U.S. patent application number 13/988254 was filed with the patent office on 2013-11-14 for surface treatment of metal objects.
This patent application is currently assigned to HARD TECHNOLOGIES PTY LTD. The applicant listed for this patent is Daniel Fabijanic. Invention is credited to Daniel Fabijanic.
Application Number | 20130299047 13/988254 |
Document ID | / |
Family ID | 46083420 |
Filed Date | 2013-11-14 |
United States Patent
Application |
20130299047 |
Kind Code |
A1 |
Fabijanic; Daniel |
November 14, 2013 |
SURFACE TREATMENT OF METAL OBJECTS
Abstract
A process for forming an outer diffusion surface layer on a
metal substrate or member includes in a first activation stage of
an inert particulate refractory material and a metal based material
including metals and metal halides. An inert gas and hydrogen
halide gas is introduced into the inert particulate refractory
material and the metal based material to activate an outer surface
of the metal based material. The metal substrate is pretreated to
form a diffusion zone extending inwardly from the outer surface of
the metal substrate having nitrogen forming an inner diffusion zone
and an outer compound or white layer of an iron nitride, an iron
carbide or an iron carbonitride compound without an oxide layer. A
subsequent diffusion stage treats the metal substrate in an inert
gas, in the absence of hydrogen halide gas to form the diffusion
surface layer on the metal substrate.
Inventors: |
Fabijanic; Daniel; (Waurn
Ponds, AU) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Fabijanic; Daniel |
Waurn Ponds |
|
AU |
|
|
Assignee: |
HARD TECHNOLOGIES PTY LTD
Launching Place, Victoria
AU
|
Family ID: |
46083420 |
Appl. No.: |
13/988254 |
Filed: |
November 17, 2011 |
PCT Filed: |
November 17, 2011 |
PCT NO: |
PCT/AU11/01479 |
371 Date: |
July 19, 2013 |
Current U.S.
Class: |
148/230 |
Current CPC
Class: |
C23C 8/24 20130101; C23C
8/02 20130101; C23C 8/26 20130101 |
Class at
Publication: |
148/230 |
International
Class: |
C23C 8/26 20060101
C23C008/26 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 17, 2010 |
AU |
2010905095 |
Claims
1. A method of forming a diffusion surface layer extending inwardly
of an outer surface of a metal substrate, said method including:
(i) in an activation stage, providing an activation treatment
furnace containing an inert particulate refractory material and a
metal based material for forming said diffusion surface layer, said
activation treatment furnace having a flow of an inert gas
introduced into the inert particulate refractory material and the
metal based material in the activation treatment furnace for a
first period of time to treat an outer surface region of said metal
based material in the presence of a hydrogen halide gas to form an
activated metal based material with an activated surface region;
and (ii) in a diffusion stage, providing a diffusion treatment
furnace and introducing said metal substrate into said diffusion
treatment furnace, said metal substrate having been pretreated to
form a diffusion zone extending inwardly from the outer surface of
the metal substrate in which nitrogen has been diffused to form an
inner diffusion zone and an outer compound or white layer formed,
at least in part, by an iron nitride, an iron carbide or an iron
carbonitride compound without an oxide layer on said outer surface
of the metal substrate, treating the metal substrate in the
diffusion treatment furnace, sealed against the ingress of
atmospheric air and under an inert gas atmosphere, in the absence
of hydrogen halide gas for at least a second period of time, in the
presence of said activated metal based material, to form said
diffusion surface layer on said metal substrate.
2. A method according to claim 1 wherein the activation treatment
furnace is the same as the diffusion treatment furnace.
3. A method according to claim 1 wherein the diffusion treatment
furnace is different to the activation treatment furnace.
4. A method according to claim 1 further including: (i) in a
pretreatment stage, forming said diffusion zone extending inwardly
from the surface of the metal substrate in which nitrogen has been
diffused to form said inner diffusion zone and said outer compound
or white layer of an iron nitride, iron carbide or carbonitride
compound; and (ii) treating the metal substrate formed in said
pretreatment stage to either prevent formation of a surface oxide
on said surface or to remove any said surface oxide formed on said
surface prior to said metal diffusion stage.
5. A method according to claim 1 wherein the inert gas flow in said
activation stage is nitrogen and/or argon.
6. A method according to claim 1 wherein the inert particulate
refractory material is aluminium oxide or silicon carbide.
7. A method according to claim 1 wherein the diffusion treatment
furnace contains an inert particulate refractory material that is
fluidized by a flow of an inert gas during said metal diffusion
stage.
8. A method according to claim 1 wherein the diffusion treatment
furnace contains an inert particulate refractory material that is,
at least partly fluidized by vibration means during said metal
diffusion stage.
9. A method according to claim 1 wherein ammonia is not supplied to
the diffusion treatment furnace during said metal diffusion
stage.
10. A method according to claim 1 wherein said second period of
time is greater than said first period of time.
11. A method according to claim 1 wherein during said second period
of time a hydrogen halide gas flow is pulsed to said diffusion
treatment furnace for periods of no hydrogen halide gas flow and at
least one period of hydrogen halide gas flow.
12. A method according to claim 1 wherein said diffusion treatment
furnace contains an inert particulate refractory material and an
inert gas flow is provided to said diffusion treatment furnace
during said second period of time, said inert gas flow being
variable between a zero flow rate and a flow rate at or above a
minimum fluidization velocity for said diffusion treatment
furnace.
13. A method according to claim 1 wherein a temperature of between
500 and 750.degree. C. is maintained in said activation treatment
furnace for said first predetermined period of time.
14. A method according to claim 1 wherein a temperature of between
500 and 750.degree. C. is maintained in said diffusion treatment
furnace for said second predetermined period of time.
15. A method according to claim 1 wherein said hydrogen halide gas
is supplied to said activation treatment furnace continuously
during said first predetermined period of time.
16. A method according to claim 1 wherein said hydrogen halide gas
is supplied to said activation treatment furnace during said first
predetermined period of time in a pulsed manner for periods of
supply separated by periods of non supply.
