WO2009053992A1 - A process for producing body centered cubic (b2) nickel aluminide (nial) coating of controlled thickness on nickel-base alloy surfaces - Google Patents

A process for producing body centered cubic (b2) nickel aluminide (nial) coating of controlled thickness on nickel-base alloy surfaces Download PDF

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Publication number
WO2009053992A1
WO2009053992A1 PCT/IN2007/000514 IN2007000514W WO2009053992A1 WO 2009053992 A1 WO2009053992 A1 WO 2009053992A1 IN 2007000514 W IN2007000514 W IN 2007000514W WO 2009053992 A1 WO2009053992 A1 WO 2009053992A1
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Prior art keywords
nickel
coating
aluminide
niai
centered cubic
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PCT/IN2007/000514
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French (fr)
Inventor
Srinivasan Ganesan
Arun Kumar Bhaduri
Baldev Raj
Gopal Venkatraman
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The Secretary, Department Of Atomic Energy, Govt. Of India,
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Application filed by The Secretary, Department Of Atomic Energy, Govt. Of India, filed Critical The Secretary, Department Of Atomic Energy, Govt. Of India,
Priority to PCT/IN2007/000514 priority Critical patent/WO2009053992A1/en
Priority to EP07870514.2A priority patent/EP2217736B1/en
Priority to US12/738,945 priority patent/US20100247793A1/en
Publication of WO2009053992A1 publication Critical patent/WO2009053992A1/en

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    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C4/00—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge
    • C23C4/04—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge characterised by the coating material
    • C23C4/06—Metallic material
    • C23C4/08—Metallic material containing only metal elements
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C4/00—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge
    • C23C4/02—Pretreatment of the material to be coated, e.g. for coating on selected surface areas
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C4/00—Coating by spraying the coating material in the molten state, e.g. by flame, plasma or electric discharge
    • C23C4/18—After-treatment

Definitions

  • TITLE A PROCESS FOR PRODUCING BODY CENTERED CUBIC (B2) NICKEL ALUMINIDE (NiAI) COATING OF CONTROLLED THICKNESS ON NICKEL-BASE ALLOY SURFACES.
  • the present invention relates to aluminizing of nickel-base alloys and, in particular, to a process for producing body centered cubic (B2) nickel aluminide (NiAI) coating of controlled thickness on nickel-base alloys surfaces.
  • B2 NiAI phase body centered cubic
  • the process of producing the nickel aluminide (B2 NiAI phase) coatings on nickel-base alloys is directed to achieve high hardness of 800 - 1100 VHN and dimensional tolerance of 80 ⁇ 30 microns coating thickness.
  • the process of the invention achieves desired nickel aluminide coatings with minimum environmental emission of toxic chemicals fumes and is thus also environment friendly.
  • the nickel aluminide coating of the present invention is possible without the need for masking of the unwanted regions of the components and more importantly can be attended both on flat and curved surfaces thereby favouring wide scale utilities of such aluminizing process.
  • the process of the invention would facilitate fabrication industries required to carry out surface modifications and engineering (protective coating) on nickel-base alloys desired for high temperatures such as nuclear industries, aircraft and gas turbine industries.
  • nickel-base alloys are required to be provided surface modified and engineered for protective coatings to adopt such nickel-base alloys for high temperature application for variety of industrial and allied uses.
  • Aluminizing on nickel-base alloys is usually known to be carried out by processes such as pack cementations, slurry spraying, brushing, dipping and chemical vapour deposition to produce nickel aluminide (NiAI) layer over the substrate adapted for providing a hard surface for fretting wear and galling resistance (1-3).
  • Pack cementations is a process which has been widely used for such aluminizing since it is inexpensive and ideally suited for batch production of small components.
  • the components to be aluminized are usually treated at temperatures between 850 0 C and 1050 0 C in a pack consisting of an aluminium source such as Ni-Al, Ti- Al, or Cr-Al, an activator (halide) and some inert filler like alumina(l).
