Method of preparing high nodule malleable iron and its named product

The invention is a malleable iron comprising about 250 to 400 nodules of graphite per square millimeter as observed in a photomicrograph at 100.times., and a Brinell hardness of about 195 to 550 BHN. Preferably, the malleable iron further comprises sulfur and manganese wherein the manganese is present in an excess amount of at least 2 times the amount of sulfur plus 0.15% and is formed by two separate quenching steps. The invention further comprises a method of preparing a malleable iron having a high nodule count comprising the steps of prenucleating a malleable iron casting at a temperature of about 600 to 900.degree. F. for about 3 to 6 hours; austenitizing the prenucleated casting at about 1680 to 1740.degree. F. for about 3 to 9 hours to form graphite nodules such that the malleable iron has about 250 to 400 nodules per mm.sup.2 ; and quenching the casting to form pearlite and a malleable iron made by this process.

BACKGROUND OF THE INVENTION 
The present invention relates to a malleable iron having high hardness and 
good lubricity and wear resistance, and more particularly, this invention 
relates to a malleable iron having at least 250 graphite nodules per 
square millimeter and a method of making such a metal. 
There are three types of cast irons: malleable, ductile and gray iron. Of 
these, malleable and ductile irons can be plastically deformed. These 
irons can be differentiated by their microstructures. Gray iron has most 
of its carbon in the form of flakes which resemble the shape of potato 
chips. Malleable iron has most of its carbon in the form of irregularly 
shaped graphite nodules also known as "temper carbon" which resemble the 
shape of popped popcorn. Ductile iron, which can also be referred to as 
"nodular" or "spheroidal" iron, contains carbon in the form of small round 
graphite spherulites. 
The carbon in malleable iron is predominantly in the form of graphite. 
Typically, malleable iron contains about 50 to 100 graphite nodules per 
mm.sup.2. 
Malleable iron is first cast as a white iron and then annealed at 
temperatures that result in the decomposition of cementite (iron carbide, 
Fe.sub.3 C) and convert the iron matrix into ferrite, pearlite, or 
combinations thereof. Ferrite is practically pure iron. Pearlite is a 
eutectoid structure comprised of alternative layers of ferrite and 
cementite. The chemical composition of malleable iron is generally 2.0 to 
2.9% carbon, 0.9 to 1.9% silicon, 0.2 to 1.0% manganese, 0.02 to 0.2% 
sulfur, and 0.02 to 0.2% phosphorus. Unless otherwise noted, all 
percentages herein are by weight. Small amounts of chromium, boron, 
copper, nickel and molybdenum may also be present. 
The iron for most present-day malleable iron is melted in coreless 
induction furnaces. The melting can be accomplished by batch cold melting 
or by duplexing. Molds are produced in green sand, silicate CO.sub.2 
bonded sand or resin-bonded sand (shell molds). Then the melted iron is 
poured into the molds. Molten iron produced under properly controlled 
melting conditions solidifies with all carbon in the combined form, 
producing white iron for ferritic or pearlitic malleable iron. After the 
casting solidifies and cools, the metal is in a white iron state and any 
gates, sprues and feeders are removed from the castings. The castings are 
then heat treated. It is known to add agents such as magnesium, cerium, 
boron, aluminum and titanium to the molten metal to enhance the nodular 
forming properties. 
The initial annealing converts the carbon that exists in combined form 
massive carbides (Fe.sub.3 C) or microconstituents in pearlite into temper 
carbon. Conventionally, the first state anneal is approximately 9-15 hours 
and up to 5 days at about 900 to 970.degree. C. (1650 to 1780.degree. F.). 
However, irons with lower silicon contents may require as much as 20 hours 
for completion of first-stage annealing. The initial anneal is followed by 
additional heat treatments that produce the desired matrix microstructures 
in the iron. 
Conventionally, such a method produces a nodule count of about 50 to 100 
discrete graphite particles per square millimeter as measured in a 
photomicrograph magnified at 100.times. (hereinafter all references to 
nodules/mm.sup.2 are assumed to be measurement in a photomicrograph at 
100.times.). The particle distribution is random, with short distances 
between the graphite particles. Temper carbon is formed predominantly at 
the interface between primary carbide and saturated austentite at the 
first stage annealing temperature, with growth around the nuclei taking 
place by a reaction involving diffusion and carbide decomposition. 
Conventional malleable iron has fewer nodules (50 to 100 nodules/mm.sup.2). 
Parts made from these irons do not exhibit sufficient lubricity for many 
applications requiring high wear. The diameter of the graphite nodules is 
large and abrasion tends to lift the nodules up causing them to pop out 
and form craters. This causes the machine parts to seize up and the parts 
fail. Thus, there is a need for a malleable iron which has an increased 
number of graphite nodules and a method of making such a metal. 
