US2474682A - Composite steel plate - Google Patents

Description

June 28, 1949. B. LIEBowlTz COMPOSITE STEEL PLATE Original Filed May 21, 1941 /IV l m /4 /v Tl.E-

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lauaiiad` June 2s, 1949 2,474,682 CoMPosrrE STEEII.4 PLATE Benjamin Liebowitz, Lewisham, N. Y. original application May-21, 1941, serial No.

This invention relates to composite steel plates and is a division of my application Serial Number 394,508, filed May'21,` 1941 for Composite steel plates and the manufacture thereof, now Patent No. 2,438,759.

I will describe this invention principally in connection with composite light armor plate,` for which it is of particular utility, although the invention is not necessarily limited to this particular field. At the present time there are two types of light armor plate in use, one known as "case hardened and the other as "homogeneous. The case hardened plate is made by using a socalled carburizing alloy steel which is carburlzed on one surface to a depth .in the neighborhood of 20 or 30% of its thickness. This plate is hardened by heating and quenching and then given a light draw which leaves the plate with a very hard outside surface and a'tough back-1 ing. Homogeneous armor plate consists of a tough steel throughout. In the lighter gauges,

, say, up to 3pinch or 1-inch in thickness, the case hardened plate has much greater stopping power for a given thickness than the homogeneous, but has a tendency to shatter and/or spall when struck by a projectile of greater energy than the stopping power of the plate. Many attempts have been made heretofore to make armor plate that is better than the case hardened` or homogeneous armor plates by welding together, face to face, a plate of hard steel and a plate of soft steel by means of roll-welding. But, so far as I am aware, this has not heretofore been accomplished. The primary object of this invention is to overcome the difficulties hitherto experienced and to produce composite armor plate, sheets, strips, and the like, which have improved properties, as will hereinafter be described.

By way of a definite example, I will describe armor plate comprisingtwo specific steels, viz., a

, hardenable die steel of the following approxi-4 mate composition:

I Per cent Carbon 0,95 Manganese 1.60 Molybdenum 0.25

and a tough steel of the following approximate composition:

i Per cent Carbon 0.20 Nickel 1.80 Molybdenum 0.25 Manganese 0.55

'I'hese percentages give the principal alloying ele- Divided and this application July 5, 1947, Serial No, 759,119 I 1 Claim. (Cl. 29-196.1)

ments and may vary, of course, according to commercialtolerances. The rst of these steels is known in the trade` as, Ryalloy, the second as S. A. E. Specification #4620. It must be definitely understood, however, that my invention is by no means limited to these specic materials.

The problem oi face-to-face Welding of these two steels or similar combinations with sufiicient 1 bond strength for ballistic purposes has been solved by this invention. One of the main features of this invention, as will hereinafter be more fully'described, consists in first. superposing the steels and forming, by rolling or otherwise, a pr'eliminary bond between the two faces, then subjecting the combined plates to a heat soaking for several hours at a temperature in the neighborhood of 2200? F., so as to establish a carbon gradient between the hard steel and the tough steel, thereby minimizing `the abrupt change in properties which would otherwise occur at the interface. After the establishment of the vcarboni gradient, the composite material is rolled to its nal thickness. This feature and other features of the `invention will become clearer from the further description and by reference to the drawings, in whichl Figure 1 is a perspective view, partly in crosssection of a "sandwich prepared for roll welding:

Figure 2 is a perspective view, partly in crosssection lof a box construction containing plates ready` for roll welding;

"Figure 3 is a diagrammatic representationin section showing the lcarbon gradient in the diffusion zone between welded plates, and

Figure 4 is a cross-sectional view of an embodiment"of my invention comprising more than one zone of union between unlike steel plates.

Referring to the Fig. 1, I-I represents a pair of slabs of tough steel, and 2-2 is a pair of slabs of hard steel. (The word hard is used throughout as a designation whether or not the steel is in its hardened state. A similar remark applies to the word tough.) As indicated, the two hard slabs are placed between two tough slabs which overhang the former all around so as` to allow space for the union bars 3. Between the two hard slabs a layer of weld-preventing material, such as magnesium oxide, is placed, as indicated at l. 'I'he side bars are peripherally welded tothe toug plates all around as indicated at 5.

