Note: Descriptions are shown in the official language in which they were submitted.
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Bac~roun_of the Invention
This invention relates to a heat treating furnace and
to a method of effecting thermochemical processing of a metal work
load within a heated furnace chamber in the presence of a gas. Typical
examples of gas heat treating processes are carburizing, decarburizing,
reduction and nitriding.
In one exemplary process, the furnace chamber is
evacuated to a high order of vacuum and then is heated to raise the
temperature of the work. Thereafter, an appropriate processing gas
is admitted into the chamber and is circulated past the work while the
chamber is maintained at a controlled and usually sub-atmospheric
pressure and while heating of the work is continued. Upon contacting
the hot metal surfaces of the work, the gas decomposes to produce
the desired surface characteristics, the gas in the chamber ordinarily
being continuously replenished so as to keep a supply of active gas in
the chamber.
Summary of the Invention
The general aim of the present invention is to provide
a new and improved furnace and method in which the processing gas
is circulated across the work in a unique manner which results in
more uniform thermochemical processing of the work surfaces than
has been possible heretofore and which, at the same time, enables
the effective processing of a comparatively dense work load with a
comparatively small volume of gas.
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A more detailed object is to achieve the foregoing by
circulating the gas past the work with a lively, multi-directional
movement, as opposed to substantially uni-directional flow, in order
to provide more uniform contact of active gas with all of the work
surfaces and to more rapidly remove the gaseous reaction products
from the surfaces.
The invention also resides in the provision of novel
means for circulating the gas within the chamber and across the work
with a back and forth reciprocating motion, and in the controlling of
10 the reciprocating motion to achieve effective circulation across work
loads of different shapes.
In summary, the invention resides in a heat treating
furnace having a walled enclosure defining a heating chamber, heating
elements within the chamber for heating a work load disposed in the
chamber, means including an inlet for admitting a substantially
continuous flow of processing gas into said chamber, means including
an outlet for exhausting a substantially continuous flow of gas from
said chamber, and power-operated means for repeatedly increasing
and decreasing the effective volume of said chamber and for imparting
20 a back and forth pulsating motion to the gas while the gas is within said
chamber and is flowing from said inlet to said outlet thereby to
circulate the gas back and forth within the chamber and past the work
load with a pulsating action as the gas flows from said inlet to said
outlet.
Also, the invention resides in a method of heat treating
a work load comprising the steps of, admitting a substantially continuous
flow of processing gas into an inlet of a heated chamber containing
the work load, exhausting a substantially continuous flow of gas from
an outlet of said chamber, and imparting a repeated back and forth
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pulsating motion to the gas in the chamber as the gas flows through
said chamber and while gas continues to flow into said inlet thereby
to circulate the gas past the work load with a reciprocating action
while substantially continuously replenishing the gas in the chamber,
said pulsating motion being imparted to the gas by repeatedly increasing
and decreasing the effective volume of the chamber.
These and other objects and advantages of the invention
will become more apparent from the following detailed description
when taken in conjunction with the accompanying drawings.
Brief Description of the Drawin~s
FIGURE 1 is a schematic cross-sectional view of a
new and improved heat treating furnace capable of carrying out the
unique method of the present invention.
FIGS. 2a and 2b are cross-sectional views which
schematically show the multl-directional gas circulation produced
by the furnace shown in FIG. 1.
FIGS. 3a and 3b are views similar to FIGS. 2a and 2b
but schematically show the circulation produced by a modified furnace.
FIGS. 4a and 4b are views which schematically show
20 an alternate method of operating the furnace shown in FIGS. 3a and 3b.
FIGS. 5a to 5d are cross-sectional views which
schematically show the circulation produced by still another
embodiment of a furnace incorporating the features of the invention.
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DetaiLe~ I)escriptLon_of the Preferred Embodiments
The invention is shown in the drawings as embodied in
a cold wall vacuum furnace 10 for heat treating a metal work load 11
in the presence of a processing gas which usually is maintained at a
sub-atmospheric pressure within the furnace, A furnace of this same
general type is disclosed in United States Patent 3,171, 759.
Briefly, the furnace 10 includes a hollow, cylindrical
vessel 13 which is supported on its side by a base 14 and is cooled
by a peripheral water jacket 15. Within the vessel is a refractory
1~ baffle 16 forming a walled enclosure whose interior defines a heat
treating chamber 17 where the work 11 is supported on a roller hearth
l9. The work is heated by suitable radiant heating elements 20 which
may be of the electrical type and which extend vertically within the
chambe r.
To evacuate the interior of the vessel 13 and hence
the chamber 17, a pump 21 communicates with the interior of the
vessel through a conduit 23. A suitable valve 24 may be disposed
in the conduit 23 and controlled by a power actuator (not shown) to
hold the vacuum in the vessel.
The processing gas is supplied from a suitable source
27 and is admitted into the chamber 17 through a line 28. The latter
herein is shown as extending into the top of the vessel 13 and
communicating with a graphite gas injector 29 which extends through
the top of the baffle 16. Valving indicated generally at 30 is connected
into the line to cause the gas to flow into the chamber at a substantlally
constant rate. Withdrawal of the gas from the chamber is effected
through a line 31 located adjacent the bottom of the chamber and
communicating with the pump 21 by way of valving 33 which serves
to maintain a substantially constant pressure within the chamber.
