted for heavier armor plate or for all heavy duty service is of the following constituents:

Per cent Zirconium 30-40 Nickel 12-18 Chromium 10-20 Carbon .20 Iron Balance By varying certain of the constituents, the hardness of the alloys may be correspondingly changed and certain of them may be rolled,

either in the hot or cold state; others, of course,

would have to be cast in the final shape in which they are to be used.

A summary of the ranges of percentages of the constituents of the alloys described above is as follows:

Per cent Zirconium 10-40 Nickel 10-20 Chromium 5-20 Carbon 0018-020 Silicon Nil-0.05 Iron Balance If a truly non-magnetic alloy is desired, about %-10% of yttrium may be added to any of the alloys described above. If approximately 15% of aluminium 99.9 is added, there is produced a highly magnetic alloy.

Forcertain uses, both the chromium and the nickel may be omitted from the alloys, as well as substantially eliminating the carbon and silicon content. This will result in a ferro-zirconium of high quality.

While some of these alloys resemble in microstructure the austenitic or gamma steels, they are really more in line with the alpha structure in which the atoms are arranged in the primary crystals according to the body crystal centered,

and are much harder than the austenitic. The grain is very dense and homogeneous throughout the entire body of the metal. The metal flows well, casts well, and rolls well and does not present the dii'flculties that many steels of the austenitic type exhibit in machining or rolling. The alloys are superior to molybdenum-chromium steels, copper steels, and carbon steels which have always been considered the most available for the toughest and hardest armor plates.

The tensile strength and resistance to impact exhibited by the alloys provided by the present invention is about twice that of the above men tioned steels which have formerly been used, for example a sheathing plate or armor plate made of the present alloy need be only half as thick as a plate constructed of the steels formerly employed.

In addition to these advantages, plates made of the alloys under the present invention have a much greater resistance to the eifects of sea water, ammonium chloride, and sulphurous acid. Also, the alloys resist the adhesion of barnacles and other marine growths, resist corrosion, and render the painting of ship plates and hulls constructed of the alloy necessary only at infrequent intervals. At the present time the ordinary vessel must be dry-docked, scraped and painted at approximately eight-month intervals.

This, of course, means that the vessel is out of service for a considerable length of time which entails financial loss in addition to the, actual cost or this servicing.

The alloys described herein may also be used for ordnance material such as gun barrels and breeches since their non-warping properties and heat resistance are very high. The alloys may also be employed in the manufacture of tanks and also in the production of all sorts of land armor whether cast or rolled.

As to the process of manufacture of these zirconium-containing alloys, it may be stated that one of the most important novel features involves a pre-melt or reduction-melt by which the ferro-zirconium is produced. Zirconium has a very great amnity for iron and also for carbon, and while it also absorbs gases including oxygen and nitrogen, these are practically eliminated in the melt due to the high temperatures employed in the operation and they also pass out.

of the melt during the volatilization of the silicon. During the various stages of the process the carbon is also practically eliminated. In attaining this pre-melt, an electric arc furnace of high voltage is employed, preferably one in which the temperatures obtainable are distributed over a wide range. The zirconium is introduced in the form of the oxide of the metal or the ore badelleyite which is preferably ground to from aacoyev 150 to 200 mesh, after which it is intimately mixed with from 20% to 35% cryolite, after which the charge is placed into the furnace. The arcs are then lowered into the charge and the current applied, starting at a very low point of approximately 150-200 amps. for about ten minutes, then advancing to 400 amps. and then to 600 amps. for approximately another ten minutes depending upon the progress of the melt. As soon as the entire mass has become fluid, the current is cut off and the charge poured into molds having a neutral lining. The initial charge, for example, might be 1000 lbs. of the ore, containing about 72% zirconium, the usual recovery in the melt being about although sometimes it is as low as 52%, much depending upon the skill in controlling the current and operation of the furnace. The resulting melt is substantially carbon-free or at least has very small percentages of carbon and silicon. There also remain very small percentages of iron and other impurities, although they do not affect the properties of the alloy for commercial purposes, since many of these impurities will be eliminated in the final melt, and of course any iron may remain therein since this is one constituent of the flnal alloy desired. Furthermore, much of the iron may be added in this pre-melt of the zirconium ore and cryolite, except that in this case about one-half of the iron used is placed in. the bottom of the furnace to provide a bath, and the balance mixed with the ore subsequently added. This produces a ferrozirconlum of exceedingly low carbon and silicon content and this procedure is one of the most important features of the present invention. In the usual process of preparation of ferro-zirconium the silicon and carbon contents have been very high.

