US2347375A - Armor plate - Google Patents

Description

Patented Apr. 25, 194% UNITED STATES PATENT OFFEQE AMIOR PLATE Albert R. Stargardter, Baltimore, Md., assignor to The Eastern Rolling Mill Company, Baltimore, Md, a corporation of Maryland No Drawing. Original application April 5, 1941,

Serial No. 387,109. Divided and this application August 29, 1941, Serial No. 408,891

2 Claims.

This invention relates to a new armor plate.

The present application is a division of prior application Serial No. 387,109, filed April 5, 1941.

The typical method of producing light armor plate, i. e., plate of a thickness from 2 in. down to about .02 in. involves among other things, a

rapid quench. In the usual method of production .of both face-hardened and homogeneous sheets, the stock is quencheclfrom an elevated temperature of the order of 1400 F. and above. Such quenching is effected either by immersion in liquid baths, such as an'oil bath, or by quickly depositing the heated sheet between quenching platens. To insure the desired physical and metallurgical characteristics in the plate it is necessary that the contained heat be abstracted not only at a rapid but also at a uniform rate over the whole surface. Since the stock treated is of thin cross section such quenching or heat abstraction is most difiicult to control. If the quenching is not sufilciently rapid the resulting product does not attain the desired hardness required by the rigorous specifications for these materials. Similarly, if the rate of heat abstraction is'non-uniform over the surface of the sheet or plate diiferential strain areas are instituted and a warped or distorted plate is produced which must either be discarded or submitted to expensive retreatment to straighten it. In view of the critical nature of the quenching operation it it apparent that in spite of careful control large losses are an inherent incident of the operation.

The present invention avoids such prior difficulties by utilizing a novel method of producing armor plate of the type described. The fundadefinite improvement in the ballistic properties of the product.

The general efficac and commercial potentialities of the improved process can best be appraised and evaluated by a direct comparison of the method of the invention with conventional practice. The analyses of-the steels employed in l the production of the novel armor plate may \generically be defined as air-hardenable steels which possess those other characteristics which qualify them as armor plate stock. Typical of the products comprehended by the invention.

available both for homogeneous and face-hardened armor, i a steel of the following composition:

Per cent C .08to .70 Cr 4.0 to7.0 Mo or W I .4 to2.0 Mn u .3 to .6 V .1 to .3

Remainder-Fe with the usual minor impurities Compositions of the above range of analysis have been found to function most satisfactorily. While vanadium is not an essential constituent in the steel it is recommended because it imparts improved structural characteristics when used in the approximate quantities indicated. In the production of face-hardened armor, involving surface carburization, it is preferred to utilize compositions of the above type having a carbon content between approximately .08% to .2%, while for homogeneous armor higher carbon ranges are preferred, i.'e., between about 25% and .7 As will be appreciated by those skilled in the art the specific carbon analysis for any specific stock will depend to a considerable degree upon the thickness of the finished product which is desired.

The comparative value of 'the new method as against earlier practice can the more readily be appreciated from a consideration of the following tests.

A heat was made of a representative product now employed of the following analysis:

Per cent C .20 Mn .54 Ni 1.90 Cr .70

The heat was made in a. high frequency induction furnace, cast and hammer forged into in. flats. These flats were face carburized on one face to a depth of 25% of-the total thickness and then hot rolled to a thickness of A; in.

Sheets were made up of the improved composition of the invention which analyzed:

Per cent C .1? Mn 52 Cr 50 Mo 1.01 V .24

This material was made up in the identical manner as the product described above which represents existing practice, namely by melting in 60 the'same high frequency. furnace, cast into the same size molds, hammer forged into in. flats, face carburlzed to a depth of 25% of the total thickness whereby the carbon content is increased to a point. above about 0.3 and hot rolled to a thickness of V4 in.

The conventional nickel-chromium-molybdenum alloy was then treated according to the method now employed in the art. The product was normalized, annealed, oil quenched from 1560 F. for core refinement, then oil quenched from 1360' I". and given a final temper at 450 F. This treatment resulted in a face hardness of 600 Brinell and a back hardness of 400 Brinell. In sheets of the described thickness these hardness figures are typical.

A compared to the above procedure involving the difilculty controllable quenching the new product was given the following treatment 'in' which quenching was eliminated. After hot rolling the sheets were annealed from 1560 F. with furnace cooling. At this stage of the treatment the plate was quite soft. on both faces displaying a Brinell hardnes of 160 Brinell on the face. It will be appreciated that in these circumstances the product at this stage may readily be machined or straightened. The plate" was then heated to 1850 F., allowed to cool in air and tempered at 950 1*. It is striking that this simple heat treatment produced the same hardness in the steel as that produced by the complicated treatment of the conventional alloy used as the control. Asa result of this heat treatment the new alloy displayed a hardness of 600 Brinell on the face and 400 Brinell on the back.

