Title of Invention

"A METHOD OF MANUFCTURING STEAM VALVE SPINDLE WITH HIGH TEMPERATURE WEAR RESISTANCE COATING"

Abstract

A method of manufacturing steam valve spindle with high temperature wear resistance coating comprising the following steps: i) Chromium Carbide based powder along with nickel chrome as a binder is fed in the plasma flame for a short duration thereby melting and accelerating the powder to attain a velocity upto 350 m/s; ii) the chromium carbide, generally in the form of Cr3C2/CrC powder along with nickel chrome in the ratio of 80: 20 is deposited on the valve spindle; iii) the valve spindle is rotated at low rpm of upto 100 rpm and the plasma torch is linearly moved upto 400 mm/min to obtain a coating thickness upto 300 micron; iv) two air jets are used to cool the valve spindle during plasma spraying so that the temperature of the spindle do not exceed more than 150ºC; v). the pores created in the plasma spraying of chromium carbide with nickel chrome as binder are filled with graphite powder which comes during graphite packing.

Full Text

The invention relates to a method of manufacturing steam valve spindle with high temperature wear resistance coating. The valve spindles are used for controlling the steam flow rates and thereby the output of the turbine. The precise movement of the valve spindle is carried by steam governor. The damages occurring on the valve spindle surface are due to the oxidizing environment. Nearly 0.20 to 0.50 mg 02/kg of steam is entrained The origin of this dissolved oxygen is due to addition of fresh water in the boiler. The free oxygen reacts with the surface at elevated temperature ( 550 C) resulting in formation of metallic oxides which later crack and spall causing expressive wear out. To minimize the surface oxidation and also to improve the wear life, hard protective layers are formed on components by various techniques as described earlier. PRIOR ART There are several method of providing suitable protective coating or surface treatment for steam valve spindle. The surface treatment or protective coatings with adequate wear and spalling resistance is designed for life of 10 to 15 years. The various methods are stelliting, nitriding and thermally sprayed overlay coating by spraying. An improved method is thermal spraying such as detonation gun/high voltage oxy fuel (HVOF) spraying. There are disadvantages associated with the present method of protective coating for steam valve spindle. Present methods are adequate but require improved wear resistance to meet the design life. One of the main disadvantage associated with the present method of protective coating is that surface treatment by nitriding lacks the high hardness retention at elevated temperature. Another disadvantage associated with the present method of protective coating is that stelliting process introduces high thermal stress during deposition which leads to cracks and warpage. Yet another disadvantage associated with the present method of protective coating is that both the process of stelliting and nitriding induces defects which are very common. Still another disadvantage associated with the present method of protective coating is that they require expensive dedicated facilities. Further disadvantage associated with the present method of protective coating by thermal spraying is that the coatings are pore free and hence their life is limited for steam valve spindles at elevated temperatures. The improved method of thermal plasma spraying involves a flame temperature more than 10,000°C and chromium carbide nickel chrome composite as a powder is fed in this temperature zone. The powder is partially melted and accelerated by plasma flame to a velocity of 150-200 m/s. It is deposited on the substrate or components for required properties. Due to this high temperature involvement, there are chances that either the substrate may be overheated. This requires cares to be generally taken in selecting plasma parameters and in proper cooling of substrate or the component where coating is given needs a lot of improvement. On the other hand detonation gun works on the principle of explosion of a mixture of oxygen and acetylene in a chamber, which produces detonation wave that travels down in a several feet long water cooled barrel. The coating powder is fed in this barrel and further accelerated to a velocity of 800-1OOOm/s. The power particle get impacted on the substrate and thereby get deposited. An important attribute of this process is that substrate does not get heated up and coatings are generally more dense and have better properties especially their abrasion, erosion and metal to metal wear resistance. One of the main disadvantage associated with the present system of thermal system is that inspite of better properties of abrasion, erosion and metal to metal wear resistance they do not achieve a minimum design life of 10 to 15 years. Therefore the main object of the present invention of method of manufacturing steam valve spindle with high temperature wear resistance coating is to have an improved plasma coated surface with solid lubricant such as graphite and molybdenum disulfide. Another objective of the invention is to have controlled porosity coating of high hardness and wear resistance for subsequent impregnation after plasma spray with solid lubricant such as graphite. Yet another objective is to have a sustained self lubricating action by impregnated solid lubricant in porosity of coated surface to have improved wear resistance at elevated temperature. This will also avoid presence of lubricant only at the interface. Still another objective of the invention is to avoid process induced defects and avoid post machining operations on components, Further objective is to develop a simple system of surface modification with a combination of material, process