Method of cryogenically hardening an insert in an article

Details Method of cryogenically hardening an insert in an article Method of cryogenically hardening an insert in an article

1980-03-03

1982-06-22

Rutledge, L. Dewayne

Metal treatment

Compositions

Heat treating

148125, 148127, 92222, 291565R, C21D 604, C21D 118, F16J 104

Patent

active

043360777

DESCRIPTION:

BRIEF SUMMARY
TECHNICAL FIELD

This invention relates generally to cryogenically treating at least a portion of an insert in an article, and has particular applicability to physically modifying a piston ring insert in a piston.


BACKGROUND ART

Because of the physical characteristics of aluminum pistons it has been the practice to manufacture pistons with built-in cast iron inserts. These annular inserts are provided with one or more peripheral grooves of precise cross sectional configuration for receiving a corresponding number of cast iron piston rings. During the manufacturing process molten aluminum material is poured around the inserts, and in order to obtain the desired degree of bond therebetween as the aluminum and insert materials cool down through a temperature range of about 770.degree. C. (1380.degree. F.) the insert material has been heretofore limited to austenitic gray cast iron. This choice is directly due to the fact that an austenitic microstructure exhibits a coefficient of thermal expansion rate more directly approaching that of the aluminum alloy material of the piston and, hence, a better bond is theoretically possible.
However, in order to extend the service life of engines the piston rings have been made harder. This has desirably resulted in a reduced rate of piston ring wear, but has undesirably resulted in an increased rate of wear and/or deformation of the insert grooves.
Thus, what is desired is an economical method of improving the hardness level of such an insert for improved service life, while still providing a highly effective bond between the material of the piston and the material of the insert during both the manufacturing stages and the dynamic operating conditions experienced in an internal combustion engine.
The present invention is directed to overcoming one or more of the problems as set forth above.


DISCLOSURE OF THE INVENTION

In accordance with one aspect of the present invention a method of hardening an insert is provided including arranging an article and an insert in intimate association, and cryogenically hardening and expanding at least a portion of the insert more tightly into engagement with the article.
In accordance with another aspect of the present invention a method of hardening a piston ring insert in a piston is provided including casting a material around the insert and providing an assembly of the piston and insert, and thereafter cryogenically hardening and expanding the insert more tightly into engagement with the piston.
In accordance with a further aspect of the present invention a method of hardening a ferrous metal piston ring insert in an aluminum piston is provided including cryogenically hardening and expanding the insert more tightly into engagement with the piston by modifying the morphology of the insert from a basically austenitic matrix to at least about 25 Vol.% martensite.
In accordance with a still further aspect of the invention a piston assembly is provided including an aluminum alloy piston and a gray cast iron insert having at least about 25 Vol.% martensite.
Advantageously, a piston assembly has been produced by spraying an austenitic gray cast iron insert with liquified nitrogen and modifying an appreciable portion thereof to martensite. This has resulted in an insert hardness increase from about 77 on the Rockwell "B" scale to about 104 on the Rockwell "B" scale, and a transformation of the material of the insert from a substantially complete austenitic matrix to a matrix of 60 Vol.% martensite.


BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagrammatic, mid-sectional elevational view of a piston assembly including an apparatus for cryogenically treating the piston ring insert of the piston assembly in accordance with one aspect of the present invention.
FIG. 2 is an enlarged, diagrammatic, fragmentary cross sectional view of the insert region of the piston assembly of FIG. 1 after cryogenic treatment thereof, showing the attraction of a magnet thereto.
FIG. 3 is a view similar to FIG. 2 showing the piston assembly including a pair of

REFERENCES:
patent: 2647847 (1953-08-01), Black et al.
patent: 3098662 (1963-07-01), Iversen
patent: 3508531 (1970-04-01), Squinto et al.
patent: 3891477 (1975-06-01), Lance et al.
patent: 4179313 (1979-12-01), Koch et al.

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