Abstract:
A process for treating a high temperature corrosion resistant composite surface is disclosed. The process includes the steps of forming a first alloy layer by coating a metallic base material with a NiCr alloy or a MCrAlY alloy (M being made of one or more selected from the group consisting of Fe, Ni and Co) with low pressure plasma spraying, forming a second alloy layer on the first alloy layer by coating the first layer with an alloy having identical composition with atmospheric plasma spraying and then subjecting these layers to thermal diffusion treatment in a vacuum furnace or an inert gas atmosphere furnace. Thus, high temperature corrosion resistance is provided for a metallic material used at high temperatures.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a process for providing high temperature corrosion resistance for a metallic material used in a high temperature, and more particularly to a process for treating a high temperature corrosion resistant surface which is suitably used for the moving and stationary blades of a gas turbine, and so on. 
     2. Description of the Related Art 
     A tremendous increase has occurred in a gas temperature, even exceeding 1300° C., at the turbine entrance of recent highly efficient industrial gas turbines typically used in combined cycle plants. Active research and development have been made for practical alloys to be used for the moving and stationary blades which are exposed to such high temperature gas, and the operating temperature has been increasing year by year. However, for practical alloys, the temperature is still limited to the level of 850° to 900° C. Accordingly, for an actual gas turbine, a thin, internal air-cooling blade is used. 
     For fuel to be used, research has been made on the utilization of LNG, by-product gas or fuel oil, and recently even on the use of coal by liquefying or gasifying it. Accordingly, the air-cooling blade has been coated with a corrosion resistant alloy made of NiCoCrAlY or CoCrAlY by low pressure plasma spraying (referred to as VPS, hereinafter) in order to prevent its high temperature oxidation or high temperature corrosion. 
     In the gas turbine in which the operating temperature is high, the rates of oxidization and corrosion increases following an increase in a gas temperature in the moving and stationary blades which come into direct contact with the combustion gas. Even when corrosion resistant coating like that described above is applied, the introduction of a high temperature corrosive component with fuel or combustion air causes conspicuous damages. Under these circumstances, a surface treating process providing much higher resistance to high temperature corrosion is required. 
     It is an object of the present invention made with the foregoing technical level and requirement in mind to present a surface treating process which provides much higher resistance to high temperature corrosion. 
     SUMMARY OF THE INVENTION 
     According to the present invention, there is disclosed a process for treating a high temperature corrosion resistant composite surface, characterized in that a first alloy layer is formed by coating a metallic base material used at a high temperatures with a NiCr alloy or a MCrAlY alloy (M being made of one or more selected from the group consisting of Fe, Ni and Co) which is deposited by low pressure plasma spraying, a second alloy layer is formed by coating the first alloy layer with an alloy having identical composition, which is deposited by atmospheric plasma spraying, and then these layers are subjected to thermal diffusion treatment in a vacuum or in inert gas atmosphere in a furnace. 
     More particularly, in order to provide high temperature corrosion resistance for a metallic base material to be used at high temperatures (simply referred to as a base material, hereinafter) represented by Fe, Ni or Co-based alloy, the surface treating process of the present invention includes the following steps: 
     (1) the material to be treated (base material) is coated with a NiCr alloy or a MCrAlY alloy (M being made of one or more selected from the group consisting of Fe, Ni and Co) by low pressure plasma spraying; 
     (2) the layer formed in step (1) is coated with an alloy having identical composition by normal atmospheric plasma spraying; and 
     (3) by performing thermal diffusion treatment between the coated layer and the base material and between the coated layers in vacuum or in inert gas (Ar, N 2 , etc.) in a furnace, excellent adhesion, uniformity and resistance to high temperature corrosion are provided for the layers. 
     Table 1 shows the general conditions of low pressure plasma and atmospheric plasma spraying for a NiCr alloy or a MCrAlY alloy on a high temperature metallic material used in a high temperature and the general range of coated layer thickness. The NiCr alloy and the MCrAlY alloy are sprayed under the same conditions. 
     
                                           TABLE 1__________________________________________________________________________         Low pressure plasma spraying                         Atmospheric                    Thermal                         plasmaItem     Division         Cleaning              Preheating                    spraying                         spraying__________________________________________________________________________Chamber  (mbar)         30-40              45-55 55-65                         None (in                         atmosphere)Spray distance    (mm) 250-275              290-320                    270-280                         100-150Ar flow rate    (liter/min)         50-60              45-55 40-50                         30-60H.sub.2 flow rate    (liter/min)         0    7-9    8-10                          8-10Current  (Amp)         490-510              590-610                    670-700                         500-800Voltage  (V)  58-62              60-65 62-67                         35-40Powder feed    (%)  --   --    12-16                             4-8(Kg/Hr)Transfer current    (A)  45-55              --    --   --Carrier gas flow rate    (liter/min)         --   1.8-2.0                    1.8-2.0                         --General coated layer thickness         100-300 μm     100-500 μm__________________________________________________________________________ 
    