Prior to making this assembly, the abutting surfaces ofthe plates to be Welded are first thoroughly freed from scale and cleaned. It has been found that the methods which are frequently em- 3 y blasting, lare very undesirable because they cause sand or other foreign material to adhere to the surfaces to be welded and thereby interfere with the welding process. A suitable method consists of grinding the surfaces with the fiat rim of a cupshaped wheel. If any oxide forms during this process on account of the heat generated in the grinding, it should be removed by means of a lighter grinding operation. Or the surfaces may be rendered clean and free of scale by machining or by other suitable methods.

In the preparation of the surface I have found it advantageous to employ promptly after the cleaning a light covering of a hydrocarbon, such as -xylene or kerosene. This thin continuous coating serves to protect the steel temporarily against rusting or formation of invisible oxides during subsequent processing and handling prior to or during the sealing of the surfaces in the sandwich.

Instead of forming a sandwich, such as shown in Fig. 1, I may make, a box of steel asshown in Fig. 2, where the tough steel is shown at lli-IIJ, and the hard steel at I2|2, with the weld-preventing layer between the two hard' steels at I4. The box consists of an upper cover plate IBA :and a lower cover plate IEB, joined by side bars I'l-l'l on all four sides of the box by means of the peripheral welds I8, thereby forming a complete enclosure for the plates inside. It will be understood that there is a weld-preventing layer also between each cover plate and its adjoining steel, as indicated at I9-l9. It will also be understood that in the box method I may enclose any number of pairs of hard and tough plates to be welded together with weld-preventing layers provided at surfaces that are not to be welded. A suitable material for the box is S. A. E. 1035 or 1045, which is relatively inexpensive straight carbon steel.

While the sandwich method of rolling is not novel, the box method illustrated in Fig. 2 is new,

so far as I am aware. In this box method, it `will be understood, the entire container is rolled-with the steel inside until the hot rolling operations have been substantially completed. In order to withstand the rolling stresses, the side bars I1 of the boxmust be of substantial thickness, and the cover plates also must be of substantial thickness. A container consisting of ordinary sheet steel will not operate because such a container will be destroyed practically in the rst pass. The total cross-sectional area of the metal of the box should not be less than about 25% of the total cross-sectional area of the box. After completion of the hot-rolling operations the box container is removedv by shearing along 'the edges.

In general, the metal of the box then becomes scrap material, but this is nota serious disadvantage in comparisonl with the advantages gained. In the first place, the box method permits the use of higher temperatures and hence shorter furnace time in the gradient forming operation. In the second place, the sandwich method of Fig. 1 exposes the tough steel to the ambient atmosphere in the furnace and, at the temperatures employed in this process, a considerable amount of scaling results, and also some decarburization, which may be very objectionable. While scaling can be reduced by covering thev surface with coke breeze or similar carbonaceous material while it is in the furnace, it is not always a convenient mill operation.

vAnother advantage of the box method is that it makes it unnecessary to perform any peripheral '4 welding operations on the plates to be bondwelded, and thi-s is advantageous particularly when small pieces are involved, and/or where the method is employed for the roll-welding of materials between which there is a large difference in the coeiiicient of thermal expansion. Another advantage in the box method lies in the fact that any number of pairs of plates may be simultaneously roll-welded in a single box, thereby reducing' the number of peripheral welds which are .ing purposes, it is usually undesirable to heat hard steel, such as the Ryalloy mentioned above, while in contact with ambient atmosphere, above about 2150 F., for otherwise it will decarburize badly and is likely to burn. Good rapid welding can, however, be obtained at rolling temperatures above 2150 F., and the employment of such temperatures ordinarily is desirable. I have'found by experiment that with the specific steel mentioned above I could u-se temperatures ashigh as 2300 F. without injuring the steel. This freedom from injury is obtained, as indicated above, by the exclusion of the ambient atmosphere from the hard steel. In practice, I

prefer to work this particular steel at temperatures between 2200 F. and 2250 F. More generally, for good rapid welding I usually prefer to work the hard steel above what may be called the safe temperature, that is above the temperature at which the hard steel can be safely worked while in contact with ambient atmosphere without undesirably affecting and/or injuring the hard steel.

