[0001] The present invention relates to a method for manufacturing a novel material of the
spheroidal graphite iron kind possessing good mechanical strength and working properties.
[0002] Obviously, it is highly beneficial to be able to cast products which exhibit good
properties, since cast products require far less machining work during manufacture
than do forged products or products produced by machine cutting operations. Consequently,
there has been developed a cast iron in which the carbon present is precipitated out
as graphite nodules upon solidification, through the action of nodulizing additives,
such as magnesium. This material is known as spheroidal graphite iron or nodular iron
and has far better strength properties than products made of grey iron with graphite
precipitated in flake form. Cast iron can also be produced with the carbon bound as
cementite, which provides a hard, brittle and extremely difficultly worked product.
[0003] Spheroidal graphite iron thus possesses good qualities, which can be further improved
by hardening. Consequently, attention has been directed towards the possibility of
hardening spheroidal graphite iron in a manner to obtain a phase having a bainite
structure. Bainite comprises a mixture of ferrite and cementite. Austempering to a
bainite structure is effected by subjecting a casting having a ferritic and perlitic
basic structure to an austenitic process (austenitization). The material is then cooled
rapidly to a point above the martensite point (M S), whereafter the austenite is converted
isothermally to bainite. There is normally obtained an austenite residue of from 10-50%,
which is considered to have a favourable effect on the mechanical properties. Spheroidal
graphite iron which has been austempered is used, for example, in the gears of vehicle
engines, and experiments are being made with the use of austempered spheroidal graphite
iron in the crank shafts of petrol driven engines, where it has previously been necessary
to forge the crank shafts. A bainite structure can also be obtained, of course, by
regulating the cooling process during a casting operation in a manner which enables
a bainite structure to be obtained.
[0004] It is normally difficult to machine or otherwise work solidified spheroidal graphite
iron, which incorporates a bainite structure. Such machining is necessary when manufacturing
components with narrow measurement tolerances, such as gears and rotating engine components,
since hardening always results in changes in dimensions. Obviously, there is a need
for a spheroidal graphite iron which can be more readily worked subsequent to being
austempered with the remaining desirable properties unchanged, and which can be used
as a substitute material for forged engine components for example.
[0005] Thus, austempering is effected by austenitization, to obtain smoother carbon distribution
and an adapted carbon content in the basic material, i.e. material between and at
a given distance from the precipitated nodular graphite. The cast austenitized component
is then rapidly cooled to a temperature above the martensite conversion M . The component
is then maintained at this temperature for a period of time sufficient to permit total
and partial isothermic conversion of the basic mass to bainite in a suitable medium.
The formation of bainite can be controlled by suitable determination of the carbon
content and the soaking temperature. Subsequent to completing isothermic bainite formation
process, the spheroidal graphite iron casting is allowed to cool in air, wherewith
a part of the residual austenite will convert to martensite. The basic material then
consists of bainite, residual austenite, and with short soaking times martensite.
Austenitization is preferably effected in a salt bath or furnaces, and the isothermic
conversion normally in a salt bath, and at lower temperatures in an oil bath.
[0006] The conversion to bainite is effected in two stages, in which there is first precipitated
ferrite as Widmanstatten ferrite at not too low temperatures, whereafter cementite
and ferrite is precipitated therebetween. The formation of ferrite causes the carbon
content of the residual austenite to increase. Silicon has a delaying effect on the
bainite conversion, which first results in a constant residual austenite content,
which is then converted to bainite at longer isothermic conversion times. The time
taken to obtain a bainite structure can be reduced by lowering the silicon content,
although a certain amount of silicon is required to obtain solidification to grey
iron.
[0007] According to GB-A-2 133 805 a ferrite-bainite cast iron with spheroidal graphite
is produced by austenizing at a temperature such that the material comprises a mixture
of ferrite and austenite after the austenitization process. According to this patent
a spheroidal graphite iron is produced by smelting an iron alloy containing 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 resultant molten bath being allowed
to cool in the presence of a nodulizing agent. The solidified cast iron is then heated
to a temperature of 855-900°C for 1-3 hours, whereafter the bath is cooled with an
effect of at least 152°C min
-1 to 205-400°C and is held at this-temperature for 0.5-4 hours, whereafter it is cooled
to room temperature. It is evident from the patent specification that a ferrite-bainite
structure was not obtained when the silicon content was below 4% or the nickel content
below 0.7%, or when the isothermic treatment exceeded or fell short of the stipulated
time interval.