17. A method according to claim 1 wherein said hydrogen halide gas
is selected from hydrogen chloride gas, hydrogen bromide gas,
hydrogen fluoride gas, and hydrogen iodide gas.
18. A method according to claim 1 wherein the hydrogen halide gas
is mixed with a said inert gas prior to entry into the activation
treatment furnace.
19. A method according to claim 1 wherein the hydrogen halide gas
is mixed with a said inert gas prior to entry into the diffusion
treatment furnace.
20. A method according to claim 1 wherein ammonium chloride is
supplied to said activation treatment furnace during said
activation stage, said ammonium chloride being heated while being
introduced to disassociate into nitrogen gas and hydrogen chloride
gas for activation of said metal based material.
21. A method according to claim 18, wherein the hydrogen halide gas
and the inert gas mixture enters the activation or diffusion
treatment furnace at a lower region thereof.
22. A method according to claim 1 wherein the metal based material
for forming the diffusion surface layer is chosen from at least one
of: (i) a solid metal or metal alloy; (ii) a metal or metal alloy
coated on a substrate carrier; (iii) a particulate or powder metal
or metal alloy; (iv) a metal or metal alloy coated on an inert
particulate refractory material; (v) a metal halide particle or
powder (anhydrous or hydrated); and (vi) a metal halide material
(anhydrous or hydrated) coated on an inert refractory particulate
material or a substrate carrier.
23. A method according to claim 1 wherein the metal based material
for forming the diffusion surface layer is selected from chromium,
titanium, vanadium, niobium, tantalum, tungsten, molybdenum,
manganese, and alloys thereof including ferrous based alloys, or
metal halides comprised of a metallic element of the aforesaid
metals and a halide selected from chlorine, bromine, iodine and
fluorine.
24. A method according to claim 1 wherein the metal substrate is a
ferrous based metal or a ferrous based metal alloy.
25. A method according to claim 1 wherein said inert gas introduced
into said diffusion treatment furnace during said second
predetermined period of time is nitrogen.
Description
[0001] The present invention relates to methods and apparatus for
treating a metal substrate to achieve a diffusion surface layer on
the substrate.
[0002] Metal surface treatments have traditionally comprised
forming a nitrided surface on the substrate followed by a physical
vapour deposition of a coating such as titanium, chromium nitride
or carbon nitrocarburising onto the surface as an adhered coating.
Some work has also been carried out where the surfacing material is
diffused into the surface zone of the substrate simultaneously as
nitrogen diffuses towards the surface making a chromium or titanium
nitride or carbon nitride layer on the surface. The published
patent specification of European Patent Nos. 0471276, 0252480,
0303191 and an International Publication Number WO/47794 disclose
such treatment methods. Such methods are capable of providing a
better performing surface treatment because, the surface layer is a
diffusion layer and not simply a coating layer adhered to the
substrate, however, practical control of the required materials and
parameters to achieve this desirable result has proven to be quite
difficult. The use of a halide gas such as HCl mixed with a
reactive gas or a combustible gas such as hydrogen and/or ammonia
leads to problems in the construction of the mixing gas panel.
Further HCl and other halide gases are relatively expensive and
extensive use of such gases can provide relatively expensive
processing of a desired product. Also the halide gas can react
instantly at low temperatures with ammonia forming solid ammonium
chloride which may block the gas pipes and even leak back into the
solenoid valves and flow meters of the gas delivery equipment
causing blockages and potential damage to the equipment.
[0003] International patent application no. PCT/AU2006/001031
discloses treatment methods and treatment apparatus enabling a
desired diffusion layer to be formed on a metal substrate product,
the methods disclosed supply halide gas throughout a lengthy period
of the processing and while the methods work satisfactorily, the
processing cost is quite expensive due to the required volume of
halide gas utilized.
[0004] The objective therefore of the present invention is to
provide a method of forming a diffusion surface layer on a metal
substrate in a more economical manner than with prior art processes
while still retaining a reliable and safe processing of the metal
substrate.
[0005] Accordingly, the present invention provides in a first
aspect, a method of forming a diffusion surface layer extending
inwardly of an outer surface of a metal substrate, said method
including: [0006] (i) in an activation stage, providing an
activation treatment furnace containing an inert particulate
refractory material and a metal based material for forming said
diffusion surface layer, said activation treatment furnace having a
flow of an inert gas introduced into the inert particulate
refractory material and the metal based material in the activation
treatment furnace for a first period of time to treat an outer
surface region of said metal based material in the presence of a
hydrogen halide gas to form an activated metal based material with
an activated surface region; and [0007] (ii) in a diffusion stage,
providing a diffusion treatment furnace and introducing said metal
substrate into said diffusion treatment furnace, said metal
substrate having been pretreated to form a diffusion zone extending
inwardly from the outer surface of the metal substrate in which
nitrogen has been diffused to form an inner diffusion zone and an
outer compound or white layer formed, at least in part, by an iron
nitride, an iron carbide or an iron carbonitride compound without
an oxide layer on said outer surface of the metal substrate,
treating the metal substrate in the diffusion treatment furnace,
sealed against the ingress of atmospheric air and under an inert
gas atmosphere, in the absence of hydrogen halide gas for at least
a second period of time, in the presence of said activated metal
based material, to form said diffusion surface layer on said metal
substrate.
[0008] Conveniently, the aforesaid method may further include:
[0009] (i) in a pretreatment stage, forming said diffusion zone
extending inwardly from the surface of the metal substrate in which
nitrogen has been diffused to form an inner diffusion zone and an
outer compound or white layer of an iron nitride, iron carbide or
carbonitride compound; and [0010] (ii) treating the metal substrate
formed in said pretreatment stage to either prevent formation of a
surface oxide on said surface or to remove any said surface oxide
formed on said surface prior to said metal diffusion stage.
[0011] In a preferred construction the aforesaid method may be
carried out in a single treatment furnace where the diffusion
treatment furnace also acts as the activation treatment furnace.
The method can however, be carried out in different furnaces acting
as the activation treatment furnace and the diffusion treatment
furnace.
