U.S. patent application number 11/234851 was filed with the patent office on 2007-03-29 for process for treating steel alloys.
Invention is credited to Harry W. Antes, Trevor M. Jones, William R. Jones, Virginia M. Osterman.
Application Number | 20070068601 11/234851 |
Document ID | / |
Family ID | 37892417 |
Filed Date | 2007-03-29 |
United States Patent
Application |
20070068601 |
Kind Code |
A1 |
Jones; William R. ; et
al. |
March 29, 2007 |
Process for treating steel alloys
Abstract
The process includes heating of steel alloy parts in the
presence of hydrogen prior to introduction of, for example,
carburizing gas, preferably acetylene, in the presence of a diluent
carrier gas. The process uses a continuous cycle involving only one
carburizing (boost) step and one diffusion step, and high velocity
gas quenching. The process is advantageously carried out in a
unique, versatile vacuum furnace (also having high internal
pressure capability) that provides very high velocity, continuous
flow gas quenching, and a furnace design, including a long, low
profile work zone configuration and quench gas recirculation
equipment and flow pattern that facilitates high velocity gas flow.
The entire process is accomplished in a single self-contained
chamber of the furnace.
Inventors: |
Jones; William R.; (Telford,
PA) ; Osterman; Virginia M.; (Collegeville, PA)
; Antes; Harry W.; (Huntingdon Valley, PA) ;
Jones; Trevor M.; (Perkasie, PA) |
Correspondence
Address: |
William R. Jones;Solar Atmospheres, Inc.
1969 Clearview Rd.
Souderton
PA
18964
US
|
Family ID: |
37892417 |
Appl. No.: |
11/234851 |
Filed: |
September 26, 2005 |
Current U.S.
Class: |
148/223 ;
148/628; 148/634 |
Current CPC
Class: |
C23C 8/22 20130101; C23C
8/02 20130101 |
Class at
Publication: |
148/223 ;
148/628; 148/634 |
International
Class: |
C23C 8/22 20060101
C23C008/22; C21D 1/773 20060101 C21D001/773 |
Claims
1. A process for treating (in a specific chamber of a heat treating
vacuum furnace having low vacuum capability, high pressure
capability, and very high gas-circulating capability), said chamber
having gas transport lines for providing gas to and drawing gas
from said chamber), surfaces of steel alloy work pieces, said
process comprising: (a) drawing a very low pressure vacuum to
evacuate gas from said chamber; (b) allowing hydrogen to flow
through a gas line into said chamber to a pressure not exceeding 10
torr; (c) heating said chamber to a temperature between 871 and
1038 degrees C. and soaking the work pieces in that heat for at
least 15 minutes, and then adjusting the pressure to at least 7.6
torr by adding gas as necessary; (d) while maintaining the pressure
at a level of at least 7.6 torr, and at a temperature at least as
high as 871 C adding to said chamber through at least one said gas
line, gas having a capability of desirably affecting said surfaces;
and (e) shutting of the gas having a capability of desirably
affecting said surface and while maintaining the temperature at
least as high as 871 C allowing nitrogen to flow through a gas line
to a pressure not exceeding 10 torr and allowing the work pieces to
diffuse and then (f) shutting off the heating mechanism and very
rapidly quenching by releasing large quantities of quench gas at
high pressure into said chamber and recycling the quench gas at a
high rate of speed.
2. A process in accordance with claim 1 wherein said gas having a
capability of desirably affecting said surfaces includes at least
one gas capable of affecting the alloy metal to form hardened
surfaces thereon and at least one diluent gas.
3. A process in accordance with claim 2 wherein said gas having a
capability of desirably affecting said surfaces includes at least
one carburizing gas.
4. A process in accordance with claim 3 wherein said gas having a
capability of desirably affecting said surfaces comprises a member
of the group consisting of propane, ethylene and acetylene.
5. A process in accordance with claim 3 wherein said gas having a
capability of desirably affecting said surfaces comprises
acetylene.
6. A process in accordance with claim 3 wherein the carburizing gas
is acetylene and is added to said chamber as a mixture of acetylene
and a diluent that is a member of the group consisting of hydrogen
and nitrogen.
7. A process in accordance with claim 1 wherein said gas having a
capability of desirably affecting said surfaces comprises a
carburizing gas.
8. A process in accordance with claim 2 wherein said diluent
comprises hydrogen or nitrogen.
9. A process in accordance with claim 8 wherein said diluent is
hydrogen.
10. A process in accordance with claim 7 wherein said diluent is
nitrogen.
11. A process in accordance with claim 1 wherein said quench gas is
nitrogen.
