FONTE A STRUCTURE SPHERO-IDALE MIXTE DE GRAPHITE A L'AUSTENITE ET AU BAINITE

CAST IRON WITH SPHEROIDAL GRAPHITE WITH AUSTENITIC- BAINITIC MIXED STRUCTURE

Abrégé anglais

ABSTRACT OF THE DISCLOSURE:
A process for the production of spheroidal
graphite cast iron with an austenitic-bainitic structure.
In this process, a cast iron containing up to 0.3 % of
manganese and from 0.2 to 0.8 % of molybdenum is subjected
to an austenitic heat treatment at a temperature of 800 to
860°C for a period of 10 to 60 minutes. The austenized
cast-iron is then fast cooled in less than 2 minutes to a
temperature of 350 to 400°C and held at this temperature
for a period of 5 to 60 minutes.

Revendications

The embodiments of the invention in which an
exclusive property or privilege is claimed are defined as
follows:
1. A process for producing a spheroidal graphite
cast iron having a mixed austenitic-bainitic structure,
comprising the steps of
subjecting a cast iron containing manganese and
molybdenum to an austenitic heat treatment,
cooling the austenitized cast iron to a bainitizing
temperature of 350 to 400°C, and
holding the cast iron at this temperature for a
period corresponding to the desired proportion of bainite,
characterized in that:
the cast iron used as starting material contains
up to 0.3 % by weight of manganese and 0.2 to 0.8 % by
weight of molybdenum;
the austenitic heat treatment is carried out at a
temperature of 800 to 860°C for a period of 10 to 60 minutes;
the cooling step is carried out in a fast manner
over a period of less than 2 minutes; and
the cast iron is held at the bainitizing tempera-
ture for a period of 5 to 60 minutes.
2. A process as claimed in claim 1, characterized
in that the austenitic heat treatment is carried out at a
temperature of 820 to 830°C.
3. A process as claimed in claim 1 or 2, charac-
terized in that the austenitic heat treatment is carried out
for a period of 10 to 25 minutes.
4. A process as claimed in claim 1, characterized
in that the austenited cast iron is cooled to a temperature
of about 375°C.
5. A process as claimed in any one of claim 1,
characterized in that the cast iron is held at the bainitiz-
ing temperature for a period of 20 to 25 minutes.

6. A process as claimed in claim 1, 2 or 5,
characterized in that the cast iron used as starting ma-
terial contains from 0.01 % to 0.25 % by weight of manganese.
7. A process as claimed in claim 1, characterized
in that the cast iron used as starting material also contains
up to 3 % by weight of nickel.
8. A process as claimed in claim 1, characterized
in that the cast iron used as starting material also contains
from 0.1 to 1.5 % by weight of copper.
9. A process as claimed in claim 8, wherein the
content of copper is ranging from 0.4 to 0.6 % by weight.
10. A process as claimed in claim 1, wherein the
content of molybdenum in the cast iron is ranging from 0.25
to 0.4 % by weight.
11. A process as claimed in claim 1, character-
ized in that the cast iron used as starting material also
contains from 2 to 3 % by weight of silicon.
12. A process as claimed in claim 11, wherein the
content of silicon is ranging from 2.2 to 2.6 % by weight.
13. A process as claimed in claim 1, character-
ized in that the cast iron used as starting material also
contains from 2.5 to 3.7 % by weight of carbon.
14. A process as claimed in claim 13, wherein
the content of carbon is ranging from 3.0 to 3.2 % by weight.