[0012] Preferably, the inert gas flow in the activation stage may
be nitrogen and/or argon. Conveniently, the inert particulate
refractory material utilized in the treatment furnace or furnaces
might be aluminium oxide or silicon carbide.
[0013] Conveniently, when the diffusion treatment furnace contains
an inert refractory particulate material, it is fluidized by a flow
of an inert gas during the metal diffusion stage. Alternatively,
such an inert refractory particulate material might be fluidized or
at least partly fluidized by vibration means. Preferably, ammonia
is not supplied to the diffusion treatment furnace during the metal
diffusion stage.
[0014] In a particularly preferred embodiment, the second period of
time is greater than the first period of time. In this manner, the
relatively expensive hydrogen halide gas is used for much shorter
periods to achieve the desired diffusion layer on the metal
substrate. During the diffusion stage, the hydrogen halide gas
might not be utilized at all but small amounts of the hydrogen
halide gas could be used for short periods of time to reactivate
the metal based material, if required. Typically, if required, the
hydrogen halide gas might be pulsed for periods of no hydrogen
halide gas provided in the retort and at least one period of
hydrogen halide gas provided during the diffusion stage.
[0015] Conveniently an inert gas flow may be provided to the
diffusion treatment furnace during the second period of time, the
inert gas flow being variable from a zero flow rate to a flow rate
at or above a minimum fluidization velocity for the diffusion
treatment furnace.
[0016] Conveniently the operating temperature for the first and
second periods for the treatment furnace or furnaces during the
activation stage and the diffusion stage is between 500 and
750.degree. C.
[0017] Preferably, in one embodiment the hydrogen halide gas flow
may be supplied continuously to the activation treatment furnace
during the first period of time. In a possible alternative, the
hydrogen halide gas might be pulsed with periods of supply and
periods of non supply during the first period of time.
Conveniently, the hydrogen halide gas used might be selected from
hydrogen chloride gas, hydrogen bromide gas, hydrogen fluoride gas
or hydrogen iodide gas. The hydrogen halide gas when supplied to
the activation treatment furnace or the diffusion treatment furnace
is preferably mixed with an inert carrier gas (e.g. nitrogen and/or
argon gas) externally of the treatment furnace or furnaces.
Conveniently, when supplied, the hydrogen halide gas and the inert
carrier gas enter the treatment furnace or furnaces at a lower
region thereof.
[0018] In a further embodiment, the hydrogen gas might be created
in the treatment furnace or furnaces by supply of ammonium chloride
(NH.sub.4Cl). Ammonium chloride might be supplied in solid or
pellet form through a delivery tube or pipe whereby it is heated in
the delivery pipe or tube to disassociate into nitrogen gas and
hydrogen chloride (HCl) gas. An inert gas such as nitrogen or argon
could also be supplied via the delivery tube such that the HCl gas
is at least partly mixed with the inert gas by the time it enters
the furnace. Such a delivery system might be used in either the
activation stage or, if required, in the diffusion stage. If this
delivery system is used, the operating temperature of the furnace
might be close to 700.degree. C. or even higher.
[0019] The metal based material for forming the diffusion surface
layer may be chosen from at least one of: [0020] (i) a solid metal
or metal alloy; [0021] (ii) a metal or metal alloy coated on a
substrate carrier; [0022] (iii) a particulate or powder metal or
metal alloy; [0023] (iv) a metal or metal alloy coated on an inert
particulate refractory material; [0024] (v) a metal halide particle
or powder (anhydrous or hydrated); and [0025] (vi) a metal halide
material (anhydrous or hydrated) coated on an inert refractory
particulate material or a substrate carrier. The metal based
material might be selected from chromium, titanium, vanadium,
niobium, tantalum, tungsten, molybdenum, manganese, and alloys
thereof including ferrous based alloys, or metal halides comprised
of a metallic element of the aforesaid metals and a halide selected
from chlorine, bromine, iodine and fluorine.
[0026] The metal substrate is conveniently a ferrous based metal or
a ferrous based metal alloy.
[0027] Conveniently nitrogen as an inert gas is introduced into the
diffusion treatment furnace during the second period of time.
[0028] The term "metal substrate" is intended to refer to any metal
part suitable for heat treatment made from ferrous based metal or
ferrous based metal alloys.
[0029] In accordance with the method of this invention where
hydrogen chloride is the halide gas used and chromium metal
particles are used to form the diffusion surface layer, it is
believed that hydrogen chloride causes an active chromium chloride
layer on the surface of the aluminium oxide (inert fluidizing
media) as well as on the chromium metal particles in the
fluidizable bed furnace during the activation stage. During the
metal diffusion stage of the process, a solid-state interaction
between the activated chromium chloride and a nitrogen-rich ferrous
surface of the metal substrate occurs to form the diffusion surface
layer on the substrate. This occurs when the treatment furnace,
typically a fluidizable bed furnace is substantially not fluidized
by a flow of inert gas and also when the bed is fluidized.
Fluidization of the bed can occur either by a suitable gas flow or
by some vibration means as is known in the art. The process has
considerable economic advantages as the hydrogen halide gas,
typically hydrogen chloride is expensive and minimizing its use
provides a much more economical process.
[0030] It is generally desirable that the outer portion of the
diffusion zone (the white layer), be substantially free from
porosity. The white layer will normally be an iron nitride, iron
carbide and/or an iron carbonitride, typically either epsilon
and/or the gamma form.