12. A process in accordance with claim 1 wherein said quench gas is
helium.
13. A process in accordance with claim 1 wherein said quench gas is
hydrogen.
14. A process in accordance with claim 1 wherein the internal
pressure during the last 10 minutes of the quench is at least 8
bar.
15. A process in accordance with claim 1 wherein the internal
pressure during the last 20 minutes of the quench is at least 9
bar.
16. A process in accordance with claim 1 wherein the internal
pressure during the last 10 minutes of the quench is at least 10
bar.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to low pressure carburization
and other heat treating processes applied to metal alloy parts and
more particularly steel parts and to high temperature capable
furnaces having the capability of providing in the same furnace
chamber alternatively, low pressure (vacuum) and high pressure (gas
quench) environments for such processes.
[0003] 2. Background Art
[0004] Vacuum (low pressure) heat treating (carburization) of steel
or high alloy-content steels has been accomplished over past
decades using various heat treating processes. Some alloys are
particularly difficult to treat and require post treatment, for
example, quenching to finish the treatment. Some metals are more
difficult to treat (for example alloys such as AISI grade 4140,
4340, 8620, and 9310). Work pieces containing such alloys are
currently heat treated and then moved to an oil or salt bath
quench. That is, the work pieces are moved, mechanically, from the
hot zone at temperature, into an outer vestibule chamber and
submerged into a tank filled with oil or salt to rapidly cool the
work pieces. The pieces thus moved and quenched have problems with
distortion. Also, cleaning the parts after they have been submerged
in oil or salt is a costly challenge. The mechanism for moving the
work pieces at temperature undesirably adds significant cost, time
and maintenance issues to the process. Gas quenching has been used
as a post treatment for carburization of steel parts. Although, gas
quenching avoids much of the finish product cleanup issues, it does
not avoid the mechanical movement of the workload from one chamber
to another. It also is not without challenges in how it affects
finished product quality. In regard to the carburization process
early and ongoing processes involve using as the carburizing gas
hydrocarbons, such as, a gaseous saturated aliphatic hydrocarbon,
e.g. methane, propane and butane. The selection as to which
hydrocarbon should be used as the carburizing agent has been an
evolving debate. The selected gas would be added at a pressure, for
example, of 10-700 torr in the carburizing chamber, and the parts
"absorb" carbon on the surface. Next, the reactive gas is removed
and the surface carbon is allowed to diffuse below the surface.
With such hydrocarbon gases, however, soot produced in the
carburizing chamber interferes with desired consistency of
carburizing quality and adds significant cost to parts cleanup and
furnace maintenance. Achieving a uniform carburized "case", a
hardened, uniform surface layer, has been difficult and costly.
Uniformity has been a major challenge. Sandblasting parts prior to
carburizing to get rid of surface oxidation prior to carburization
became a routine requirement. Atmosphere carburization suffers from
the added problem of surface oxidation during heat treatment. The
use of moderately higher carburizing temperatures, compared to
atmospheric carburizing conditions, over shorter carburizing times
has, for example, been found to provide a more uniform oxide free
carburized case depth, cleaner parts, less part distortion, and the
elimination of post process machining. Over the years vacuum
carburizing has become cost effective as compared to traditional
atmosphere carburization. Conventional high temperature vacuum
furnaces have been described in numerous prior art patents.
Carburizing furnaces are in many respects similar to those
conventional high temperature vacuum furnaces. In general, such
furnaces are commonly of a substantially cylindrical shape having a
substantially circular internal cross-section. Such a furnace is
closed at its forward end by a releasable door, regularly with
hinges so that the door swings out of the way for loading and
unloading the furnace. The furnace doors have vacuum seals when
closed to support the vacuum capability of the furnace. Also the
doors regularly have insulation placed and formed to mate with
insulation lining of the circular cross section furnace walls.
Although the furnace of this invention has the above-mentioned
features of prior art furnaces, and others, (See for example, U.S.
Pat. No. 4,499,369, wherein a series of cylindrical resistance
graphite heating elements are spaced longitudinally along the
furnace interior and spaced from the walls.) key differences will
be revealed in the following.