  • an aluminium source such as Ni-Al, Ti- Al, or Cr-Al
  • an activator (halide) halide
  • Vapour phase aluminizing (2) on the other hand largely eliminates the disadvantages of the pack cementation processes, but required specialized vacuum furnaces and fixtures. Moreover, both these processes involve exposure of operator to corrosive halide activators which is unsafe and undesired from the safety of the operator. Apart from the above discussed limitations and complexities of the conventional methods of aluminizing on nickel-base a'loys, it is also important that following such processes of aluminizing it has been always essential to mask the surfaces which were not requi ⁇ d to be coated requiring special precautions and added complexities of such aluminizing processes.
  • Another object of the present invention is directed to a process for NiAI aluminide coating of nickel-base alloys which would on one hand be cost effective and at the same time on the other hand would involve safe technology without the problems/disadvantages of handling of large quantity of alumina and metal powder, long furnace time cycle due to large thermal inertia and inherent reduced throughput.
  • Further object of the present invention is directed to a simple process for NiAI aluminide coating of nickel-base alloys which would also not require any specialized vacuum furnaces and fixtures and thus can be readily applied and used for producing aluminide nickel-based alloys.
  • a further object of the present invention is directed to an aluminizing process which would favour achieving NiAI aluminide coating on nickel-base alloys and which would be suitable for mass production of large sized components to a tolerance of 80 ⁇ 30 microns with hardness range of 800-1100 VHN with reduced cycle time than the conventional pack cementation and vapour phase diffusion coating.
  • Yet another object of the present invention is directed to favour production of selectively body centered cubic (B2) nickel aluminide (NiAI) coated nickel-base alloys of desired thickness without the complexities of masking of surfaces not to be coated thereby facilitating controlled and simple production of such nickel aluminide coated nickel-base alloys.
  • B2 body centered cubic
  • NiAI nickel aluminide
  • a further object of the present invention is to produce ordered body centered cubic (B2) nickel aluminide (NiAI) coated nickel-base alloys which can be advantageously used to form the coating only on the desired_ surfaces without any special steps to masking the areas where aluminizing is not desired.
  • B2 ordered body centered cubic
  • NiAI nickel aluminide
  • Yet another object of the present invention is directed to. produce ordered body centered cubic (B2) nickel aluminide (NiAI) coated nickel-base alloys with higher productivity, environmental cleanliness and suitability for mass production of large sized components.
  • B2 ordered body centered cubic
  • NiAI nickel aluminide
  • Yet another object of the present invention is directed to a process which would achieve NiAI aluminide coating on nickel-base alloys involving flat and/or corrugated surfaces to thereby favour production of variety of NiAI aluminide coated nickel-base alloys.
  • a further object of the present invention is directed to produce surface protective coating for high temperature application on nickel-base alloys with selectively desired coating thickness, micro structure and hardness which can be consistently reproduced.
  • Yet further object of the present invention is to provide for a simple yet effective process for producing surface protective coating for high temperature application on nickel-base alloys which would favour overall reduction of time and cost for aluminizing vis-a-vis the conventional processes of aluminizing of such nickel-base alloys presently in use.
  • NiAI nickel aluminide
  • B2 body centered cubic
  • NiAI nickel aluminide
  • the aluminium thus sprayed onto the nickel-base alloy substrate to diffusion heat treatment such that the aluminium sprayed reacts with the nickel-base alloy substrate and form the desired aluminum coating on the entire nickel-base alloy substrate or selective required surface thereof to be coated and with controlled thickness.
  • NiAI nickel aiuminide
  • B2 nickel aluminide
  • NiAI nickel aluminide
  • nickel alum ' .nide (NiAI) coating the said surface activation by grit blasting is carried out using alumina grit of specific size and distribution preferably in the range of 30-120 mesh size at an air pressure of 5.5 +/- 0.5 kg/cm 2 .
  • the thermal spray parameters comprise (i) aluminum wire diameter in the range of 3.0 to 3.2mm at a Wire Feed Rate in the range of 2.0-2.5 mm/sec;(ii) Pneumatic pressure in the range of 5.5 +/- 0.5 kg/cm 2 ; (iii) Oxygen Pressure in the range of 2.5 +/- 0.3 kg/cm 2 ; and Acetylene pressure in the range of 1.0 +/- 0.2 kg/ cm 2 .
  • the said diffusion heat treatment in said vacuum atmosphere is carried out preferably (10 ⁇ 5 ) level at temperature in the range of 950-1100 0 C for a duration of 1.0 to 1.5 hours.