     General conditions for thermal diffusion treatment performed in the vacuum furnace or in inert gas atmosphere furnace after low pressure plasma spraying and atmosphere spraying are respectively as follows. 
     Vacuum furnace: 900° to 1150° C., 2 to 24 hours 10 to 50 Torr (N 2  or Ar atmosphere) 
     Inert gas atmosphere furnace: 900° to 1150° C., 2 to 24 hours atmospheric pressure to 2 ata. (Ar or H 2  atmosphere) 
     The NiCr alloy or the MCrAlY alloy and the base material constitutional element plasma-sprayed by low pressure are mutually diffused and thus adhesion between the base material and the coated layer is maintained. In addition, since the surface of the layer formed by low pressure plasma spraying has proper surface roughness necessary for atmospheric plasma spraying, blasting as treatment performed prior to atmospheric plasma spraying is made unnecessary. Accordingly, the intrusion of a foreign matter such as a blasting material or the like can be prevented between low pressure plasma spraying and atmospheric plasma spraying. Further, the formation of layers by low pressure and atmospheric plasma spraying makes it possible to prevent peeling caused by a thermal expansion coefficient difference between these two sprayed layers. 
     The surface of an atmospheric plasma spraying particle is oxidized during spraying and covered by an oxide (Cr 2  O 3 , Al 2  O 3 , and so on) coating film. Since this oxide coating film has excellent resistance against corrosion caused by fused salt or corrosive gas, the progress of corrosion can be controlled. The layer formed by atmospheric plasma spraying has through-holes. The intrusion of a corrosive component (e.g., gas of oxygen, and so on, or liquid of fuel ash, and so on) through such holes produces corrosion (internal oxidation or corrosion) in the boundary with a material to be treated. This corrosion may cause peeling of the sprayed layers. However, in the case of the present invention, since the layer coated with a NiCr alloy or a MCrAlY alloy having excellent resistance to oxidation and corrosion by low pressure plasma spraying is used as a substrate for the layer formed by atmospheric plasma spraying, the progress of such internal oxidation or corrosion is retarded and thus peeling of the sprayed layers can be controlled. 
     With its actual use, cracks may occur in the coated layer which contains a large amount of Cr or Cr·Al. These cracks may result in the great reduction of a base material strength. In the case of the present invention, however, such cracks occur only in the layer formed by atmospheric plasm spraying and thus adverse effects on the base material can be prevented. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a section view of a composite surface treated layer of Example 1 of the present invention; and 
     FIG. 2 is a section view of a composite surface treated layer of Example 2 of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The effects of the present invention will become more apparent with reference to the following specific examples. 
     Example 1 
     Referring to FIG. 1, a reference numeral 1 denotes a base material, which is composed of a gas turbine moving blade Ni-based alloy IN738LC (by wt. %, its composition is Co: 8.3, Cr: 15.9, Ti: 1.75, W: 2.54, Ta: 1.73, C: 0.09. Al: 3.42, Zr: 0.03, B: 0.008, Fe: 0.1, Si&lt;0.05, Mn&lt;0.05, S&lt;0.005 and Ni: remaining part). This base material 1 was subjected to blasting by alumina and then installed in a low pressure plasma spraying canister (simply referred to as a spraying canister, hereinafter). Then, a low pressure plasma-sprayed layer was formed by applying a 50 Ni-50 Cr alloy 2 with low pressure plasma spraying so as to have a film thickness of 100 μm. Then, after dry air was introduced in the spraying canister, an atmospheric plasma-sprayed layer was formed by applying a 50 Ni-50 Cr alloy 3 with atmospheric plasma spraying so as to have a film thickness of 500 μm. After spraying was over, thermal diffusion treatment of 1050° C.×4 hours was performed in an Ar gas atmosphere furnace. The conditions for such low pressure and atmospheric plasma spraying are shown in Table 2, later described. 
     Example 2 
     Referring to FIG. 2, a reference numeral 4 denotes a base material, which is composed of a gas turbine stationary blade Co-based alloy ECY768 (by wt. %, its composition is Cr: 23.5, Ni: 9.86, Ti: 0.22, W: 7.18, Ta: 3.75, C: 0.61, Al: 0.21, Zr: 0.01, B: 0.001, Fe: 0.06, Si&lt;0.1, Mn&lt;0.1, S&lt;0.001 and Co: remaining part). The base material 4 was subjected to blasting by alumina and then installed in the spraying canister. Then, a low pressure plasma-sprayed layer was formed by applying a Co--30 wt. % Cr--8 wt. % Al--0.5 wt. % alloy 5 with low pressure plasma spraying so as to have a film thickness of 200 μm. Then, after dry air was introduced in the spraying canister, an atmospheric plasma-sprayed layer was formed by applying a Co--30 Wt. % Cr--8 wt. % Al--0.5 wt. % Y alloy 6 with atmospheric plasma spraying so as to have a film thickness of 300 μm. After spraying was over, thermal diffusion treatment of 1150° C.×2 hours was performed in a vacuum furnace. 
     The conditions of the low pressure and atmospheric plasma spraying described in the foregoing Examples 1 and 2 are shown below in the Table 2. 
     