[0005] Consideration of the explosive and fire dangers associated
with low molecular weight unsaturated hydrocarbons no doubt
dissuaded some early carburization developers from attempting to
use gasses such as acetylene and ethylene in carburizing
applications. A relatively recent patent, U.S. Pat. No. 6,187,111
B1, (hereinafter the 111 patent) teaches away from the concept of
using "acetylenic gas" as presenting "safety problems due to the
combustibility of the gas." That teaching is significant, in part
because it apparently takes issue with earlier studies and patents
much of which apparently does not deal with the dangers so
conspicuous to the 111 patent authors. The 111 patent also teaches
away from using hydrogen in carburizing applications, for example,
as described in U.S. Pat. No. 5,205,873, also because of the safety
issue. An early study, 1982 Jelle Kassperma and Robert H. Shay
(Metall. Trans. B 13B, 1982 267), presented an intensive study of
the use of hydrocarbon gases as carburizing agents. The paper
reveals investigation of the carburization reaction rates for
methane, ethane, propane, ethylene and acetylene. The hydrocarbons
were used in a conjunction with nitrogen as the carrier gas and
hydrogen as an additive. The data supported acetylene as having the
fastest rate for carburization and that propane is faster than
ethylene. The investigators also provided an assessment of soot
formation and the benefits of hydrogen in the mixture. An even
earlier use of unsaturated hydrocarbons for carburizing, including
acetylene, was disclosed in U.S. Pat. No. 3,988,955, issued Nov. 2,
1976: "Suitable carbonizing gases include methane, natural gas,
propane, acetylene and benzene." U.S. Pat. No. 4,035,203 also
discloses the use of acetylene as an "active" gas for carburizing.
About the same time Russian developers, recognizing problems
associated with the use of aliphatic hydrocarbons in carburizing
and the dangers of poor furnace construction, nonetheless looked to
acetylene as the hydrocarbon of choice for carburizing. USSR Patent
Specification No. 668978 (published patent specification date: Jun.
28, 1979, and referred to hereinafter as "USSR patent") disclosed
vacuum carburizing using acetylene at a pressure in the range of
"0.01-0.95 atm." (that is, 7.6 torr to 722 torr.). Interestingly,
U.S. Pat. No. 5,702,540, (filed 15 plus years later, without
referencing the USSR patent) claims using an acetylenic gas as the
carburizing gas at a vacuum of not more than "1 kPa" (that is, not
more than 7.5 torr). More recently, US patent application,
US2003/0168125, disclosed a method for vacuum carburizing utilizing
acetylene as the carburizing gas in the presence of a neutral
carrier gas (N.sub.2 or H.sub.2) and requiring a pulsing sequence
(i.e. boost/diffuse cycles). Reference is also made to the patent
application filed on this date by William R. Jones entitled
"Versatile High Velocity Integral Vacuum Furnace" which is
incorporated by reference in its entirety.
BRIEF SUMMARY OF THE INVENTION
[0006] Applicants have found that a carburizing process including
heating of steel parts in the presence of hydrogen prior to
introduction of carburizing/diluent gas, can provide substantial
improvement in carburizing in accordance with the present
invention. The process uses a continuous cycle involving only one
carburizing (boost) step and one diffusion step, and carburizing
gas, preferably acetylene in the presence of a diluent carrier gas.
The carburizing is desirably carried out in a furnace having high
velocity quenching capability. The process according to the instant
invention uses hydrogen as a pretreatment gas with significant soak
time under heat, then, after the pretreatment, carburizing,
followed by a high pressure, high velocity gas quench. The process
provides a method that avoids the need for: (a) a highly programmed
cycle; and (b) a complex sequential boost/diffuse process. The
process also substantially avoids the requirement for sand blasting
the steel parts prior to carburizing. The process is advantageously
carried out in a unique, versatile furnace that provides a novel,
high velocity, continuous flow gas quenching capability, and a
furnace design, including an effective work zone configuration that
contributes to more effective carburization. The entire process is
advantageously accomplished in a single self-contained chamber of
the unique furnace. The advantages of a high velocity gas quench
are substantial. For example, with the gas quench there is far less
work piece distortion and no oil cleanup following heat treatment.
Also, the cost of having a separate chambers and equipment for
moving workloads from one chamber to another are completely
avoided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 depicts in perspective a partial front, open door
cross section view of cylindrical resistance heating vacuum furnace
100 having high velocity quenching capability.
[0008] FIG. 2 depicts in partial side view cross section the front
or treatment end of furnace 100.
[0009] FIG. 3 depicts in partial cutaway a side cross section view
revealing features in the gas supply and movement end of the
furnace 100.
[0010] FIG. 4 is a side view schematic illustration of furnace 100
depicting (with emphasis) carburizing gas connections (rotated 90
degrees for illustration only) and gauges of the present
invention.
[0011] FIG. 5A depicts in partial cutaway of furnace (autoclave)
door 51 inside (as viewed from the interior of furnace 100)
illustrating connected carburizing gas nozzle arrangement in the
door and schematically illustrating the gas supply tubes. FIG. 5 B
depicts an end view of carburizing nozzle 18) as viewed from
furnace 100 interior. FIG. 5C depicts in cross section the taper of
carburizing nozzle 18.
