US 3676907 A
A method of making a mortar shell of AISI 06 steel, and the product produced thereby, comprising forging the steel to a rough shape, rough machining the forged article, heat treating said article to approximately 1,750 DEG F for approximately 2 hours, air-cooling said article, tempering said article at approximately 500 DEG F for approximately 1 hour and finish machining said article to completion.
Description (OCR text may contain errors)
Unitefi States Patent Magis [451 July 18, 1972  NATURALLY FRAGMENTING 2,368,418 1/1945 McCarroll et a] ..14s/134 PROJECTILES 0F A181 06 STEEL 2,438,267 3/1948 Bonte ..75/l28.l  Inventor: Sigfred F. Magis, Dahlgren, Va. OTHER PUBLICATIONS Assigneel The United, States of America as Roberts et al., "r661 Steels," Third Edition, American represented y the Secretary of the y Society for Metals, Ohio, 1952, T8320 G47, 1962, 02 pp. 22 Filed: Feb. 18, 1966  Appl. No.: 530,251 Primary Examiner-Verlin R. Pendegrass Attorney-E. J. Brower and A. L. Branning  US. Cl ..29/l.21 102/56 s 1 161. ct ..B2l k 21/06 7 ABSTRACT  Field of Search 148/2, 12, 134; 75/123; A method of making a mortar Shell of 1 1 ()6 Steel, and the 29/121; 102/56 product produced thereby, comprising forging the steel to a rough shape, rough machining the forged article, heat treating  References cued said article to approximately 1,750" F for approximately 2 UNITED STATES PATENTS hours, air-cooling said article, tempering said article at approximately 500 F for approximately 1 hour and finish 1,0 l 9 l 2 Potter machining aid article to completion 2,087,764 7/1937 Bonte..... .l48/l34 X Bonte ..75/1 23 1 Claim, N0 Drawings NATURALLY FRAGMENTING PROJECTILES OF A181 06 STEEL The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
The present invention relates to the production of fracturable projectiles and mortar shell casings and, more particularly, to the selection of the steel and the subsequent heat treatment of that steel to produce the desired fragmentation.
Steel previously used in the making of fracturable projectiles and mortar shell casings has shown a poor fragmentation pattern, and, therefore, the majority of fracturable projectiles and mortar shell casings are made from cast iron. Pearlitic Malleable Cast Iron (PMCl) used in the making of mortar shell casings produces the desirable fragmentation behavior, but has strength and ductility limitations and is not forgeable which makes it inadequate for gun projectiles. Also, PMCI must be cast in halves which are afterward joined together by brazing. This joining often causes premature failure of the projectile during gun firing. Ductile Cast Iron (Modular Cast Iron) is also used and produces a fragmentation efficiency equal to that obtained with Pearlitic Malleable Cast Iron. However, neither of these cast irons can produce a fragment size of 4ll grainsm without impairing both their strength and ductility. Also, the economic production of one-piece forged projectiles of cast iron is yet to be developed.
The object of the present invention is to provide a method of heat treating a selected steel to produce a desired fragmentation behavior.
Another object of the present invention is to provide a method of heat treating a selected steel to produce a fragmentation behavior which is desirable, without impairing the strength or ductility of the steel.
Still another object of the present invention is to provide a method of making projectile shell casings which have a desired fragmentation behavior and which are formed of a metal which can be forged to substantially a finished shape.
A further object of the present invention is to provide a method of heat treating AlSl 06 steel to produce a desired fragmentation behavior. (M51 06 steel is a steel containing substantially 95.8 percent iron; 1.45 percent carbon; [.0 percent manganese; 1.25 percent silicon; 0.25 percent chromium; and 0.25 percent molybdenum, by weight.)
A still further object of the present invention is to provide a method of heat treating AlSl 06 steel in such a manner as to produce a fragmentation behavior of from 4-ll grainsm without impairing its strength or ductility.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention.
A151 06 steel used in this invention has not been previously used in naturally fracturable munitions. This steel contains graphite nodules and stringers which serve as crack starters in initiating fragmentation under explosive load and enhance machinability which is important in projectile production. The graphite is precipitated from the total carbon content in the steel during mill processing and remains undisturbed by the heat treatment employed in this invention. The form of graphite coupled with a range of microstructures and mechanical properties developed by the heat treatment produces a 4-! l --m in the AIS] steel. (The fragment size, -m, used throughout this application is determined from fragmentation tests in which the fragments are collected after exploding a cylinder 5.0 inches O.D. X 7.0 inches long X 0.625 inch wall thickness with Composition B. Composition B explosive is substantially a mixture of 60 percent RDX and 40 percent TNT. The ratio of explosive weight to steel cylinder weight, C/M, is 0.27. m is the average fragment mass measured in grains and is equal to the total weight in grains of fragments recovered divided by the total number of fragments recovered in a test.
AISI 06 steel is first forged into the desired projectile pattern, such as a mortar shell casing. After forging, the casing is rough machined to within close tolerances of the finished pro uct. It 15 then subjected to heat of l,750 F for a period of 2 hours, after which it is air cooled and tempered at 500 F for 1 hour. After cooling, the shell casing is machined to the finished product.
The A151 06 steel hardened and tempered to a martensitic structure with a range of mechanical properties will produce, under explosive force, a fragment size of 4-1 I grainsm. m decreases with increase in strength and hardness and with decrease in ductility and impact strength.
The fragment size of 4 grainsm has never been attained before using a forgeable metal for shell casings. This fragment size (4 grainsm) which enhances the effectiveness of naturally fragmenting anti-personnel projectiles is produced from AlSl 06 steel. The natural microstructure of AlSI 06 steel is enhanced by normalizing and tempering A181 06 steel to change this microstructure to produce this average fragment.
Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.
What is claimed is:
l. A method of producing a high capacity explosive shell casing of AlSl 06 steel having a mixed microstructure which upon explosion of the shell will produce fragments of 4ll grainsm comprising the steps of:
forging said shell easing into a desired rough shape;
rough machining said shell casing to within close tolerances of its desired finished shape;
normalizing said shell casing by heating at approximately 1,750 F for a period of about 2 hours;
air cooling said shell casing;
tempering said shell casing by heating at approximately 500 F for a period of about 1 hour;
whereby said mixed microstructure is produced;
and then finish machining said shell casing.