                                           TABLE 2__________________________________________________________________________         Low pressure plasma spraying                         Atmospheric                    Thermal                         plasmaItem     Division         Cleaning              Preheating                    spraying                         spraying__________________________________________________________________________Chamber  (mbar)         30   55    60   --Spray distance    (mm) 260  300   280  120Ar flow rate    (liter/min)         50   50    50   40H.sub.2 flow rate    (liter/min)         0    8     10   8Current  (Amp)         500  600   650  600Voltage  (V)  60   62    65   40Powder feed    (%)  --   --    12   5(Kg/Hr)Transfer current    (A)  50   --    --Carrier gas flow rate    (liter/min)         --   2.0   2.0  --Coated layer thickness         Example 1              50Ni50Cr                    100 μm                         500 μm         Example 2              CoCrAlY                    200 μm                         300 μm__________________________________________________________________________ 
    
     Comparison Example 
     By using the test pieces obtained in the Examples 1 and 2 and test pieces (base materials: IN738LC and ECY768) coated with a 50 wt. % Ni--50 wt. % Cr alloy and a Co--30 wt. % Cr--8 wt. % Al--0.5 wt. % Y alloy by singly performing low pressure plasma spraying or atmospheric plasma spraying so as to have a film thickness of 500 μm, evaluation was made for corrosion resistance by a Na 2  SO 4  --V 2  O 5  synthetic ash coating high temperature corrosion test and for adhesion by a heat cycle test performed by repeating 1150° C. and RT (room temperature). 
     (1) Result of the Synthetic Ash Coating High Temperature Corrosion Test 
     Referring to Table 3, it can be understood that as compared with the low pressure plasma-sprayed material of the Comparison Example, the corrosion reduction rates in the Examples 1 and 2 of the present invention were about 60% for 50 Ni-50 Cr and about 65% for CoCrAlY respectively. 
     On the other hand, as compared with the atmospheric plasma-sprayed material, the rates were almost equal or slightly smaller. In evaluation made in terms of maximum erosion depth, the result was almost the same as that in the case of corrosion reduction rate. In the Table 3, in the evaluation of the corrosion testing result by low pressure plasma spraying, the corrosion reduction rate and the maximum erosion depth of the test piece coated with a 50 Ni-50 Cr alloy are shown being set to 100 respectively. 
     
                                           TABLE 3__________________________________________________________________________RESULT OF SYNTHETIC ASH COATING HIGH TEMPERATURECORROSION TESTMethod ofexecution        Low pressure plasma                       Atmospheric plasmaCoating Example 1       Example 2            spraying   sprayingmaterial 50Ni--50Cr       CoCrAlY            50Ni--50Cr                  CoCrAlY                       50Ni--50Cr                             CoCrAlY__________________________________________________________________________EvaluationCorrosion 57    52   100   80   60    56reductionrateMaximum 60    56   100   82   65    58erosiondepth__________________________________________________________________________ *1 For 50Ni--50Cr, a base material (material to be treated) was IN738LC. For CoCrAlY (Co  30 Cr  8Al  0.5Y), a base material was ECY768. *2 Test conditions Synthetic ash: 80 wt. % Na.sub.2 SO.sub.4 - 20 wt. % V.sub.2 O.sub.5 Atmosphere: N.sub.2 --CO.sub.2 --O.sub.2 --SO.sub.2 mixed gas Temperature: 850° C. Time: 100 hr *3 For evaluation, the values of the test piece coated with 50Ni--50Cr by low pressure plasma spraying were respectivety set to 100. 
    
     (2) Result of the Heat Cycle Test 
     Referring to Table 4 which shows the gist of a test result, it can be understood that no special abnormality except slight color changes occurred in the test pieces of the Examples 1 and 2 as in the case of the low pressure plasma-sprayed material while cracks or peeling occurred in the atmospheric plasma-sprayed material. 
     
                       TABLE 4______________________________________RESULT OF HEAT CYCLE TESTMethod of  Coating materialexecution  (base material)                    Test result______________________________________Example 1  50Ni-50Cr     Appearance was black, but      (IN738LC)     no abnormality, such as                    cracks or peeling,                    occurred.Example 2  Co-30Cr-8A1-0.5Y                    No abnormality.      (ECY768)Low        50Ni-50Cr     Appearance was slightlypressure   (IN738LC)     black, but no abnormalityplasma                   occurred.spraying   Co-30Cr-8A1-0.5Y                    No abnormality.      (ECY768)Atmospheric      50Ni-50Cr     Small cracks occurredplasma     (IN738LC)     after 5 cycles.spraying                 Cracks gradually                    increased in size after 6                    cycles and partial peeling                    occurred after 10 cycles.      Co-30Cr-8A1-0.5Y                    Cracks occurred at 1      (ECY768)      cycle.                    Small peeling occurred                    after 5 cycles.                    Peeling range expanded                    after 10 cycles.______________________________________ 
    
     Heat cycle condition: atmosphere=air 1150° C. (15 min.)≠RT (room temperature) 10 cycles 
     The process for treating a high temperature corrosion resistant composite surface of the present invention is remarkably effective for industrial purpose in that excellent high temperature corrosion resistance can be provided for a metallic material used in a high temperature.