It should be emphasized that the surfaces of the hard and tough steel which contact at the interface, after' having once been cleaned, are kept clean and free from scale or oxide throughout all the subsequent operations by virtue of the fact that they are sealed in the pack. It h as been pointed out above that Sandblasting, for example, does not produce a desirable surface for contact at the interface because of the impurities which it introduces in the form of sand. As a result of the initial cleaning operations and of the exclusion of the ambient atmosphere throughout all the heating and rolling operations, the interface is substantially clean, that is, substantially free from impurities (except of the previous casting methods, the deposited y metal itself could hardly be expected to be clean, and in addition it is to/be expected that a segregated core would' develop in the cast or deposited metal close to the interface. After the sandwich or box has been prepared, as shown in Fig. 1. or 2, the assembly is slowly asti-.08,0

heated in a furnace to a temperature of about 22001". and is then rolled down by theusual methods until its thickness has been reduced to. roughly.' half (in this flrs't rolling the amount of reduction may be varied over a fairly wide` range) Afterthis first preliminary rolling I put the assembly' backv into the furnace and maintain it at atemperature of approximately 2200 F. for several hours. Under these conditions there is a` fairly rapid migration of the carbon` from vthe high carbon hard steel into the low carbon tough steel. Fig. 3 is a diagrammatic representation corresponding to a microphotograph showing the extent to which carbon migrates from4 the hard high carbon steel plate to the soft lower carbon steel 2| in aperiod of 31/2'hours at a temperature of 2200 F. to form a diffusion zone of material and substantial thickness. The scale of the drawing is such that the region labeled diffusion zone is approximately .080" to .100" thick, as was actually ascertained in this' case. A The arrows indicate the approximate'position Vof the original interface between plates 20 and 2|.

. The existence of this diffusion zone is of great.4

importance both for the subsequent operations in manufacture and for the properties of the final to the body portion of the soft steel.

steel 'inthe region or me welded interface and forms a .diiluslon zone in this region. the result uall decrease or gradient of carbon content from thebody portion of the hard steel The vresulting`composite varmor plate isV thereby enabled towithstand the very severe treatment for which being agrad it-.is designed withoutl failure at the zone of' strength, since such articles ordinarily are manufactured with a good safety factor, and the bond between the two metals, therefore, is not subplate. One of the difficulties which has not hitherto been recognized in the roll-Welding. of com- K posite plates arises from the fact that the hard steel'is much less plastic at final rolling temperatures .than the tough steel. As a result of this there is a tendency during subsequent rolling for creepage to occur at the welded interface and this creepage in turn tends to weaken or even destroy the weld that. had Apreviously been made. By virtue of the carbon gradient'however, there is a more or less gradual change in plastic properties from the hard steel to the tough steel and the diillculties arising from differential plasticity duringsubsequent 'rollingto final thickness areA thereby minimized or obviated. By actualexperiment I have found it possible to make welds in this manner whose strength is, as nearly as could be determined, approximately equal to that of the hard steel and, therefore, greater than that of the tough steel. A weld whose strength is at least equal to that of the weaker of the two components welded together may Properly be called isosthenic (Greek, combining` form iso meaning equal, and the Green noun ,"stenthos" meaning strength), and this term is used in the specification and claim with this meaning. Furthermore, the avoidance yof an abrupt change in mechanical properties at the welded interface is of value for ballistic purposes also.` It is known that discontinuities of any sort create concentrations of stress. By avoiding the discontinuity or abrupt changeV in mechanical properties whichhigh resistance to armor piercing projectiles and is highly resistant to shattering or spalling. In

order that these properties may be attained it is important that the steels of the hard and soft plates be in effect blended together in the region of the welded interface with essential continuity of metal structure. During the heat soak the jected to great stress. In the case of armor plate, however, there is no safety factor and the weld, or zone of union, between hard and soft steels should be such that the weld, or zone of union,

will `not fail under stresses up to the maximum that the steel will withstand. According to the present invention, I have produced composite armor plate wherein the hard and soft plate elements are united in this way. So far as I am aware, this has not previously been accomplished.