[0012] FIG. 6A depicts an end view of radial hot zone gas
carburizing gas nozzle 11. FIG. 6B illustrates a side view cross
sectional of radial hot zone gas carburizing gas nozzle 11, while
FIG. 6C illustrates a 90 degree lateral rotational view of the
lower segment of carburizing gas nozzle 11 illustrated in FIG. 6B.
FIG. 6D is a cross sectional view along line Z-Z of carburizing gas
nozzle 11 connection.
DETAILED DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0013] A front, cross section view (looking toward the door end) of
high temperature, vacuum furnace 100, is depicted in perspective in
FIG. 1 revealing outer furnace wall 101 and inner wall 102 which
form the radial boundaries of furnace water jacket 110 used for
cooling the furnace. The outer chamber of furnace 100, thus, is a
cylindrical double walled water-cooled vessel, and is manufactured
from 304 stainless steel. The water jacket width is approximately
1'' maximum, with large oversized water inlet and exit ports (not
shown) located around the chamber to allow for convenient periodic
flushing of the water jacket to reduce sediment build-up. Inner
wall 102 also forms the outer wall of spacious gas plenum chamber
13 (see FIG. 2), a large annular cavity important to high velocity
(very rapid) quenching. Cylindrical shaped resistance elements 1,
each desirably graphite heating elements, each forming a complete
circle, are supported in place by molybdenum standoff assemblies
107 (described in U.S. Pat. Nos. 6,111,908, 6,021,155, 6,023,155)
attached to and in a nonconducting relationship to inner wall 103
which is the support wall for the hot zone ring assembly. The hot
zone ring assembly comprises inner wall 103, insulation 104 held in
place by anchors 105, heat reflective inner surface material 106,
heating elements 1, and the various assemblies and connectors
anchoring them in position. Insulation layer 104 is desirably
comprised of low mass insulation utilizing a 2.54 cm (one inch)
thick, highly durable graphite board having bonded thereto on the
board side facing the heating elements 1 a heat reflecting graphite
foil, for example, 0.38 mm (0.0150 inch) thickness. The heating
elements are connected to water-cooled power terminal assemblies
(not shown). The furnace system is designed to operate in
conjunction with: a vacuum pump capable of operating in a vacuum
range of about 10.sup.-3 torr and at least one high pressure system
(including, for example a surge tank) for achieving in the furnace
a high pressure of at least 10 bar. Such pumps are commercially
available.
[0014] To maximize carburizing furnace efficiency the effective
work zone dimensions desirably will fit with and complement other
furnace features and provide flexibility by accommodating a variety
of target parts (workload to be carburized). The process of
carburization also desirably would complement and be complemented
by the furnace and its work zone dimensions. According to the
present invention effective work zone 120 of furnace 100 finds a
fit with and is complemented by mammoth quench gas duct 17. Duct
17, which is very large compared to ducts emptying into prior art
furnaces, especially for comparable purposes, accommodates very
high velocity of flow in the direction of the furnace work zone for
quenching the workload placed therein, particularly with its lower
angled arc that allows for lower pressure drop for the gas it
passes to plenum chamber 13. Advantageously, the smallest diameter
of the interior of duct 17 should be at least 50 percent as long as
the diameter of the furnace hot zone (distance between an element
on one side of the hot zone and the same or corresponding element
on the opposing side of the hot zone). In one embodiment of the
invention herein, the smallest diameter of the interior of duct 17
should be at least 70 percent (advantageously 90 percent) as long
as the shortest distance across the furnace effective work zone
(distance from one side of the furnace work zone to the opposing
side of the work zone.) The latter relationship is illustrated in
FIG. 1 wherein the diameter of duct 17 is significantly longer than
90 percent of the width or height of work zone 120. In another
important embodiment of the invention, for high volume transport of
the quenching gas into the furnace, the perpendicular cross section
area of the duct (like duct 17) feeding from the supply source into
the furnace desirably is at least as large as 50 percent (desirably
at least 70 percent) of the perpendicular cross section area of the
work zone of the furnace. In a particularly desirable embodiment of
the invention disclosed herein, the effective work area of the
furnace, work zone 120 dimensions are 24''.times.24''.times.72''
(0.6.times.0.6.times.1.8 meters). An improvement over earlier
furnaces results from the overall geometry of the furnace. Work
zone 120 is narrow and long with a 3 to 1 ratio of length to the
width or height. This allows the carburizing gas to interact with
the work more efficiently, and the geometry allows greater exposure
of the parts to the cooling gases, further facilitating rapid
quenching. Another complementing relationship in the furnace and
process according to the present invention is the minimal
interruption of flow of quenching gas as it moves from mammoth duct
17 through port 5 into large gas plenum chamber 13 and then across
flow director 10 so that flow of gas in gas plenum 13 is semi
circumferential as well as flowing laterally, and then to the
plurality of gas quenching nozzles 9. The base of each nozzle 9
(the proximal end) is: (a) radially anchored to a matching aperture
in and for gas flow through inner wall 103; and (b) projects in the
direction of effective work area 120. The end of each nozzle 9
proximal to inner wall 103 is attached to inner wall 103 in a
location so that it extends radially into the furnace a distance so
that its distal nozzle end is at least a short distance closer to
effective work zone 120 than heating elements 1, thereby providing
free, or only minimally obstructed, gas flow through its interior.