The improvements insured by the invention similarly apply to homogeneous plate as is evidenced by the following test. An alloy of the type contemplated herein and a conventional alloy were produced under identical conditions, namely by melting in the same furnace, casting into the same mold, forged by the same procedure to in. flats and then hot rolled into in. plates.

The conventional alloy sheet thus produced was of the following analysis:

Per cent C .50 Mn .80 Ni 1.85 Cr .90 V .25 Mo .27

The alloy of the invention was of the following analysis:

' Per cent C .36 Mn. .57 Cr 5.15 Mo 1.01 V .26

The conventional nickel-chromium-molybdenum alloy was then given the treatment which is representative of current practice, that is to say it was normalized from 1750 F. followed by reheating to 1420' F. with air cooling, then heated to 1600 F., oil quenched and finally tempered at 920 1". As a result of such treatment the material developed a hardness of 460 Brineil.

The improved alloy of the invention on the other hand was treated by the new method without quenching. The alloy was heated to 1560 F., allowed to cool in the furnace, reheated to 1850 F., withdrawn from the furnace. allowed to cool in air and was given a final temper at 1100 1''.

This material as thus treated without quenching displayed a hardness of 460 Brinell.

As noted previously, operations under the novel procedure not only eliminate the quenching heretofore conceived to be essential but also insures a definite improvement in ballistic properties of the resulting products. The samples which were prepared as described herein were tested for pro- Jectile striking velocities employing the standard army rifle and thirty caliber armor piercing ammunition. The results of such tests were as follows:

F. -p. s. Conventional plate-face hardened; ballistic limit 2,150

New alloy plate, described herein-face hardened; ballistic limit 2,550 Conventional plate-homogeneous; ballistic limit New alloy plate, described herein-homogeneous; ballistic limit 1,600

It is thus apparent that the novel procedure of the present invention not only establishes'a simplified procedure but also positively insures an improved product.

Microg'raphic examination discloses that the structure of the products of the invention is similar to that of the earlier products. Under the old procedure the structure produced is essentially martensitic tempered to produce a large amount of sorbite. The new alloys described produce a similar microstructure by a much simpler procedure and display a fine granulation. Such random dispersion promotes high impact toughness o valuable in armor stock.

It has been determined that light armor plate may be produced under the novel procedure described herein in thicknesses up to 2 in., that is to say the air hardening characteristics of steel of the type contemplated are effective up to such thickness.

In the claims compositions of the type herein defined containing from approximately 4 to 7% chromium will be designated as steels of the 5% chromium type. when such steels contain the stated range of chromium a critical cooling rate is secured which is sufficiently low to yield adequate hardening by means of simple'ltill air cool-- ing in lieu of the conventional rapid heat removal. The addition of the stated percentages of molybdenum or tungsten functions beneficially to accentuate the effect of random carbide dispersion of a very fine carbide particle size which imparts markedly improved ballistic properties as well as definitely increased toughness.

It will be understood that the present invention provides a very simple and effective method of producing light armor plate of an improved quality. The elimination of quenching operations avoids a critical and wasteful step heretofore utilized. The utilization of the air hardenable steels of the type defined permits a simplified procedure and insures a product of very desirable ballistic properties.

While preferred embodiments of the invention have been described it will be understood that these are given didactically to explain the underlying principles involved and not as limiting the useful scope of the invention to the examples chosen for illustration.

I claim:

1. A light armor plate characterized by a high ballistic limit having a body portion formed from a plate which is a substantially silicon and nickel Tl free alloy and comprising from substantially .08%

to .7% carbon, from substantially 4% to 7% chromium, from substantially .3% to .6% manganese, and from substantially .4% to 2.0% of a metal chosen from the group consisting of molybdenum and tungsten and the remainder substantially iron and having a Brinell hardness in the armor plate range and a face hardened carburized surface portion having a higher Brinell hardness than the body portion.

2. An air quenched light armor plate having a thickness of between about .02 and 2.5 inches and characterized by a high ballistic limit at least above substantially 1450 t. p. 5. formed from a plate which is substantially silicon free and devoid of nickel alloy and comprising from substantially .25 to 37% carbon, from substantially 4% to 7% chromium, from substantially 3% to .6% manganese, and from substantially .4% to 2.0% of a metal chosen from the group consisting of molybdenum and tungsten and the remainder substantially iron and having a Brinell hardness 10 in the armor plate range.