SUMMARY 
In accordance with the present invention, a malleable iron is provided 
having about 250 to 400 nodules of graphite per square millimeter (as 
determined by examination of a 100.times. photomicrograph), and a Brinell 
hardness of about 195 to 550 BHN. The Brinell hardness test is the 
standard of measuring the hardness of metal. The smooth surface of the 
metal is dented by a 10 mm steel ball under force. The standard load and 
time is 3000 kilograms for 30 seconds for steel and other hard metals. The 
diameter of the resulting dent is measured and the hardness determined 
from a chart or formula. Preferably, the malleable iron further comprises 
sulfur and manganese wherein the manganese is present in an amount which 
significantly exceeds two times the amount of sulfur (expressed as weight 
percent) plus 0.15%. 
The invention further comprises a method of preparing a malleable iron 
having a high nodule count comprising the steps of prenucleating a casting 
of an iron capable of forming a malleable iron by heating at a temperature 
of about 600 to 900.degree. F. for about 3 to 6 hours; austenitizing the 
prenucleated casting at about 1680 to 1740.degree. F. for about 4 to 9 
hours to malleablilize the casting and form graphite nodules; and 
quenching the casting to form pearlite, such that the malleable iron has 
about 250 to 400 nodules per mm.sup.2. In a preferred embodiment, the 
method further comprises the steps of melting an iron containing carbon, 
silicon, manganese and sulfur, and pouring the melt into a mold to form a 
casting, prior to the step of prenucleation. The quench is preferably 
performed using forced air and is carried out so as to reduce the 
temperature of the casting to about 700 to 1000.degree. F. The method 
further may comprise the step of heating at a temperature capable of 
stabilizing the casting and performing a second quench to form tempered 
martensite, wherein said second quench is conducted in oil. 
In a further embodiment, the invention is a malleable iron made by the 
above-referenced process.

DETAILED DESCRIPTION OF THE INVENTION 
In accordance with the present invention, a malleable iron is provided 
which has a higher nodule content than that of conventional malleable 
irons. The malleable iron of the present invention is produced from a 
white cast iron and is heat treated to form a martinsitic matrix having a 
nodule count which equals that of some ductile irons. This results in a 
material with a high hardness, high lubricity and high temperature and 
wear resistance. This can be used for bearings, journals for air 
conditioning parts, or other applications which require high lubricity, 
high hardness and high temperature resistance. The malleable iron of the 
present invention has a nodule count of about 250 to 400 nodules/mm.sup.2 
and a hardness of about 195 to 550 BHN. 
The method of making the malleable iron of the present invention is 
basically as follows. In a melt furnace, metal is liquified. The molten 
metal is poured into a sand mold having an impression of the casting, and 
cooled to about room temperature. The casting is separated from the mold 
and desprued. In accordance with the present invention, the casting is 
prenucleated in a heat treat furnace before heating to the austenitizing 
temperature. The casting is then air quenched. 
The steel starting material which is placed in the melting furnace is 
preferably 60/40 steel (60% returns, sprue, castings, etc.; 40% steel). In 
addition, to the steel, other additives are added to the molten metal. 
These additives include carbon, manganese, silicon, and sulfur, and may 
additionally include one or more of phosphorus, chromium or bismuth. 
Typical additions are about 2.2 to 2.8% carbon, about 1.35 to 2.0% 
silicon, and about 0.30 to 0.85% manganese. Preferably, the additives are 
present in the following amounts: about 2.40 to 2.60% carbon, about 1.35 
to 1.55% silicon, about 0.45 to 0.65% manganese, about 0.02 to 0.05% 
sulfur. 
The amount of manganese should be such that there is a significant excess 
balance of manganese with respect to the sulfur in the melt. In 
conventional malleable iron, manganese is present in an amount of two 
times the percentage of the sulfur plus 0.15%. The iron used in the 
present invention should contain in excess of that amount of manganese. 
Preferably, the excess or free manganese should be present in an amount 
about at least 0.30% free manganese. Typical amounts of sulfur are about 
0.02 to 0.05% and up to about 0.45 to 0.65% total manganese can be used 
for harder malleable iron. This gives a ratio of approximately 14 to 1 
which is 325% in excess of industry standard ratios of 3 or 4 to 1. 
A typical heat treatment that can be used to form the malleable iron of 
this invention is diagramed in FIG. 1. 