Nozzles 9 are positioned on wall 103 so that as fixed to wall 103
their radial extensions reach between heating elements, or (in the
case of the elements at the ends of the element banks) between the
end element and the respective end of the furnace hot zone, to
deliver from large gas plenum area 13 high velocity, balanced, even
and direct flow toward and into work zone 120. The quench gas
nozzles 9 are a unique high velocity threaded graphite tube, which
is designed for ease of replacement. The number of quenching
nozzles 9 can vary with the size of the furnace, the effective work
zone volume, workload surface area and size, and spacing of the
heating elements. For high velocity quenching according to the
present invention for effective work zone sized: 2 feet wide, 2
feet high, and 6 feet long (0.6 m.times.0.6 m.times.1.8 m),
advantageously, about 50 to 80, desirably about 70, or 71 quenching
nozzles 9 are distributed in the furnace for such balanced, direct
and even flow. There are additionally up to 8 quenching nozzles 9
anchored to the furnace (autoclave) door 124 for gas quench flow
from the door to furnace 100 interior toward the work zone 120.
Each quenching nozzle 9 is, desirably, capable of carrying
quenching gas flowing at least about 322 km (200 miles) per hour.
Carburizing gas nozzles 11 are also anchored in inner wall 103, and
are made from graphite (or ceramic), which prevents clogging due to
carbon pick-up from the carburizing source and are desirably
threaded for simple replacement if necessary. Carburizing gas
nozzles 11 are located at 2:00, 4:00, 8:00, and 10:00 within the
cylindrical array (as viewing a clock face). The gas jet tubes 30
of carburizing nozzles 11, (see FIG. 6) are centered with a chamfer
to give a more streamlined laminar flow as opposed to a turbulent
flow. The flow characteristics affect the distribution of the
carburizing gas to the workload. A laminar flow will give a more
even distribution of the gas throughout the workload providing more
efficient reactivity between the gas and the workload. The
carburizing gas tube connection 42 (see FIG. 6) furnishing
carburizing gas to the jet is at a 90 degree angle in order to
reduce or block heat. Carburizing gas nozzles 11 are fed through
smaller diameter tubing, desirably stainless steel tubing, leading
from a gas source, for example, containers of highly purified
hydrogen or acetylene outside furnace 100 (See FIG. 4). Desirably
there are a total of twelve to sixteen carburizing gas nozzles 11
in the furnace. Four additional unique carburizing jets, nozzles 18
are anchored in furnace (autoclave) door 124 for carburizing gas
flow from the door interior toward work zone 120 (see FIG. 5). Also
visible in FIG. 1 are gas plenum restrictor or closure plates 121
having specific orifices 122 which, during furnace operation,
advantageously channel, for example, quenching gasses into the
front head quenching nozzles 9 located in the furnace door.
Molybdenum work support pins 2 are fixed at their lower end to
inner wall 102, and support at their upper end molybdenum work
rails 3 which support hearth 7. The hearth assembly follows the pin
and rail design and is completely removable. Alternatively, the
hearth assembly materials include carbon fiber carbon work support
pins, advanced design sculptured graphite support rails and
molybdenum rod inserts. The hearth will support a gross weight up
to 1500 lbs at 1316 C (2400 F) and advantageously accommodates an
effective work zone 0.6 meter (2 feet) wide, 0.6 meter (2 feet)
high, and 1.8 meter (6 feet) long. Desirably the effective work
zone should have a length to width, and length to height ratios of
at least 2.5:1, desirably at least 2.9:1, and preferably at least
3.0:1. Large quench gas port 5 and quench gas flow director 10 are
further described in the context of FIG. 2. Instrumentation ports
111 and 112 as the name indicates are for connecting various
aspects of the equipment internal to the furnace and, for example
the vacuum carburizing control panel. Vacuum pumping port 113 is,
as the name indicates, to be connected, through high stress
tolerant piping, to a vacuum pump and, alternately, through an
alternate high stress tolerant piping system to a high pressure
system, e.g. a high pressure surge tank.