[0031] A preferred embodiment of the process of this invention will
now be described with reference to the accompanying drawings, in
which:
[0032] FIG. 1 is a cross-sectional schematic view of a fluidizable
bed furnace capable of use in the performance of the present
invention;
[0033] FIGS. 2 and 3 are detailed cross-sectional views of seal
arrangements capable of use with the fluidizable bed furnace shown
in FIG. 1;
[0034] FIG. 4 is a graph showing Nitrogen (N), Chromium (Cr) and
Iron (Fe) wt % concentrations against depth in the treated metal
sample of Example 1 produced according to the present
invention;
[0035] FIG. 5 is a graph showing Nitrogen (N), Chromium (Cr), Iron
(Fe) and Copper (Cu) against depth in the activated chromium coated
copper carrier substrate of Example 1;
[0036] FIG. 6 is a graph showing Nitrogen (N), Chromium (Cr) and
Iron (Fe) wt % concentrations against depth in the treated metal
sample of Example 1 not produced according to the present
invention;
[0037] FIG. 7 is a graph showing Nitrogen (N), Chromium (Cr), Iron
(Fe) and Copper (Cu) wt % concentrations against depth in the non
activated chromium coated copper carrier substrate of Example
1;
[0038] FIG. 8 is a graph showing Chromium (Cr), Iron (Fe) and
Nitrogen (N) wt % concentrations against depth in the treated metal
sample of Example 2 produced according to the present
invention;
[0039] FIG. 9 is a graph showing Chromium (Cr), Iron (Fe) and
Nitrogen (N) wt % concentrations against depth in the treated
activated Chromium sample utilized in Example 2 resulting from
carrying out the process of this invention;
[0040] FIG. 10 is a graph showing Iron (Fe), Nitrogen (N) and
Chromium (Cr) wt % concentrations against depth in the metal sample
when not treated with a preactivated chromium sample as described
in Example 2;
[0041] FIG. 11 is a quantitative depth profile showing Iron (Fe),
Chromium (Cr), Nitrogen (N), Carbon (C) and Oxygen (O) wt %
concentrations against depth in the metal sample treated according
to the present invention utilising activated chromium powder as
described in Example 3;
[0042] FIG. 12 shows the microstructure of the treated metal sample
represented in FIG. 11 (Example 3);
[0043] FIG. 13 is an x-ray diffraction analysis showing the
diffusion layer in the treated metal sample (Example 3) was
predominantly CrN;
[0044] FIGS. 14 and 15 are a quantitative depth profile showing
Chromium (Cr), Iron (Fe), Nitrogen (N), Carbon (C) and Oxygen (O)
against depth in the respective treated metal sample as described
in Example 4; and
[0045] FIG. 16 shows the microstructure of the treated metal
samples as described in Example 4.
[0046] Reference will now be made to the drawings which
schematically illustrate relevant parts of a fluidized bed
treatment apparatus according to a preferred form of this
invention, it being understood from the preceding disclosure that
at least the pre-treatment stage of the heat treatment process need
not be completed in fluidized bed heat treatment equipment and any
other known heat treatment equipment could be used in this stage.
Moreover, although it is desirable that the activation stage and
the diffusion stage be carried out in the same fluidized bed heat
treatment furnace, it is equally possible for separate fluidized
bed heat treatment furnaces to be used for the activation and
diffusion stages.
[0047] As illustrated in FIG. 1, the apparatus comprises a
fluidized bed furnace 10 having an inner retort 11 containing a
particulate inert refractory material 12 such as aluminium oxide
(Al.sub.2O.sub.3), however, other such inert refractory materials
can be employed. The furnace includes an outer insulating layer 13
and a heating zone 14 that might be heated in any conventional
manner by combusting a fuel gas, by electrical resistance heating
or by any other suitable means. In the drawings, the heating zone
14 is heated by a fuel gas supplied burner 16. At the bottom of the
retort 11, a primary inert gas supply line 17 is provided for
fluidizing the refractory material 12 when required. The gas supply
line 17 leads to a gas distribution system comprised of a primary
distributor 18 and a secondary distributor 19 typically of a porous
material construction that is aimed at preventing streaming of the
gas flow within the retort and thereby even fluidization and heat
treatment. A further gas delivery line 20 is provided so that a
halide gas and an inert carrier gas can be introduced into the
bottom of the retort via a further distributor 21 separate from the
distributors 18/19. A carrier inert gas line (e.g. nitrogen and/or
argon) might be supplied via a line 70 with a hydrogen halide gas
supplied via line 71 and mixed in a valve 72 before being delivered
via line 20. The amount of inert gas delivered via lines 70 and 17
and the amount of hydrogen halide gas delivered via line 71 may be
metered such that the gas quantity delivered to the furnace 10 is
known. The distributor 21 might be positioned in the coarse
refractory material zone 80 in the lower region of the retort 11.
As an alternative, the delivery line 20 may enter through the
bottom of the retort as shown in broken outline or elsewhere
subject to the distributor 21 being located in the lower region of
the retort. In this arrangement the delivery line 20 might pass
upwardly as shown at 20' and include one or more heating coils 81
before returning the halide and inert carrier gas to the
distributor 21 in the lower region of the retort 11. The heating
coil(s) 81 are conveniently just above or just within the coarse
refractory material zone 80. It is preferred that the halide gas
and the inert carrier gas be thoroughly mixed externally of the
retort 11 and further that it be heated before the mixed gases
enter the retort. Conveniently heating occurs by heat exchange with
a region of the fluidized bed treatment furnace. With the
illustrated arrangement in full line, heating of the externally
mixed gases occurs as the line 20 passes downwardly through the
heated refractory material in the retort. Other arrangements are
equally possible. For example one or more coils of the delivery
pipe might be provided in the line 20 within the retort.
Alternatively, the delivery line 20 might pass through the heating
zone 14 with one or more coils located in the zone 14. In yet
another possible arrangement the premixed inert carrier gas and
hydrogen halide gas might enter the furnace directly to be
discharged via the distributor 21 without being preheated. Metering
and mixing equipment (not illustrated in detail) is used to ensure
proper proportions of halide gas and inert carrier/fluidizing gases
are used in the activation stage of the treatment process.
[0048] An exhaust passage 22 leads from an upper region of the
retort 11 whereby exhaust gases can escape in a controlled manner
and be treated downstream (not shown) for safety purposes. It is
possible for some of the refractory material to escape along this
path and this material is conveniently collected in a grit
collection box or container 23. From time to time it is possible
for certain reaction products to solidify in this passage 22 which
might lead ultimately to the passage becoming blocked. A scraper
mechanism 24 may be provided to scrape such materials, preferably
back into the collection box 23. Other approaches could be utilized
rather than the illustrated physical scraper. For example, pulsed
bursts of inert gas might be used from time to time to break up or
move material in the exhaust passage 22 back into the retort 11.
