Metal treatment – Process of modifying or maintaining internal physical... – Producing or treating layered – bonded – welded – or...
Reexamination Certificate
2002-03-05
2003-09-02
Yee, Deborah (Department: 1742)
Metal treatment
Process of modifying or maintaining internal physical...
Producing or treating layered, bonded, welded, or...
C148S518000, C148S516000, C148S527000, C102S516000
Reexamination Certificate
active
06613165
ABSTRACT:
FIELD
Described below are (1) a method for heat treating parts comprising multiple dissimilar metals and/or alloys to simultaneously soften some metals and/or alloys and harden others, and (2) parts made by the method. More particularly, specific implementations concern heat treating a bullet having a core and a jacket on at least a portion of an external surface of the core, wherein the heat treatment simultaneously hardens the core and softens the jacket.
BACKGROUND
A typical cartridge includes a casing and a bullet. The casing is a generally cylindrical container that houses gun powder and incorporates a primer on one end and an opening on the other end. A bullet is fitted into the open end. A firing pin strikes and detonates the primer to ignite the gun powder, which produces a large gas pressure that forces the bullet outwardly and away from the casing, down and out of the firearm's barrel.
Bullets are known that have an inner core and an outer jacket on at least some portion of the core's outer surface. The inner core usually is made from a high-density, high-ductility, low-cost material, such as lead. The outer jacket is made of a harder material, such as copper or copper alloys. Bullets having this structure are referred to herein as ball ammunition bullets.
Bullets are jacketed for several reasons. First, lead and most lead alloys have yield strengths of 10,000 psi or less. Pressures produced within the casing exceed these yield strengths, and can be at least as high as 50,000 psi. Such high pressures can deform non-jacketed lead bullets and thereby deleteriously affect the projectile's performance. Second, the temperature of the burning gun powder can be greater than the melting point of lead and lead alloys used to make bullet cores. That portion of a core exposed to a temperature greater than its melting point can melt, and can be deposited on the wall of the barrel. Over time, such deposits can alter the flight of subsequently fired bullets. Bullet jackets can protect the core from damaging temperatures. Third, when the bullet is fired down the barrel, rifling (inner-facing helical grooves within the barrel) forces the bullet to spin. This spin stabilizes and increases the accuracy of a bullet. Lead and lead alloys can be stripped off the unprotected surface of the lead bullet by the rifling. Stripping deposits material in the barrel, which can result in inadequate bullet rotation and, consequently, decreased accuracy. Jackets protect the lead alloy core from rifling damage.
Jackets also increase bullet rigidity and can increase the ability of a bullet to penetrate a given target. A bullet's penetrating capability predominantly is a function of its impact velocity, the shape of the bullet, and the hardness, strength, and resistance to deformation of a bullet's component parts. Because both the hardness of the jacket and the core affect the ability of a bullet to penetrate a target, penetrator ammunition bullets have a steel core tip with the remainder of the core being a lead alloy. However, these additional parts included in penetrator ammunition bullets typically decrease their accuracy.
A major factor in a bullet's trajectory and retained velocity at any given point is the bullet's ballistic coefficient. The ballistic coefficient is a measure of the bullet's ability to resist atmospheric frictional drag, and primarily is a function of a bullet's form and density. A bullet having a greater density will have a better ballistic coefficient, a greater retained velocity at a given point, and a flatter trajectory than bullets that are less dense. Armor-piercing ammunition bullets and penetrator ammunition bullets typically replace at least part of the lead core with steel, which is less dense than lead or lead alloys, and therefore have poorer ballistic coefficients than otherwise equivalent ball ammunition bullets. Thus, although armor-piercing and penetrator ammunition bullets have greater penetration performance, their in-flight performance is poor when compared to denser ball ammunition bullets.
Knowing the effects of core hardness on a bullet's penetrating capability, methods are known by which bullets are heat treated without applying a jacket to the core. Precipitation hardening is one heat-treating process that has been used to harden bullets made from lead alloys. Precipitation-hardening has four steps: first, the metal is heated to a sufficient temperature; second, the metal is held at this temperature for a period of time (commonly referred to as “soak time”); third, the metal is quenched in a liquid; and fourth, the metal is aged, which refers to the period subsequent to quenching and prior to any further mechanical or heat processing, and during which the physical property in question is changing, e.g., during which period the hardness of the bullet increases. The metal may initially be softer than it was originally after the heating and quenching process, but the hardness soon increases as the metal ages. For a description of precipitation-hardening and a discussion of possible explanations for the resulting increase in hardness, see William Howard Clapp and Donald Sherman Clark,
Engineering Materials and Processes
183-84 (1954).
The hardness of a bullet typically is measured by a Brinell Hardness Number (BHN). This value is useful because it is directly proportional to the yield strength of the metal tested. A BHN value for an article is obtained using a test device. The test device includes a spring-loaded plunger that screws into a loading press and applies a known load on a ball bearing, which then creates a small crater in the object being measured. If the dimensions of all components in the test device are known, the BHN is determined by the equation:
BHN=
0.0004485*
F
/{(&pgr;/2)*
D
2
*[1−(1−(
d/D
))
2
]}
where
F=load, pounds
D=ball diameter, inches
d=diameter of crater in sample, inches
&pgr;=3.14159
BHN values of lead cores produced by conventional bullet processing methods are typically reported in units of kg/mm
2
(such values also can be stated simply as Brinell Hardness Numbers, e.g., BHN values of 8 kg/mm
2
are reported as 8 or 8 Brinell). From the BHN, the core yield strength of the measured object is then determined by multiplying the BHN by about 515. For a review of how to construct a test device to measure the Brinell Hardness Number and core yield strength, see Harold R. Vaughn's
Rifle Accuracy Facts,
Precision Shooting, Inc. Press, Manchester, Conn., (1998).
Prior heat-treated bullets typically have been mechanically worked after the core heat treatment to resize and shape the bullet and apply the jacket to the core. Mechanical working or re-heating steps realign the grains of the core, allow slipping within the metal, and thus decrease the original core hardness obtained by heat-treating. Working after heat treatment can be advantageous, e.g., when it is done to selectively lower the hardness of a nose portion of a bullet that is already jacketed. When a core must be reworked or reheated as a whole after precipitation heat treatment, however, some of the desired increase in hardness of the core is lost, even in portions of the core where high hardness is advantageous (e.g., the base).
Another problem that has been encountered in maximizing the penetrating capability of bullets has been the brittle nature of most jackets. During the process of applying the jacket to the bullet and resizing or reshaping the bullet, the jacket becomes brittle as the orientation, shape, and/or size of the grains are altered. Because the jacket is brittle, it tends to crack and fractures quickly upon impact, which decreases its penetrating capability.
The decrease in a bullet's penetration capability due to lead cores that are not sufficiently hard and jackets that are too brittle may be offset to some extent by increasing the jacket thickness. A thick jacket is less prone to fracture easily upon impact and is more likely
Klarquist & Sparkman, LLP
Yee Deborah
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