Methods and devices for obtaining a homogeneous austenite structure

A method and device for thermally treating at least one carbon steel wire such a way as to obtain a homogenous austenite structure, characterized by the fact that the wire is heated in a tube containing a gas which has practically no forced ventilation, the gas being directly in contact with the wire and the time of heating of the wire being less than 4 seconds per millimeter of diameter of the wire. Pearlitization installation using such a method and device.

BACKGROUND OF THE INVENTION 
The present invention concerns methods and devices for heat treating carbon 
steel wires to obtain a homogeneous austenite structure and, if desired, 
of subjecting these wires to a subsequent thermal treatment to obtain a 
fine pearlitic structure. 
The known methods of austenitization of travelling steel wires are in 
particular as follows: 
heating by induction, in which the wire is subjected to a magnetic field 
having a frequency of 5,000 to 200,000 Hz; this method is applied under 
good conditions only to wires of a diameter larger than 3 mm and at 
temperatures lower than the Curie point. 
heating in a muffle furnace by means of electric resistors; this method 
avoids the inconveniences of heating by induction, but it requires 
important heating times on the order of 10 to 15 seconds per millimeter of 
diameter of the wire to achieve the desired result. 
heating in a gas furnace; this method also requires important heating times 
on the same order as those of the muffle furnace since the temperature of 
the gases at the outlet of the oven must be low if it is desired to obtain 
a suitable thermal yield; on the other hand, the thermal conductivity of 
the combustion gases is not as good as that of the gases which can be used 
in a muffle furnace (hydrogen, mixture of hydrogen and nitrogen, helium); 
it is possible in gas furnaces to control the deoxidizing power of the 
combustion gases, but this requires very careful supervision of the 
adjustment of the gas burners. 
SUMMARY OF THE INVENTION 
The object of the present invention is to achieve the desired 
austenitization treatment with heating times of less than 4 seconds per 
millimeter of diameter of the wire, which makes it possible to have higher 
rates of production than with the known installations, and which also 
makes it possible to decrease the lengths of the installations. 
Accordingly, the method of the present invention for the heat treatment of 
at least one carbon steel wire, so as to obtain a homogeneous austenite 
structure is characterized by the following features: 
a) the wire is heated by passing it through at least one tube containing a 
gas which is practically without forced ventilation, the gas being 
directly in contact with the wire, the wire heating time being less than 4 
seconds per millimeter of diameter of the wire; 
b) the characteristics of the tube, the wire and the gas are so selected 
that the following relationships are satisfied: 
EQU 1.05.ltoreq.R.ltoreq.7 (1) 
EQU 0.6.ltoreq.K.ltoreq.8 (2) 
with, by definition 
EQU R=D.sub.ti /D.sub.f 
EQU K=[Log(D.sub.ti /D.sub.f)].times.D.sub.f.sup.2 /.lambda. 
D.sub.ti being the inside diameter of the tube expressed in millimeters, 
D.sub.f being the diameter of the wire expressed in millimeters, .lambda. 
being the conductivity of the gas determined at 800.degree. C., this 
conductivity being expressed in watts.m .sup.-1..degree.k.sup.-1, Log 
being the natural logarithm. 
The invention also concerns a device which makes it possible to heat treat 
at least one carbon steel wire so as to obtain a homogeneous austenite 
structure, the device being characterized by the following features: 
a) it comprises at least one tube and means making it possible to pass the 
wire through the tube; the tube contains a gas which is practically 
without forced ventilation, the gas being directly in contact with the 
wire, the device comprising means for heating the gas; the means which 
make it possible to pass the wire through the tube are such that the time 
of contact of the wire with the gas is less than 4 seconds per millimeter 
of diameter of the wire; 
b) the characteristics of the tube, the wire and gas are so selected that 
relationships (1) and (2) above are satisfied, D.sub.ti, D.sub.f, .lambda. 
and Log having the same meanings as indicated above. 
The expression "practically without forced ventilation" means that the gas 
in the tube is either stationary or subjected to low ventilation which 
does not substantially modify the heat exchanges between the wire and the 
gas, this low ventilation being, for instance, due solely to the 
displacement of the wire itself. 
The invention also concerns the methods and complete installations for the 
heat treatment of carbon steel wires employing the methods and/or devices 
previously described.