Conveniently particulate metal or metal alloy (when used in a
treatment process) can also be introduced via the exhaust passage
22. A storage zone 25 for such particulate metal is provided with a
metering valve or the like 26 to deliver a desired quantity of
metal powder or metal coated particulate material into the passage
22. The scraper mechanism 24 if used or some pusher device might
then be used to push this metal into the retort 11 when required.
This is preferably done when the bed is slumped (i.e. not in
operation) such that there is no or minimal gas flow in an outward
direction along the passage 22.
[0049] As shown in FIG. 1, a first seal means 27 associated with a
cover member 29 is provided around the upper access opening 28
leading to the inner zones of the retort 11. The first seal means
27 enables the retort 11 to be sealed against the ingress of
atmospheric air during a treatment process. Features of the first
seal means 27 are better seen in FIG. 2 or 3 where they are shown
operationally with the cover member 29 for the upper access opening
28. The first seal means 27 comprises a first outer seal part 30
formed by a circumferential flange 31 on the cover member 29
engaging with a seal material 32 positioned between two
circumferential and radially spaced flanges 33, 34 on a member 35
secured to the retort 11 and surrounding the access opening 28. The
first seal means 27 further includes a second inner seal part 36
formed by circumferential flange 37 supported on the member 35 and
engaging with a seal material 38 positioned between the outer
flange 31 on the cover member 29 and a more inwardly located
circumferential flange 39 carried by the cover member 29. The seal
materials 32 or 38 may be any compressible seal material capable of
operation at the relevant operating temperatures for the furnace,
but may include ceramic fibre or VITON (registered trade mark)
rubber material. When the first seal means 27 is operationally
engaged as illustrated in FIG. 2a, a seal zone 40 is established
between the flanges 31 and 37. A gas distributor tube 41 is located
in this zone 40 and is fed externally via a line schematically
shown at 42 to deliver nitrogen, argon or some other inert gas to
the zone 40 at a pressure whereby such gas will leak towards the
retort opening 28 if leakage is possible thereby preventing ingress
of atmospheric oxygen into the retort 11. The seal means 27 further
includes a third seal part 43 formed by the inner circumferential
flange 39 being engaged in a zone 44 containing inert refractory
particulate material 45 typically of the same type as contained
within the retort 11. The particulate material 45 may be fluidized
by an inert gas supply delivered via line 46 to a distributor 47
therefor to assist at least entry of the flange 39 into the
particulate material 45 as the cover member 29 moves to the
illustrated closed position. To enable access to the retort 11, the
cover member 29 is removed. This would occur, for example, when a
treatment member (e.g. metal substrate) is introduced or withdrawn
from the retort.
[0050] In the seal arrangement shown in FIG. 3, two annular flanges
82, 83 are provided upstanding from the peripheral retort part or
member 35 defining a seal zone 84 therebetween. The flanges 82, 83
are welded or otherwise secured to the retort part 35 and are of
differing heights to achieve the seal zone 84. The upper edges 85,
86 of the flanges 82 press into and seal with a suitable seal
material 87 within an annular recess 88 in the cover member or lid
29. Preferably the upper edge 85 of flange 82 is marginally lower
than the upper edge 86 of flange 83 whereby if gas leakage from the
seal zone 84 occurs it will preferentially leak towards the inside
of the retort 11 rather than externally of same. The seal material
87 might be the same type of material discussed above for seal
material 32, 38 of FIG. 2a. An inert gas delivery tube 42 is
provided to deliver inert gas (eg nitrogen) to a distributor ring
41 within the seal zone 84 such that when the furnace 10 is in use
and the cover member 29 is closed, the seal zone 84 is pressurized
with an inert gas at a pressure higher than atmosphere and higher
than within the retort. Gas leakage from the seal zone 84 "may"
occur in both directions past the upper flange edges 85, 86 but
preferentially, if leakage does occur at all, it will occur past
the edge 85 back towards the retort. Thus the required atmosphere
is maintained within the retort without permitting unwanted oxygen
to enter same from the external atmosphere. Inwardly of the seal
zone 84 a further annular flange 89 is provided with a heat
insulating material 87 therebetween which can be the same material
as the seal material 87 discussed above. Refractory particle
material 91 can build up as shown in FIG. 3, but at a point where
the slope of this material is about 60.degree. to the horizontal,
further such material will fall by gravity back into the retort 11,
helped by any inert gas leakage inwardly past the flange edge 85.
Thus escaping of refractory material from the retort is prevented
or kept to a very low level. Conveniently the volume of the seal
zone 84 is kept to a minimum to minimize inert gas usage. The lid
or cover member 29 carries a treatment basket (or similar) support
device 92 and the cover member 29 is conveniently at least
insulated against heat loss. In some applications, particularly
when batch processing, it may also be desirable to include cooling
coils or tubes in the lid or cover member 29 to cool down the
furnace 10 when desired at the end of a treatment operation. The
lid or cover member 29 might also carry optionally, a plug 93 to
minimize space above the treatment bed.
[0051] The process of this invention according to a number of
preferred aspects will now be described. In a pre-treatment stage,
a metal part (or substrate) to be treated is, subjected to a
surface treatment known generally as nitriding or nitrocarburising.
This can be achieved in a variety of different apparatus including
salt baths, gas heat treatment apparatus, vacuum plasma equipment
and fluidized bed furnaces. It is, however, desirable that the
so-called white layer established via this first stage is
substantially without significant porosity. Other desirable factors
also relate to the concentration, depth and microstructure of the
white layer including the lack of porosity therein.
[0052] When producing a nitrided or nitro carburised structure, two
zones are produced. The first inner zone is the diffusion zone
where nitrogen diffuses into the substrate through the diffusion
zone from the substrate surface and increases the hardness of the
substrate, and the second outer zone is the white layer which can
consist of either the epsilon and/or the gamma layer as
illustrated, for example, in international patent application no.
