Method of making high strength ferritic ductile iron parts

Details Method of making high strength ferritic ductile iron parts Method of making high strength ferritic ductile iron parts

1983-01-24

1984-10-09

Skiff, Peter K.

Metal treatment

Compositions

Heat treating

148 35, 148139, C21D 514, C22C 3704, C22C 3710

Patent

active

044759568

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT

It is conventional in the art of making ductile iron castings that when maximum ductility and the best machinability is desired, and high strength is not required, nodular iron castings are given a conventional full anneal. The microstructure is converted to ferrite and spheroidal graphite. This microstructure is called a ferritic nodular iron (the term nodular being interchangeable with ductile herein, although ductile irons can include some forms of graphite other than spherulitic); it typically possesses a yield strength of 40,000 psi, a tensile strength of 60,000 psi, an elongation of 18%, and a hardness of 137-170 BHN.
However, such ferritic nodular irons do not offer sufficient strength (at room temperature and at elevated temperatures) and corrosion resistance (at 1200.degree. F.) to be used in many automotive applications such as engine components. It would be desirable if such irons could be enhanced in such physical properties since the casting would offer considerable manufacturing economy as compared to steel forgings which consume considerable thermal and mechanical energy in forming the final product. In addition, such casting would offer weight savings due to the presence of graphite in significant amounts.
The prior art has not attempted to achieve these enhanced physical properties (see U.S. Pat. Nos. 3,954,133 and 3,549,430).
The use of higher amounts of silicon has been investigated and it has been determined that higher quantities of silicon, up to 4%, tend to stabilize the ferritic iron against phase change at elevated temperatures; and higher quantities of silicon tend to reduce oxidation, but is limited by the uniformity of silicon microsegregation gradient. Silicon, however, as generally accepted in the art, reduces the ductility of ferritic irons at room temperature. Therefore, the prior art, for maximum toughness at ambient temperatures, has kept silicon to the lower possible level. Consequently, the maximum level of silicon for practical production has been limited by the ability to process the iron without excessive difficulty and this has usually been in the area of 2-3%.


SUMMARY OF THE INVENTION

This invention has discovered a method by which the strength of ferritic ductile iron castings can be dramatically increased and at the same time maintain ductility at a high level. The method is an economical way of making high strength ferritic ductile iron parts by essentially increasing the silicon content, far in excess of that used in normal standard chemistry for ferritic ductile iron castings, reducing the amount of manganese normally used with a ferritic ductile iron casting to a level which is essentially one-half, and adding molybdenum and nickel in quantities that provide significant solution strengthening of the casting.
The method comprises: (a) casting an iron alloy melt into substantially the shape of the desired part, the melt consisting essentially of by weight 3.9-6.0% silicon, 3.0-3.5% carbon, 0.1-0.3% manganese, 0-0.35% molybdenum, sulphur no greater than 0.015%, phosphorus no greater than 0.06%, nickel of at least 1.25%, and the remainder iron, the melt having been subjected to a nodularizing agent to form nodules of the graphite upon solidification; and (b) heat treating the cast part to provide a fully ferritic iron microstructure with 9-14% by volume graphite and having a yield strength of at least 75,000 psi, a tensile strength of at least 95,000 psi, and an elongation of at least 17%.
It is advantageous if the chemistry of said melt is limited to having 4.0-4.2% silicon, nickel in an amount of about 1.25%, and molybdenum in an amount of about 0.3%. With this chemistry the physical charactistics can be improved to levels of 80 ksi for yield strength and 100 ksi for tensile strength. It is preferred that the carbon level be in the range of 3.0-3.5 to promote spheriodal nodular iron.
Preferably, the iron is heat treated to promote a hardness of at least 220 BHN, by heating to 1600.degree. F. for two hours,

REFERENCES:
patent: 2809888 (1957-10-01), Schelleng et al.
patent: 3055756 (1962-09-01), Kanter
patent: 3210183 (1965-10-01), Ototani et al.
patent: 3549430 (1970-12-01), Kies et al.
patent: 3954133 (1976-05-01), Miyaoka et al.

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