  • nickel aluminide (NiAI) coating comprise removing adherent oxide scale using pickling solution following the steps of:
  • the pickling solution preferably comprising (a) concentrated nitric acid (specific gravity :1.41 g/cm 3 ) in amounts of 16% (by volume);(b) Hydrofluoric acid (specific gravity 1.61 g/cm 3 ) in amounts of 3.5 % (by volume); and (c ) Water in amounts of 80.5% (by volume); (ii) soaking the diffusion treated strips in the solution for a period of about 30 minutes; followed by (iii) further cleaning the strips involving emery papers of 400 grit size.
  • NiAI aluminide coating on nickel-base alloys involving selectively flat and/or corrugated surfaces.
  • the said NiAI aluminide coating is carried out on nickel-base alloys with minimum 40weight % nickel content on both flat and corrugated surfaces.
  • the process is adapted -to produce surface protective coatings selectively free of any masking of the uncoated regions for desired high temperature applications on nickel-base alloys with minimum 40 weight % nickel content and having hardness of 800-1100 VHN and a coating thickness of 80+/- 30 micron thickness.
  • the surface activity, spraying procedures and diffusion parameters are critical in optimization of the coating thickness and hardness.
  • Surface activity is achieved by selection of suitable grade grit size (30-120 mesh size) during grit blasting.
  • Thermal spraying of aluminium is carried out using 3.2 mm diameter aluminium wires and by maintaining a torch-work distance of 200-300 mm and wire feed rate of 2.0- 2.5 mm/sec. Thickness of sprayed layer is monitored on 100% of the qualification strips and on a certain percentage of production strips at random to ensure sprayed layer thickness of 100-200 microns.
  • the optimized diffusion treatment is carried out at 950-1100 0 C for 1.0- 1.5h.
  • the selective diffusion treatment after thermal spray in vacuum atmosphere is involved whereby the aluminium melts and reacts with the nickel/iron base alloys to thereby form the desired B2 phase.
  • the major advantage in the above process of the invention involving the —combination of thermal spraying and diffusion treatment is that the same avoids the complexities of masking of uncoated surfaces and yet can favour achieving the coating only on the desired surfaces of the nickel-base alloys.
  • Figure 1 is a schematic illustration of the stages involved in the aluminizing of Ni-base alloys in accordance with the present invention involving thermal spray and diffusion treatment.
  • the basic steps involved in such process of aluminizing comprises of carrying out sequentially (a) dimensional checks, (b) degreasing ,(c ) grit blasting, (d) thermal spraying , (e) diffusion treatment ( f) surface finish and (g) dimensional check.
  • a process of aluminizing involving the thermal spray and diffusion treatment of the invention is illustrated further by way of the following example.
  • the present aluminizing process can achieve NiAI aluminide coating on nickel-base alloys with minimum 40 wt% nickel content on both flat and corrugated surfaces.
  • the processes found suitable for mass production of large sized components to a tolerance of 80 ⁇ 30 microns of NiAl aluminide coating with hardness range of -800 - 1100 VHN with reduced cycle time as compared to the conventional processes such as the pack cementation and vapour phase diffusion coating.
  • the process of the invention involving the selective combination of thermal spray and diffusion heat treatment would favour coating on selected areas of the components both on flat and curved surfaces without the need for masking and its related complexities.
  • the process offers overall reduction of time and cost of aluminizing which is found to be four time faster than the conventional processes of aluminizing nickel-base alloys.
  • the present process would thus enable producing surface protective coatings for high temperature applications on nickel-base alloys with desired coating thickness, microstructure and hardness which importantly can be consistently reproduced. Moreover, the process would favour the desired surface areas alone to be aluminized without rhe need for complex masking of uncoated areas.
  • the process offers overall reduction of time and cost for aluminizing. The duration of aluminizing for other conventional processes is four times higher than the present invention and the process would therefore favour much simpler and faster generation of NiAI coated Ni-base alloys both on flat and/or corrugated surfaces apart from being environment friendly and safe to carry out.