and impregnation to have high wear resistance surface at elevated temperature. Yet a further object of the present invention is to propose a process for high temperature wear resistance coating is that it does not lead to distortion/warpage of the steam valve spindles which generally occurs in the present processes. Still further object of the present invention is to propose a process for high temperature wear resistance coating is that a controlled layer of thickness is achieved during spraying and does not require further machining. According to the present invention there is provided method of manufacturing steam valve spindle with high temerature wear resistance coating comprising the following steps: i) Chromium carbide based powder along with nickel chrome as a binder is fed in the plasma flame for a short duration thereby melting and accelerating the powder to attain a velocity upto 350 m/s; ii) the chromium carbide, generally in the form of Cr3C2/CrC powder alongwith nickel chrome in the ratio of P0:20 is deposited on the valve spindle; iii) the valve spindle is rotated at low rpm of upto 100 rpm and the plasma torch is linearly moved upto 400 mm/min to obtain a coating thickness upto 300 micron; iv) two air jets are used to cool the valve spindle during plasma spraying so that the temperature of the spindle do not exceed more than 150°C; v ) the pores created in the plasma spraying of chromium carbide with nickel chrome as binder are filled with graphite powder which comes during graphite packing. The nature of the invention, its objective and further advantages residing in the same will be apparent from the following description made with reference to non-limiting exemplary embodiments of the invention represented in the accompanying drawings. Figure 1 - shows the state of the art in valve Engineering. Figure 2 - microstructure of chromium carbide by plasma (CCP) Figure 3 - microstructure of chromium carbide by detonation (CCD) Figure 1 - shows the Section of a valve used in governing and regulating of steam in a turbine. The drawing shows the valve spindle coated or stellited or Nitrided and the temperature of the various components. The inventive process involves applying a hard chromium carbide coating by plasma spray capable of producing wear resistant protective layers which retain the wear resistance at elevated temperature in excess of 550°C as in steam valve spindle. The process is capable of producing controlled porosity hard chromium carbide coating so that controlled quantity of graphite as a lubricant can be impregnated producng sustained lubrication effect at elevated temperature when in use. Filling up of the pores with graphite in the chromium carbide hard layer results in improved friction characteristics compared to conventional D gun and HVOF sprayed deposits along with other techniques such as stelliting and nitriding since the conventional deposits are impervious in nature. The process is simple and easy in handling and operation. The process/technology can be adapted for similar application for other components in gas turbines or nuclear application. DETAILED DESCRIPTION OF THE INVENTION; This invention details development of controlled porosity in chromium carbide coating on valve spindles to which self lubricating properties has been introduced to the hard coatings by making use of selfimpregnating at high temperature by self lubricating materials such as graphite. The viability of presence of self lubricating materials are brought out by modifying the plasma spray parameters to introduce controlled porosity, resulting in many fold improvement in wear resistance compared to conventional techniques. This also avoids process induced defects. CHOICE OF COATING MATERIALS; To introduce modified hard wear resistance surface on valve spindles, a number of materials were evaluated. The following materials by plasma spray were found to be suitable for this application. These are as under: 1. Chromium carbide 2. Chromium Oxide, 3. Nickel Chromium aluminium Yttrium (NiCrAlY), 4. A1203 NiAl These were experimented for their friction and wear properties. The materials are classified based on their performance at elevated temperature wear testing and are tabulated in table 1. The present invention by way of example is described with details for evaluation for high temperature wear in valve spindle. This invention will lead to a steam valve spindle of thermal power plant with high hardness, self lubricating coating with assured wear life greater than 15 years. DESCRIPTION; Plasma spraying process involves temeratures more than 10,000°C. In this process the chromium carbide based powder along with nickel chrome as a binder is fed in the plasma flame for a short duration thereby melting and accelerating this powder to attain a velocity upto 350 m/s. The chromium carbide generally in the form of Cr3C2/CrC powder along with nickel chrome in he ratio of (80:20) is deposited on the valve spindle. The valve spindles are rotated at low rpm ( up to 100 rpm) and the plasma torch is linearly moved up to 400 mm/min to obtain a coating thickness up to 300 micron. To minimize the rise in temperature of these spindles which occurs due to high temerature of plasma flame during spraying, are cooled with air jets (generally two numbers) so that the temperature does not exceed more than 150°C. In the present technique pores of certain size and numbers are created by optimizing the plasma parameters. Figures 2 and 3 shows microstructure of chromium carbide by plasma (CCP) and chromium carbide by detonation (CCD). Creation of these pores in stelliting/nitriding are impossible. These pores are filled with graphite powder which comes due to graphite packing during operation. This graphite packing is used as a lubricant at high temperature. The graphite filled pores in