Valve bonded with corrosion and wear proof alloy and apparatus using said valve

Abstract: The object of the present invention is to provide a valve characterized by excellent corrosion and wear resistance and maintainability by bonding corrosion and wear proof alloy containing non-continuously distributed eutectic carbide to the sliding portions of various types of apparatuses and valves by diffusion bonding. Another object of the present invention is to improve the maintainability of a thermal and nuclear power plant and to provide a nuclear power plant comprising the recirculating water which ensures excellent working safety, in particular. A valve and apparatus of a power plant using said valve characterized in that corrosion and wear proof alloy is bonded to the sliding portion, wherein said corrosion and wear proof alloy is further characterized in that network-formed eutectic carbide in the alloy containing the cast structure base metal and eutectic carbide is formed into( multiple) granules or lumps having the particle size of 30 microns or less so that said eutectic carbide is non-continuously distributed.

Description: BACKGROUD OF THE INVENTION 1.Field of the Invention The present invention relates to a valve with corrosion and wear proof alloy bonded to the sliding portion and apparatuses using said valve. 2.Description of the Prior Art In recent years, chemicals such as hydrogen peroxide and hydrazine have come to be injected into the recirculation system of the turbine generation facilities in order to adjust the quality of recirculating water. This increases the volume of dissolved oxygen on the downstream side of the inlet of said chemicals, resulting in damages of the valve seat, etc. due to erosion and scoring. Official Gazette of Japanese Patent Laid-Open NO. 179283/1984 discloses a valve wherein a valve seat comprising 30 to 45 wt% of Cr, 3.0 to 8.0 wt% of Ti, 0 to 10 wt% of Mo and remaining Ni is diffusion-bonded to the valve disc and valve body. In such valves as globe valve, gate valve and butterfly valve, alloys of high hardness characterized by excellent corrosion and wear resistance such as cobalt (Co) base alloy, nickel (Ni) base alloy, and iron (Fe) base alloy are padded onto the sliding portion of the valve and valve seat in order to avoid damage of erosion caused by fluid close to the valve seat during operation, or to prevent scoring at the time of valve operation. However, eutectic carbide of cobalt (Co) base, nickel (Ni)base or iron (Fe) base alloy and others padded onto the sliding portion and valve seat of the valve steam or hot water is selectively corroded and damaged. Fluid in the recirculating water system has a high temperature and high speed. After the said damage due to corrosion, the base metal of the cast structure may be separated to cause erosion. (Thermal nuclear power generation, Vol. 30, No.5, P.67 and "Machine damage": 1982 No.2 P.90) This will lead to reduced leakage resistance of the valve, resulting in failures such as changes in controllability and operation characteristics. This is accompanied by more frequent inspection and disassembling of the valve, hence poorer plant maintainability. If the above failure occurs to the sliding portion in contact with reactor water and the valve of the facilities in the system to supply coolant into the reactor in the nuclear power generation facilities, cobalt, nickel or iron alloy of high hardness padded to these positions may be separated to dissolve into the system due to corrosion and erosion. In the welded overlay alloy comprising a conventional cast structure base metal and eutectic carbide of a network structure, resistance to corrosion and erosion is not sufficient. To solve these problems in the present invention, the corrosion and wear proof alloy with non-continuously distributed eutectic carbide is bonded to the sliding portions of the valve and others by diffusion bonding, thereby providing a valve characterized by excellent resistance to corrosion and erosion and superb maintainability. SUMMARY OF THE INVENTION The object of the present invention is to improve the corrosion and erosion resistance of the sliding portions and valve seat of valves used in the thermal and nuclear power plant or the like. The following describes the summary of the present invention: The present invention relates to a valve comprising a valve disc and valve body with a valve seat installed on each of the sliding portions of said valve disc and box; said valve characterized in that the surface of said valve seat is made of the material selected from one of cobalt base alloy, nickel base alloy and iron base alloy, and said alloys have a granular or lumpy eutectic carbide in the matrix having a metallic micro-structure wherein the particle size of said eutectic carbide does not exceed 30 microns or more preferably 10 microns. The alloy selected from one of cobalt base alloy, nickel base alloy and iron base alloy is preferred to be bonded to the surface of the valve seat by diffusion bonding, especially by transient liquid phase diffusion bonding. Resistance to corrosion can be improved