Abstract

Aluminizing of nickel-base alloys and a process for producing body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloys surfaces. Importantly, the process of producing the nickel aluminide (B2 NiAl phase) coatings on nickel-base alloys is directed to achieve high hardness of 800 - 1100 VHN and dimensional tolerance of 80 ± 30 microns coating thickness. The process achieves desired nickel aluminide coatings with minimum environmental emission of toxic chemicals fumes and is also environment friendly. Obtaining the nickel aluminide coating is specifically possible without the need for masking of the unwanted regions of the components and more importantly can be attended both on flat and curved surfaces thereby favouring wide scale utilities of such aluminizing process, facilitate fabrication industries required to carry out surface modifications and engineering (protective coating) on nickel-base alloys desired for high service temperatures such as nuclear industries, aircraft and gas turbine industries.

Description

TITLE: A PROCESS FOR PRODUCING BODY CENTERED CUBIC (B2) NICKEL ALUMINIDE (NiAI) COATING OF CONTROLLED THICKNESS ON NICKEL-BASE ALLOY SURFACES.
FIELD OF THE INVENTION
The present invention relates to aluminizing of nickel-base alloys and, in particular, to a process for producing body centered cubic (B2) nickel aluminide (NiAI) coating of controlled thickness on nickel-base alloys surfaces. Importantly, the process of producing the nickel aluminide (B2 NiAI phase) coatings on nickel-base alloys is directed to achieve high hardness of 800 - 1100 VHN and dimensional tolerance of 80 ± 30 microns coating thickness. Advantageously, the process of the invention achieves desired nickel aluminide coatings with minimum environmental emission of toxic chemicals fumes and is thus also environment friendly. Moreovsr, the nickel aluminide coating of the present invention is possible without the need for masking of the unwanted regions of the components and more importantly can be attended both on flat and curved surfaces thereby favouring wide scale utilities of such aluminizing process. The process of the invention would facilitate fabrication industries required to carry out surface modifications and engineering (protective coating) on nickel-base alloys desired for high temperatures such as nuclear industries, aircraft and gas turbine industries.
BACKGROUND ART
It is well known that nickel-base alloys are required to be provided surface modified and engineered for protective coatings to adopt such nickel-base alloys for high temperature application for variety of industrial and allied uses. Aluminizing on nickel-base alloys is usually known to be carried out by processes such as pack cementations, slurry spraying, brushing, dipping and chemical vapour deposition to produce nickel aluminide (NiAI) layer over the substrate adapted for providing a hard surface for fretting wear and galling resistance (1-3).
It also known to carry out thermal spray processes alone for deposition of metals on worn out surfaces of steel components (4, 5). Also, it is well known to achieve pack diffusion and vapour phase diffusion coatings for protection against wear and fretting of fast reactor components (6-7). Aluminium paint - based processes for NiAl coating have also been used previously (8). While, the above existing state of the art prevails for the nickel aiuminide coating on nickel base alloys there has been some inherent limitations/disadvantages of such hitherto known processes and devices.
Pack cementations is a process which has been widely used for such aluminizing since it is inexpensive and ideally suited for batch production of small components. In such pack cementation process, the components to be aluminized are usually treated at temperatures between 8500C and 10500C in a pack consisting of an aluminium source such as Ni-Al, Ti- Al, or Cr-Al, an activator (halide) and some inert filler like alumina(l). It is however, experienced that such pack cementation technology involves handling of large quantities of alumina and metal powders, long furnace time cycles due to a large thermal inertia and inherently reduced throughput.
Vapour phase aluminizing (2) on the other hand largely eliminates the disadvantages of the pack cementation processes, but required specialized vacuum furnaces and fixtures. Moreover, both these processes involve exposure of operator to corrosive halide activators which is unsafe and undesired from the safety of the operator. Apart from the above discussed limitations and complexities of the conventional methods of aluminizing on nickel-base a'loys, it is also important that following such processes of aluminizing it has been always essential to mask the surfaces which were not requi^d to be coated requiring special precautions and added complexities of such aluminizing processes.
OBJECTS OF THE INVENTION
It is thus the basic object of the present invention to provide for a process for NiAI aluminide coating of nickel-base alloys which would avoid the afore discussed limitations and complexities of the present processes/devices for carrying out surface modification and engineering (protective coating) of-oickel-hase alloys.
