US3513038A - Method for producing fragmenting steel - Google Patents

Description

United States Patent 3,513,038 METHOD FOR PROgUCIIlJYG FRAGMEN] ING TEE Rolf Weil, Hoboken, NJ., assignor to the United States Kt. America as represented by the Secretary of the rmy N0 Drawing. Filed Nov. 18, 1965, Ser. No. 508,606

Int. Cl. C21d 9/16 US. Cl. 148-49 4 Claims ABSTRACT OF THE DISCLOSURE The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to me of any royalty thereon.

This invention relates to fragmentation of ordnance articles. More particularly, the invention relates to ordnance articles exhibiting improved fragmentation properties and to a method for improving fragmentation properties of forged steel ordnance articles.

The problem of fragmentation control in ordnance articles such as shells, bombs, grenades and the like is a significant one since it materially affects tactical use and, employment and substantially influences technical and economical aspects of production. Existence of the problem has not gone unnoticed and to date several methods of solution have been investigated while others are being considered. These methods are based on controlling the fracture path by such mechanical techniques as machining grooves in the surface of the article or by such metallugrical techniques as using cast iron in which the graphite distribution is determinative of fracture occurrence. While seemingly a solution to fragmentation control is offered by such methods, they are attended by collateral disadvantages, particularly in the area of production and inability to withstand high velocity firing, which tend to offset and mitigate the benefits conferred.

Accordingly, a principal object of the present invention is to provide a method for improving the fragmentation properties of ordnance articles which is unattended by the foregoing disadvantages of the prior art.

Another object of the invention is to provide a method for improving the fragmentation properties of forged steel ordnance articles.

Still another object of the invention is to provide a forged steel ordnance article exhibiting improved fragmentation properties.

Another object of the invention is to provide a forged ordnance article useable at high velocities exhibiting improved fragmentation properties.

Other objects of the invention will in part be obvious and in part appear hereinafter in the following description of the invention and in the appended claims.

The present invention solves the problem of fragmentation control in ordnance articles, e.g., mortar sheels, by empolying forged steel therein and involves the formation of a relatively brittle grain boundary network by proper heat treatment of such articles. The formation of such network can be accomplished either in a steel initially of hypereutectoid composition or in a hypoeutectoid steel carburized to the hypereutectoid composition. Inthe latter case, improved fragmentation may be realized even by carburization of a part of the wall of an article to the hypereutectoid composition. Proper heat treatment involves the precipitation of carbide on the austenite grain boundaries by heating in or passing thru at suitable speeds the austenite-iron carbide region of the iron-iron carbide equilibrium diagram, quenching to form martensite from the remaining austenite, and tempering to stabilize the structure and dimensions, reach the desired hardness level and relieve residual stresses.

Articles made in accordance with the disclosed invention exhibit fragmentation comparable to prior art articles made of malleable cast iron and are attended by a better and wider range of mechanical properties. Additional advantages include reduced inspection requirements and shorter heat treating periods. As an economy factor, the latter looms significant when it is considered that the inventive method may require heat treating for a period of about four hours as contrasted with the production of malleable cast iron which necessitates heat treating white cast iron for about hours.

In the course of the investigation leading to the present invention the following experimental procedure was conducted:

EXPERIMENTAL PROCEDURE Steel cylinders 1" outside diameter with a /a" wall and 2 /2" long were heat treated to obtain a large austenite grain size with ferrite and cementite grain boundaries and a tempered martensite matrix. This martensite matrix was to give the grains the toughest possible structure. The large austentite grains were grown by heating the steel to various high austenitizing temperatures and holding therefor various times.

The ferrite network was obtained by slowly cooling samples of 1045 steel to 1350 F., a temperature at which some of the austenite, particularly that in the grain boundary, transforms to ferrite. Subsequently the steels were oil quenched to transform the remaining austenite to martensite. The martensite was later tempered at 800 F. for one hour.

A carbide network in a hypereutectoid steel was obtained by a heat treatment similar to that which causes a ferrite network in a hypoeutectoid steel. However, as hypereutectoid steels have lower melting points than those containing less carbon, it was necessary to grow large grains, which must be done at high austenitizing temperature, in a 1045 steel and later carburize at a lower temperature.

Cylinders of the same dimension as the steel ones were made of malleable cast iron for tests to compare them with 3 the steel of various structures. A forged 1045 untreated steel cylinder was also tested for comparsion.

Three cylinders were heat treated simultaneously under any given condition. One was examined metallographically and the other two were fragmented. The fragmentation test consisted of casting Composition B cylinders, in diameter and long. The charge was then inserted in an assembly consisting of two brass cylinders, 1%" and A" long and of the same inside and outside diameter as the test cylinders, which were placed below and above each iron or steel cylinder respectively in order to eliminate end effects.

This assembly, consisting of the brass and steel'tubes and the Composition B cylinder, was placed in sawdust and the explosive initiated with a tetryl pellet. The fragments were collected magnetically and sorted according to size. Representative fragments from each size group of the various steels and cast irons tested were examined microscopically.

