US 3078194 A
Description (OCR text may contain errors)
Feb. 19, 1963 E. A. THOMPSON 3,078,194
TAPPET WITH CAST IRON BASE AND TUBULAR STEEL BODY 2 Sheets-Sheet 1 Filed June 23, 1955 Feb. 19, 1963 E. A. THOMPSON TAPPET WITH CAST IRON BASE AND TUBULAR STEEL BODY Filed June 23, 1955 2 Sheets-Sheet 2 FIG. 5
United States Patent 3,078,194 TAPPET WITH CAST IRON BASE AND TUBULAR STEEL BODY Earl A. Thompson, Ferndale, Mich. (1300 Hilton Road, Ferndale Station, Detroit 20, Mich.) Filed June 23, 1955, Ser. No. 517,520 9 Claims. (Cl. 148-31) This invention relates to a tappet and more particularly to a tappet comprising a cast iron base and a tubular steel body bonded, fused, welded, brazed or otherwise permanently joined together, such as shown in my Patent 2,887,098 of May 19, 1959.
One of the objects of this invention is that of producing a tappet for an internal combustion engine wherein the cast iron base or wear surface which contacts the customary cam for operating such a tappet has excellent wear resistance properties and strength. This object is achieved in part by fabricating the base of the tappet independently of its body from. an alloy cast iron which is hardenable by heat treatment. By fabricating the base or wear face of the tappet separately from the tubular steel body one is able to control the rnicrostructure of the base and thereby achieve the above mentioned wear resistance properties and strength.
My above mentioned copending applications show the preferred method for joining the tubular steel body to the cast iron base. The present invention is particularly concerned with a tappet such as shown in my copending applications wherein the cast iron base or button has a uniform microstructure across substantially the entire wear face of the tappet. This object is accomplished by fabricating the base from a cast iron alloy which is hardenable by heat treatment. By way of example, the base of .the wear surface of the tappet can be made from any typical castiron alloy the elements of which are expressed in percentages by weight of the total composition; such Carbon d 2.9.
Manganese .70. Chromium; .70.
Copper n .50.
Nickel .25. Sulphur About .1 maximum. Phosphorus; About .2 maximum. Iron Balance.
The percentages of these constituents will vary slightly depending on the control exercised in the foundry.
I prefer to melt the iron and alloying constituents in an electric furnace and cast a plurality of individual base castings or buttons in a shell mold. I prefer a shell mold over a sand mold because with a shell mold one can obtain a more uniform and rapid freezing rate than with an ordinary green sand mold. The cast iron is left in the shell mold for only suflicient time to allow all of the metal in the castings to solidify. This, by way of example, is about two minutes. At the end of this time the castings are quickly removed from the molds and the base castings clipped off of the runners and immediately quenched at sufiicient rate to produce at least a partially complete martensitic structure commonly referred to as mottled iron. This may be done by air quenching when the cast bases are sufficiently separated or they may be dropped into a suitable liquid quenching media, such as oil.
After a cleaning operation the castings are then introduced into a furnace at a temperature of l700-1800 F. This furnace must have sufficient heating capacity to 3,078,194 Patented Feb. 19, 1963 quickly bring the temperature of the base castings to the temperature of the furnace; for example, in approximately two and one-half to four minutes. The base castings should be distributed in the furnace so that they may be quickly heated to this temperature. Where the base castings are made from the above typical alloy, the time they are held at this temperature of 1700-1800 F. is about eight minutes. However, where the percentage of-carbide forming alloy constituents such as chromium and molybdenum .or even vanadium is varied, the length of time which they are held at this range of temperature will be varied to produce the desired result. 1
if the above analysis is varied to increase the carbide forming constituents, that is, chromium, molybdenum, then the length of time that the alloy will be held at furnace temperature can be increased to produce the dev sired microstructure whereas if the amount of carbide forming alloy constituents is decreased over that stated in the above example, then the time that the base castings are held at furnace temperature will be correspondingly decreased-or the temperature decreased.
After the base castings have been left in the furnace I at the 1700-l800 F. zone for the above specified time,
they are then immediately and quickly transferred to an adjacent zone in the furnace having a lower temperature of approximately 1300 F. The base castings are allowed to remain in this zone about twelve minutes which is sufiicient time to insure the transforming of the austenite content of the castings to pearlite. The base c ast-' ings may then be air cooled, or, if desired, transferred to a third zone in the furnace at around 750 F.'for a that- 1 ter of ten to fifteen minutes and then air cooled.
The above described casting procedure and heat treatment will bring out changes in the microstructure in the castings as illustrated in the attached photomicrographsi FIG. 1 is a photomicrograph showing the structure of the casting as eastand quenched to room temperature.
Photomicrograph FIG. 2 shows the changesinthe structure brought about the time at heat in the 1700 1800 -F. zone. This photograph is taken from a'sp'eeimenthat was removed from this zone after having been in this zone for the specified time and then air cooled.
Photomicrograph FIG. 3 shows the structure of the base castingafter having been treated as above described in zone 1 (1700-1800 F.) and zone 2 (1300 F.), the specimen being taken out at that point and air cooled.