Another object of the present invention is directed to a process for NiAI aluminide coating of nickel-base alloys which would on one hand be cost effective and at the same time on the other hand would involve safe technology without the problems/disadvantages of handling of large quantity of alumina and metal powder, long furnace time cycle due to large thermal inertia and inherent reduced throughput.
Further object of the present invention is directed to a simple process for NiAI aluminide coating of nickel-base alloys which would also not require any specialized vacuum furnaces and fixtures and thus can be readily applied and used for producing aluminide nickel-based alloys.
A further object of the present invention is directed to an aluminizing process which would favour achieving NiAI aluminide coating on nickel-base alloys and which would be suitable for mass production of large sized components to a tolerance of 80± 30 microns with hardness range of 800-1100 VHN with reduced cycle time than the conventional pack cementation and vapour phase diffusion coating.
Yet another object of the present invention is directed to favour production of selectively body centered cubic (B2) nickel aluminide (NiAI) coated nickel-base alloys of desired thickness without the complexities of masking of surfaces not to be coated thereby facilitating controlled and simple production of such nickel aluminide coated nickel-base alloys.
A further object of the present invention is to produce ordered body centered cubic (B2) nickel aluminide (NiAI) coated nickel-base alloys which can be advantageously used to form the coating only on the desired_ surfaces without any special steps to masking the areas where aluminizing is not desired.
Yet another object of the present invention is directed to. produce ordered body centered cubic (B2) nickel aluminide (NiAI) coated nickel-base alloys with higher productivity, environmental cleanliness and suitability for mass production of large sized components.
Yet another object of the present invention is directed to a process which would achieve NiAI aluminide coating on nickel-base alloys involving flat and/or corrugated surfaces to thereby favour production of variety of NiAI aluminide coated nickel-base alloys.
A further object of the present invention is directed to produce surface protective coating for high temperature application on nickel-base alloys with selectively desired coating thickness, micro structure and hardness which can be consistently reproduced.
Yet further object of the present invention is to provide for a simple yet effective process for producing surface protective coating for high temperature application on nickel-base alloys which would favour overall reduction of time and cost for aluminizing vis-a-vis the conventional processes of aluminizing of such nickel-base alloys presently in use.
SUMMARY OF THE INVENTION
Thus according to the basic aspect of the present invention there is provided a process for producing nickel aluminide (NiAI) coating preferably body centered cubic (B2) nickel aluminide (NiAI) coating of controlled thickness on nickel-base alloy surfaces comprising:
thermal spraying of the required surfaces with commercial pure aluminium wires to desired thickness;
subjecting the aluminium thus sprayed onto the nickel-base alloy substrate to diffusion heat treatment such that the aluminium sprayed reacts with the nickel-base alloy substrate and form the desired aluminum coating on the entire nickel-base alloy substrate or selective required surface thereof to be coated and with controlled thickness.
In accordance with a preferred aspect of the present invention there is provided a process for producing nickel aiuminide (NiAI) coating body centered cubic (B2) nickel aluminide (NiAI) coating of controlled thickness on nickel-base alloy surfaces comprising:
activating the surfaces to be coated by grit blasting preferably following dimensional check and degreasing;
thermal spraying of the required surfaces with commercial pure aluminium to desired specific thickness in the range of 100 to 200 microns; subjecting the aluminium thus sprayed onto the nickel-base alloy substrate to diffusion heat treatment in vacuum atmosphere such that the aluminium sprayed reacts with the nickel- base alloy substrate and form the desired aluminum coating on the entire nickel-base alloy substrate or selective required surface thereof to be coated and with controlled thickness; and subjecting the coated surface to surface cleaning.
Importantly, in the above process for producing body centered cubic (B2) nickel alum'.nide (NiAI) coating the said surface activation by grit blasting is carried out using alumina grit of specific size and distribution preferably in the range of 30-120 mesh size at an air pressure of 5.5 +/- 0.5 kg/cm2.
In accordance with another aspect of the invention, the thermal spray parameters comprise (i) aluminum wire diameter in the range of 3.0 to 3.2mm at a Wire Feed Rate in the range of 2.0-2.5 mm/sec;(ii) Pneumatic pressure in the range of 5.5 +/- 0.5 kg/cm2; (iii) Oxygen Pressure in the range of 2.5 +/- 0.3 kg/cm2; and Acetylene pressure in the range of 1.0 +/- 0.2 kg/ cm2.