In the main melt wherein the desired alloy is produced, the same or similar type of arc furnace may be employed as in the zirconium premelt. The iron, which should have a very small carbon and silicon content, is charged into the furnace: except in the case that the pre-melt has been a ferro-zirconium melt in which case the ferro-zirconium is placed in the furnace, together with approximately 2% of iron oxide to substantially eliminate any remaining carbon or silicon which might be present. Then the necessary 'amount of nickel and chrome are placed in the furnace. Then the furnace is sealed and the arcs are lowered and the current applied, the'input of current being slowly advanced as the melt progresses. For the first twenty minutes the currentwillbeincreased from 300 amps. to 1000 amps. which will raise the temperature of the furnace to approximately 4200" 1''. After about twenty-five minutes the current is turned 01! and the melt allowed to remain for approximately five minutes. Then the current is re-applied very abruptly in order to shock the mass and there ensues a considerable turbulence which effects a thorough mixture of the melt, after which the arcs are raised and the furnace opened and tilted. The

. metal is poured into suitable molds having neutral linings. The pouring temperature is gengll'lally at approximately 3800 F. for the ferrous oys.

In the production 01' the non-ferrous alloy substantially the same conditions prevail except for the fact that the iron content being much lower the procedure does not require as long a time nor as high temperatures to produce the melt.

are furnace is charged with boric acid anhy-' If the nickel and chromium contents are increased to any great extent the periods of the successive melts is about twenty minutes and the final temperature approximately 3350" F. On the contrary, if the zirconium is increased and the iron, nickel, and chromium decreased reciprocally, then the temperatures are higher and the time longer. The same principle will apply if the nickel and chromium are eliminated altogether and the zirconium and iron are alloyed bon and quartz in the reduction processes and a highly impure product of high silicon and carbon content has been attained. In fact, the commercial products now known as "ferro-zirconium consists largely if not entirely of a mixture of zirconium and iron carbides and silicides'.

and exhibit an inferior micro-structure, at least for the purposes of the .present invention. It has been the usual procedure to first obtain the oxides of zirconium by means of chemical processes, then employ electric furnaces with carbon and quartz for reduction. This procedure has been eliminated in the present invention, the reduction being effected upon the ore directly.

However, as an alternative to the use of cryolite in the reduction process, when for example some small amount of carbon content is desired, or if it is difficult to obtain the cryolite, flour of carbon maybe used. In this case, in order to keep the carbon content down, iron oxide is charged into the melt, this driving much of the excess carbon into the slag. This process is a little more expensive, not as rapid, and does not produce as great a yield as with the use of the cryolitea Furthermore, the process is also preferably varied in the following way when the flour of carbon is used. The pulverized ore is mixed with flour of carbon in the ratio of approximately 65 parts of the mineral to 35 parts of the carbon and the mixture is charged into the furnace. Then the material is covered with the calcium containing compound, preferably in the form of calcium carbonate and upon the surface of this material small scrap machinings or punchings of aluminium are scattered. During the process the lime and the aluminium ignite, thus intensifying the heat. The lime serves to draw whatever silicon may be present to the top of the bath and into the slag, incidentally carrying much of the excess carbon with it. However. if there should remain too much carbon in the melt. iron oxide must be added and the current applied for an additional period of about five minutes. This serves to eliminate all except a small percentage of the carbon.

One reason for eliminating the carbon and silicon from the alloy is to el minate brittleness and provide a tougher product ofgreat tensile strength and resistance to impact. such as would be necessary in the provision of an effective armor plate. The addition of chromium and nickel also serves to increase the toughness and .reduce brittleness.