by as much as about 300 times by changing eutectic carbide from a continuous network form to a non-continuous granular or lumpy form having the particle size not exceeding 30 microns. The particle size of the eutectic carbide is preferred to be smaller, and this improves mechanical characteristics including ductility, toughness and resistance to impact. Preferably, a member comprising the cobalt, nickel or iron base alloy having said granular or lumpy eutectic carbide is bonded on the surface of the sliding surface of a valve seat composed of stainless steel made of carbon steel, low alloy steel or stainless steel. The cobalt base alloy is preferred to contain 0.6 to 3 wt% or preferably 1.0 to 3 wt% of C, 2 wt% or less of Si, 25 to 32 wt% of Cr, 15 wt% or less or preferably 4 wt% to 15 of W, 0 to 3 wt% of Fe, 0 to 3 wt% or less of Ni, and 0 to 6 wt% of Mo. The nickel base alloy is preferred to contain 1 wt% or less, or preferably 0.2 to 0.5 wt% of C, 7% or less of wt% of Si, 7 to 20 wt% of Cr, 5% or preferably 0.5 to 2 wt% of W, and 3.5 or less wt% of B. The ion base alloy is preferred to contain 1.5 wt% or less, or 0.3 to 1.0 wt% of C, 4 wt% or less, or 0.5 to 5 wt% of S, 15 to 27 wt% of Cr, and 8 wt% or less, or preferably 0.5 to 2 wt% of Mo Corrosion and wear proof alloy used in the invention is typically made by plastic deformation of a base alloy containing a eutectic carbide in a continuous network structure, for example in order to make the eutectic carbide non-continuous or granular. Preferably, the corrosion and wear proof alloy is bonded to the valve seat by diffusion bonding. Furthermore, said diffusion bonding is preferred to be transient liquid phase diffusion bonding wherein the insert material having a melting point lower than that of the base metal is inserted into the boundary of bonding, and the melting point lowering element is diffused in the base metal, thereby promoting coagulation and bonding. Said insert material is preferred to be the nickel base alloy containing 1 to 8 wt% of Si and 1 to 5 wt% of B, or the nickel alloy further containing 5 to 20 wt% of Cr in addition. Particularly, said insert material is preferred to be the nickel base alloy containing 3 to 6 wt% of Si and 2 to 4 wt% of B, or the nickel base alloy further containing 5 to 10 wt% of Cr in addition. Said base metal is preferred to be cobalt base corrosion and wear proof alloy. The present invention provides a nuclear power plant comprising piping equipment having water feed valves in the recirculating system of coolant of nuclear reactor water, characterized in that the valve seat of said valve is formed bonded with the corrosion and wear proof alloy, wherein said eutectic carbide contained in the alloy comprising the cast structure base metal and eutectic carbide is formed into granules and lump, and said eutectic carbide is non-continuously distributed. It also provides a power plant characterized in that piping equipment with the valve according to the present invention is installed in the recirculating system where water or high hot water recirculates. As discussed above, the apparatus and valve according to the present invention provides a reliable method of reducing the damages on sliding portion of the valve or valve seat resulting from damages due to corrosion of the eutectic carbide caused by dissolved oxygen in fluid, and of avoiding increase in frictional resistance on the sliding portion and valve seat and leakage from the valve seat. Furthermore, the nuclear power plant according to the present invention prolongs the service life of apparatuses and valves in said plant. The present invention is characterized in that the alloy composed of Co base alloy, Ni base alloy or Fe base alloy, and comprising the cast structure base metal and eutectic carbide continuously distributed in a network form is provided plastic processing by forging or rolling, and, thereby, the eutectic carbide continuously distributed in a network structure is made non-continuously distributed in granules or lumps formed. Thus the continuously distributing corrosion and wear proof alloy is bonded to the sliding portions of various apparatuses and valves by diffusion bonding, especially by transient liquid phase diffusion bonding. This invention controls increase in frictional drag of the sliding portions of the apparatuses and valves. Especially this invention controls reduction in leakage resistance due to roughing of valve seat surface and the like. The method of converting eutectic carbide continuously distributed in network structure into the one non-continuously distributed in granules or lumps is exemplified by the method where the alloy obtained by forging is subjected to plastic formation by forging or rolling and heat treatment (annealing), but is not restricted to the method thereof. Corrosion and wear proof alloy with non-continuously distributed eutectic carbide can be bonded to the sliding portion by diffusion bonding, above all, by transient liquid phase diffusion bonding. In transient liquid phase diffusion bonding, the insert material having a melting point lower than that of