DESCRIPTION OF PREFERRED EMBODIMENTS 
FIGS. 1 and 2 show a device 100 according to the invention for the carrying 
out of the method of the invention. FIG. 1 is a section through the device 
100 along the axis xx' of the device; FIG. 2 is a section perpendicular to 
this axis xx', the section of FIG. 2 being indicated diagrammatically by 
the straight line segments II--II in FIG. 1. The device 100 has a tube 2, 
for instance of ceramic, refractory steel or tungsten carbide, through 
which the wire 1 of carbon steel passes in the direction indicated by the 
arrow F along the axis xx'. 
The means for the driving of the wire 1 are known means, not shown in FIGS. 
1 and 2 for purposes of simplification, these means comprising, for 
instance, a winder actuated by a motor in order to wind the wire up after 
treatment. 
The space 3 between the wire 1 and the inner wall 20 of the tube 2 is 
filled by a gas 4. This gas 4 is directly in contact with the wire 1 and 
the inner wall 20. The gas 4 remains in the space 3 during the treatment 
of the wire 1, the device 100 being without means capable of permitting 
forced ventilation of the gas 4, that is to say, the gas 4, which is 
without forced ventilation, is possibly placed in movement in the space 3 
only by the displacement of the wire 1 in the direction indicated by the 
arrow F. This gas is, for instance, hydrogen, a mixture of hydrogen and 
nitrogen, a mixture of hydrogen and methane, a mixture of hydrogen, 
nitrogen and methane, helium, or a mixture of helium and methane. 
The wire 1 is guided by two wire guides 5, for instance of ceramic or 
tungsten carbide, located at the entrance and exit of the wire 1 in the 
tube 2. The tube 2 is heated on the outside by an electric resistor 6 
wound around the tube 2 on the outside of this tube 2 against the outer 
wall 21 of the tube 2. The tube 2 is heat insulated from the outside by 
the sleeve 7 surrounding the tube 2 and by the two plates 8 located at the 
ends of the tube 2. The tube 2 is also electrically insulated, in the 
event that it is metallic. The plates 8 and the sleeve 7 are, for 
instance, made of fritted refractory fibers. The tube 2, the heating 
resistor 6, the sleeve 7 and the plates 8 are placed within a metal tube 
9, which is cooled by a hollow tube 10 wound around the tube 9, said 
hollow tube 10 being traversed by a cooling fluid 11, for instance water. 
The device 100 is closed at its two ends by circular plates 12 which rest 
against the flanges 90 of the tube 9 through gas-tight joints 13. The 
electric supply to the resistor 6 is through a gas-tight passage 14 
through which pass two electric wires 15, each connected to one end of the 
resistor 6 (this connection has not been shown in the drawing for purposes 
of simplification). This gas-tight passage 14 is formed in a plug having 
gas-tight joints 16 and inserted in one of the two circular plates 12. 
The device 100 has an expansion play 17, the springs 18, which act on the 
plate 19, serving for the distribution of the forces, which makes it 
possible to maintain the tube 2 in the middle of the sleeve 7, whatever 
its temperature. 
In FIG. 2, D.sub.f represents the diameter of the wire 1, D.sub.ti 
represents the inside diameter of the tube 2 (diameter of the inner wall 
20), D.sub.te represents the outside diameter of the tube 2 (diameter of 
the outer wall 21). .lambda. is the conductivity of the gas 4 determined 
at 800.degree. C., this conductivity being expressed in 
watts.m.sup.-1..degree.K.sup.-1. 
In accordance with the invention, D.sub.ti, D.sub.f, and .lambda. are 
selected so as to satisfy the following relationships: 
EQU 1.05.ltoreq.R.ltoreq.7 (1) 
EQU 0.6.ltoreq.K.ltoreq.8 (2) 
with, by definition 
EQU R=D.sub.ti /D.sub.f 
EQU K=[Log(D.sub.ti /D.sub.f)].times.D.sub.f.sup.2 /.lambda. 
D.sub.ti and D.sub.f being expressed in millimeters and Log being the 
natural logarithm. 
The invention thus unexpectedly makes it possible to heat the wire 1 from a 
temperature below the AC3 transformation temperature, for instance from 
ambient temperature up to a temperature above the AC3 transformation 
temperature so as to obtain a homogenous austenite structure, and this for 
a very short period of time of less than 4 seconds per millimeter of 
diameter of the wire D.sub.f. Furthermore, if desired, the nature of the 
gas 4 can be so selected that it exerts a chemical action on the surface 
of the wire, for instance a deoxidizing, carburizing, or decarburizing 
action. 