The casting is prenucleated at about 600 to 900.degree. F. for about 3 to 6 
hours. This prenucleation step is designed to increase the nucleation 
sites for the graphite nodules thus leading to a greater number of nodules 
in the final product. The increase is due to the creation of vast areas of 
austenite/carbide interfaces. These interfaces act as favorable nucleation 
sites for graphite as well as providing shorter diffusion paths for 
carbon. In turn, the prenucleation decreases the size of the nodules. The 
prenucleation step is generally not effective if it is only carried out 
for about 1 to 2 hours. However, if the prenucleation step is 
substantially longer than about 6 hours, the carbon shape may start to 
deteriorate and become flaky. 
After the prenucleation step, the casting is heated to about 1680 to 
1740.degree. F. and the casting is austenitized for about 3 to 9 hours. 
Temperatures in excess of this range are not recommended because they can 
lead to warped castings or scale. This treatment breaks down the primary 
carbides (Fe.sub.3 C). Austenitizing forces the carbon out of solution and 
into the graphite nodules at the nucleation sites formed during the 
prenucleation. After austenitizing for at least 3 hours, the iron is 
essentially free of carbide and contains about 250 to 400 
nodules/mm.sup.2. If the iron is austenitized too long surface 
decarbonization can result as ambient oxygen depletes the casting of 
carbon. 
After austenitizing the casting is preferably air quenched to form 
pearlite. The forced-air quench is carried out to cool the metal to about 
700 to 1000.degree. F. This typically takes about 10 minutes. An air 
quenched structure prior to a subsequent oil quench provides a dispersion 
of graphite nodules in a matrix of iron carbide lamellae (pearlite). 
After air quenching, the casting is heated and reaustenitized at about 
1650.degree. F. for 30 minutes and then cooled slightly to about 
1575.degree. F. and held for another 30 minutes to stabilize the 
microstructure. Upon heating during reaustenitizing, the carbon goes into 
solution faster from the air quenched structure since it has less 
diffusion distance to travel due to the iron carbide lamellae. Carbon 
diffusion is further enhanced by the small but highly dispersed high count 
graphite nodules. 
The casting is then quenched in oil held at 125.degree. F. for about 15 to 
20 minutes. This results in a structure of quenched martensite. Martensite 
is a very hard needle like structure with a hardness approaching 600 BHN. 
The higher carbon content austenite is transformed to a higher carbon 
content martensite during the quench. The higher carbon content matrix 
with more carbide will result in increased wear resistance due to a higher 
micro-hardness. In place of an oil quench, a molten salt quench may be 
used such as potassium nitrate/sodium nitrite 
The iron is then tempered or drawn by reheating to a temperature below the 
critical range to secure final properties; typical temperatures are about 
1100 to 1300.degree. F. This tempering step relieves internal stresses, 
and depending on tempering temperature, spheroidizes the martensite 
needles. The resultant product is tempered martensite with typical BHN 
hardness of about 187 to 355. This hardness is advantageous for articles 
which must be machined since machinability is maximized in the 187 to 285 
BHN range. Lower tempering temperatures reduces spheriodization of the 
martensite and can result in an extremely hard iron of 550 BHN. This is 
advantageous for high strength severe wear applications. 
EXAMPLE 
A charge of 60% returns, 40% iron is liquified in a melting furnace at 
2700.degree. F. The steel contains: 
2.40 to 2.60% carbon 
1.35 to 1.55% silicon 
0.025-0.05% sulfur 
0.45 to 0.65% manganese (&gt;0.3 excess or free manganese) 
0.0015 boron 
0.015 titanium 
0.015 aluminum 
The metal is poured into a sand mold having the impression of a casting and 
cooled to room temperature. The mold goes through a shake out process that 
separates the sand from the metal and removes the casting from the mold 
and sprues. The casting has a length of about 3 inches and a thickness of 
about 3/4 inches. After it has been separated, the casting is prenucleated 
in a heat treat furnace at about 800.degree. F. for about 4 hours, then 
heated to about 1720.degree. F. for about 5 hours. Next the casting is 
quickly air quenched with forced air for about 10 minutes until it reaches 
about 700 to 1000.degree. F. Following the first quench, the casting is 
reheated at about 1650.degree. F. and cooled slightly to about 
1575.degree. and held at that temperature. The casting is cooled by an oil 
quench having a temperature of about 125.degree. F. oil for about 15 to 20 
minutes and tempered at 1200.degree. F. for 11/2 h and cooled to room 
temperature. The resulting malleable iron has a microstructure of tempered 
martinsitie having about 300 nodules/mm.sup.2 and a Brinell hardness of 
about 300 BHN. 
Having described the invention in detail and by reference to preferred 
embodiments thereof, it will be apparent that modifications and variations 
are possible without departing from the scope of the invention defined in 
the appended claims.