However. should it be found necessary to in crease the hardness of the product while st ll retaining a high degree of tensile strength, the following procedure may be carried out. A small dride and red phosphorus, in the ratio of about 70:30. The current is applied at about 350 amps. and from 75 to 100 volts. In about ten minutes a quantity of amorphous boron is produced, which is removed from the furnace and placed around the sides of the crucible in the arc furnace and either zirconium 'or nickel charged into the central part of the crucible. With substantially the same amperage and voltage, after about fifteen minutes operation of the'furnace, zirconium or nickel boride is produced. This product is extremely hard and when added to the desired alloy, especially where a low caigbon product is desired, the hardness of the alloy is increased to the desired degree. Preferably this ingredient is added in the range of 3%-10%, although the proportion'may be as high as 20% for alloys where extreme hardness is preferred rather than toughness or high tensile strength.

From the furnace. the-final alloy material is cast into very hard cast iron molds having machined surfaces. The'alloy casts well with a very satisfactory dense grain when the conditions described herein are maintained. If a permanent production mold is desired, the pattern also is made of cast iron and finished off to a reasonable degree of smoothness. The molds are preheated to approximately a cherry red heat and maintained at this heat until the alloy is poured into them. The molds are painted before each pouring with 'a parting oil refractory solution.

Other molds for receiving these alloys may be made of refractory material. In preparing such a mold a pattern of the desired product is formed and the mold constructed around it. The mold is made entirely from refractory material which is of higher heat resistance than the alloy to be poured such as chrome or zircon or similar material. These are mixed together with a suitable binder which may consist of diatomaceous earth and sodium silicate and the mold is subjected to great pressure (as for ex- .ample from '7 to 20 tons) after which it is placed in an oven and slowly baked until it has become hard. It is then heated and glazed at a temperature of approximately 1800 F. The mold parts are then assembled and preheated to from 800-1200 F. before the alloy is poured into them.

After the pour, the molds and castings are transferred to an aging furnace or they may be covered with hot sand or lime and allowed to cool slowly and gradually. If a furnace is used, the initial heat is at approximately 800F. and after the molds have. been charged into them, the heat is turned off and the molds and castin s allowed to cool to normal room temperature.

The castings produced in this manner are substantially free of cold spots or scabs caused by uneven cooling. Chill spots and blow holes. due

due to gas inclusions or any other cause. In this and are perfectly smooth and ready for the blooming mill without any preliminary treatment. The ingots will not show any pits or seams in the rolled sheet or plate. The-soaking pit temperature will ordinarily be from 1900" I".-2300 F., usually about 2150 F. of course, during the passage through the blooming mill the alloy will be subject to gradual cooling, but will not loseits heat as rapidly as the average stainless steels. Furthermore, the alloy product does not require to be pickled.

In the bar heating furnace, the roughing mill,

the sheet furnaces and finishing mill, including doubling operations, the practices customarily employed in rolling stainless steels of the 18:8 class may be employed, since the cooling rate is approximately the same, although the toughness and hardness of the alloy is considerably higher. These latter factors must be taken into consideration in the choice of the mill capacity and strength. For deep drawing the sheets, the operation should be carried out cold, or at least the'flnishing passes through the rolls should be,

long life are required, to the carbon steels and stainless steels which are now in wide use. Although one of the most effective uses for the present alloy is in the field of armor plate and the like for protection of military and naval ordnance; vehicles and other devices, the utility of the invention is not limited to these fields but the novel and improved product may be applied in commerce and industry wherever a metal of high tensile strength and resistance to corrosion and impact is desired.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

1. A high tensile strength and non-corrosive alloy comprising from about 20% to about 40% zirconium, from about 10% to about 20% nickel, from about 5% to about 20% chromium, less than 1lrtothcarbon, less than 1% silicon, and thebalance 2. A high tensile strength and non-corrosive alloy comprising from about 80% to about 40% zirconium, from about 12% to about 18% nickel, from about 10% to about 20% chromium, less than 1% carbon, less than 1% silicon, and the balance iron.

3. A high tensile strength and non-corrosive alloy comprising from about 20% to about 25% zirconium, from about 10% to about 15% nickel, from about 10% to about 12% chromium, less than 1% carbon, less than 1% silicon and the balance iron.

CHARLES J. SCHAFER.