The invention therefore has the following advantages: 
simplicity, low investment and operating expenses since no compressors or 
turbines are used as would be necessary with a forced gas circulation; 
a precise heating law can be obtained; 
the heating is rapid, which makes it possible to increase the rates of 
manufacture and to decrease the length of the installation; 
the rapid heating can be applied to wires, the diameter D.sub.f of which 
varies within wide limits, the same device making it possible, in 
particular, to treat wires having diameters D.sub.f which vary in a ratio 
of 1 to 5. 
For wires of large diameter D.sub.f, more than 4 mm, the ratio R is close 
to 1 and the use of a gas which is a very good conductor of heat, for 
instance hydrogen, then becomes necessary. 
The diameter D.sub.f of the wire is preferably at least equal to 0.4 mm and 
at most equal to 6 mm. 
FIGS. 3 and 4 show another device 200 in accordance with the invention, 
this device making it possible to treat several wires 1, for instance 6, 
simultaneously, FIG. 3 being a section through this device along the axis 
yy' of this device and FIG. 4 being a section perpendicular to the axis of 
this device, the axis yy' being represented by the reference letter "y" in 
FIG. 4. 
The structure of this device 200 is similar to that of the device 100, with 
the difference that six tubes 2 are arranged in the enclosure 9 formed by 
a steel tube around the axis yy', which is the axis of this tube 9. A wire 
1 passes through each tube 2, the gas 4 being arranged within the tubes 2 
each of which is heated by a resistor 6, as previously described in the 
case of the device 100, the insulating sleeve 7 being arranged around the 
six tubes 2. 
The following examples will make it possible better to understand the 
invention. 
EXAMPLES 1 to 4 
Four examples of the treatment of a carbon steel wire 1 with the device 100 
previously described will be given. The characteristics of the wire 1 and 
of the device 100 are given in the following Table 1. 
TABLE 1 
______________________________________ 
Example No. 
1 2 3 4 
______________________________________ 
Properties of Wire 1 
Carbon content of the 
0.70 0.85 0.75 0.80 
steel (% by weight) 
D.sub.f (mm) 0.53 1.75 1.75 5.50 
Properties of the Device 100 
Nature of the tube 2 
alumina alumina alumina 
refrac- 
tory steel 
D.sub.ti (mm) 
1.5 2.5 3 6 
D.sub.te (mm) 
5 6 6 12 
Power of the resistor 
3.6 27 20 110 
6 (kw) 
Temperature of the 
1100 1100 1100 1100 
outer face 21 of the 
tube 2 (.degree.C.): 
Speed of travel of the 
2.9 2.02 1.52 0.81 
wire 1 (meters per 
second) 
Length of the tube 
2 6 6 5 
2 (meters) 
Heating time T.sub.c 
0.69 2.97 3.96 6.15 
(seconds) 
Production of the 
17.9 136 102 540 
device (kilograms of 
wire 1/hour 
Temperature of the 
20 20 20 20 
wire 1 at the entrance 
to the tube 2 (C..degree.) 
Temperature of the 
980 980 980 980 
wire 1 at the outlet of 
the tube 2 (.degree.C.) 
.lambda. (watts .multidot. m.sup.-1 .multidot. 
0.328 0.328 0.328 0.345 
.degree.K..sup.-1) 
R 2.83 1.43 1.71 1.09 
K 0.89 3.33 5.03 7.63 
Heating time per milli- 
1.30 1.70 2.26 1.12 
meter of diameter of 
wire 1 (seconds/mm) 
(T.sub.c /D.sub.f) 
______________________________________ 
The nature of the gas 4 was the following for the examples: 
Examples 1, 2, 3: cracked ammonia (75% hydrogen, 25% nitrogen, these 
percentages being expressed by volume) 
Example 4: 78% hydrogen, 2% methane (percent by volume) 
The heating time T.sub.c corresponds to the time necessary for the wire to 
pass from the ambient temperature (about 20.degree. C.) which it had at 
the entrance of the tube to the temperature which it has at the outlet of 
the tube (980.degree. C.), this temperature being sufficient to place the 
carbides in solution. 