It will be obvious that in'composite plates made according to my process a section taken at any place inthe plate would show under microscopic examination substantially the same diffusion zone as in any other section. That is to say, the

diffusion zone in my composite plates is lsubstantially uniform laterally throughout the plates. Of course, there may be some differences between the depth of the diffusion zone in the center as compared with the edges, due to the fact that the edges will heat up more rapidly than the center in bringing the piece up to the temperature of the heat soak, but these differences will be of a minor magnitude, and I may therefore describe these diffusion zones as laterally uniform, with the implication that it is not strictly uniform but substantially so.

TheV two steels selected for 'description illustrate a further factor which is usually present, viz., that the hard steel and the tough steel will differ in their compositionin other respects, besides the dierence in carbon content. Other alloy elements, like manganese, nickel, chromium, molybdenum andvanadium, will also diffuse under the heat soak. While such diffusion is very slow, compared with the rate of carbon diffusion, neverthelessfthere is some diiiusion olz these other ingredients which is valso beneficial to the bond. Y e 'Ihe thickness of the carbondiffusion zone or gradient region is determined by various factors, e. g., the temperature of the heat soak, the timeV of the heatsoak, and the subsequent reduction after the heat soak. It also depends upon the difference in concentration of the substance in question between `the united plates. cificI limits on the ultimate thickness of this -diffusion zone can .be given as it wil1 naturally vary to a very great extent with the properties desired and with the iinal thickness of the plate, but, in any case, a diffusion zone of substantial and ma- 'high-carbon steel loses carbon to the low-carbon 75 terial thickness is produced. The specific times No speand temperatures mentioned above are intended for illustrative purposes only. A

It will be understood that after the rolling operations are complete the sides of the sandwich, shown in Fig. l. are trimmed away so as to remove the side bars 3 and the overhanging portions of the tough plates l-l. The sandwich then separates into two composite plates which are then fabricated to desired form and size, after which the material-is heat treated, as will be understood by those skilled in the art, so as to harden the hard steel to the desired degree. Ordinarily a Brinell of about 600 is wanted on the 4hard face, and a Brinell of about 250 to 350 on the tough face. Likewise, in the event that the box method is used, after the completion of the hot rolling operations, the sides are trimmed away and the cover plates removed so as t9' remove the rolled composite plates inside, which can then be fabricated and heat treated as described above.

I have described my invention in connection with compositev plates comprising a single plate oi tough steel and a single plate of hard steel; it wil1 be understood, however, that composite plates may be made in accordance with my invention, which comprise any number of alternate layersof Ahard steel and tough steel; for example, I may make composite ,armor plate which con'- sists of a layer of hard steel, then a layer of tough steel, then another layer of hard stee1 .and then another layer of tough steel. Such, multi-ply steels offer ballistic advantages', as has been shown in actual ballistic tests. Moreover, such ,in the art.

Among other advantages of my invention, it should be mentionedA that the hard-face steels available at present, that is, carburized armor plates, are produced very slowly (and hence expensively) because of the extremely prolonged heating necessary for the carburization.. My method should be materially less expensive than the carburizing-method, because no such prolonged heatings are required. The cost of making the peripheral welds and the cost of the extra steel required for the sandwich or for the box are relatively small, calculated on a per-pound basis of finished product, where the initial slabs are of good commercial size. For example, it is good commercial practice to rol1 a completed sandwich or box weighing 5,000 to 10,000 pounds or more. Hence, when proper facilities. are used for making the peripheral welds, their cost per pound oi' nished product is small.