The said diffusion heat treatment in said vacuum atmosphere is carried out preferably (10~5) level at temperature in the range of 950-11000C for a duration of 1.0 to 1.5 hours.
Preferably, in the above process for producing body centered cubic (B2) nickel aluminide (NiAI) coating the said surface cleaning of the coated surface comprise removing adherent oxide scale using pickling solution following the steps of:
(i) providing the pickling solution preferably comprising (a) concentrated nitric acid (specific gravity :1.41 g/cm3 ) in amounts of 16% (by volume);(b) Hydrofluoric acid (specific gravity 1.61 g/cm3 ) in amounts of 3.5 % (by volume); and (c ) Water in amounts of 80.5% (by volume); (ii) soaking the diffusion treated strips in the solution for a period of about 30 minutes; followed by (iii) further cleaning the strips involving emery papers of 400 grit size.
Following the above disclosed process of the invention it is possible to carry out the NiAI aluminide coating on nickel-base alloys involving selectively flat and/or corrugated surfaces. Importantly, the said NiAI aluminide coating is carried out on nickel-base alloys with minimum 40weight % nickel content on both flat and corrugated surfaces.
The process is adapted -to produce surface protective coatings selectively free of any masking of the uncoated regions for desired high temperature applications on nickel-base alloys with minimum 40 weight % nickel content and having hardness of 800-1100 VHN and a coating thickness of 80+/- 30 micron thickness.
Importantly, in the above process the surface activity, spraying procedures and diffusion parameters are critical in optimization of the coating thickness and hardness. Surface activity is achieved by selection of suitable grade grit size (30-120 mesh size) during grit blasting. Thermal spraying of aluminium is carried out using 3.2 mm diameter aluminium wires and by maintaining a torch-work distance of 200-300 mm and wire feed rate of 2.0- 2.5 mm/sec. Thickness of sprayed layer is monitored on 100% of the qualification strips and on a certain percentage of production strips at random to ensure sprayed layer thickness of 100-200 microns. The optimized diffusion treatment is carried out at 950-11000C for 1.0- 1.5h.
It is thus possible by way of the present invention discussed above to produce ordered oody centered cubic (B2) nickel aluminide (NiAI) coating of controlled thickness on nickel-base alloys involving a selective thermal spray-diffusion process. Importantly, the process of the invention achieves selective advantages in producing such coated nickel-base alloys by involving the thermal spraying which is a process in which molten or semi-molten particles are applied by impact on to the surface at uniform rate using mechanise or manually operated spray gun.
The selective diffusion treatment after thermal spray in vacuum atmosphere is involved whereby the aluminium melts and reacts with the nickel/iron base alloys to thereby form the desired B2 phase.
Importantly, the major advantage in the above process of the invention involving the —combination of thermal spraying and diffusion treatment is that the same avoids the complexities of masking of uncoated surfaces and yet can favour achieving the coating only on the desired surfaces of the nickel-base alloys. This is an important advantage in the present process of producing the nickel aluminide coated nickel-base alloys of the invention since the conventional coating process of such alloys such as the conventional pack cementation process essentially required coating the entire strip unless special steps are taken to mask the area where aluminizing is not desired.
The process being simple to operate and apply favours for higher productivity and is importantly environment friendly and safe from the operators point of view since the same does not require the operator to be exposed to corrosive environment such as corrosive halide activators involved in the conventional pack cementation and vapour phase aluminizing processes.
The details of the invention, its object and advantages are explained hereunder in greater detail in relation to non-limiting exemplary illustration of the process of the invention as per the following accompanying figure and examples :- BRIEF DESCRIPTION OF THE ACCOMPANYING FIGURE:
Figure 1: is a schematic illustration of the stages involved in the aluminizing of Ni-base alloys in accordance with the present invention involving thermal spray and diffusion treatment.
As shown in said figure the basic steps involved in such process of aluminizing comprises of carrying out sequentially (a) dimensional checks, (b) degreasing ,(c ) grit blasting, (d) thermal spraying , (e) diffusion treatment ( f) surface finish and (g) dimensional check. Such a process of aluminizing involving the thermal spray and diffusion treatment of the invention is illustrated further by way of the following example.