EXAMPLE 5 
In this example, the diameter D.sub.f of the wire 1 is varied, as is the 
nature of the gas 4, which is a mixture of hydrogen and nitrogen, and 
therefore the values of .lambda., R and K are changed. The properties of 
the wire 1 and of the device 100 are as follows: carbon content of the 
steel of the wire 1 =0.85%; tube 2 of alumina, D.sub.ti =2.5 mm, D.sub.te= 
6mm; the outer face 21 of the tube 2 is heated to 1100.degree. C. with an 
electric resistor 6 having a power of 33 kw; speed of travel of the wire 
1: 2.35 meters per second; length of the tube 2: 6 meters; heating time: 
2.55 seconds; temperature of the wire 1 at the entrance to the tube 2: 
20.degree. C., at the outlet from the tube 2: 980.degree. C. 
The following Table 2 gives the values of D.sub.f, the volumetric percent 
of the gas 4 of hydrogen, the values of .lambda., R and K, as well as the 
production of wire 1. For all the tests corresponding to this example, the 
heating time per millimeter of diameter of wire (T.sub.c /D.sub.f) varies 
from 1.46 to 3.1 sec/mm. 
TABLE 2 
______________________________________ 
Diameter Produc- 
of the tion of 
wire 1 .lambda. at 800.degree. C. 
wire 1 
(mm) (D.sub.f) 
R % H.sub.2 
(w.m.sup.-1 .multidot. .degree.K..sup.-1) 
K in kg/hr 
______________________________________ 
1.75 1.43 100 0.487 2.24 158.0 
1.55 1.61 98 0.472 2.43 124.0 
1.30 1.92 90 0.418 2.64 87.0 
0.94 2.66 69 0.297 2.91 45.8 
0.82 3.05 62 0.263 2.85 35.0 
______________________________________ 
EXAMPLE 6 
A multi-tube device similar to the device 200 previously described is used, 
but this time having ten tubes 2. The properties of the example are as 
follows: 
Carbon content of the steel of the wire 1: 0.70%; diameter D.sub.f of the 
wire: 1.75 mm; identical tubes 2 of alumina, D.sub.ti =2.5 mm, D.sub.te =6 
mm; the outer faces 21 of the tubes are heated to 1100.degree. C. by means 
of 10 resistors 6 (one resistor per tube 2), each resistor having a unit 
power of 27kw (total power 270 kw); gas 4: cracked ammonia; speed of 
travel of the wire: 2.02 meters per second; length of each tube 2: 6 
meters; heating time 2.97 seconds; production of wire 1: 1360 kg/hour; 
temperature of the wire at the entrance to each tube 2: 20.degree. C., at 
the outlet from each tube 2: 980.degree. C.; .lambda.=0.328; R=1.43; 
K=3.33. The heating time per millimeter of diameter of the wire (T.sub.c 
/D.sub.f) is equal to l.70 sec/mm. 
EXAMPLE 7 
This example is carried out under the same conditions and with the same 
results as Example 2, but replacing the cracked ammonia, by a gas 4 which 
maintains the thermodynamic equilibrium with the carbon of the steel at 
800.degree. C., this gas 4 having the following composition (% by volume): 
74% hydrogen, 24% nitrogen; 2% methane. 
EXAMPLE 8 
This example is carried out under the same conditions as Example 2, but the 
cracked ammonia is replaced by a carburizing gas which makes it possible 
to correct a decarburization which took place in the preceding operations. 
The composition of the gas 4 is as follows in this example (% by volume): 
85% hydrogen, 15% methane. The other conditions and results are the same 
as in Example 2, with the following differences: The heating time changes 
from 2.97 to 2.75 seconds, the ratio T.sub.c /D.sub.f being then equal to 
1.57 sec/mm, the speed of travel of the wire is 2.18 m/sec, and a surface 
recarburization thickness on the order of 2.mu.m is obtained. No deposit 
of graphite is observed on the wire 1. 
The invention makes it possible to obtain a very precise temperature of the 
wire at the outlet of the treatment, this temperature not varying by more 
than 1.5.degree. C. plus or minus from the temperature indicated at the 
outlet of the tubes 2 in the case of Examples 1 to 8, which makes it 
possible to guarantee good constancy of the quality of the wire. 
Examples 9 to 12 which follow are carried out in a device similar to the 
device 100 previously described, but these examples are not in accord with 
the invention. The characteristics of the wire 1 and of this device are 
given in the following Table 3. These examples are characterized by a 
T.sub.c /D.sub.f ratio which is substantially greater than 4 seconds per 
millimeter of diameter of wire, the values of the ratios R and K not 
corresponding to the whole of the relationships (1) and (2) previously 
indicated, and the austenitization cannot then be carried out with the 
advantages previously described. 
TABLE 3 
______________________________________ 
Example No. 