the base metal (bonded material) is inserted into the boundary of bonding, and such melting point lowering element as boron (B), silicon (Si) or phosphorus (P) contained in the insert material at the time of bonding process is diffused in the base metal. This makes it possible to reduce the pressure at the time of bonding below the level in solidus diffusion bonding, and to reduce the deformation of the base metal(bonded material). Furthermore, machining accuracy of the bonded surface before the bonding process can be improved to about 40 microns in Rmax, or more preferably 20 microns in Rmax, and satisfactory bonding is obtained. As a diffusion bonding of base metal (bonded material) through transient liquid phase, transient liquid phase diffusion bonding is suitable in that it is superior in the reliability of the bonded portion such as bonding strength. For example, the conventional brazing method utilizes chemical bondage and physical adhesion between the brazing filler metal and the base metal, and mutual diffusion among them is smaller than that in transient liquid phase diffusion bonding. Welded overlay alloy such as Co base, Ni base or Fe base alloy with eutectic carbide continuously distributed in network structure in the normal state can be converted into corrosion and wear proof alloy containing non-continuously distributed eutectic carbide when eutectic carbide is formed in granules or lumps. By diffusion bonding of said corrosion and wear proof alloy to the sliding portion of the valve and apparatus. Such chemical as hydrogen peroxide or hydrazine is injected into the recirculating water system. Even if this increases the volume of dissolved oxygen, it is possible to control the corrosion damage of eutectic carbide on the sliding portion of valve and apparatus present in said system. Furthermore, it also controls increase in the frictional resistance on the sliding portion and reduction in leakage resistance due to roughing of the valve seat surface, thereby making it possible to provide a valve and apparatus characterized by outstanding maintainability. According to the present invention, Co base, Ni base or Fe base corrosion and wear proof alloy containing eutectic carbide in granules or lumps obtained from the sheet or rod material subjected to plastic formation by forging or rolling and heat treatment (annealing) is bonded to the valve seat or the like by diffusion bonding, especially, by transient liquid phase diffusion bonding. This makes it possible to provide safety valves, gate valves, globe valves and others valves characterized by reduced possibility of damages on the valve seat due to corrosion or erosion caused by dissolved oxygen. Furthermore, the present invention makes it possible to control scattering or diffusion of cobalt resulting from corrosion, erosion or wear of the Co base alloy used in the valves of nuclear power generation facilities, thereby providing a nuclear power plant characterized by outstanding maintainability. Conventionally, stellite alloy of high hardness, for example, Co base alloy containing 1.3 wt% of C, 27.7 wt% of Cr and 4.9 wt% of W has been used on the sliding portion of the valve seat of the valve and apparatus in order to prevent erosion or scoring. They are padded by gas welding or electric arc welding such as PTA (Plasma Transfer Arc). In said cobalt base alloy, however, eutectic carbide mainly comprising Cr and C is continuously distributed in network structure in the cast structure base metal mainly comprising cobalt. As a result of this corrosion or erosion, separation of the cast structure base metal is promoted and caused by corrosion of eutectic carbide of network structure by dissolved oxygen. According to the present invention, the alloy comprising the cast structure base metal and the eutectic carbide continuous in a network form is subjected to the process of plastic formation by forging and rolling, and subsequent heat treatment (annealing) or the process of hot forging or rolling at a high temperature. This allows eutectic carbide to be non-continuously distributed in granules or lumps. Damage due to corrosion of carbide caused by dissolved oxygen is limited to the carbide on the surface in contact with dissolved oxygen, with the result that separation of the cast structure base metal is controlled. The above effect can be verified, for example, in the intergranular corrosion test (Strauss test JIS G0575) on the Co base alloy with 1.3 wt% of C, 27.7 wt% of Cr and 4.9 wt% of W containing eutectic carbide in network structure or eutectic carbide in granules or lumps. When eutectic carbide is formed in granules or lump, corrosion depth is reduced down to about 1/300 in the Strauss test, compared to the case where eutectic carbide is formed in the network structure. