The manufacture of armor plate for protection in battle has for centuries required much time. even in the last 100 years little progress was made in speeding up the process, except for metallurgical improvements in raw steel, until a far superior light armor for aircraft was developed and is now being made in a fraction of the time required prior to World War II.
Light body armor had not been generally used in war from the late Middle Ages to the Spanish Civil War. The old armorer turned out a custom-built suit by hand. Later, when ironclad ships fought in our own Civil War, armor for warships became an established means of protection. But even now long hours are required to produce heavy plates for naval vessels. And there was little hope until recently that even light armor, introduced by Russian and German aircraft in the Spanish War, could be produced rapidly enough to keep pace with accelerated airplane production.
Breeze Corporations, Inc, Newark, NJ, developed the solution to the problem of. increasing the production rate of light armor when by a revolutionary process it carries out three heats of carburized plate in 24 hours where by older, more conventional methods 50 hours are required to complete one heat of average ¼-in plate.
Light armor is used to protect the pilot and crew of nearly every type of warplanes built in the United States for its own air force and that of the United Nations. It is also used to shield instrument panels, engines, radio installations, gun turrets, and the backs, sides, top and bottom of aircraft to protect personnel and equipment from attack from nearly every quarter. Surveys have shown that use of such armor greatly increases the confidence of pilot and crew in hard-fought battles. And airmen who fought in World War I and are active in current hostilities agree that modern armor is a vast improvement over the French stove lids on which pilots rode forth to battle in 1917-18.
There are two general types of armor plate known to the trade as face-hardened and/or carburized armor plate, so-called due to the addition of carbon on one side of plate, which in tempering causes the face to become extremely hard and resistant even to armor-piercing shells. The other type of armor, called homogeneous plate, is uniformly hard throughout, but it is not so hard nor resistant to machine gun or shell fire as the face-hardened plate. Both types of armor employ accurately controlled electric furnace alloy steel.
Production of face-hardened plate is a highly specialized and costly heat treating and fabricating process involving problems of temperature, time, quenching, drawing, tolerance allowances for warpage, straightening, bending, welding and, finally, of rigid ballistic acceptance tests.
The standard and older method of carburizing or face-hardening armor plate consists, in general, of coating one side of the plate with a vitreous material so that it will not absorb carbon and then tack welding two such plates together with their uncoated faces exposed. A group of such twin plates is carefully packed, with charred bone dust or one of the many types of carburizing materials, into a steel box, sealed up and then placed in a gas or oil fired furnace.
The furnace is gradually brought up to a temperature and so held for a period of hours, depending on the thickness of the plate which absorbs the carbon gases liberated to the desired depth. The steel boxes containing the plates are then allowed to cool, the plates are removed and annealed, straightened and cut to shape, edges ground, drilled to specifications, tempered, drawn, again straightened, face and back hardness recorded and finally ballistically tested.
The Breeze process requires in all some 50 separate labor operations and it has proved to be a much faster method of producing face-hardened or carburized armor plate for aircraft. Raw alloy steel plate, as received from the mill, is first inspected and the Brinell hardness of each plate is tested. Before any actual work is started on the plates, several 36-in by 36-in sheets of each thickness desired are selected by the government inspector and carburized for ballistic and physical acceptance tests.
Such plates are first tested by the Breeze firing range and in the company's physical and chemical laboratories. If the plates pass these rigid tests, they are sent to government proving grounds for further testing. Not until such primary plates are approved can the material in stock be put into factory production. This consists of first flame cutting the plates to specified shape, grinding or machining the edges for final finish, and then sand-blasting to insure a smooth surface. The plates are then put through the degreaser for cleaning so that the face to be carburized can be spray painted with lacquer and the opposite side copper plated to prevent carbon penetration during the carburizing process.
In some cases, where specified holes are required, they are drilled prior to the last operation. Lugs are then welded to the plates so they can be suspended from an overhead trolley in the liquid salts electrically heated carburizing furnaces where they are subjected to heat for a period of hours, depending on the thickness of the plate and the depth of carbon penetration desired.