[0008] It has now been found possible, in accordance with the invention, to obtain an austempered
spheroidal graphite iron having a microstructure of bainite, ferrite, and residual
austenite although with negligible martensite content, and possessing extremely good
working properties and extremely good strength properties, which enable austempered
spheroidal graphite iron to be utilized economically as a material for the manufacture
of machine and engine components, such as gears and crank shafts, with respect to
both mechanical strength and economic manufacturability. In accordance with the invention
there is austenitized a cast product manufactured from an iron alloy containing 2-3.5%
silicon, 0.2% manganese, 0-5% nickel, 0-0.3% tin, 0-0.3% chromium, 0-3% molybdenum,
0-1.5% copper, 3.3-3.8§ carbon, and at most 0.08% phosphorus and 0.015% sulphur, partly
by heat treating the casting at a temperature beneath the level at which austenite
and graphite, or just austenite are in equilibrium, but within the temperature range
in which ferrite, austenite and graphite are in equilibrium, and then rapidly lowering
the temperature of the casting by at least 100 Kmin
-1, and thereafter holding the casting at a constant temperature for 0.5 to 4 hours,
this tempperature being between 235 and 4250C, wherewith part of the austenite formed
is converted to bainite, to provide a casting containing a bainite structure with
a residual austenite content of 5-40% and a ferrite content of 10-50%.
[0009] Figure 1 is an iron-carbon diagram of an alloy containing 2.7% silicon and 0.2% manganese.
As is normal practice, the sign α represents ferrite and the sign austenite. It will
be seen from the diagram that an area is formed in which ferrite, austenite and graphite
are in equilibrium within an area corresponding to the line A, in the iron-carbon
diagram. The magnitude and precise location of this three-phase area varies with the
composition of the iron, and silicon and the manganese content are therewith of particular
significance. The existence of this area is well known to those skilled in this art,
although the realization of its significance when austempering spheroidal graphite
iron, to obtain a bainite structure, is an important contribution to the art. It is
possible with knowledge of Gibbs free energies to calculate thermodynamically the
location of the three-phase area for a given composition.
[0010] A higher silicon content results in a broader three-phase area, and hence the silicon
content suitably lies within 2.7-4.5% by weight, preferably within 3.0-3.6%. The higher
silicon contents, however, render the material more difficult to machine or cut, due
to solution annealing of the ferrite present. The manganese content increases the
three-phase area and, in accordance with the invention, is preferably held between
0 and 2%. Subsequent to austenitization, the temperature of the casting is lowered
at a rate corresponding to at least 100 K min-l, and is thereafter held at a constant
temperature of between 285 and 425
0C for sufficient length of time for part of the austenite formed to convert to bainite,
therewith providing a casting which incorporates a bainite structure with a residual
austenite content of 5-40%, preferably 15-20%, and a ferrite content of 10-50%, preferably
20-40%.
1. A method for producing an austempered spheroidal graphite iron having a microstructure
comprising bainite, ferrite and austenite, characterized in that a spheroidal graphite
iron casting containing 2-3.5% Si, 0-2%Mn, 0-5% Ni, 0-0.3% Sn, 0-0.3% Cr, 0-3% Mo,
0-1.5% Cu and 3.3-3.8% C and at most 0.08% P and 0.015% S is partially austenitized
by heat treatment at a temperature beneath the temperature at which austenite and
graphite or austenite are in equilibrium, but within the temperature range in which
ferrite, austenite and graphite are in equilibrium, whereafter the temperature of
the casting is rapidly lowered by at least 100 K min-1, and is then held at a constant temperature for 0.5 to 4.0 hours, this temperature
being 235 to 4250C, wherewith part of the austenite formed is converted to bainite to provide a casting
that includes a bainite structure with a residual austenite content of 5-40% and a
ferrite content of 10-50%,
2. A method according to Claim 1, characterized in that the residual austenite content
is caused to lie between 15 and 20%.
3. A method according to Claim 1, characterized in that subsequent to austenitization
there remains a ferrite content of 20-40%.