chromium carbide coating acts as a lubricant and do not allow it to wear out due to the inherent high hardness of chromium carbide at elevated temperature. This reduces the frictional force along with wear and tear of spindle at 550 to 580°C thereby increasing the life span of these spindles. COATING MATERIALS AND PARAMETERS; Commercially available chromium carbide powder can be used for this process. The plasma spray system used for this process produces normal specified coating parameters yield coatings with a hardness in the range of 650 to 700HV, and porosity limited to less than 2%. The parameters were modified to get porosity upto 8%. A special fixture for cooling the coating area to a limiting value below 150 °C is employed. EVALUATION OF FRICTION AND WEAR PROPERTIES; The friction and wear characteristics of hard facing deposits such as stelliting, surface treatment such as nitriding and thermal sprayed coatings such as D-gun sprayed and plasma sprayed coatings on valve spindle material X22CrMoV 121 were evaluated at 550°C in graphiate as lubricate using SRV Friction and Wear test rig. Table 1 provides their friction and wear characteristics. From the evaluation, it has been observed that Chromium carbide with controlled porosity UP TO 6 percent gives the best friction and wear properties at 550°C in the presence of graphite as a lubricating media. This evaluation also indicates that the chromium Oxide coating is another candidate for this application however, lacks the bond strength. This is because chromium oxide is ceramic in nature. FIELD TRIAL; After evaluating the friction and wear characteristics of all these coatings, Chromium carbide coating is adopted on a valve spindle. This is due to its stability, antisiezure and excellent wear resistance at high temperature of the Cr.,C2 coating along with NiCr as a binder. The field results are encouraging and agreed with laboratory. Figure 1 shows the assembly drawing of a steam valve spindle. The spindles are lubricated by high grade graphite glands packing. The coated areas are also indicated in the figure. The field results are encouraging and agree with laboratory evaluation. We thereby claim that chromium carbide based plasma sprayed protective layers on steam valve spindle which generally operate in excess of 550°C is ideal for this application. We also claim that this protective layer is much superior to conventionally used protective layers like stelliting, nitriding and recently developed D gun and HVOF based chromium carbide hard layers. Table 1 Friction and Wear Characteristics of Different Coating Materials Test Conditions Environmental temperature 550°C/Air Humidity 40% Load 100N Stroke length 2mm Cycle frequency 25 Hz Test duration 3Hrs (Table Removed) ST CRV - Stellited valve spindle material CCP - Chromium carbide by plasma CCD - Chromium carbide by deonation CR0106 - Chromium oxide by plasma CR0136 - Chromium oxide+titania+silica by plasma CRVN - Nitrided valve spindle material NiCrAlY NiCraly by plasma Al203NiAl - Al203NiAl by plasma The invention described hereinabove is in relation to non-limiting embodiments and as defined by the accompanying claims. WE CLAIM: 1 . A method of manufacturing steam valve spindle with high temerature wear resistance coating comrising the following steps:- i) Chromium Carbide based powder along with nickel chrome as a binder is fed in the plasma flame for a short duration thereby melting and accelerating the powder to attain a velocity upto 350 m/s; ii) the chromium carbide, generally in the form of Cr3C2/CrC powder alongwith nickel chrome in the ratio of 80:20 is deposited on the valve spindle; iii) the valve spindle is rotated at low rpm of upto 100 rpm and the plasma torch is linearly moved upto 400 mm/min to obtain a coating thickness upto 300 micron; iv) two air jets are used to cool the valve spindle during plasma spraying so that the temperature of the spindle do not exceed more than 150°C; v) the pores created in the plasma spraying of chromium carbide with nickel chrome as binder are filled with graphite powder which comes during graphite packing. 2. A method of manufacturing steam valve spindle as claimed in claim 1 , wherein the process deposits a thin hard layer of chromium carbide coating by atmospheric plasma spraying said layer is having a porosity up to 8%. 3. A method of manufacturing steam valve spindle as claimed in claim 2 wherein the plasma spraying process involves temperatures more than 10,000°C. 4. A method of manufacturing steam valve spindle as claimed in claim 1 wherein the Plasma process spray system have parameters of yield coatings with a hardness in the range of 650 to 700HV. 5. A method of manufacturng steam valve spindle as claimed in claim 4 wherein the porosity of the plasma coating is limited to less than 8%. 6. A method of manufacturng steam valve spindle as claimed in claim 3 wherein a special fixture for cooling the coating area comprising two air jets are used to limit the temperature of the spindle below 150°c. 7. A method of manufacturing steam valve spindle as claimed in claim 1 wherein the chromium carbide {Cr3C2) plasma sprayed coating along with Nickel Chrome (N1Cr ) as binder on steam valve spindle having an operative temperature of 550°C enhances useful life upto 15 years. 8. A method of manufacturing steam valve spindle with high temperature wear resistant coating as herein described and illustrated in the accompanying drawings.

Documents:

1079-del-1999-abstract.pdf

1079-del-1999-claims.pdf

1079-del-1999-correspondence-others.pdf

1079-del-1999-correspondence-po.pdf

1079-del-1999-description (complete).pdf

1079-del-1999-drawings.pdf

1079-del-1999-form-1.pdf

1079-del-1999-form-19.pdf

1079-del-1999-form-2.pdf

1079-del-1999-form-3.pdf

1079-del-1999-form-5.pdf

1079-del-1999-gpa.pdf

1079-del-1999-petition-138.pdf