While this invention has been described in connection with the composite armor plate having different degrees of hardness or toughness, the

component layers may diier from each other in structuresl have two other very significant advantages. In'the first place, a four ply'plate, for example, has much less tendency to warp'in heat treating, than a corresponding carburiz'ed plate, a tendency which has hitherto been a constant source offabricating-dificulty. In the second place, in a multi-ply structure such as a four ply, the type of steel and its thickness can be chosen in the most advantageous manner. for each layer.

A composite multi-ply plate-of the character above mentioned isexemplied in Fig. 4.wherein the component plates or layers 22 and 23 are of hard steel and the component plates or .layers 24 and 25gare lcomposed of tough steel. The common interfaces at the interior of the composite plate between the hard and soft steels are welded together and in the region of each of the common interfaces is adiffusion zone of material thickness which is of the character hereinabove described, and illustrated on a larger scale in Fig. 3, these zones being indicated by the reference characters 26, 21 and 28. In the example shown in Fig. 4, the component plates 22 and 23 may be of Ryalloy and `the plates 24 and 25 may be S. A. E. Specication 4620, as above mentioned. However, it is not essential that the plates 22 and 23 be identical, or that the plates 24 and 25 be identical. In fact, for certain purposes, it may be desirable to usev different hard steels for the hard layers and different tough steels for the tough layers.

In the making of composite plates of more than two layers of steel, the yhard steel on the inside will, in general, lose carbon faster than the hard steel on the outer face, because the inner layer of hard steel will diffuse carbon to a tough layer on both its faces'. In an experiment on a 4-ply plate, it was found that a l/g-hour soak at a temperature of about 2200 F. to 2250 F. had caused suicient migration of carbon to make an appreciable reduction in the carbon content of the internal hard plate. In such cases care must other respects. For example, the component lay` ers may diier from each other in resistance to chemicals, in thermal expansion, or the like, depending upon special purposes for which the plate is intended. In such case differences in the concentration of alloying elements other than carbon between the component layers or plates may be of primary significance, and the production of a diffusion zone of material thickness in which such element varies from the concentration of the element in the body portion of one component layer to the concentration of the element in the body portion ofanother component layer united therewith, may be achieved according to this invention. The concentration or proportion of an alloying element may vary from zero in one'component layer or plate to some substantial concentration or proportion in a co'mponent layer or plate united therewith; orthe alloying element in question may be present in both, but in different concentrations or proportions.

While this invention has been described primarily in connection with armor plate, the features and advantages of this invention are of utility whenever a union of great strength and permanence is desired between steel plates, sheets, strips, or the like, which differ from each other in chemical composition by at least one chemical element or substance.

In the speciiication and in the claims, the word plate is used in a broad sense as embracing plate, sheet, strip, or the like.

While this invention has been described in connection with specific examples of the practice thereof, it is to be understood that this has been done for the purpose of illustration and that the scope of this invention is to be governed by the language of the following claim.

I claim: l

A welded composite plate comprising at least one pair of steel plates consisting of one plate vof soft steel and another plate of hard steel having a substantially higher carbon content than the carbon 4content of the soft steel, said plates being welded together at their common interface and there being a carbon diusion zone of material thickness in said region of said interface which is substantially uniform laterally and in which the carbon content varies from its concentration in the body portion of the hard steel plate to its concentration in the body portion of the soft steel plate and decreases in its concentration continuously from the interface extending into the soft steel plate thereby providing a weld having approximately the strength of the steel inthe hard steel plate, the metal of-said plates in the region of said interfaceV being substantially free of impurities other than impurities in the metal of said respective plates and there being no intermingling of the metals as such of said respective platesat said interface as distinguished from said carbon diffusion extending from .the metal of said hard steel plate into the metal of said soit steel plate, and there being relatively little diffusion in the region of said interface of alloying elements other than carbon.

BENJAm LIEBWKTZ.

IREFERENQES @MED Plihe following references are of record inthe le of this patent: i Y

UNITED STATES PATENTS Y Date

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