PCT/AU2006/001031.
[0053] When the pretreatment stage of this process is carried out
in a fluidized bed heat treatment furnace, control of same requires
the supply to the bed of ammonia/nitrogen (for nitriding) and a
carbon bearing gas (e.g. natural gas and/or carbon dioxide) for
nitrocarburising. During nitrocarburising, it is important that
some oxygen is involved in the process which may be contributed by
a hydrocarbon gas, carbon dioxide and/or oxygen. Once this
pre-treatment stage is completed satisfactorily, the part or
substrate to be processed needs to be treated to ensure a surface
oxide does not exist on the surface into which a metal is to be
diffused. To obtain (or maintain) a suitable surface finish, one of
the following options may be followed: [0054] (i) the surface of
the part or substrate might be mechanically treated such as by
repolishing and then kept under an inert atmosphere before
proceeding with the second stage; [0055] (ii) the surface of the
part or substrate could be maintained fully under an inert
atmosphere or within a vacuum between the pretreatment stage up to
and including the activation and metal diffusion stages; [0056]
(iii) any surface oxide formed on the surface of the part or
substrate could be removed in the activation stage with a
combination of halide gas and hydrogen; or [0057] (iv) the surface
of the part may be subjected to a wet abrasion process where grit
and air and water pressure can be varied to blast the surface. This
process selectively removes any cover layer while retaining the
desirable white layer.
[0058] In the activation stage of the process, the metal or metal
based material to be surface diffused may be placed into and held
in a fluidized bed furnace operated at a temperature below
750.degree. C. and preferably no higher than 700.degree. C.
Conveniently the temperature is in the range of 500.degree. to
700.degree. C., typically about 575.degree. C. The bed itself may
include an inert refractory particulate material such as
Al.sub.2O.sub.3 with the desired metal to be diffused into the
surface in particulate or powder form in the bed or alternatively
coating the inert refractory particles. Such metal should
preferably comprise between 5 to 30 weight percent of the bed
materials, i.e. the balance being the inert refractory material.
The bed is then fluidized by a flow of halide gas (e.g. hydrogen
chloride) and inert gas for a first period of time without the
metal substrate to be treated. The inert gas may be argon and/or
nitrogen in the presence of a separately introduced halide gas
(e.g. HCl) premixed into an inert carrier gas stream (e.g. nitrogen
and/or argon).
[0059] Preferably, the metal powders introduced into the bed should
be of high purity and conveniently without a surface oxide. Thus
measures need to be taken to prevent air contact before the powders
enter the bed and while they remain in the bed itself. The gases
used also need to be of high purity. Common inert gases capable of
use in the process are high purity nitrogen (less than 10 ppm
oxygen), high purity argon (less than 5 ppm oxygen), and for the
first pretreatment stage processing, technical grade ammonia which
has no more than 500 ppm water vapour and is further dried, for
example by passing same through a desiccant before use. The
hydrogen halide gas used may typically be a technical grade HCl
although other hydrogen halide gases might be used.
[0060] The hydrogen halide gas typically will constitute between
0.2 and 3 percent of the total gas flow to the fluidized heat
treatment bed furnace. The hydrogen halide gas flow needs to be
closely regulated and mixed thoroughly with the inert carrier gas
before it enters the bed. This is important to avoid non uniformity
within the bed. The hydrogen halide gas may be preheated before it
enters the bed to ensure that it is in its most reactive stage when
it enters the bed. Preheating of the halide gas and the inert
carrier gas has the benefit of enabling a further reduction in the
amount of hydrogen halide gas required. The first period might
typically be between 45 and 120 minutes, preferably between 60 and
90 minutes to produce an active layer on the diffusion metal and on
the inert fluidizing media (aluminium oxide) in the bed. When
chromium is used and the hydrogen halide gas is hydrogen chloride,
the active layer will be chromium chloride.
[0061] At the end of this initial activating period, the pretreated
metal substrate (pretreated as described above) is immediately
introduced into the furnace bed or a furnace bed containing the
activated metal based material and the flow of halide gas is then
stopped. During this subsequent metal diffusion stage, the metal
substrate on which the diffusion layer is to be formed is then held
within the preactivated bed for a second period (typically 1 to 8
hours and preferably 4 to 8 hours) under an inert gas atmosphere.
The bed is conveniently held at a temperature below 750.degree. C.
and conveniently in the range of 500.degree. C. to 700.degree. C.,
typically about 575.degree. C. The fluidized bed in the metal
diffusion stage may have minimal inert gas flow such that it is
substantially slumped up to a high inert gas flow such that it is
highly fluidized. The inert gas might be nitrogen. In some cases it
might be desirable to include a pulsed halide gas flow during the
second stage, if it is deemed the bed needs some reactivation.
[0062] It is generally desirable during treatment processes to
maintain relatively uniform temperature levels in the bed, i.e.
between the various heights in the bed. This may be achieved by
including temperature monitoring means and varying the flow of the
inert gases to the bed in response to sensed temperatures.
[0063] The metal or metal based material used to provide a metal to
be diffused into the diffusion surface layer of the metal substrate
to be treated may be chosen from at least one of a solid metal or
metal alloy either in particulate form or one or more solid block
members, a metal or metal alloy coated on a substrate carrier where
the substrate carrier is in particulate form or as one or more
solid block members where the substrate carrier will not, within
the treatment conditions, react with the coating metal or metal
alloy or the metal substrate being treated, a metal halide particle
or powder (anhydrous or hydrated), and a metal halide material
(anhydrous or hydrated) coated on a substrate carrier where the
substrate carrier is in particulate form or as one or more solid
block members where the substrate carrier will not, within the
treatment conditions, react with the coating material or the metal
substrate being treated. Conveniently the metal of the metal based
material used to provide a metal to be diffused can be selected
from chromium, titanium, vanadium, niobium, tantalum, tungsten,
molybdenum, manganese, and alloys thereof including ferrous based
alloys. Conveniently, the above referred to metal halides may be
comprised of a selected metal as set out above and a halide
selected from chlorine, bromine, iodine or fluorine. For example,
CrCl.sub.2 and CrCl.sub.3 are soluble in water and ethanol to form
a slurry whereby it could be painted on a suitable carrier
substrate or the carrier substrate could be dipped into the slurry
to form a suitable coating.