EXAMPLE
An Exemplary process of aluminizing of nickel-base alloys following the process of the invention involving the thermal spraying and diffusion treatment was carried out as detailed hereunder: i) The nickel-base alloys surface to be coated was first activated , after dimensional check and degreasing , by grit blasting using alumina grit of specific size and size distribution i.e., in the range of 30-120 mesh size at a air pressure of 5.5 ± 0.5 kg/cm2; ii) thereafter thermal spraying of the required surfaces using commercially pure aluminium (dia 3.2 mm) to specific thickness in the range 100 to 200 microns was carried out using the following thermal spray parameters
thermal spray parameters:
a) wire feed rate : 2.0 - 2.5 mm/sec;
b) pneumatic pressure: 5.5±0.5 kg/cm2;
c) oxygen pressure: 2.5 ± 0.3 kg/cm2; and
d) acetylene pressure : 1.0 ± 0.2 kg/cm2.
iii) subsequently diffusion heat treatment was carried out in vacuum atmosphere (10"5) level at specified temperature in the range 950 - 11000C for a duration of 1.0-1.5 h to enable the aluminium sprayed to react with the nickel - iron - chromium alloys substrate and form the aluminide coating;
iv) subsequently surface cleaning procedure was carried out to remove adherent oxide scale using a pickling solution as per the following details : a) concentrated nitric acid (specific gravity : 1.41 g/cm3) : 16% (by volume);
b) hydrofluoric acid (specific gravity, 1.61 g/cm3): 3.5 % (by volume);and
c) water : 80.5% (by volume). The pickling solution obtained as above was used to soak the diffusion treated strips for 30 minutes followed by hand cleaning of the strips such as by using 400 grit size emery papers.
By way of the above disclosed process of the invention it was possible to produce the ordered body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloys involving the simple yet effective thermal spray-diffusion process. Advantageously, the present aluminizing process can achieve NiAI aluminide coating on nickel-base alloys with minimum 40 wt% nickel content on both flat and corrugated surfaces. Moreover, the processes found suitable for mass production of large sized components to a tolerance of 80± 30 microns of NiAl aluminide coating with hardness range of -800 - 1100 VHN with reduced cycle time as compared to the conventional processes such as the pack cementation and vapour phase diffusion coating. Moreover, the process of the invention involving the selective combination of thermal spray and diffusion heat treatment would favour coating on selected areas of the components both on flat and curved surfaces without the need for masking and its related complexities. Thus, the process offers overall reduction of time and cost of aluminizing which is found to be four time faster than the conventional processes of aluminizing nickel-base alloys.
The present process would thus enable producing surface protective coatings for high temperature applications on nickel-base alloys with desired coating thickness, microstructure and hardness which importantly can be consistently reproduced. Moreover, the process would favour the desired surface areas alone to be aluminized without rhe need for complex masking of uncoated areas. The process offers overall reduction of time and cost for aluminizing. The duration of aluminizing for other conventional processes is four times higher than the present invention and the process would therefore favour much simpler and faster generation of NiAI coated Ni-base alloys both on flat and/or corrugated surfaces apart from being environment friendly and safe to carry out.

Claims

WE CLAIM:
1. A process for producing nickel aluminide (NiAI) coating body centered cubic (B2) nickel aluminide (NiAl) coating of controlled thickness on nickel-base alloy surfaces comprising:
thermal spraying of the required surfaces with commercial pure aluminium to desired thickness;
subjecting the aluminium thus sprayed onto the nickel-base alloy substrate to diffusion heat treatment such that the aluminium sprayed reacts with the nickel-base alloy substrate and form the desired aluminum coating on the entire nickel-base alloy substrate or selective required surface thereof to be coated and with controlled thickness.
2. A process for producing nickel aluminide (NiAI) coating body centered cubic (B2) nickel aluminide (NiAI) coating of controlled thickness on nickel-base alloy surfaces comprising:
activating the surfaces to be coated by grit blasting preferably following dimensional check and degreasing;
thermal spraying of the required surfaces with commercial pure aluminium to desired specific thickness in the range of 100 to 200 microns;
subjecting the aluminium Thus sprayed onto the nickel-base alloy substrate to diffusion heat treatment in vacuum atmosphere such that the aluminium sprayed reacts with the nickel-base alloy substrate and form the desired aluminum coating on the entire nickel- base alloy substrate or selective required surface thereof to be coated and with controlled thickness; and subjecting the coated surface to surface cleaning.
