Method of making ductile cast iron with improved strength

Metal treatment – Compositions – Heat treating

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75123CB, 75123L, 148 35, 148139, C21D 500, C22C 3710

Patent

active

044849532

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION AND PRIOR ART STATEMENT

Ductile cast iron, also known as nodular iron or spherulitic iron, is cast iron in which the graphite is present as tiny balls or spherulites, instead of as flakes normally present in grey iron, or instead of compacted aggregates present in malleable iron.
The composition of unalloyed ductile iron is similar to that of grey iron, containing similar amounts of carbon, silicon, manganese, and phosphorus. The spheroidal graphite structure is produced by the addition of one or more elements to the molten metal, such elements commonly being referred to as nodularizing agents; on a commercial basis the agent is magnesium and/or cerium.
Ductile iron can be produced as-cast, or given an annealing treatment such as a ferritizing anneal, or can be quenched and tempered. The microstructure of as-cast ductile iron is pearlitic in the matrix along with a small amount of cementite, and has considerable ferrite surrounding each graphite nodule (commonly referred to as a bulls-eye ferrite configuration). The relative amounts of pearlite, ferrite, and cementite are dependent on the composition, type of inoculant, inoculation practice, and, most importantly, the cooling rate.
The microstructure of annealed ductile cast iron, particularly in the case of ferritized annealed cast iron, is a ferrite matrix in which are nestled graphite nodules along with a small or negligible amount of cementite. The microstructure of austempered ductile cast iron is a mixed phase matrix composed of austenite and martensite or bainite (see U.S. Pat. Nos. 2,324,322 and 3,860,457). The microstructure of quenched and tempered ductile iron is tempered martensite and/or bainite (see U.S. Pat. No. 3,702,269).
Each of these types of ductile cast iron microstructures leaves something to be desired in terms of the total combination of physical characteristics. For example, in a conventional as-cast ductile iron the yield strength is typically about 60 ksi, the tensile strength is about 80 ksi, accompanied by an elongation of about 3%. This type of iron is not particularly strong nor is it particularly ductile. An annealed ductile cast iron, particularly one having been subjected to a ferritizing anneal, will have a yield strength of about 40 ksi, a tensile strength of 60 ksi, and an elongation of 10-18%. This latter iron is not particularly strong, although excellent is ductility. A conventional quenched and tempered ductile cast iron will typically have a yield strength of about 90 ksi, a tensile strength of 120 ksi, and an elongation of 2% or less. The quenched and tempered ductile iron is exceptionally strong but poor in ductility.
What is needed by the prior art is a method and ability to produce ductile iron with an improved combination of physical characteristics, including a yield strength of at least 80 ksi, a tensile strength of at least 140 ksi, an elongation of 6-10% as well as exceptional hardness in the range of 275-290 BHN.


SUMMARY OF THE INVENTION

The invention is an improved method of making a ductile cast iron (and an improved casting iron composition), the ductile cast iron having a microstructure with a matrix consisting of acicular ferrite and bainite, said cast iron exhibiting an elongation of 6-10%, a yield strength of at least 80,000 psi, and tensile strength of at least 140,000 psi.
The process comprises: (a) forming a ductile cast iron by melting a ferrous alloy consisting essentially of by weight 3.0-3.6% carbon, 3.5-5.0% silicon, 0.7-5.0% nickel, 0-0.3% molybdenum, 0.2-0.4% manganese, less than 0.015% sulphur and 0.06% phosphorus, the remainder essentially iron, said melt being subjected to a nodularizing agent for effecting graphitic aggregates upon cooling and solidification to form said ductile cast iron; (b) heating said ductile cast iron to 1575.degree.-1650.degree. F. for a period of 1-3 hours and immediately quenching to 400.degree.-775.degree. F. at a rate of at least 275.degree. F./min.; and (c) holding the ductile cast at said latter temperature for a period of

REFERENCES:
patent: 2324322 (1943-07-01), Reese et al.
patent: 3549430 (1970-12-01), Kies et al.
patent: 3600159 (1971-08-01), Moore et al.
patent: 3673004 (1972-06-01), Dumitrescu
patent: 3702269 (1972-11-01), Church

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