US2092283A - Piston alloy - Google Patents

Description

Patented Sept. 7, 1937 UNITED STATES PISTON ALLOY Russell H. McCarroll, Dearborn, Mich., assignor to Ford Motor Company, Dearborn, Mich, a

corporation of Delaware No Drawing. Application January 30, 1935,

Serial No. 4,126

1 Claim.

The object of my invention is to provide a steel alloy especially adapted for constructing internal-combustion engine pistons, which pistons will have most of the advantages of aluminum pistons without several of the disadvantages inherent in such aluminum pistons. Aluminum pistons are advantageous in that the metal aluminum is an exceptionallyogood heat conductor so that the transfer of heat from one part of the piston to another part is rapid enough to allow the piston to remain cool even though comparatively high compression pressures are used. Furthermore, aluminum being very light in weight minimizes the reciprocating strains on the wrist pin and connecting rod bearings, and also requires aminimum of counter-weighting in the crank-shaft to producea running balance. These advantages are sumciently important to make an aluminum piston preferable to the ordinary cast iron piston for high speed engine use. However, they are obtained only with certain inherent disadvantages. These disadvantages are that aluminum has a high co-efiicient of expansion so that compensating slots, invar struts, or some other means must be provided to prevent piston slap during the warming up period of the motor. Furthermore, aluminum has a comparativelyhigh co-efficient of friction with cast iron so that excessive piston wear results, and further the cylinder surface attains a somewhat hotter temperature than when cast iron pistons are used, due to the increased friction.

Perhaps the greatest disadvantage inherent' in aluminum pistons is that the metal is comparatively soft so that the piston ring grooves pound out or increase in width appreciably after only a few hundred hours of use. Furthermore, wear on the piston skirt causes the piston to wobble as it reciprocates, which motion wears off the sharp edges of the ring grooves. These two conditions cause the engine to pump considerable oil. One of the reasons for oil passing by the pistons in internal-combustion engines is that excessive clearance between the ring grooves and the rings, allows the oil upon the down stroke of the piston to be forced into the space inba'ck of the piston rings so that upon the succeeding down stroke of the piston this oil is deposited on the cylinder walls above the ring: To prevent oil pumping, itis essential that there be a minimum clearance between the ring grooves and the piston rings. The applicants piston alloy has substantially the same co-efficient of expansion as have cast iron piston rings and it is sufnot be commercially machined.

ficiently hard and wear resistant that the ring grooves are prevented from appreciably increasing in width. For this reason the rings, even after hundreds of hours of use, fit very closely in the ring grooves.

The applicants metal is believed unique among steel alloys in that it is an exceptionally good heat conductor to thereby allow a minimum thickness of the piston head. The improved heat conductivity is obtained from a copper matrix which is provided in the alloy.

Still further, the applicants alloy is an exceptionally free flowing metal to thereby permit the commercial casting of relatively thin ribs and piston walls.

Still further, the sulphur content is particularly high in the applicant's alloy, it being three to four times as high as the maximum usually permissible in cast-iron. Experience has shown with this combination of elements a high sul- 20 phur content provides for freer machining which is essential when thin wall castings are to be machined.

It is believed that all of these characteristics are essential inasmuch as without an increased 25 heat conductivity, a piston having a light weight piston head, ribs and skirt, could not be used 'in high compression motors, and without free flowing of the metal, such light weight construction could not be cast, and without freer machining 30 characteristics, such thin walled pistons could With these and other objects in view, my invention consists in thecomposition and combination of elements in my improved alloy, as 35 described in the specification and claimed in my claim.

My improved alloy consists of the following:

Carbon 1.40-1.70 per cent Manganese .90-1.10 per cent 40 Sulphur .12- .16 per cent Silicon .90-1.10 per cent Copper 1.50-2.00 per cent Phosphorus .12 (max) Iron balance 5 a Brinell of 07-286 is readily attained, the ma- 1 terial having a tensile strength of about 100,000

pounds per square inch.

With the above mentioned heat treatment, the carbon is in the form of temper carbon and not as graphite flakes. The abovementioned heat treatment differs from the conventional normalizing treatment in that heating the castings to 1650 F. breaks up the grains and starts secondary graphitization, while the 1400 draw completes this graphitization while at the same time spheroidizing the pearlite. In this way secondary graphite is formed which result is not obtained by normalizing. This accounts, to a great extent, for the increased strength of the alloy and the increased wear resistance of the metal. Furthermore, a characteristic of my improved alloy is that the copper is held in solution by the silicon. I

An important advantage obtained with my improved piston alloy is that the copper content forms a matrix or network throughout the casting which improves the heat conductivity to such extent that the cross-sectional area or thickness of the piston head may be reduced to substantially that required to carry the structural load upon the head, Inasmuch as this material has a tensile strength of approximately 100,000 pounds per square inch a comparatively thin piston head, much thinner than required with cast iron pistons, is sufficient to carry the stresses imposed on same, and due to the improved heat conductivity; pre-ignition does not occur. Excellent results with no pre-ignition have been obtained using a three inch diameter piston having a head-thickness of only .090 and operating at 6.5 atmospheres of compression; whereas, when an ordinary cast iron piston is used, the piston head must be several times this thickness to give the necessary structural strength. If an ordinary steel piston is used, over 50% increase in head thickness must be provided to prevent pre-ignition, due to the lower heat conductivity of steel.

A further advantage obtained with my improved alloy is that the coefiicient of friction with a cast iron cylinder is considerably less than the co-efilcient of friction between cast iron and cast iron and considerably less than the co-eflicient of aluminum on cast iron. Consequently, less wear results on the piston walls and less heat is ing internal-combustionv engine pistons, comprising,

Carbon 1.40-1.70 per cent Manganese .90-1.10 per cent Sulphur .12- .16 per cent Silicon .901.10 per cent Copper 1.50-2.00 per cent Phosphorus .12 (max Iron balance RUSSELL H. MCCARROLL.