9 10 11 12 
______________________________________ 
Properties of wire 1 
Carbon content of the 
0.70 0.85 0.75 0.80 
steel (% by weight) 
D.sub.f (mm) 0.53 1.75 1.75 5.50 
Properties of the device 
Nature of the tube 2 
alumina alumina alumina 
refrac- 
tory steel 
D.sub.ti (mm) 
5 5 3 7 
D.sub.te (mm) 
10 10 6 14 
Power of the resistor 
0.5 6 9 25 
6 (kw) 
Temperature of the 
1100 1100 1100 1100 
outer face 21 of the 
tube 2 (.degree.C.): 
Speed of travel of the 
0.24 0.46 0.65 0.187 
wire 1 (meters per 
second) 
Length of the tube 
2 6 6 5 
2 (meters) 
Heating time T.sub.c 
8.3 13 9.2 26.7 
(seconds) 
Production of the 
1.5 31.3 44.3 12.6 
device (kilograms of 
wire 1/hour) 
Temperature of the 
20 20 20 20 
wire 1 at the entrance 
to the tube 2 (.degree.C.) 
Temperature of the 
980 980 980 980 
wire 1 at the outlet of 
the tube 2 (.degree.C.) 
.lambda. (watts .multidot. m.sup.-1 .multidot. 
0.059 0.220 0.160 0.220 
.degree.K..sup.-1) 
R 9.43 2.86 1.71 1.27 
K 10.68 14.60 10.31 33.16 
Heating time per milli- 
15.7 7.43 5.26 4.85 
meter of diameter of 
wire 1 (second/mm) 
(T.sub.c /D.sub.f) 
______________________________________ 
The nature of the gas 4 was as follows in Examples 9 to 12: 
Example 9: pure N.sub.2 
Example 10: N.sub.2 =50% H.sub.2 =50% 
Example 11: N.sub.2 =65% H.sub.2 =35% 
Example 12: N.sub.2 =50% H.sub.2 l32 50% (% by volume) 
In all the examples according to the invention, a homogeneous austenite 
structure is obtained. 
FIG. 5 shows a complete installation for the heat treatment of a carbon 
steel wire 1 in order to obtain a fine pearlitic structure. This 
installation 300 comprises the zones Z.sub.1, Z.sub.2, Z.sub.3, Z.sub.4, 
Z.sub.5, the wire 1 passing through these zones in the direction indicated 
by the arrow F from the starting bobbin 30 to the bobbin 31 on which the 
treated wire 1 is wound. The bobbin 31 is driven in rotation by the motor 
310 which therefore imparts travel to the wire 1 in the direction 
indicated by the arrow F. The wire 1 passes in succession through the 
zones Z.sub.1 to Z.sub.5 in that order. 
The zone Z.sub.1 corresponds to the heating of the wire 1 in order to 
obtain a homogeneous austenite structure; 
the zone Z.sub.2 corresponds to the cooling of the wire 1 to a temperature 
of 500.degree. C. to 600.degree. C. so as to obtain a metastable 
austenite; 
Zone Z.sub.3 corresponds to the transformation of metastable austenite into 
pearlite; 
Zone Z.sub.4 corresponds to a cooling of the wire 1 after pearlitization to 
a temperature, for instance, of about 300.degree. C.; 
Zone Z.sub.5 corresponds to a final cooling of the wire 1 in order to bring 
it to a temperature close to ambient temperature, for instance, 20.degree. 
C. to 50.degree. C. 
FIG. 6 shows the curve .phi. which indicates the change in temperature of 
the steel wire 1 as a function of time when this wire passes through zones 
Z.sub.2 to Z.sub.5. This figure also shows the curve x.sub.1 corresponding 
to the start of the transformation of metastable austenite into pearlite 
and the curve x.sub.2 corresponding to the end of the transformation of 
metastable austenite into pearlite for the steel of this wire. In this 
FIG. 6, the abscissa axis corresponds to the time T and the ordinate axis 
corresponds to the temperature .theta., the time origin corresponding to 
the point A. 
Prior to the pearlitization treatment, the wire 1 is heated and maintained 
at a temperature above the AC3 transformation temperature so as to obtain 
a homogeneous austenite, this temperature .theta..sub.A, which, for 
instance, is between 900.degree. C. and 1000.degree. C., corresponds to 
the point A of FIG. 6. The point known as "pearlite nose" corresponds to 
the minimum time T.sub.m of the curve x.sub.1, the temperature of this 
pearlite nose being indicated as .theta.p. 