[0015] The processing end of furnace 100 as illustrated in cross
section in FIG. 2 of the invention evidences emphatically some
further complementary aspects of the instant invention. Effective
work area 120 shown as the elongated rectangle with corner to
corner diagonal lines, the bottom of which is along the surface of
hearth 7. As shown in FIG. 1, effective work area 120 has a
generally square cross section completely surrounded by heating
elements 1. As shown in FIG. 2, that "surround" continues
completely along the length of the furnace hot zone. Long and
narrow effective work zone 120 as described in the context of FIG.
1 is of significant benefit in promoting uniform carburizing. Large
gas plenum chamber 13 (about 90% as large as the interior of
furnace 100 hot zone) is also of significant benefit. Its size and
configuration and outlets provide substantial opportunity for
directed but relatively free flow of quench gas. Additional
advantages in quench gas flow uniformity is provided by gas quench
nozzles 9 extending form the interior of hollow furnace door
through the interior wall of the door and the insulation anchored
to the door to communicate with the door interior. As mentioned
above, when quench gas enters cylindrical gas plenum chamber 13 at
high velocity through port 5 some of that gas passes from plenum
chamber 13 into door 124 through orifices 122 in gas plenum
restrictor 121. From door 124 the gas passes through 8 additional
gas quench nozzles 9 to provide yet more flow toward work zone 120.
The hot zone of furnace 100 when it is in the heating mode
advantageously operates in the range of 260 degrees C. (500 degrees
F.) and 1316 degrees C. (2400 F) with a temperature uniformity
within the furnace of 427 to 93 degrees C+/-5.6 C (800 to 200
degrees F+/-10 F). The system is designed to operate in conjunction
with a roughing pump (commercially available). In the heating mode
of the furnace, reflective heat radiation baffle 8 reflects heat
back to the furnace hot zone and away from equipment in the furnace
end opposite the door end. Behind reflective heat radiation baffle
8 is generally circular gas exit port 55 which in operation, for
example in quenching mode allows gas to pass from the furnace
interior through gas diffuser baffle 6 into the all copper, gas to
water, vacuum tight fin tube heat exchanger 14 included in FIG.
3.
[0016] Important additional embodiments of the instant invention
which also complement the overall effectiveness of the furnace are
revealed by the equipment and the processes used in conjunction
with the operation of vacuum tight fin tube heat exchanger 14 and
the other equipment depicted in FIG. 3. It is particularly so as
the process moves from a heating and or vacuum mode to a very high
pressure, cooling mode. Because the capital invested in such
furnaces is significant, furnace owners want to keep the overall
treatment time for each workload as brief as is practical while
producing high quality product. Thus, decreasing the quench time
provides significant advantage in cost, and has been found to be
another advantage provided by the instant invention. The use of
large heat exchanger 14 together with gas quench blower 16 having a
300 horsepower motor provide additional significant complement to
the furnace design. Heat exchanger 14 provides cooled gas through
gas inlet collector, which focuses the gas flow from the heat
exchanger into fan scroll housing 33 (see FIG. 7) for recirculation
forced by gas quench blower (fan) 16. The cooling system comprising
a blower (fan) that has a 300 hp motor for very high velocity gas
flow, a low resistance to flow, vacuum tight, straight through, all
copper water cooled fin and tube designed heat exchanger, is
designed to support a 10 bar gas quenching system. Blower 16 has a
radial fan wheel and a fan scroll which acts as a pump or
compressor which pushes the gas straight up toward low pressure
drop, high volume gas return duct 17. The design of the low
pressure drop high volume duct is significant due to the fact that
there are no sharp right angles the gas has to pass through. This
gently curving, large radius, duct 17 prevents large pressure drops
from occurring, thus minimizing the potential for turbulence. The
ability to maintain the pressure as the gas is passing through the
heat exchanger, blower and return duct allows the gas to be driven
at a very high velocity, upwards of 327 kmph (200 mph) through the
quenching nozzles and toward the workload at such speeds to provide
faster quenching than in the prior art. Quench gas nozzles 9 are a
unique high velocity, threaded graphite tube, which is designed for
ease of replacement. Advantageously, a total of 8 quench gas
nozzles are functionally anchored in the autoclave door and at
least 60 to 70+gas nozzles evenly distributed throughout the ring
assembly to surround the workload. Quench gas nozzles 9 are
directed toward the workload to maximize the cooling capability
while giving a uniform quench. The delivery gas jets have an
internal taper to maximize the gas velocity. Gas flow plenum
chamber 13 is also cylindrical.
