Steel ballistic shot and production method

Details Steel ballistic shot and production method Steel ballistic shot and production method

2001-07-09

2004-06-15

Wyszomierski, George (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Producing solid particulate free metal directly from liquid...

C075S351000, C148S208000, C148S513000

Reexamination Certificate

active

06749662

ABSTRACT:

BACKGROUND OF THE INVENTION
(1) Field of the Invention
This invention relates to ammunition, and more particularly to steel shot utilized in shotshells.
(2) Description of the Related Art
Steel shot is utilized extensively in industry. Such shot may be used for surface treatment of metal parts by spraying a stream of the shot onto the surface in a process known as “shot peening”. The shot may also be used as an abrasive.
One method of manufacturing industrial shot is by impinging a jet of water or other fluid onto a stream of molten steel. Upon contact with the water, the molten steel is atomized, forming spheroidal particles. By spheroidal it is meant “sphere-like” but not necessarily spherical or round. The particles fall into a water tank, cool and then are dried and sorted (by size and to segregate significantly out-of-round particles) and subjected to any further treatment. Particles which are either: too irregular in shape; or of a size exceeding the useful range, are crushed to form grit used for abrasive purposes (e.g., grit blasting). Industrial steel shot is typically very hard, with a Vickers hardness usually in excess of 400 DPH (all mechanical measurements are at room temperature, nominally 21° C.). To provide the desired hardness, the manufacturing process may utilize a relatively high carbon steel which may also include additional hardening elements such as silicon and manganese in quantities on the order of 1% by weight (all compositions are in weight percent unless otherwise indicated). One example of a process for manufacturing industrial shot is shown in U.S. Pat. No. 4,023,985 of Dunkerely et al., the disclosure of which is incorporated herein by reference in its entirety.
Steel shot is also utilized for ballistic purposes (i.e., to be loaded into shotshells for expulsion from shotguns). Steel shot has increasingly displaced lead-containing shot in various applications as the latter has become more strictly regulated. Ballistic steel shot is typically formed from a wire of a low carbon steel (e.g., SAE-AISI 1006 steel having a carbon content of less than 0.08%, a manganese content of 0.25-0.40%, a phosphorus content of less than 0.04% and a sulfur content of less than 0.05%). To prepare the ballistic shot, the wire is first cut to size (i.e., into approximately cylindrical pieces having the volume of the desired spherical shot pellets). Each piece is then mechanically deformed (“headed”) in a die to partially form the piece into a sphere. A highly spherical (round) pellet is traditionally regarded as necessary to provide uniformity and consistency of dispersion when the shot is ultimately fired. Accordingly, the pieces are then placed in a groove between counter-rotating plates and formed into spheres, a grinding process akin to the formation of ball bearings. This produces a highly round shot pellet having a Vickers hardness of 200-250 DPH. The shot is then annealed to reduce the hardness to from about 90 to about 110 DPH, a level generally regarded as desirable to avoid wear of the gun barrel used to discharge the shot.
One key application for which steel shot has become popular is use in hunting waterfowl. Waterfowl loads (commonly known as duck loads) typically utilize American Standard #2 and #4 shot, having respective nominal diameters of 0.15 in and 0.13 in. Waterfowl loads are regarded as a relatively high performance use for which the market often demands high quality steel shot and is able to bear the associated costs of such shot. Upland game (dove and quail) loads and target loads typically utilize smaller pellets than waterfowl loads and still commonly utilize lead shot. Common lead shot utilized in upland game loads is typically between #6 and #8. The market for shotshells for these applications is such that the loaded shotshells retail for between about one-fourth and one-half of the price of waterfowl loads.
Industrial shot is typically smaller than ballistic shot. The diameter of industrial steel shot is typically from about 0.005 inch to about 0.08 inch. Ballistic steel shot is typically between about 0.09 inch (#8 shot) and about 0.20 inch (T-size) in diameter. These American Standard shot sizes convert to about 0.23 cm and 0.51 cm, respectively. Industrial shot is typically more irregular than ballistic shot. The atomization processes used to produce industrial shot end up producing a wide range of particle sizes and shapes potentially well off spherical. Sieving allows for size segregation and a spiral (helical) rolling process may be utilized to screen out the more egregiously misshapen particles and particles with density-reducing voids. Nevertheless, even with such quality control, atomized shot is generally very noticeably out of round.
BRIEF SUMMARY OF THE INVENTION
We have realized that common processes used to manufacture industrial shot produce a by-product which includes pellets too large for typical industrial use but of appropriate size for ballistic use. Such pellets have heretofore been crushed and used as lower value grit. We seek to take such pellets, soften them (as described below), and utilize them as ballistic shot (a higher value product). Broadly, this entails obtaining relatively high carbon steel shot of a composition suitable for industrial use and softening such shot to render it suitable for ballistic use at lower cost than that of traditional steel shot. The hardness which may be preferred or may be tolerated depends on a number of factors including pellet size. Other factors being equal, a relatively high level of hardness may be acceptable for relatively small diameter pellets. It may be possible to express the maximum acceptable hardness as a function of pellet diameter (e.g., by a linear approximation) for given circumstances or ranges thereof. For smaller pellets, an acceptable hardness may be achievable by an annealing process without substantial carbon removal. For larger pellets, obtaining acceptable hardness may require annealing with substantial to near total decarburization at least from a surface layer.
Accordingly, in one aspect, the invention is directed to a method for manufacturing shot useful for discharge from a shotgun. There is provided a source of molten steel having an initial carbon content. The molten steel is subjected to an atomization process so as to produce substantially spheroidal pellets. These pellets are annealed in a decarburizing atmosphere effective to decrease the carbon content in at least a surface layer of each of the pellets. The pellets are cooled, whereupon the surface layer has a median (median measured radially across the layer) Knoop hardness of less than 225 at 21° C.
In various embodiments, the surface layer may be at least 0.1 mm thick. The surface layer may be at least 0.3 mm thick. The surface layer may have a thickness of at least 1% of an average diameter of the associated pellet. The surface layer may have a thickness of 5%-10% of an average diameter of the associated pellet and the carbon removal may be effective to provide the surface layer with a Knoop hardness of less than 225 at 21° C. over substantially the entire surface layer. After annealing, a core region of each pellet may retain sufficient carbon so that the core region has a Knoop hardness in excess of 225 at 21° C. The core region may have an average diameter of at least 50% of an average diameter of the associated pellet.
The carbon removal may be effective to provide the surface layer with a Vickers hardness of no more than 180 at 21° C. over a majority of the surface layer. The carbon removal may be effective to provide the pellets with a Vickers hardness of between 130 and 180 at 21° C. substantially throughout.
The spheroidal pellets may have characteristic diameters between about 0.08 inch and about 0.23 inch. The spheroidal pellets may have preferably characteristic diameters between about 0.09 inch and about 0.16 inch. The spheroidal pellets may be #4 pellets and the atomization process may produce additional pellets and the method may further comprise separati

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