The wire 1 is then cooled until it reaches a temperature below the AC1 
transformation temperature, the state of the wire after this cooling 
corresponding to the point B and the temperature obtained at this point B 
at the end of the time T.sub.B being marked .theta..sub.B. This 
temperature .theta..sub.B has been represented in FIG. 6 as higher than 
the temperature .theta..sub.P of the pearlite nose, as is most frequent in 
practice, without being absolutely necessary. During this cooling of the 
wire between the points A and B there is a transformation of stable 
austenite into metastable austenite as soon as the temperature of the wire 
drops below the AC3 transformation point, and "seeds" appear at the grain 
joints of the metastable austenite. The zone between the curves x.sub.1, 
x.sub.2 is marked .omega.. The pearlitization consists in causing the wire 
to pass from the state represented by the point B at the left of the zone 
.omega. to a state represented by the point C at the right of the zone 
.omega.. This transformation of the wire is diagrammatically indicated, 
for instance, by the straight line segment BC which intersects the curve 
x.sub.1 at B.sub.x and the curve x.sub.2 at C.sub.x, but the invention 
also applies to cases in which the variation in the temperature of the 
wire between the points B and C is not linear. 
The formation of the seeds continues in the part of the segment BC located 
to the left of the zone .omega., that is to say, in the segment BB.sub.x. 
In the part of the segment BC within the zone .omega., the segment B.sub.x 
C.sub.x, there is a transformation of metastable austenite into pearlite, 
that is to say, pearlitization. The pearlitization time is susceptible to 
variation from one steel to another, therefore the treatment represented 
by the segment C.sub.x C has the purpose of avoiding the application of 
premature cooling to the wire in the event that the pearlitization should 
completed. In fact, residual metastable austenite which would be subjected 
to rapid cooling would be transformed into bainite, which is not a 
structure favorable for drawing after heat treatment or for the value in 
use and the mechanical properties of the final product. 
A rapid cooling between the points A and B followed by isothermal holding 
in the metastable austenite domain, that is to say, between the points B 
and B.sub.x, permits an increase in the number of seeds and a decrease in 
their size. These seeds are the starting points of the further 
transformation of the metastable austenite into pearlite and it is well 
known that the fineness of the pearlite and therefore the utilitarian 
value of the wire will be greater the more numerous and smaller these 
seeds are. 
After the pearlitization treatment, the wire is cooled, for instance, to 
ambient temperature; this cooling, which is preferably rapid, is 
diagrammatically indicated for example by the curved line CD, the 
temperature D being marked .theta..sub.D. 
In the installation 300, the zone Z.sub.1 corresponds to the heating of the 
wire 1 in order to bring it to the condition corresponding to point A, the 
zone Z.sub.2 corresponds to the cooling represented by the portion AB of 
the curve .phi., the zone Z.sub.3 corresponds to the portion BC of the 
curve .theta., the zones Z.sub.4 and Z.sub.5 together corresponding to the 
cooling represented by the portion CD of the curve .phi.. 
The zone Z.sub.1 is produced, for example, with the device 100 according to 
the invention, which has been previously described. 
The zone Z.sub.2 is produced, for instance, in accordance with French 
Patent Application No. 88/00904. The device 32 corresponding to this zone 
Z.sub.2 is shown in FIGS. 7 and 8. 
This device 32 is a heat exchanger having an enclosure 33 in the form of a 
tube of inside diameter D'.sub.ti and an outside diameter D'.sub.te in 
which the wire 1 to be treated, of diameter D.sub.f, passes in the 
direction indicated by the arrow F. 
FIG. 7 is a section taken along the axis xx' of the wire 1, which is also 
the axis of the device 32, and FIG. 8 is a section taken perpendicular to 
said axis xx', the section of FIG. 8 being diagrammatically indicated by 
the straight line segments VIII--VIII in FIG. 7, the axis xx' being 
diagrammatically indicated by the letter "x" in FIG. 8. The space 34 
between the wire 1 and the tube 33 is filled with a gas 35 which is in 
direct contact with the wire 1 and with the inner wall 330 of the tube 33. 