[0017] The process for carburizing in accordance with one
embodiment of the invention herein involves loading high integrity
furnace 100 with pieces to be carburized by placing the pieces in
furnace work zone 120 and closing furnace door 124. Furnace 100, is
thereafter evacuated, i.e., removing substantially all gas (or
"drawing a vacuum", in vacuum furnace parlance) from furnace. The
high integrity furnace must have a leak rate of 5 microns (Hg) or
less per hour. In addition all gases used in the process must be of
the highest purity, The purest grade commercially available.
Impurities found in lower grade gases, according to the present
invention have been found to contribute to soot formation and
product contamination. Also, before each carburizing run all gas
feed lines are to be bubble tested to ensure they are effectively
leak free. After the vacuum has been drawn, in accordance with an
important embodiment of the present invention, highly purified
hydrogen (again, the purest quality available) is piped into
furnace 100 through leak free conduit to a relatively low pressure
(for example, partial pressure 4.5 torr). Because the furnace is
under intense, practically complete vacuum at the start of the
hydrogen transfer into the furnace the transfer and distribution
occurs quickly, uniformly and completely as hydrogen seeks to fill
the furnace. When the pressure reaches at most 8 torr, the furnace
temperature is then increased to about 954 C (1750 F). The pieces
to be carburized are in that heated hydrogen environment (hydrogen
soaked) for about one hour, typically 60-65 minutes. The hydrogen
soak has been found to be particularly effective in oxide removal
prior to carburizing. The hydrogen also serves to activate or open
the surface of the pieces thereby facilitating carburizing.
Additional hydrogen or other high purity diluent is then added to a
pressure of at least 8 torr. (The term "at least 8 torr" herein
means the pressure is not lower than 8 torr.) High purity
carburizing gas, advantageously acetylene, is then inserted into
the furnace having hydrogen therein, thus gradually displacing some
but not all of the hydrogen to carburize the workload at a pressure
of at least 8 torr, desirably between 8 and 15 torr, and
advantageously between 8 and 10 torr. Utilizing established data
based on a solution of Fick's Law of Diffusion and the known ratio
R, which relates diffuse time to carburizing time) carburizing
cycles can readily be developed to result in case depths in the
range of 0.035'', a surface carbon content of approximately 0.8%,
and Rockwell hardness values in the low to mid 60's.
[0018] Because furnace 100 according to the present invention is
versatile and will be used for treating several different metals
(alloys) it is desirable to have piped connections for channeling
various gasses into and out of the furnace. The specific needs for
many furnaces according to the instant invention may vary. Drawing
4 is not to scale, and is for illustration of desirable components
of the furnace rather than a precise engineering drawing, that is,
a schematic of furnace 100 illustrating by example the array of
controllers, meters, motors, etc. that provide some detail as to
the complexity of such equipment confronting the personnel of
ordinary skill in this art. Furnace 100 with its mammoth duct 17 is
shown in partial phantom with carburizing nozzles 11 (rotated for
illustration only) and carburizing nozzles 18 extending through
door 124. Carburizing gas line 23 connects the gas cylinder to gas
manifold 179 via a high accuracy mass flow controller 47. Hydrogen
gas line 22 is also connected to gas manifold 170 via mass flow
controller 46. Nitrogen gas line 21 and hydrogen gas line 22 are
also connected to gas manifold 179 via partial pressure flow valves
44 for hydrogen and 45 for nitrogen. Carburizing gas mixtures are
fed through gas manifold 179 to the gas carburizing nozzles with a
separate line 180 directing the carburizing gas to interior door
nozzles 18. Trim valves allow the control of the carburizing gas
distribution between the different carburizing gas nozzles. 19. A
particularly effective carburizing process in accordance with this
invention includes varying the flow rate of the carburizing gas at
regular intervals, for example, every five to ten minutes, in a
descending direction and increasing the flow rate of the diluent
gas correspondingly, thereby maintaining an absolute pressure of at
least 8 torr.
[0019] To further improve carburizing efficiency the design of
carburizing gas nozzles 18 shown in FIG. 5A and their arrangement
within the autoclave door desirably fit in a uniform arrangement at
12:00, 3:00, 6:00, and 9:00 as on the face of a clock. Nozzles 18
are designed as graphite threaded units for ease of replacement and
freedom from clogging. Alternatively the nozzles can be
manufactured with ceramic materials. The gas mixtures, which are
delivered from stainless steel line 180, enter manifold 143 before
being evenly distributed over the 4 carburizing nozzles 18. The
nozzle heads have an internal jet tube 20 shown in FIGS. 5B and 5C
which is not centered but angled. This angled design controls the
carburizing gas flow from the interior of the door toward the
center of the workload. The aperture size can be 1.59 mm-3.96 mm,
desirably 3.96 mm.