[0064] Several examples of preferred embodiments of the process of
this invention will be described in the following.
EXAMPLE 1
[0065] A specimen of hardened and tempered (1020.degree. C.
autenitised and air cooled, double tempered at 575.degree. C.) AISI
H13 hot work tool steel with a diameter of 38 mm and thickness of 5
mm was nitrocarburised in a 35% ammonia, 5% carbon dioxide, 60%
nitrogen atmosphere for 3.5 hours at 575.degree. C. Prior to
nitrocarburising the surface of this specimen was prepared using
1200 grade SiC abrasive to ensure good surface finish. This
produced a surface structure consisting of a 1 micron oxygen-rich
surface layer directly above a 10 micron compound layer composed of
.epsilon.-iron carbonitride, and finally an inner diffusion zone of
70-90 microns. The surface of this nitrocarburised sample was then
wet grit blasted to remove the oxide layer, while retaining the
compound layer and diffusion zone. The composition of chromium in
the compound layer was determined to be about 4 wt %.
[0066] A 38 mm diameter 5 mm thick piece of pure copper was
polished to a 1200 grade SiC finish prior to electrolytic hard
chromium plating from a commercial supplier. A 2 micron pure
chromium layer was produced by this method. Copper was chosen as a
substrate carrier as Cr and Cu are essentially insoluble, and
therefore the chromium layer will not decompose by diffusion into
the copper specimen during heating. This chromium-plated sample was
then immersed in a fluid bed heat treatment reactor of diameter 90
mm and depth 250 mm containing 3 kg of 99.99% purity alumina oxide
powder of average particle size 125 microns. This fluid bed was
heated to 575.degree. C. under nitrogen and at this temperature
hydrogen chloride gas was added to the input gas stream to a
concentration of 1% of the total gas flow. This "activation" stage
continued for a duration of 1 hour. After this activation stage the
chromium plated copper sample was cooled to room temperature in a
flow of nitrogen.
[0067] Immediately after removal from the fluid bed reactor into
ambient air conditions the hydrogen chloride activated chromium
plated copper sample was physically coupled to the nitrocarburised
sample, and a clamping pressure applied. This coupling was then
placed in a fluid bed furnace and heated to 575.degree. C. under
nitrogen flow, held at this temperature for 4 hours, then cooled to
room temperature under a nitrogen flow. This experiment was
repeated, where the coupling consisted of chromium-plated copper
without hydrogen chloride treatment as aforesaid and a
nitrocarburised specimen. Upon uncoupling the two contacting
surfaces were analysed for chemical composition using Glow
Discharge Optical Emission Spectroscopy (GDOES).
[0068] It was found that by activating the surface of the chromium
plated copper sample by use if hydrogen chloride gas, this surface
reacted with the nitrocarburised specimen. Chromium transferred
from the activated chromium-plated sample to the nitrocarburised
specimen (FIG. 4), depleting chromium on the chromium-plated copper
specimen (FIG. 5). In response to the enrichment of chromium on the
nitrocarburised surface, nitrogen diffused to the surface to create
a peak coinciding with the chromium peak (FIG. 4). Iron transferred
from the nitrocarburised sample to the chromium-plated sample (FIG.
5). Correspondingly, the iron concentration was depleted on the
nitrocarburised specimen (FIG. 4). In contrast, no reaction
occurred between the non activated chromium plated copper and the
nitrocarburised surface. No chromium enrichment of the
nitrocarburised surface was observed (FIG. 6) or depletion of
chromium from the chromium-plated copper specimen (FIG. 7). This
example indicates the importance of hydrogen chloride surface
activation of chromium to the transfer of chromium metal from a
chromium source to a nitrogen-rich surface zone.
EXAMPLE 2
[0069] A specimen of hardened and tempered (1020.degree. C.
autenitised and air cooled, double tempered at 575.degree. C.) AISI
H13 hot work tool steel with a diameter of 38 mm and thickness of 5
mm was nitrocarburised in a 35% ammonia, 5% carbon dioxide, 60%
nitrogen atmosphere for 3.5 hours at 575.degree. C. Prior to
nitrocarburising the surface of this specimen was prepared using
1200 grade SiC abrasive to ensure good surface finish. This
produced a surface structure consisting of a 1 micron oxygen-rich
surface layer directly above a 10 micron compound layer composed of
.epsilon.-iron carbonitride, and finally an inner diffusion zone of
70-90 microns. The surface of this nitrocarburised sample was then
wet grit blasted to remove the oxide layer, while retaining the
compound layer and diffusion zone. The composition of chromium in
the compound layer was determined to be about 4 wt %.
[0070] A 38 mm diameter 5 mm thick piece of 99.99% purity chromium
was polished to a 1200 grade SiC was immersed in a fluid bed
reactor of diameter 90 mm and depth 250 mm containing 3 kg of
99.99% purity alumina powder of average particle size 125 microns.
This fluid bed was heated to 575.degree. C. under nitrogen and at
this temperature hydrogen chloride gas was added to the input gas
stream to a concentration of 1% flow. This "activation" stage
continued for duration of 1 hour. After this activation stage the
chromium sample was cooled to room temperature in a flow of
nitrogen.
[0071] Immediately after removal from the fluid bed reactor in to
ambient air conditions the hydrogen chloride activated chromium
sample was physically coupled to the nitrocarburised sample and a
clamping pressure applied. This coupling was then placed in a fluid
bed furnace and heated to 575.degree. C. under nitrogen flow, held
at this temperature for 4 hours, then cooled to room temperature
under a nitrogen flow. This experiment was repeated, where the
coupling consisted of chromium without hydrogen chloride treatment
and a nitrocarburised specimen. Upon uncoupling the two contacting
surfaces were analysed for chemical composition using Glow
Discharge Optical Emission Spectroscopy (GDOES).