The gas 35 remains in the space 34 during the treatment of the wire 1, the 
device 32 being without means capable of permitting forced ventilation of 
the gas 35, that is to say, the gas 35, which is substantially without 
forced ventilation, is possibly placed in movement within the space 34 
only by the displacement of the wire 1 in the direction indicated by the 
arrow F. Upon the heat treatment of the wire 1, a transfer of heat takes 
place from the wire 1 toward the gas 35. .lambda. ' is the conductivity of 
the gas 35, determined at 600.degree. C. This conductivity is expressed in 
watts.m.sup.-1..degree.K.sup.-1. The wire 1 is guided by two wire guides 
36 made, for instance, of ceramics or tungsten carbide, these guides 36 
being located one at the entrance and the other at the exit of the wire 1 
in the tube 33. The tube 33 is cooled on the outside by a heat transport 
fluid 37, for instance water, flowing in an annular sleeve 38 which 
surrounds the tube 33. This sleeve 38 has a length L'.sub.m, an inside 
diameter D'.sub.mi and an outside diameter D'.sub.me. The sleeve 38 is fed 
with water 37 through the connection 39; the water 37 emerges from the 
sleeve 38 via the connection 40, the flow of the water 37 along the tube 
33 thus taking place in the direction opposite the direction F. The seal 
between the zone 41 containing the water 37 (inside volume of the sleeve 
38) and the space 34 containing the gas 35 is obtained by means of joints 
42 made, for instance, of elastomers. The length of the tube 33 in contact 
with the fluid 37 is marked L'.sub.t in FIG. 7. 
The exchanger 32 can by itself constitute a device for the zone Z.sub.2. 
One can also assemble several exchangers 32 along the axis xx' by means of 
the flanges 43 constituting the ends of the sleeve 38, the wire 1 then 
passing through several exchangers 32 arranged in series along the axis 
xx'. 
The characteristics of the tube 33, the wire 1 and the gas 35 are so 
selected that the following relationships are satisfied upon the cooling 
preceding the pearlitization, which is indicated diagrammatically by the 
part AB of the curve .phi.: 
EQU 1.05.ltoreq.R'.ltoreq.15 (3) 
EQU 5.ltoreq.K'.ltoreq.10 (4) 
with, by definition: 
EQU R'=D' .sub.ti /D.sub.f 
EQU K'=[Log (D'.sub.ti /D.sub.f)].times.D.sub.f.sup.2 /.lambda.' 
D'.sub.ti and D.sub.f being expressed in millimeters, .lambda.' being the 
conductivity of the gas determined at 600.degree. C. and expressed in 
watts.m.sup.-1 ..degree.K.sup.-1, Log being the natural logarithm. 
The gas 35 is, for example, hydrogen, nitrogen, helium, a mixture of 
hydrogen and nitrogen, of hydrogen and methane, of nitrogen and methane, 
of helium and methane, and of hydrogen, nitrogen and methane. 
In the case of wires 1 of large diameter, the ratio R' between the inside 
diameter D'.sub.ti and the diameter D.sub.f of the wire must be close to 
1, and the use of a very conductive gas 35, for instance hydrogen, becomes 
necessary. 
The zone Z.sub.3 of the installation 300 is developed, for instance, by the 
use of several exchangers 32 arranged in series under the conditions 
described below. 
In order to obtain a transformation from austenite into pearlite under the 
best conditions, it is preferable that the transformation steps of the 
wire 1, indicated diagrammatically by the line BC in FIG. 1, take place at 
a temperature which varies as little as possible, the temperature of the 
wire 1, for instance, not differing by more than 10.degree. C. plus or 
minus from the temperature .theta..sub.B obtained after the cooling 
indicated diagrammatically by the line AB. This limitation on the 
variation of the temperature is therefore effected for a period of time 
greater than the pearlitization time, this pearlitization time 
corresponding to the segment BxCx. The temperature of the wire 1 
advantageously does not differ by more than 5.degree. C. plus or minus 
from the temperature .theta..sub.B on this line BC. FIG. 6 shows, for 
instance, the ideal case in which the temperature is constant and equal to 
.theta..sub.B during diagrammatically indicated by the line BC which is 
therefore a straight line segment parallel to the abscissa axis. 
The transformation of austenite into pearlite which takes place in the 
region .omega. liberates an amount of heat of about 100,000 J.Kg.sup.-1, 
with a transformation rate which varies in this region as a function of 
the time, this speed being low in the vicinity of the points B.sub.x and 
C.sub.x and maximum toward the middle of the segment B.sub.x C.sub.x. 
Under these conditions, if a practically constant temperature upon this 
transformation is desired, it is necessary to effect modulated heat 
exchanges, that is to say, heat exchanges the power of which per unit of 
length of the wire 1 varies along the device in which this transformation 
takes place, the cooling due to the gas 35 being maximum when the rate of 
pearlitization is maximum, so as to avoid the phenomenon of recalescence 
due to an excessive increase in temperature of the wire 1 upon 
pearlitization. 
