Armor plate and method for making same

July 31,-1951 j. w. kin near, jr 2,562,467

ARMOR PLATE AND METHOD FOR MAKING SAME

Filed May 14, 1946

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. INVENTOR.

James W. Kinnear , Jr.

Patented July 31, 1951

2,562,467

UNITED STATES PATENT OFFICE

2,562,467

ARMOR PLATE AND METHOD FOR MAKING . SAME

James W. Kinnear, Jr., Pittsburgh, Pa., assignor to United States Steel Company, a corporation of New Jersey

Application May 14, 1946, Serial No. 669,574

Claims.

The present invention relates to heavy armor plate hardened to the center thereof and a method for making same.

Modern designers of ballistically-resistant equipment frequently call for armor of 'considerable thickness, and specify certain minimum prerequisites in respect to its hardness and mechanical strength. Within certain limits of thickness, these properties are achieved by heat treatment or a combination of alloying and heat treatment. Once a specific thickness for each composition of plate is exceeded, present conventional methods fail to achieve the objects sought, and a recourse to specific methods becomes necessary at the expense of complicated and financially burdensome processing steps.

When a comparatively large steel body is quenched under best normal conditions, it does not become hard throughout its mass, but a given level of hardness is established at a certain distance from the quenched surface. This distance varies with the method of quenching, size of the body being quenched, character of the cooling medium and, most pronouncedly, with the composition of steel used. The depth of hardening can be increased for a given steel only by rendering quenching practice more rigorous, but the remedy frequently results in cracking of the metal under the influence of thermal and allotropic strains.

It is an object of the present invention to produce heavy-gage armor plate which have a hardenability adequate for fully hardening throughout by conventional quenching methods while circumventing the danger of cracking. This object is accomplished by making the armor plate having both outside surfaces made of steel capable of being quenched cold without cracking, and a core possessing a sufficient hardenability to be hardened throughout to a predominantly martensitic structure by quenching rates taking place at the center of heavy-gage plates.

Differential composition of plates is provided in this invention by an improved method of casting ingots. In accordance with the process of the invention, steel having a composition suitable for the outside layers of the plate, is teemed into ingot molds supplied with appropriate hot tops. The steel is allowed to rise in the molds until a few inches of the lower portion of the hot tops are immersed in the metal. Ingot molds teemed in the manner described are held at the pouring platform until there has occurred a partial solidification of the metal in contact with the walls and bottom of the molds, the top surface being prol. 29—196.6) tected from solidification by a proper cover. The holding time at this stage is determined by the desired ratio between the thickness of the core and the outside layers of the finished plate. On expiration of the required time, the hot tops of ingots are filled with molten metal having a composition which, after dilution with the still liquid core of the ingot, produces a concentration of hardening elements required for imparting the i desired hardenability to the core. The refilled ingot is then allowed to solidify completely.

A uniform distribution of the alloying elements throughout the whole length of the core is effected in the present invention in any suitable manner either by utilizing the dynamic force of the stream of metal coming from the ladle, or by holding the ladle during teeming in the closest possible position above the upper edge of the hot tops.

• The solidified ingots are then heated to substantially equalize their temperature and bring the temperature within the proper hot-deforming range. Forging is the preferred method for reducing the ingots of the present invention to • suitable dimensions, but rolling can be substituted for it when desired. The resulting hot reduced plates are then finished in a substantially conventional manner, and heat treated by quenching and drawing to cause the desired hardening of the core simultaneously with the production of necessary properties in the outside layers.

The single figure of the drawing is a perspective view of the section of armor plate made in accordance with the present invention. The section includes outside layers 10 and 12 and a core 13. The composition of these layers and the different heat-treating characteristics is hereinafter explained.

An illustrative, but not limiting, example of the preferred method for carrying out the present invention is given below as applied to the manufacture of 10½ inch thick armor plate, having a core equal to about one-half the total thickness.

The composition of the steel selected for the outside layers was intended to be 0.25% C, 0.60% Mn, 0.25% Si, 3.50% Ni, and 1.75% Cr, and steel for the core was intended to contain 0.30% C, 0.60% Mn, 0.20% Si, 3.50% Ni, 1.75% Cr, and 0.85% Mo. The analysis selected for the outside layers had to assure quenching cold without cracking, while the composition of the core was directed towards a hardenability adequate for fully hardening the center of the finished plate.

These steels were made in basic open-hearth furnaces, two heats, for purposes of identification

3,662,467 being referred to as 24M115 and 26M076, being required for the outside layers, and one heat, identified as 14M134, for the core. Their compositions, determined by ladle analysis, are given as:

<td> Heat No.</td><td> O</td><td> Mn</td><td> P</td><td> s</td><td> Si</td><td> Ni</td><td> Cr</td><td> Mo</td>

<td> 24M115.........</td><td> .25</td><td> .61</td><td> .012</td><td> .020</td><td> .24</td><td> 3.60</td><td> 1.86</td><td></td>

<td> 26M076.........</td><td> .25</td><td> .60</td><td> .012</td><td> .021</td><td> .25</td><td> 3.60</td><td> 1.64</td><td></td>

<td> 14M134.........</td><td> .38</td><td> .53</td><td> .012</td><td> .024</td><td> .18</td><td> 3.61</td><td> 1.95</td><td> 1.46</td>

All steels were made by the best practice for fully developed allow steel grades using a 75 lb. aluminum addition to the ladle to produce a fully killed steel. Low carbon nickel-chromium heats ¹⁶were cast simultaneously into a big-end-up 45'736 x 135/131 x 132” ingot mold provided with an appropriate refactory hot top, to a level 3 inches above the hot top joint, and the surface of the metal was covered with a layer 6 inches ²⁰thick of low-carbon insulating proprietary material known as Therm-O-Cel. Eighty minutes after casting the body of ingot, the 0.38% carbonnickel-chromium-molybdenum heat was cast from a ladle raised about ten feet above the hot ²⁶top to assure a sufficient driving force for a thorough mixing of added steel with remaining liquid metal of the ingot.