[0072] As per Example 1, by activating the surface of chromium by
use if hydrogen chloride gas, this surface reacted with the
nitrocarburised specimen. Chromium transferred from the activated
chromium sample to the nitrocarburised specimen and nitrogen
diffused to the surface to create a peak coinciding with the
chromium peak (FIG. 8). Iron transferred from the nitrocarburised
sample to the chromium-plated sample (FIG. 9). Correspondingly, the
iron concentration was depleted on the nitrocarburised specimen
(FIG. 8). In contrast, no reaction occurred between the chromium
without prior activation and the nitrocarburised surface. No
chromium enrichment of the nitrocarburised surface was observed
(FIG. 10)
EXAMPLE 3
[0073] Two specimens of hardened and tempered (1020.degree. C.
autenitised and air cooled, double tempered at 575.degree. C.) AISI
H13 hot work tool steel with a diameter of 38 mm and thickness of 5
mm were nitrocarburised in a 35% ammonia, 5% carbon dioxide, 60%
nitrogen atmosphere for 3.5 hours at 575.degree. C. Prior to
nitrocarburising the surface of each specimen was prepared using
1200 grade SiC abrasive to ensure good surface finish. This
produced a surface structure consisting of a 1 micron oxygen-rich
surface layer directly above a 10 micron compound layer composed of
.epsilon.-iron carbonitride, and finally an inner diffusion zone of
70-90 microns. The surface of the nitrocarburised samples was then
wet grit blasted to remove the oxide layer, while retaining the
compound layer and diffusion zone. The composition of chromium in
the compound layer was determined to be about 4 wt %.
[0074] In a fluid bed reactor of diameter 90 mm and depth 250 mm
380 g of 99.99% purity chromium powder of average particle size 80
microns was mixed with 3.4 kg of 99.99% purity alumina powder of
average particle size 125 microns.
[0075] This fluid bed was heated to 575.degree. C. under high
purity nitrogen with sufficient flow for fluidisation and at this
temperature a sample of nitrocarburised AISH13, as prepared above,
was immersed in the heated fluidising powder for a period of 4
hours. The sample was cooled in the fluid bed to 350.degree. C.
under nitrogen flow and cooled in air. No chromium enrichment of
the nitrocarburised surface was experienced as a result of this
process.
[0076] In a fluid bed reactor of diameter 90 mm and depth 250 mm
380 g of 99.99% purity chromium powder of average particle size 80
microns was mixed with 3.4 kg of 99.99% purity alumina powder of
average particle size 125 microns. This fluid bed was heated to
575.degree. C. under high purity nitrogen with sufficient flow for
fluidisation and at this temperature hydrogen chloride gas was
added to the input gas stream to a concentration of 1% flow. This
"activation" stage was for a duration of 1 hour. After activation a
sample of nitrocarburised AISH13, as prepared above, was immersed
in the heated fluidising powder with the hydrogen chloride gas flow
being stopped, the heat treatment being at 575.degree. C. for a
period of 4 hours. The sample was then cooled in the fluid bed to
350.degree. C. under nitrogen flow and cooled in air. In this trial
significant chromium-enrichment (about 70 wt %, refer to
quantitative depth profile in FIG. 12) of the nitrocarburised
surface was experienced forming a distinct, uniform and continuous
2.5 micron thick layer (FIG. 12). X-ray diffraction analysis
indicated the layer was predominately CrN (FIG. 13).
EXAMPLE 4
[0077] To assess the potential to increase the process temperature
above 575.degree. C. two steel grades were selected having higher
tempering resistance than AISI H13 hot work tool steel. Specimens
of hardened and tempered (1050.degree. C. autenitised and oil
quenched, double tempered at 575.degree. C.) powder metallurgy tool
steel Crucible CPM 1V.RTM. and conventional ingot metallurgy
Bohler-Uddeholm QRO.RTM. 90 with a diameter of 38 mm and thickness
of 5 mm were nitrocarburised in a 35% ammonia, 5% carbon dioxide,
60% nitrogen atmosphere for 3.5 hours at 575.degree. C. Prior to
nitrocarburising the surface of each specimen was prepared using
1200 grade SiC abrasive to ensure good surface finish. This
produced a surface structure consisting of a 1 micron oxygen-rich
surface layer directly above a 10 micron compound layer composed of
.epsilon.-iron carbonitride, and finally an inner diffusion zone of
70-90 microns. The surface of the nitrocarburised samples was then
wet grit blasted to remove the oxide layer, while retaining the
compound layer and diffusion zone. The composition of chromium in
the compound layer was determined to be about 4 wt %.
[0078] In a fluid bed reactor of diameter 90 mm and depth 250 mm
380 g of 99.99% purity chromium powder of average particle size 80
microns was mixed with 3.4 kg of 99.99% purity alumina oxide powder
of average particle size 125 microns. This fluid bed was heated to
625.degree. C. under high purity nitrogen with sufficient flow for
fluidisation and at this temperature hydrogen chloride gas was
added to the input gas stream to a concentration of 1% flow. This
"activation" stage was for a duration of 1 hour. After activation
one nitrocarburised sample of each grade, as prepared above, was
immersed in the heated fluidising powder for a period of 4 hours
under high purity nitrogen. The samples were cooled in the fluid
bed to 350.degree. C. under nitrogen flow and then removed from the
fluid bed reactor and cooled in air. In this trial significant
chromium-enrichment (about 70 wt %, refer to quantitative depth
profile in FIGS. 14 and 15) of the nitrocarburised surface was
experienced, with a corresponding nitrogen peak. Compared to
processing at 575.degree. C., performing the chromium deposition
stage at 625.degree. C. resulted in an increase in layer thickness
to approximately 4-6 microns (FIG. 15). Beneath the CrN layer the
diffusion zone and core hardness is substantially retained.
* * * * *