This modulation can be effected preferably by varying either the inside 
diameter D'.sub.ti of the tubes 33 through which the wire passes, or the 
length L'.sub.t of the various tubes 33 through which the wire passes, as 
described in the aforementioned French Patent Application No. 88/00904. 
In the zone Z.sub.3, the exchanger 32, the cooling power of which is the 
greatest, corresponds to the region where the rate of pearlitization is 
the highest. Under these conditions: 
if the modulation is effected by varying the inside diameter D'.sub.ti of 
the tubes 33, this diameter decreases from the entrance of the zone 
Z.sub.3 up to the exchanger 32 where the speed of pearlitization is the 
highest, whereupon this diameter then increases in the direction toward 
the outlet of the zone Z.sub.3, in the direction indicated by the arrow F; 
if modulation is effected by varying the length L'.sub.t of the tubes 33, 
this length increases from the entrance of the zone Z.sub.3 up to the 
exchanger 32 where the rate of pearlitization is the greatest, and then 
this length decreases in the direction toward the outlet of the zone 
Z.sub.3 in the direction of the arrow F. 
In both cases there is produced, in the direction of the arrow F, an 
increase in the cooling power from the entrance of the zone Z.sub.3 up to 
the exchanger 32 where the rate of pearlitization is the fastest, and then 
this power decreases in the direction toward the outlet of the zone 
Z.sub.3. 
In this exchanger 32 in which the rate of pearlitization is the fastest, 
the following relationships preferably apply: 
EQU 1.05.ltoreq.R'.ltoreq.8 (5) 
EQU 3.ltoreq.K'.ltoreq.8 (6) 
R' and K' having the same meanings as previously. 
The zone Z.sub.4 is formed, for instance, by an exchanger 32 which 
satisfies the relations (3) and (4) previously defined. 
The wire 1 then penetrates into the zone Z.sub.5 where it is brought to a 
temperature approaching ambient temperature, for instance, 20.degree. to 
50.degree. C., by immersion in water. 
The wire 1 treated in the installation 300 has the same structure as that 
obtained by the known patenting method with lead, that is to say, a fine 
pearlitic structure. This structure comprises lamellae of cementite 
separated by lamellae of ferrite. By way of example, FIG. 9 shows, in 
cross-section, a portion 50 of such a fine pearlitic structure. This 
portion 50 has two cementite lamellae 51 which are practically parallel to 
each other, separated by a ferrite lamellae 52. The thickness of the 
cementite lamellae 51 is represented by "i" and the thickness of the 
ferrite lamellae 52 is represented by "e". The pearlitic structure is 
fine, that is to say, the average value i+e is at most equal to 1000 
.ANG., with a standard deviation of 250 .ANG.. 
Such a wire can serve, for instance, to reinforce articles of plastic or 
rubber, in particular, tires. 
The installation 300 makes it possible furthermore to obtain at least one 
of the following results: 
After heat treatment and before drawing, the wire has an ultimate tensile 
strength at least equal to 1300 MPa; 
The wire can be drawn in such a manner as to have a ratio of the sections 
at least equal to 40; 
After drawing, the wire has an ultimate tensile strength at least equal to 
3000 MPa. 
The ratio of the sections corresponds by definition to the ratio: 
##EQU1## 
The installation 300 has the following advantages: 
simplicity, low investment and operating expenses, since: 
the use of molten salts or metals is avoided; 
the use of compressors or turbines which would be necessary with a forced 
gas circulation is avoided; 
a precise law of cooling can be obtained and the phenomenon of recalescence 
can be avoided; 
possibility of carrying out with the same installation a pearlitization 
treatment on wire diameters D.sub.f which may vary within wide limits; 
any problem of hygiene is avoided and cleaning of the wire is not necessary 
since one avoids the use of molten salts or metals. 
These advantages are obtained only when relationships (3) and (4) are 
satisfied upon the cooling indicated by the portion AB of the curve .phi. 
(FIG. 6). When tubes containing a gas without forced ventilation are used, 
the tube being surrounded by a heat transport fluid, but when 
relationships (3) and (4) are not satisfied upon the cooling preceding the 
pearlitization corresponding to the portion AB of the curve .phi., it is 
not possible to effect a correct pearlitization. 
The invention is not limited to the embodiments which have been described 
above.