Photomicrograph FlG. 4 shows the structure of the base casting after it has been heat treated in zones land 2 referred to above and then additionally treated in the furnace zone at 750 F. for a periodoften to fiftee minutes and then air cooled.
. After the base casting has been heat treated as above described to produce, the structure shown in hotomicrographs FIG. 3 or FIG. 4, the base casting is then joined to the tubular steel body, preferably as shown and de scribed in my above specified copending applications.
'- This produces a combined fusion bond and mechanical connection between the base casting and the tubular steel body. The tappet as thus formed is then subjected to a final carburizing treatment in a gas carburizing or carbonitriding furnace at a temperature of from 1525 to 1550" F. and held at the temperature for a sufiicient time to produce a case depth in the steel tubing of around .012 to 0.16. The thus carburized or carbo-nitrided tappet is direct quenched in oil and drawn to a temperature of approximately 400 F. This final carburizing or carbonitriding treatment improves the fusion bonded joint.
A photomicrograph FIG. 5 shows the structure of the base after the final heat treatment in the carburizing or carbo-nitriding atmosphere.
This quenching from 1550 F. produces a fully mattensitic matrix in the casting and further refines the shape and size of the cementite or iron carbide particles. It will be noted that the final microstructure of the base casting is characterized by uniformly distributed cementite particles with predominantly rounded contours and the matrix is martensitic. A minor amount of the carbon content exists as free graphite mostly concentrated in mottled areas.
A study of the final microstructure of the base casting shows that these mottled areas are located on an average over the entire wear face of the base casting of approximately one mottled area per square inch at 100 diameter magnification, that is, about 10,000 mottled areas per square inch of actual surface. The average of mottled areas may range as high as one and one-half mottled areas per square inch at 100 diameter magnification, but preferably should average less than about one per square inch at 100 magnification. The unconnected cementite or iron carbide particles may, in the vicinity of high graphite concentration, be as low locally as approximately ten particles per square inch at 100 diameter magnification. In areas where there is a low concentration of graphite, the cementite particles may range as high in 190 particles per square inch at 100 diameter magnification.
A highly desirable or preferred base casting will contain in the minimum areas of cementite particles of the order of 40-50 particles of cementite per square inch at 100 diameter magnification and in the maximum areas of cementite particles a concentration of the order of 120-440 cementite particles per square inch at 100 diameter magnification.
As a modification of the above method, the base casting after undergoing the treatment in zone 1 at 1700- 1800 F. for the time specified, can be transferred into a zone at about 900 F. and maintained in this zone until the casting falls to a temperature of 1300 F. or a little below, whereupon the casting can be cooled in air to room temperature. After reheating to 1550" F. and quenching in oil a microstructure, such as shown in the photomicrograph FIG. 6, is obtained which comprises substantially uniformly distributed discrete cementite particles in a martensitic matrix with some free graphite. The reheating step at 1550 F. can be accomplished simultaneously with the carburizing ofthe tappet after the base has been permanently joined to the tubular steel body.
FIG. 7 is a photomicrograph having the same disclosure as that of FIG. but at a magnification of 1000 diameters.
1. A valve tappet comprising a tubular steel body and a base of alloy cast ironwhich has been hardened by heat treatment characterized by a martensitic matrixhaving substantially uniformly distributed unconnected cementite particles predominantly with rounded contours, the number of discrete cementite particles being present within a range of from about 10 to 190 per square inch at 100. diameter magnification and a minor amount of the carbon in the alloy cast iron being present as free graphite.
2. A valve tappet as claimed in claim 1 wherein the cementite particles have their lowest concentration in the vicinity of the areas of higher graphite concentration and their highest concentration in the vicinity of the areas of lowest graphite concentration.
3. A valve tappet as claimed in claim 2 wherein the carbon existing as free graphite is concentrated mostly in mottled areas.
4. The valve tappet as claimed in claim 2 wherein the cementite particles in their lowest concentration areas fall within a range of from 40 to 50 cementite particles per square inch at diameter magnification and in their highest concentration areas fall within a range of from to cementite particles per square inch at 100 diameter magnification.
5. A valve tappet as claimed in claim 3 wherein the mottled areas of free graphite are present in an average amount of less than about 1.5 per square inch at 100 diameter magnification.
6. The valve tappet as claimed in claim 4 wherein the carbon existing as free graphite is concentrated mostly in mottled areas.
7. A valve tappet comprising a tubular steel body and a base of alloy cast iron which has been hardened by heat treatment characterized by a martensitic matrix having substantially uniformly distributed, unconnected, relatively small cementite particles predominantly with rounded corners and very small primary graphite particles, said graphite particles appearing primarily in generally isolated areas having a mettle-like pattern, said cementite particles being present in said mettle-like areas in a range of about at least 40 to 50 cementite particles per square inch at a 100 diameter magnification.
8. A valve tappet as claimed in claim 7 wherein said cementite particles are present in the areas of lowest graphite concentration in an amount of about at least 90 particles per square inch at a 100 diameter magnification.
9. A valve tappet as claimed in claim 8 wherein the cast iron alloy has substantially the following composition by weight: carbon 2.9%, silicon 2.10%, manganese .70%, chromium .70%, molybdenum 50%, copper 50%, nickel .25%, sulphur about .1% maximum, phosphorus about .2% maximum, and the balance iron.
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