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a vertical sectional view of a gate valve and an enlarged illustrative view of the valve seat; FIG.2 is a vertical sectional view of a check valve; and FIG.3 is a piping system diagram of the nuclear power plant where the valve according to the present invention is used in a feed water system. DESCRIPTION OF THE PREFERRED EMBODIMENTS [Embodiment 1] In the present embodiment, corrosion and wear proof cobalt alloy of high hardness containing 1.1 wt% of C, 29.7 wt% of Cr and 4.5 wt% of W is bonded to the valve seat of the gate valve used in dissolved oxygen atmosphere by transient liquid phase diffusion bonding, wherein said corrosion and wear proof cobalt alloy is characterized in that eutectic carbide is made into granules or lumps having a diameter not exceeding 30 microns by hot rolling at the same temperature as hot forging at 1050 to 1100 degrees Celsius. Figure 1 is a vertical sectional view of a gate valve composed of casting of SCPH2 (equivalent to S25C). (a) is an enlarged view of the valve disc and (b) is an enlarged view of the valve body. The valve disc 50 and valve body 51 a gate valve 2, and valve seat 4a and 4b were washed. Insert materials were placed on these valve seat surfaces, and said corrosion and wear proof cobalt base alloy sheet in the forms of a 5 mm thick ring sheet with eutectic carbide formed in granules or lumps was placed on said insert materials to carry out transient liquid phase diffusion bonding. Said insert materials consist of 40 microns thick Ni base alloy containing 4.5 wt% of Si and 3.2 wt% of B. The solidus line temperature and transient liquid phase line temperature thereof are about 980 and 1040 degrees Celsius, respectively. Transient liquid phase diffusion bonding was carried out at bonding temperature of 1100 degrees Celsius for a holding time of one hour at a degree of vacuum of 2 × 10 -4 Torr and pressure of 80 g/cm 2 . Since melting point lowering elements of Si and B are contained, the melting point of the insert material is lower than that of the base metal (bonded material). When held at the bonding temperature, Si and B are diffused into the base metal to increase the melting point, so coagulation of the insert material proceeds during bonding process. In this way, Co base alloy 100a is bonded on the surface of the valve seat 4a to get a valve seat la. Co base alloy 100b is bonded on the surface of valve seat 4b to get a valve seat 1b. After bonding process, the cross section of the bonding boundary was observed to find out that there were almost no bonding defects such as voids. Excellent bonding performance was verified. For the gate valve manufactured according to the present embodiment, continuous corrosion of eutectic carbide caused by dissolved oxygen is held down, thereby reducing separation of the base metal of the cast structure. This retards the progress of corrosion or erosion of the valve seats 4a and 4b and prevents deterioration of leakage resistance. Brazing is one of the methods for bonding base metal (bonded material) through transient liquid phase. When brazing is used, brazing filler material is mainly in transient liquid phase even in the completion of heating at the bonding temperature. Mutual diffusion between the brazing filler material and base metal is smaller than mutual diffusion between the insert material and base metal in transient liquid phase diffusion bonding. Reliability of the bonded portion such as bonding strength is inferior to that of transient liquid phase diffusion bonding. [Embodiment 2] In Embodiment 2, corrosion and wear proof cobalt base alloy of high hardness containing 1.1 wt% of C, 29.7 wt% of Cr and 4.5 wt% of W is bonded to the valve seat of check valve 3 used in the dissolved oxygen atmosphere by transient liquid phase diffusion bonding, wherein said corrosion and wear proof cobalt base alloy is characterized in that eutectic carbide is formed in granules or lumps by hot forging at 1050 to 1100 degrees Celsius. In this way, valve seats 1a and 2b were manufactured. Figure 2 is a vertical sectional view/ vertical section of a check valve. The insert material used for bonding is Ni base alloy containing 7 wt% of Cr, 3 wt% of Fe, 4.5 wt% of Si and 3.2 wt% of B, and solidus line temperature and transient liquid phase line temperature are about 970 and about 1000 degrees Celsius, respectively. Transient liquid phase diffusion bonding is carried out at the bonding temperature of 1090 degrees Celsius for the holding time of one hour at a degree of vacuum of 2 × 10 -4 Torr and pressure of 50 g/cm 2 . After bonding, the cross section of the bonding boundary was observed to find out that there were almost no bonding defects such as voids. Excellent bonding performance was verified. Furthermore, for the check valve manufactured according to the present embodiment, continuous corrosion of eutectic carbide caused by dissolved oxygen is held down, thereby reducing separation of the base metal of the cast structure. This retards the corrosion or erosion of the valve seat 1a and 1b and prevents deterioration of leakage resistance. In Embodiments 1 and 2, Ni base alloy of lower melting point was used as the insert material. It is also possible to use Fe base or Co base of lower melting point, instead. Furthermore, the insert material is preferred to contain Cr effective to corrosion resistance as shown in Embodiment 2. This makes it possible to maintain the corrosion