Roller-hearth furnace with shielded rollers

A roller-hearth furnace has workpiece-supporting rollers which are thermally shielded at the sides and from below. Intermediate the rollers the workpieces are exposed to heat action from below.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to roller-hearth furnaces. 
2. The Prior Art 
Conventional roller-hearth furnaces are generally used for the 
heat-treatment of workpieces where rapid heating is either not possible or 
not desirable. They cannot be used for heat-treatments where rapid heating 
of workpieces, such as plates or strips, to high temperatures is required. 
For this latter type of heat-treatment it is typically necessary to employ 
soaking-pit and similar furnaces in which rapid heating can take place. 
Also, in many instances it is undesirable--from the viewpoint of the end 
results desired for the workpieces--to heat too rapidly. For such 
applications the relatively slow-heating roller-hearth furnaces have 
heretofore been used in which the differential between furnace heat and 
workpiece temperature is limited to not very high values at all points of 
the furnace interior, in contrast to other heat-treating furnaces where it 
is often immaterial for the qualities of the treated workpieces that 
substantial temperature differences exist between the workpieces and the 
furnace heat. 
However, for a variety of reasons know to those conversant with the art it 
is desirable to be able to employ roller-hearth furnaces also for the 
rapid heating of workpieces. In fact, repeated attempts have been made to 
construct special high-performance roller-hearth furnaces for this 
purpose. These special furnaces are characterized in that substantial 
differences can develop locally between the furnace temperature and the 
workpiece temperature, especially in the region where the workpiece enters 
the furnace and are rapidly heated to high temperatures. These attempts 
have never been successful because the rollers of the hearth were unable 
to withstand the prevailing conditions and became damaged so rapidly that 
their reduced service life made the proposition uneconomical. Attempts at 
providing rollers of improved construction and/or materials have 
heretofore always failed. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to overcome the prior-art 
disadvantages. 
More particularly, it is an object of the invention to provide an improved 
roller-hearth furnace which is capable of heating workpieces rapidly to 
high temperatures, but without accepting a deterioration in the service 
life of the hearth rollers. 
In keeping with these and other objects which will become apparent 
hereafter, one aspect of the invention resides in a roller-hearth furnace 
for high-temperature use which, briefly state, may comprise a plurality of 
axially parallel rollers; and means for shielding each of the rollers 
laterally and from below against direct exposure to the heated atmosphere 
in the furnace, so as to prevent heating of successive roller-surface 
increments to a temperature which is so much higher than the workpiece 
temperature as to cause premature deterioration of the roller material due 
to alternating heating and cooling of the roller-surface increments. 
The novel features which are considered as characteristic for the invention 
are set forth in particular in the appended claims. The invention itself, 
however, both as to its construction and its method of operation, together 
with additional objects and advantages thereof, will be best understood 
from the following description of specific embodiments when read in 
connection with the accompanying drawing.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to the drawing, it should be understood that no attempt has 
been made to illustrate those aspects of a roller-hearth furnace which are 
conventional, i.e., known per se. 
With this in mind it will be seen that FIG. 1 only diagrammatically shows 
the heating chamber of a roller-hearth furnace having an upper heating 
zone Z.sub.1 and a lower heating zone Z.sub.2. There will, of course, be 
the requisite conventional heating devices, such as burners, and the 
hearth will be constructed in known manner as an array of parallel rollers 
1 (only two shown) on which workpieces 2 travel (either leftward or 
rightward in FIG. 1). Rollers 1 are journalled for rotation, also in a 
manner known per se. 
In accordance with the invention, however, each of the rollers 1 is 
provided with a thermal shield 3 which here is in form of a substantially 
U-shaped baffle which surrounds the roller 1 at the sides thereof and from 
below. Because of this shield 3 the successive movements of the roller 
surface are shielded against exposure to strongly varying temperatures 
during rotation of the roller. This avoids the prior-art disadvantage 
which is primarily responsible for early deterioration of the rollers, 
namely the fact that as each roller-surface increment sequentially faces 
laterally or downwardly (and is thus exposed to the zone Z.sub.2) it is 
strongly heated. The workpieces 2, especially as they traverse the 
upstream half of the furnace chamber, have a temperature which is 
substantially lower than the furnace temperature. Therefore, the 
increments of the roller surface are cooled drastically when they come 
into contact with the relatively cold workpiece; this results in a rapid 
succession of heating and cooling of the rotating rollers and causes their 
premature destruction. 