[0020] Even further improvements in carburizing efficiency within
the furnace chamber derive from the design of internal furnace
carburizing nozzles 11, which were designed as graphite threaded
nozzles for ease of replacement and freedom from clogging. Internal
jet tube 30, as shown in FIGS. 6A and 6B, is centered with a
chamfer to give a more streamlined laminar flow as opposed to a
turbulent flow. The centered alignment gives efficient and direct
gas flow from nozzle 11 to the workload. The internal diameter of
the gas jet tubes can be 1.59 mm-3.96 mm, with 3.96 mm as a desired
diameter. Carburizing gas connect tube 42, as shown in 6B-6D, is at
a 90 degree angle in order to reduce or block heat.
[0021] As noted above, versatile furnace 100 and the investigation
of how to use it most beneficially has opened the way for different
and economic processes for heat iron-containing alloys and
especially for carburizing. For metal treatment that reaction can
be sensitive to a number of different interactions with impurities.
Having the metal cleaned by chemical purification in the same
chamber in which it is to be subjected to later treatment by heat
and, and, or chemicals and, or pressure change would not be
acceptable UNLESS, as is the case with the instant invention, the
undesirable bi-products of the cleaning were completely removed
from the chamber after the cleaning and before the treatment. The
chemical and physical (high and low pressure and temperature
environments, as well as high velocity gas flow) that are necessary
and desirable for metal treatment are also fraught with the
potential for adding additional undesirable impurities during the
various physical and chemical changes taking place in or on the
metal surface. The following list includes some of the important
factors according to the present invention helping to tame this
very complex technical challenge in addition to the high quality
furnace described above:
1. high purity source of gases such as hydrogen, acetylene,
ethylene, propane, nitrogen and argon that can supplied through gas
lines into the chamber to a controlled level.
[0022] 2. low vacuum capability, e.g., to evacuate the chamber,
high pressure capability to operate at a pressure level of 10 bar,
and very high gas-circulating capability, and gas transport lines
for providing gas to and drawing gas from the chamber with the
ability to control low pressures, for example to at least within
0.1 torr
[0023] 3. heating capability and instruments for controlling
temperatures for heating in the range of 30 C up to at least 1316 C
with a temperature, including the ability to heat the furnace to
954 C and hold that temperature for 60 to 65 minutes for example to
soak the workload in hydrogen for that length of time.
4. The capability to quench very rapidly by releasing quench gas
into the chamber and recycling the gas at a high rate.
[0024] 5. Having well articulated processes with well defined
guidelines that include, for example: treating (in a specific
chamber of a heat treating vacuum furnace having low vacuum
capability, high pressure capability, and very high gas-circulating
capability), with gas transport lines for providing gas to and
drawing gas from said chamber, surfaces of steel alloy work pieces,
by: [0025] (a) drawing a very low pressure vacuum to evacuate gas
from the chamber; [0026] (b) allowing hydrogen to flow through a
gas line into the chamber to a pressure not exceeding 10 torr;
[0027] (c) heating the chamber to a temperature up to, desirably,
954 C and soaking the work pieces in that heat for at least 60 to
65 minutes, and then adjusting the pressure to at least 7.6 torr.
by adding gas as necessary; [0028] (d) while maintaining the
pressure at a level of at least 7.6 torr, (at a pressure no lower
than 7.6 torr) and at a temperature at 954 C adding to said chamber
through at least one said gas line, gas having a capability of
desirably affecting said surfaces 9 for example a carburizing gas
and then [0029] (e) shutting of the gases and introducing nitrogen
through a gas line up to 10 torr and allowing the work pieces to
diffuse for a time dependent upon, the work load composition, e.g.,
the alloy make up and then [0030] (f) shutting off the heating
mechanism and very rapidly quenching by releasing large quantities
of quench gas at high pressure into said chamber and recycling the
quench gas at a high rate of speed.
[0031] Although specific embodiments of the present invention have
been described above in detail, it will be understood that this
description is merely for purposes of illustration. Various
modifications of, and equivalent steps corresponding to, the
aspects of the preferred embodiments, in addition to those
described above, may be made by those skilled in the art without
departing from the spirit of the present invention defined in the
following claims, the scope of which is to be accorded the broadest
interpretation so as to encompass such modifications and equivalent
embodiments.
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