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A furnace 10 of the foregoing character is particularly
suitable for use in performing a thermochemical process such as
vacuum carburizing in which carbon from a hydrocarbon gas is
transferred to the hot metal work surfaces in order to ~nable case
hardening of the work 11. In a typical vacuum carburizing process,
the loaded chamber 17 is evacuated to a relatively high order of
vacuum and then is heated to subject the work to a brief vacuum
conditioning cycle. Thereafter, the chamber is raised to a temperature
in the neighborhood of 2, 000 F. and then is backfilled through the
line 28 with an appropriate gas such as methane (CH4~. By way of
example, the methane may be admitted continuously into the chamber
at the rate of 25 cubic feet per hour and the flow of gas out of the
chamber may be regulated so as to maintain the pressure in the
chamber at approximately one-eighth atmosphere.
The flow of gas through the chamber is continued
after the desired pressure level has been reached. The methane is
clrculated past the work 11 and decomposes upon contacting the hot
work surfaces. The controlling decomposition reaction is:
CH4(g~ ~ C + 2H2(g~
wherein the carbon is absorbed by the hot surfaces and the hydrogen
is displaced. To obtain a uniform case depth with a controlled carbon
gradient, carbon must be made uniformly available to all of the work
surfaces. Accordingly, the gas must be circulated past the work in
such a manner as to uniformly replace the hydrogen decomposition
product with active methane in the vicinity of all of the work surfaces.
The present invention is based upon my discovery
that the surfaces of the work ll can be treated more uniformly than
previously has been possible by circulating the gas across -the ~rork
surfaces with a lively pulsating, multi-directional motion rather than
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the conventional unidirectional circulation produced by presently
used rotary fans and the like. By circulating the gas back and forth
past the work, a supply of active gas is more readily brought into
uniform contact with all sides of the work and the gaseous reaction
products are more quickly displaced from the work surfaces so as
to not only produce a more uniform surface chemistry response but
also to allow the processing of denser work with a comparatively
- small volume of gas.
In the preferred manner of carrying out the invention,
the gas is circulated past the work 11 with a back and forth reciprocating
motion. For this purpose, a plunger 35 covered with heat insulating
material is received with a close fit within a cavity or compartment
36 which communicates with the chamber 17 and which herein is shown
in FIG. 1 as being located at the lower side o~ the chamber. An
actuator such as a pneumatic cylinder 37 is attached to the underside
of the vessel 13 and includes a rod 39 which is connected to the plunger.
When the rod 39 is retracted, the plunger 35 is shifted downwardly
away from the chamber 17 and draws gas into the compartment 36
to cause the gas to flow downwardly past the work as shown in FIG.
2a. Conversely, upward extension of the rod shifts the plunger
upwardly out of the compartment and toward the chamber so as to
force gas from the compartment and cause the gas to flow upwardly
across the work as illustrated in FIG. 2b.
With the foregoing arrangement, the gas is admitted
continuously into the chamber 17 and may be reciprocated back and
forth past the work 11 at a desired frequency and velocity by varying
- the cycle time and velocity of the plunger 35. By virtue of the back
and forth motion imparted to the gas, the gas circulates with various
eddying effects and comes into substantially uniform contact with all
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of the work surfaces. When used in a vacuum conditioned carburizing
process, the reciprocating circulation system rapidly replaces the
hydrogen with active methane and causes the gas surrounding all of
the work surfaces to be of a more uniform nature so as to effect a
more uniforrn case depth. In addition, the system enables the use
of smaller quantities of gas to treat a work lbad of a given size and
allows the effective treating of a comparatively large or dense load
in a chamber of relatively small volume. The system is effective
for pressures ranging from highly negative (e. g., 50 to lO0 microns)
10 to highly positive ~e. g., at or above atmospheric) and does not rapidly
deteriorate under high temperature conditions in the presence of a
reactive gas. That is, the plunger 35 may be made of or covered
with the same refractory material as the baffle 16 and thus will
experience a long service life even though exposed to a hot reactive
gas .
The furnace 10' shown in FIGS. 3a, 3b and 4a, 4b is
identical to the furnace 10 except that reciprocating plungers 35' are
provided in compartments 36' located at both the upper and lower
sides of the chamber 17'. As shown in FIGS, 3a and 3b, the plungers
20 35' can be reciprocated in unison but in the same direction whereby
one plunger moves toward the chamber 17' while the other moves
away from the chamber, and vice versa, thereby to effect substantially
bi-directional circulation by drawing gas into one compartment 36'
while forcing gas from the other compartment. Alternatively, a
more random multi-directional circulation may be effected by
operating the plungers 35' as shown in FIGS, 4a and 4b wherein the
plungers are reciprocated in unison but in opposite directions so as
to simultaneously draw gas into both compartments 361 and then
simultaneously force gas from both compartments,
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In FIGS. 5a to 5d, there is shown a furnace l0" in
which one plunger 35" is located in a compartment 36" at one sid~
of the chamber 17" while an additional plunger 35" is disposed at
right angles to the plunger and is located in a second compartment
36". By operating the plungers in various sequences, the gas can be
circulated in several patterns so as to best be brought into contact
with irregularly shaped workpieces whose surfaces otherwise might
- not be effectively reached by simple back and forth circulation, One
exemplary sequence is shown in FIGS. 5a to 5d wherein a complete
10 cycle involves forcing the gas downwardly, then to the right, back
upwardly and then to the left.