The resulting solidified 228,920 lb. ingot was reheated substantially uniformly at 1250° C. forged ³⁰to 20 thickness, reheated to the same temperature, and forged to 10|i thickness. The second forging was then followed by the following heat treatment: .,

<td rowspan="2"></td><td rowspan="2"> Operation</td><td rowspan="2"> Temperature</td><td colspan="2"> Time (Hrs.)</td><td rowspan="2"> Final Step</td>

<td> Heating</td><td> Soaking</td>

<td> 1</td><td> Equalizing...</td><td>⁰ C. 328</td><td></td><td> 62½</td><td> Heat to 652° C.</td>

<td> 2</td><td> Anneal........</td><td> G52</td><td> 24</td><td> 8</td><td> Air cool.</td>

<td> 3</td><td> .....do.........</td><td> 652</td><td> 21</td><td> 8</td><td> Do.</td>

<td> 4</td><td> Equalizing...</td><td> 350</td><td> 12</td><td></td><td> Heat to 953° C.</td>

<td> 5</td><td> First quench..</td><td> 953</td><td> 44</td><td> 2</td><td> Water spray for 30</td>

<td> 6</td><td> Equalizing....</td><td> 350</td><td></td><td> 74</td><td> minutes. Heat to 653’ C.</td>

<td> 7</td><td> Anneal........</td><td> 653</td><td> 13½</td><td> 2½</td><td> Air Cool,</td>

<td> 8</td><td> Second quench</td><td> 873</td><td> 44</td><td> 2</td><td> Water spray 45 min-</td>

<td> 9</td><td> Equalizing....</td><td> 325</td><td> 18</td><td> 48</td><td> utes to 200“ C. Heat to 617° C.</td>

<td> 10</td><td> First draw....</td><td> 617</td><td> 18½</td><td> 4</td><td> Water spray until</td>

<td> 11</td><td> Second draw..</td><td> 615</td><td> 12</td><td> 4</td><td> cold. Do.</td>

<td> 12</td><td> Third draw...</td><td> 635</td><td> 11</td><td> 2½</td><td> Do.</td>

This heat treatment developes a martensitic structure in the core and a sorbitic structure in 65 the outside layers.

The outside layers of the finished plate analyzed 0.25% C, 0.61% Mn, 0.013% P, 0.021% S, 0.23% Si, 3.63% Ni, 1.60% Cr, 0.07% Mo with variations within a maximum range of 0.02% carbon, and 60 0.04% chromium, and the core had a composition of 0.33% C, 0.58% Mn, 0.014% P, 0.22% S, 0.22% Si, 3.67% Ni, 1.79% Cr, and 0.87% Mo, with variations within the same compositional ranges, due provisions being made for the usual axial 65 segregations of elements. The core averaged 55.4% of the plate thickness, indicating that an average shell of nine inches of metal had solidified in the eighty minutes between castings. The ultimate juncture between two steels, as revealed 70 by the microscope,· presented a perfect weld characterized by complete absence of non-metallic inclusions, deep interpenetration of the metals, and the absence of perceptible dividing line.

Mechanical testing of specimens taken from the 75 .

core and the outside layers of the plate yielded the following results:

<td rowspan="2"></td><td colspan="2"> Outside Layers</td><td colspan="2"> Core</td>

<td> Top</td><td> Bottom</td><td> Top</td><td> Bottom</td>

<td> Tensile strength, lbs./In.&gt;..........</td><td> 112,900</td><td> 113,400</td><td> 138,000</td><td> 137,600</td>

<td> Yield, point........</td><td> 92,060</td><td> 91,520</td><td> 116,400</td><td> 114,200</td>

<td> Elongation, Per Cent in 2.......</td><td> 23.0</td><td> 25.0</td><td> 17.5</td><td> 18.0</td>

<td> Reduction of area, Per Cent.........</td><td> 70,3</td><td> 67.9</td><td> 49.9</td><td> 50.3</td>

<td> Hardness, Rockwell C...........</td><td> 23.5-25.0</td><td> 22.0-24.0</td><td> 32.5-35.5</td><td> 33.0-34.0</td>

Ballistic testing using a 12 armor-piercing shell Mark 18 at 35 degrees’ obliquity led to ballistic limit being estimated at 96%. The plate behaved in a ductile manner without spalling or splitting at the transition zone of the two analyses.

Adequate mixing of added metal and the steel of the molten core does not require lifting the ladle to the height above the hot top recited in the exemplary case. Experimental evidence demonstrated a four-foot elevation being fully sufficient to equalize the composition of the cores obtained in 34 x 60 x 46 big-end-up ingots, while lower elevations appear to be capable of good mixing under specific conditions.

The present process for producing composite armor plate facilitates joining any two steels possessing different composition in layers ranging within comparatively wide thickness limits under conditions of an ideal fusion of the contacting zone. The method practically eliminates primary piping and minimizes heterogeneity of steel forming the core of the plate. Plates can be made by this method combining full advantages of respective compositions of outside layers and core improved by optimum heat treatment in a simple and meritorious manner, affording furthermore, composite products hitherto deemed unfeasible.

While certain exemplary methods for reducing the present invention to practice have been described, many modifications within the processing steps employed will become readily apparent to those skilled in the art without departing from the spirit and scope of the present invention as described by the appended claims.