4 characteristics. A partial network, such as possessed by Specimens D-2300-1 and D-2300-4, is considerably less effective and shows only a slight improvement over the forged, untreated steel. Of the two types of intercrystalline networks, the carbide appears to result in better fragmenting characteristics.

Metallographic examinations prior to fragmentation showed that the grain size of the specimens are inhomogeneous and that each piece contained a considerable number of small crystallites.

Microscopic study of the fragments revealed that fracture occurred when network was present practically always along the grain boundaries but not necessarily along each grain boundary.

Fracture due to the hoop stress occurred readily regardless of the network. However, with the possible exception of one cylinder possessing a complete ferrite network, only those samples with a complete or nearly complete carbide network broke into relatively equiaxed fragments TABLE I.-SUMMARIZED PROPERTIES OF FRACTURE SPECIMENS Steel Specimen Heat treatment Heated in dry hydrogen at 3,100 F. for 1% hrs. Cooled slowly in hydrogen to 1,350 F. and held for 1 hr.

Quenched in oil. Tempered at 800 F. for an hour.

Heated at 2,250 F. in carbon block for 1% hrs. Trzgiisi ierred in block to furnace at 1,700 F. with atmosphere. Cooled to 1,350 F. and held for 1 hr.

Quenched in oil and tempered at 800 F. for 1 hr.

Heated at 2,300 F. in dry hydrogen for 1% hrs. Cooled slowly in hydrogen to l,350 F. Quenched in oil.

Tempered at 800 F. for 1 hr.

Heated at 2,350 F. for 1% hrs. in carbon block, transferred to furnace at 700 F. with RX atmosphere and cooled at 1,350 F. Held for 2 hrs. Quenched oil. Tempered at 800 F. for 1 hr.

Heated at 2,400 F. in dry hydrogen for 1 hr. Cooled to 1,350 F. and held for hr. Oil quenched. Tempered 1 hr. at 800 F.

ferred to packed carburizing box at 1,700 F Heated at 2,200 F. in dry hydrogen for 1% hrs. Transi held for 21 hrs. Cooled to 1350 F. and held for 1 hr.

Quenched in oil and tempered at 800 F. for 1 hr.

Heated in dry hydrogen to 2,300 F. for 2% hrs. Transferred to carburizing box at 1,700 F. and held for 1% hrs. Quenched in oil. Tempered at 800 F. for 1 hr.

Packed in carburizing box with carburizing compound-Austlnitized at 1,900 F. for 19 hrs. Brought down to 1,350 F. and held for 1 hr. Quenehed in oil.

Tempered for 1 hr. at 800 F.

Packed in carburizing box with carburizing c0111- pound-Austinitized at 2,200 F. for 1% hrs. Brought down to 1,400 F. in furnace and held for 16 hr.

Quenched in water. Tempered at 500 F. for hr.

Heated at 2,100 F. in dry hydrogen for 13/ hrs. Transterred to packed carburizing box at 1,700 F. and held for 19 hrs. Cooled to 1,350 F. and held for 1 hr.

Quenched in oil and tempered for 1 hr. at 800 F.

EXPERIMENTAL RESULTS Table I outlines the various heat treatments and describes the resulting microstructures and hardnesses. It can be noted that steels with complete or nearly complete ferrite and carbide grain-boundary networks and with partial networks were tested. For comparison, cast iron and untreated steel samples were also fragmented. The results of the fragmentation tests are summarized in Table II. (Table in C01. 5.)

DISCUSSION OF RESULTS The test results show that a complete or nearly complete grain-boundary network improves the fragmentation Grain dia. (average) cm.

Network continuity, percent Hardness N o.

Grain boundary Background structure Ferrite.

Tempered martinsete 0.199 .do do 29.4

0. Carbide--- 10 d0 0.125 do 10 d0 0.110 d0 95 do 0. do 100 do 0.232 .do 95 do 42.5

tenite-iron carbide region of the iron-iron carbide equilibrium diagram,

holding said article in said temperature range until carbide precipitation occur on the austenite grain boundaries,

quenching said article thereafter to form martensite,

tempering the quenched article to stabilize the resultant structure and dimensions and relieve residual stresses, said article being heated to about 2100 F. in dry hydrogen for a period of about 1 /2 hours,

transferring said heated article to a packed carburizing box at about 1700 F. and holding at this temperature for about 19 hours,

cooling said carburized article at about 1350 F. for a period of about 1 hour, said cooled article being quenched in oil, and said quenched article being tempered at about 800 F. for about 1 hour, said article yielding a carbide grain boundary having about 95% network continuity.

References Cited UNITED STATES PATENTS 2,825,669 3/1958 Herzog 148-143 OTHER REFERENCES A.S.M. Transactions, 1946, Preprint No. (pp. 18

and -39 CARL D. QUARFORTH, Primary Examiner A. J. STEINER, Assistant Examiner US. Cl. X.R.