US2611690A - Ball-peening and cleaning shot - Google Patents

Description

Patented Sept. 23, 1952 enthusiasm A" NIN fG'sHoTif H v Richard Schneidewind, Ann Arbor, Mich.,.as.i-.

' signor to Industrial Metal Abrasive;- Company; ..l'. a .ckso:11, Micln. a corporation ofM-ichigan;

so mavens;

- Thepresent invention relatesto improvements in iron shot and gritused in' peening, cleaning, abrasion and for similar purposes, and the method of producing the same; i

The object of my invention is to produceshot andgrit' characterized by both hardnessi and greattoughness and resistance to shattering. This-fshot-"i'sof increasedms'efulness for the'pur poses of removing burned-in sand from iron or steel castings, to produce a clean finish on metals and other objects, and to increase the surface hardness and strength of hardened steels by a process of severe localized cold working. The fatigue resistance ofsuch steels is improved not only by the increased surface properties but also by virtue of the fact that microscopic surface cracks, which are foci for localized high stresses, are peened shut.

Ball-peening shot now used in the trade is a white cast iron made by atomizing molten iron with a blast of water, steam or other suitable agent and rapidly cooling the resultant globules of molten iron. Grit is simply the product formed by crushing, grinding, or otherwise comminuting white iron shot. Usually only oversize, nonspherical, or otherwise unsalable shot is used for grit.

The metallographic structure of such materials is very fine and consists of dendrites or needles of cementite, a hard, brittle carbide of iron and martensite, which is likewise brittle, or a decomposition product of martensite. The chemical composition of present-day material is not critical and usually falls within the following ranges:

Per cent by weight Carbon 3.0 -3.5 Silicon 1.4 -2.4 Manganese 0.4. -0.6 Sulphur 0.06-0.15 Phosphorus 0.10-0.40

In using the shot, the'object to be treated is placed in a chamber and shot is directed at it at very high velocities. The momentum of the shot 7 enables it to perform its desired functions men- Application February 1611046: Serial No. 648,199 1: Claim.. (o1. 51 -3099 to increase-"thetoughness orresistanceto shattering of ball-peeningshot'. Alloying hasbeen-suggested in an effort. to makea stronger martensite.

:Whereas some increase-in toughness mayf lbie anticipated, the actual practical benefit is slight and'does not justify 'the added costof'the alloy i'n'g= constituents; Heat treatmenthas also been resorted"to in -the past in orderto increase -sh'ot 'Iif'e. 1 Due to the presence of appreciable amounts of silicon in the irons, the iron carbides decompose under the influence of high temperatures and the carbon thereof is deposited as temper carbon, a nodular graphite constituent. The matrix of martensite is also transformed during heat treatment and, dependent upon the final cooling rate of the heat treatment cycle, results in a matrix of ferrite, or pearlite, or a mixture thereof, or martensite at the will of the heat treater. Since elemental carbon or graphite is more brittle and weaker than iron carbide, heattreated shot does not possess superior toughness if a martensite matrix is used. If a ferritic or. pearlitic matrix is used, the shot is too soft to function properly since ferrite, pearlite and graphite are all softer by far than iron carbide or martensite.

Since hardness i one of the principal requisites of ball-peening shot, iron carbides are a desirable constituent. Considerable increase in toughness and shatter-resistance can be secured by altering the physical size and shape of the cementite or iron carbide, which upon casting is in the form of long, needle-like dendrites. This may be accomplished by subjecting the iron as cast to high temperaturesof the order of 1,500 F. to 1,800 F. for several hours to bring about the solution of a portion of the carbides into the matrix of the iron. Surface tension forces at the same time are tending to spheroidize the remaining free carbides. lowed by slow cooling to the critical temperature (about 1,350 F.-1,380 F.), which permits the precipitation of some of the carbides previously absorbed. The precipitated carbides tend to be spherical in shape. Slow cooling to below the critical temperature results in the precipitation of the remainder of the carbides as spheroidized pearlite.

The heat treatment may be varied by rapidly quenching the material from the critical temperature. This would give a structure of spheroidized cementite, some unaffected original cementite, and martensite.

As a third variation, the quenched product may be tempered at some suitable temperature below This treatment is fol- I 3 the critical temperature to produce a matrix of decomposed martensite.

Treatments of this nature have not been performed on ball-peening shot heretofore because iron carbide decomposes to form temper carbon when heated to high temperatures due to the presence of'silicon. In order to use high temperature treatments, it is found necessary to add sufiicient carbide-stabilizing alloys to counteract the oarbide-destroying influence of silicon. Such alloys are chromium, vanadium, tellurium, tungsten, molybdenum, boron, and others or mixtures thereof. The quantities needed for stabilization are well known to those skilled in the art.

A secondary benefit from the use of a carbidestabilizing alloy such as vanadium is the wellknown action of vanadium to refine the crystal may be applied in conjunction with the'disclosure 01 my co-pending application Serial No. 644,431,

filed January 25,1946, now abandoned, dealing with the production of ball-peening shot having anaustenitic matrix.

Care must be exercised lest the austenite-forming alloy be at the same time a carbide-destroying alloy such as nickel. .Also alloys must not be combined which prevent 4 the production of austenite at room temperatures.

Having thus described my invention, what I claim as new and desire to protect by Letters Patent is:

An improved cast-iron ball-peening shot of high hardness and toughness characterized by a microstructure containing a large portion 01 stabilized, hard, metallic carbides in spheroidized form in a suitable matrix in the substantial absence of graphitic carbon.

RICHARD SCHNEIDEWIND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Comstock Sept.