METHOD FOR AT LEAST PARTIAL ISOTHERMAL BAINITIZING OF WORKPIECES MADE OF FERROUS MATERIALS

METHODE BAINITISATION ISOTHERME AU MOINS PARTIELLE DE PIECES FERREUSES

English Abstract

ABSTRACT OF THE DISCLOSURE


A method is suggested for at least partial isothermal
bainitizing, the said method being particularly suitable for
thick-walled workpieces made of ferrous materials. After
austenitizing, the workpieces are cooled in an intermediate
cooling bath and are then adjusted to the bainitizing tempera-
ture, where they are left until the desired transition takes
place.

Claims

The embodiments of the invention in which an
exclusive property or privilege is claimed are defined
as follows:-


1. A method for at least partial isothermal bainitizing
of workpieces made of ferrous materials, characterized in
that, after austenitizing, the workpieces are quenched in an
intermediate cooling bath and are held, for a predetermined
length of time, supercooled, in the austenitic condition,
the transition to bainite occurring when the temperature is
raised to, and held at, the transition temperature, so that
supercooling of the austenite is assured, even in the interior
of the workpiece, by the cooling accelerated by the lower
intermediate cooling-bath temperature.


2. A method according to claim 1, characterized in that
the workpiece is held in the intermediate cooling bath until
temperature equalization is attained.


3. A method according to claim 1, characterized in that
in the case of thick-walled workpieces, having a corresponding
thermal capacity, the temperature of the intermediate cooling
bath is below the martensite starting line in the time-
temperature transformation (TTT) diagram, with no martensite
occurring during the predetermined holding period.


4. A method according to claim 1, characterized in that
the bainitizing temperature is equal to, or higher than, that
of the intermediate cooling bath.



5. A method according to claim 1 or 2, characterized in
that vaporizing cooling media, e.g. oil, low-melting metals,
are used for intermediate cooling.


6. A method according to claim 1 or 2, characterized in
that non-vaporizing cooling media, e.g. air, salts, high-



melting metals, are used for intermediate cooling.


7. A method according to claim 1, characterized in that
the bainite transition is carried out in air.


8. A method according to claim 1, characterized in that
the bainite transition is carried out in a bath, e.g. a bath
of oil, salt or metal.


9. Use of the method according to one of claims 1 to 3,
for workpieces which, with no intermediate cooling, form a
pearlitic structure.

Description

6~Z

The present invention relates to a method for at
least partial isothermal bainitizing of workpieces made of
ferrous materials, and to the use thereof for workpieces
which, because o-f their inadequate cooling velocity, would
have fully pearlitic cross-sections.
~ Iitherto-known bainite heat-treatment methods
depend upon a critical cooliny velocity arising from the
temperature-gradient between the austenitizing and cooling
bath and the diameter of the workpiece.
French Patent 76 21 520 discloses a method whereby
the workpieces are carried past the pearlite peak by rapid
cooling to a selected intermediate-stage temperature and
thereafter past the intermediate-stage rang~ quasi isother-
mally~ i.e. the transition must be complete before the
martensite starting line is reached. The continuous cooling
pattern gives a mi~ed structure of upper and lower bainite
stage.
It is the purpose of the invention to propose a
bainite heat-treatment method whereby, in the case of
thick-walled workpieces, the cooling velocity is increased
to such an extent that the pearlite transition can be at
least partly avoided. Furthermore, specific properties of
the wor~pieces in the upper and lower bainite stage can be
adjusted separately by isothermal treatment methods.
According to the present invention, this purpose
is achieved in that there is provided a method for at least
partial isothermal bainitizing of workpieces made of ferrous
materials, characterized in that, after austenitizing, the
workpieces are quenched in an intermediate cooling bath and
are held7 for a predetermined length of time, supercooled,
in the austenitic condition, the transition to bainite

occurring when the temperature is raised to, and held at,




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72

the transition temperature, so that supercooling of the
austenite is assured, even in the interior of the workpiece,
by the cooling accelerated by the low0r intermediate
cooling-bath temperature.
Additional advantayeous features will be evident
frorn the description hereinafter.
The invention will be better understood by an
examination of the following description, together with
the accompanying drawings, in which
Figs- 1, 2 and 3 illustrate cooling curves
obtained with workpieces having differing wall thicknesses
as discussed below.
If a thin-walled workpiece is immersed in a
bainitizi~g bath, the outer layer is rapidly cooled to the
temperature of the bath. Because of ~he short distance the
heat must travel to the surface, and because of the ~avour-
able ratio between the cooling surface and the total mass
of the workpiece, the core will also rapidly reach the
bainitizing temperature (Fig. 1).
Both of the cooiing curves shown in Fig. 1 remain
in the austenitizing range until the temperature of the bath
is reached. After the transition, therefore, bainite is
obtained over the entire cross-section~ In the said figure,
cooling curve 1 represents the cooling of the core, while
cooling curve 2 represents that of the outer layerO
Conditions ar~ different in the case of workpieces
in which, because of the wall-thickness ratios, cooling
passes through the pearlite pea~- In this case, because of
the larger volume of heat, the outer layer and the core cool
so slcwly that a pearlite structure is obtained instead of
bainite ~Fig. 2).

Because of the small amount of heat dissipated,

:;~2~67~

the core will cool considerably more slowly than in thin-
walled pieces, and the cooling curve therefore passes through
the pearlite range.
It is now proposed to immerse thick-walled work-
pieces into an intermediate cooling bath be~ore they are
adjusted to the hainitizing temperature, the temperature
of the said bath being below the temperature at which the
desired bainite structure forms (Fig. 3). The workpiece
is therefore, as it were, supercooled.
The temperature of the intermediate cooling bath
is adapted to the critical wall-thickness of the workpiece.
The workpiece is cooled so rapidly in this bath
that it passes through the pearlite peak and is thereafter
trans~erred for transition of the structure at the bainiti-
zing temperature.
This method makes it possible to control the
cooling velocity in such a manner that even greater wall-
thicknesses do not pass through a pearlite transition, at
least in the edge areas.
After the transition, therefore, the desired
bainitic structure, with its known ad~antageous mechanical
properties, is obtained at least partly o~er the cross-
section of the workpiece.




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