Description

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The present invention is concerned with a process
for the production of spheroidal graphite cast iron having a
mixed austenitic-bainitic structure.
A spheroidal cast iron with the associated basic
structure and mechanical properties - besides the usual
spheres of application - is suitable as a production ma-
terial for areas where until now heat treated steels had
been used exclusively - for example crankshafts for motor-
vehicle engines, segmented shafts and pistons for power
assisted steering systems, fifth wheel kingpins for commer-
cial vehicle couplings (semi-trailed), driveshaft heads for
motor-vehicle driveshafts, and coupling bodies for trailer
couplings.
Among spheroidal graphite cast irons, types with
a bainitic structure have been known and in use for a long
time, as being materials with a high tensile strength an~
high resistance to abrasion. During manufacture these
castings are generally heated to an austenisation tempera-
ture of 920 to 950C and maintained at this temperature for
between 2 and 5 hours, until the carbon content of the
matrix has become more or less evenly distributed and any
ledeburite, which might be present after casting, has been
dlssolved.
After the austenisation, the cast irons are cooled
down so fast that a premature austenite transformation into
pearlite iron is prevented prior to reaching the isothermal
bainite transformation temperature. The cast iron is held
at the bainitizing temperature until the baini-te reaction
has stopped. The casting is then cooled - as usual - to
room temperature (Giesserei (Foundry) 65 (brochure) (1978)
No. 4, pages 73 to 80).
There is also spheroidal graphite cast iron with
a bainite structure, which is based on alloyed cast iron,
where directly in connection with the cooling process -
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a bainite reaction is produced because of the con-tent of
nickel, molybdenum, copper and possibly also chromium and
manganese, depending on the wall thickness of the casting
(German published application D.O.S. 18 08 515).
A slightly alloyed spheroidal graphite cast iron
with conventional quantities of carbon, silicon, phosphorus,
sulphur and magnesium, a molybdenum content of 0,10 to
0,26 % and a manganese content of 0,3 to 1,~ % is also
known from German published application D.O.S. 23 34 992.
The casting which consis-ts of the above is hea-t
treated to an austenisation temperature of 900C and after
being held at this temperature for a period of two hours,
is cooled in a bath at 300C for a period of ]0 minutes to
4 hours. As a result of these treatments it is possible
to obtain an austenite-bainite structure, which gives a
tensile strength of 1100 N/mm , an elongation at rupture at
10 ~ and a hardness of 270 to 300 HB.
According to the present invention, there is
provided a process for producing a spheroidal graphite cast
iron having an austenitic-bainitic structure and a manganese
content of up to 0.3 ~, whi.ch does not interfer with favor-
able properties.
The process according to the invention which is
of the type comprising the steps of:
subjecting a cast iron containing manganese and
molybdenum to an austenitic heat treatment,
cooling the austenitized cast iron to a bainitizing
temperature of 350 to 400C, preferably 375C, and
holding the cast iron at this temperature for a
period corresponding to the desired proportion of bainite,
is characterized in that:
the cast iron used as starting material contains
up to 0.3 % by weight of manganese and 0.2 to 0.8 ~ by
weight of molybdenum;
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the austenitic heat -treatment is carried out a~
a temperature of 800 to 860C,preferably 820 to 830C,for
a period of 10 to 60 minutes, preferably 10 to 25 minutes;
the cooling step is carried out in a fast manner
over a period of less than 2 minutes; and
the cast iron is held at the bainitizing tempera-
ture for a period of 5 to 60 minutes, preferably 20 to 25
minutes.
The manganese content in the cast iron used as
starting material may preferably lie in the range 0,01 to
0,25 % by weight. At least one of the following elements
may be contained in the cast iron in the proportions
specified:
nickel up to 3 ~ by weight
molybdenum 0.2 to 0.8 ~ by weight
copper 0.1 to 1 5 % by weight.
Molybdenum, if present, is preferably present in
the proportion 0.25 to 0.4 % by weight. Copper, if present,
is preferably present in the proportion 0.4 to 0.6 ~ by
welght.
The cast iron used as starting material may also
contain carbon and/or silicon. The carbon content may
amount to 2,5 to 3,7 % - preferably 3,0 to 3,2 % by weight -
and the silicon content 2,0 to 3,0 % - for preference 2,2
to 2,6 % by weight.
One of the advantages achieved with the process
according to the invention is that - in comparison to the
prior art processes - heat treatment is much less costly
without impairing the good material properties shown by an
austenite-balnite structure cast iron with spheroidal
graphite.
The accompanying photographic illustration is of
a metallographic specimen of a type of spheroidal graphite
cast iron having a bainite austeni-te structure and enclosed
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spherulites, as obtained by carrying out the process
according to the invention. In this illustration, the
dark needles are bainite and the light structural components
are austenite.
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