3. A process for producing body centered cubic (B2) nickel aluminide (NiAI) coating according to claim 2 wherein the said surface activation by grit blasting is carried out using alumina grit of specific size and distribution preferably in the range of 30-120 mesh size at an air pressure of 5.5 +/- 0.5 kg/cm2.
4. A process for producing body centered cubic (B2) nickel aluminide (NiAl) coating according to anyone of claims 1 or 2 wherein said thermal spray parameters comprise (i) aluminum wire diameter in the range of 3.0 to 3.2 mm at a Wire Feed Rate in the range of 2.0-2.5 mm/sec; (ii) Pneumatic pressure in the range of 5.5 +/- 0.5 kg/cm2; (iii) Oxygen Pressure in the range of 2.5 +/- 0.3 kg/cm2; and Acetylene pressure in the range of 1.0 +/- 0.2 kg/ cm2.
5. A process for producing body centered cubic (B2) nickel aluminide (NiAI) coating according to anyone of claims 1 or 2 wherein said diffusion heat treatment in said vacuum atmosphere is caried out preferably (10"5) level at temperature in the range of 950-11000C for a duration of 1.0 to 1.5 hours.
6. A process for producing body centered cubic (B2) nickel aluminide (NiAI) coating according to claim 2 wherein the said surface cleaning of the coated surface comprise removing adherent oxide scale using pickling solution following the steps of:
(i) providing the pickling solution preferably comprising (a) concentrated nitric acid
(specific gravity :1.41 g/cm3 ) in amounts of 16% (by volume) ;(b) Hydrofluoric acid (specific gravity 1.61 g/cm3 ) in amounts of 3.5 % (by volume); and (c )
Water in amounts oF 80.5% (by volume);
(ii) soaking the diffusion treated strips in the solution for a period of about 30 minutes; followed by
(iii) further cleaning the strips involving emery paper.
7. A process for producing body centered cubic (B2) nickel aluminide (NiAI) coating according to anyone of claims 1 to 6 wherein the NiAI aluminide coating is carried out on nickel-base alloys involving selectively flat and/or corrugated surfaces.
8. A process for producing body centered cubic (B2) nickel aluminide (NiAI) coating according to claim 7 wherein said NiAI aluminide coating is carried out on πicκel-base alloys with minimum 40 weight % nickel content on both flat and corrugated surfaces.
9. A process for producing body centered cubic (B2) nickel aluminide (NiAI) coating according to anyone of claims 1 to 8 adapted to produce surface protective coatings selectively free of any masking of the uncoated regions for desired high temperature applications on nickel-base alloys with minimum 40 weight % nickel content and having hardness of 800-1100 VHN and a coating thickness of 80+/- 30 micron thickness.
10. A process for producing body centered cubic (B2) nickel aluminide (NiAI) coating of controlled thickness on nickel-base alloy surfaces substantially as herein described and illustrated with reference to the accompanying figures.
PCT/IN2007/000514 2007-10-26 2007-10-26 A process for producing body centered cubic (b2) nickel aluminide (nial) coating of controlled thickness on nickel-base alloy surfaces WO2009053992A1 (en)

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PCT/IN2007/000514 WO2009053992A1 (en) 2007-10-26 2007-10-26 A process for producing body centered cubic (b2) nickel aluminide (nial) coating of controlled thickness on nickel-base alloy surfaces
EP07870514.2A EP2217736B1 (en) 2007-10-26 2007-10-26 A process for producing body centered cubic (b2) nickel aluminide (nial) coating of controlled thickness on nickel-base alloy surfaces
US12/738,945 US20100247793A1 (en) 2007-10-26 2007-10-26 Process for producing body centered cubic (b2) nickel aluminide (nial) coating of controlled thickness on nickle-based alloy surfaces

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CN107164716B (en) * 2017-06-13 2020-03-17 中国石油天然气集团公司 Powder core wire and method for preparing high-speed electric arc spraying coating

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