The lacquer covering the areas of the plate where carbon penetration is sought is instantly burned off in the electrically heated furnaces, and the plates quickly come up to heat without affecting the copper-plated areas where carburization is not desired. With this procedure, only the mass of the plate has to be heated and the long period of preheating for the older method of pack hardening is thereby done away with.
The plate is uniformly heated and not from the out perimeter inward as in pack-carburizing. Likewise, the whole surface of the plate is subjected to a constant degree of carbon penetration effecting uniform and consistent results as indicated by the superior ballistic properties obtained.
Such plates as have been prefabricated are then quenched in a ram that holds them practically flat. They are then drawn in an oil bath for several hours before being degreased to remove the oil. Hot water is used to wash off any remaining carburizing salts before both sides of the plate are tested for their degree of hardness. They then go to brakes or roll equipment for final straightening, sand-blasting, painting, final inspection and shipment.
Plates that require bending and forming, which are widely used in aircraft, are annealed after being carburized. They are formed to shape and again brought up to heat and quenched in a jig to hold prescribed curvatures. Thereafter, such plates proceed as do the flat plates referred to above.
Liquid carburizing prior to its development and use by Breeze as a means of accelerating production of armor plate for aircraft had been confined to very light depths of penetration.
Throughout the process, which is speeded by transfer of the plates from point to point via a specially designed crane and rail system, an accurate record is kept of the performance of each plate at various stages of its production. These records are constantly checked by both government and Breeze inspectors.
The handling of carburized plate is far more difficult than that of homogeneous plate or the working of mild steel. The different carbon contents of the two sides of the carburized plate present the problems of holding to tolerances and curvatures which can be predetermined. But it is safe to say that no two plates of the same heat will distort the same under the high temperatures to which they are subjected for obtaining armor hardness.
Various intricate shapes and designs of armor required by aircraft manufacturers have made it necessary to produce welded assemblies. In order to save weight, all-important in warplanes, Breeze often employs varying thicknesses of plate, the heavier armor being used in the most vulnerable locations. The company has specialized largely in producing intricately curved and welded designs because of its development of special processes allowing for such welding without loss of hardness. Tests have demonstrated that the welded portion is to all practical purposes as strong as the armor plate itself.
Adjoining the Breeze plant is its complete small arms range equipped with facilities for measuring the velocity and striking power of .30- and .50-caliber and 37-mm armor piercing ammunition. Exhaustive tests are conducted here with both primary plates and samples of those rapidly coming off production lines. The plates are secured to a heavy frame which can be moved on rails set into the floor. Huge bolts and screws hold the plates vertically or at any angle desired to simulate aircraft tilted in flight. When high velocity armor piercing ammunition hits the armored plates, it smudges out in a blob of metal which barely dents the plate, but when an occasional stray shot hits the heavy bolts, a neat hole is drilled as easily as though the untreated metal were made of sponge rubber.
Breeze Corporations first became intensely interested in developing short cuts without loss of hardness in armor plate after J J Mascuch, its founder and general manager, returned from a tour of Germany just prior to the outbreak of World War II. As an airman of the previous war, he was particularly interested in the Nazi air force and its wide use of armor plate for protecting personnel and equipment.
Upon his return, Mascuch and A L Johnston, Jr, engineer in charge of Breeze research and development, went to work on the problem of speeding production of armor plate. After two years of intensive effort and careful tests, the method of carburizing with a liquid salt bath in an electric furnace under constantly controlled temperatures was evolved. Mass production quickly followed, together with the development of the special welding process which makes Breeze armor turn the corners and assume the angles which better protect American and United Nations' air crews and their equipment.
This article was originally published in the September, 1942, issue of Aviation magazine, vol 41, no 9, pp 120-122.
The PDF of this article includes photos of the armor in production and under test.
Photos credited to Dimitri Kessel, Press Association.