The inventive heat shield 3, however, prevents this hot-cold-hot cycle 
since it protects the roller 1 against direct exposure to the heat in the 
zone Z.sub.2. As a result, the temperature of each roller-surface 
increment--at the time it contacts the workpiece 2--is much closer to the 
workpiece temperature than in the prior art so that the deleterious 
effects of the hot-cold-hot cycle are eliminated or at least so 
substantially reduced that the service life of the rollers 1 is 
drastically increased, as compared to the prior art. Since the sheilds 3 
of successive rollers are spaced from one another, as shown, the full heat 
from the zone Z.sub.2 can impinge upon the workpiece 2 in the space S 
between successive shields. The shields may in all embodiments be made of 
metallic (e.g., steel, titane, molybdane, cobalt-alloys, nickel-chromium 
alloys) or ceramic material (aluminium-silicum-oxides, chromium oxide, 
silicum carbide). 
According to the embodiment of FIG. 2, the shield 3a may be provided with 
an, e.g. embedded carrier 4 having a passage 4a through which a cooling 
medium (e.g., water) may circulate when passage 4a is connected to a 
known-per-se source of such cooling medium. The carrier 4 of course also 
serves to support and reinforce the shield 3a. 
Particles (e.g., rust or other substances) may in some instances drop off 
the workpieces 2. These could clog the clearance between the shield and 
the roller 1. To avoid this, a construction such as the one illustrated in 
FIG. 3 may be employed, i.e., the shield may be provided with an opening 
through which such particles can drop out. 
In the particular exemplary embodiment of FIG. 3 the hearth roller is 
identified with reference numeral 5, the workpiece with reference numeral 
7 and a transporting chain for the workpiece with reference numeral 6. 
The shield is here composed of two shield sections 8 and 9 which are 
provided with respective embedded carriers 10, 11 having passages for 
circulation of a cooling medium. The two shield sections 8, 9 define with 
one another an opening 12 which should advantageously be inclined as shown 
to prevent, or at least limit, the exposure of roller 5 to heat radiation 
from the zone Z.sub.2 (see FIG. 1). With this construction any particles 
falling from above into the clearance between the roller 5 and the shield 
sections 8, 9 can drop out through the channel-shaped opening 12 whose 
presence, however, decreases the shielding effectiveness only to a very 
slight extent. 
FIG. 4 and 5, finally, show an embodiment in which the hearth rollers 13 
(one shown) have circular disks 14 (one shown) mounted on them. In this 
embodiment there will, of course, be two or more of these disks 14 mounted 
on the roller 13 at axially spaced locations (this is known per se) and 
the workpiece 17 will be supported on the disks 14 rather than on the core 
of the roller 13 directly. It is, therefore, not the core of the roller 13 
per se which must be protected against the hot-cold-hot cycle, but the 
disks 14 which contact the workpiece and which form part of the roller. 
For this purpose the heat shield 15, which may be of heat-resistant sheet 
metal, surrounds the respective disk laterally and from below as well as 
at its axial ends. It is mounted on a sidewall (not shown) of the furnace 
by means of one or more (two shown) tubular members 16 which themselves 
are connected to axial end walls 15a of the heat shield 15, so as to 
remain stationary while the core of the roller 13 (which extends through 
the members 16) and the disk 14 rotate. 
All embodiments have, of course, in common that the increments of the 
roller or disk surface are protected against direct exposure to the heat 
in zone Z.sub.2 so that, when they subsequently contact the workpiece, the 
temperature differential between the workpiece and these surface 
increments will be relatively small. This protects the rollers (or disks) 
and increases their service life, making the use of a roller-hearth 
furnace for rapid heating of workpieces to high temperatures, for the 
first time an economically viable proposition. 
While the invention has been illustrated and described as embodied in a 
roller-hearth furnace, it is not intended to be limited to the details 
shown, since various modifications and structural changes may be made 
without departing in any way from the spirit of the present invention. 
Without further analysis, the foregoing will so fully reveal the gist of 
the present invention that others can, by applying current knowledge, 
readily adapt it for various applications without omitting features that, 
from the standpoint of prior art, fiarly constitute essential 
characteristics of the generic or specific aspects of this invention.