resistance of the bonded portion, especially the corrosion resistance of the bonded portion in the steam atmosphere of high temperature and bonding weighting with a great deal of dissolved oxygen. Figure 3 shows an example of the piping system diagram of the nuclear power plant where the valve according to the present invention is used in the facilies of a primary coolant recirculating system. A great number of valves according to the present invention are present in the piping of the facilies of a primary coolant recirculating system, and are not illustrated. In the nuclear power plant, coolant is heated by nuclear reactor pressure vessel 14 to become the steam of high temperature and pressure and is led into the high pressure turbine 18 through main steam line 15. Then steam discharged from high pressure turbine 18 is led to low pressure turbine 19 to drive generator 20. After passing through main condenser 10, steam discharged from the high pressure turbine 18 and low pressure turbine 19 is fed by the feed water valve system 6 equipped with many valves according to the present invention to pass through the feed water pump 30 and high pressure feed water heater 31, then through the feed water line 9. Then it is condensed in the nuclear reactor pressure vessel 14. The following shows the other numerals in the Figure 3: Numeral 5 denotes a coolant cleanup system heat exchanger, 7 a feed water heater, 8 a recirculating system piping, 10 a main condenser, 11 an SLC tank for boric acid spray system, 12 an SLC pump, 13 a fuel containment vessel, 16 a feed water line, and 17 a humidity analyzer, 21 a main transformer, 22 an stack, 23 an offgas disposal system, 24 an air ejector, 25 a low pressure condensate pump, 27 a condensate storage tank, 28 a condensate filter, 29 a condensate demineralizer (CD), 32 a control rod drive system, 33 and 36 a heat exchanger, 34 a filter demineralizing facility, 35 a reactor core isolation cooling system and 36 a high pressure condensation pump. The present invention is effective in prolonging the service life of the apparatus and valve in the nuclear power generation plant. The method according to the present invention reduces deterioration in leakage resistance, controllability and operation performances when applied to the sliding portion of the valve seats of the safety valve and globe valve in addition to said gate valve and check valve. Combined use of above valves reduces corrosion or erosion on the sliding portions of the valve seats and others of valves caused by the influence of dissolved oxygen by injection of hydrogen peroxide performed for water quality adjustment in the plant equipment such as turbine generation facilities. This is effective in improving safety of the plant facilities such as turbine generation facilities. Especially in the nuclear power generation facilities, the present invention provides a nuclear power generation plant characterized in that it is possible to reduce corrosion and separation of eutectic carbide of cobalt base alloy on the sliding portion in contact with reactor water and the valve seat, case or the like of the valve in system facilities to feed coolant into the reactor, and to reduce accompanying discharge and diffusion of cobalt of the eutectic carbide into the system. Said nuclear power generation plant is further characterized in that it is possible to reduce the separation of cast structure base metal mainly composed of cobalt wherein erosion resistance is deteriorated by corrosion and separation of eutectic carbide.

Claims: A valve comprising a valve member and valve body each of said valve member and valve body having a valve seat portion, characterised in that the surface of at least one of said valve seat portions is made of a material selected from cobalt base alloy, nickel base alloy and iron base alloy, said alloy having a non-continuous granular or lumpy eutectic carbide in the matrix wherein the particle size of said eutectic carbide does not exceed 30 microns. A valve according to claim 1, wherein said cobalt base alloy, nickel base alloy or iron base alloy is bonded to the base metal of said valve seat. A valve according to claim 2, wherein said base alloy is bonded to the base metal of the valve seat by diffusion bonding. A valve according to claim 3, wherein said base alloy is bonded to the base metal of the valve seat by transient liquid phase diffusion bonding. A method of manufacturing a valve comprising a valve member and a valve body, each of said valve member and valve body having a valve seat portion, comprising the steps of subjecting cobalt, nickel or iron base alloy containing a eutectic carbide in network form to plastic deformation, to provide a non-continuous granular or lumpy eutectic carbide distributed in the matrix, wherein the particle size of said eutectic carbide does not exceed 30 microns, and bonding a sheet of said base alloy to the surface of one of said valve seat portions. A method of manufacturing a valve according to claim 5, wherein said sheet of base alloy is bonded to said valve seat portion by transient liquid phase diffusion bonding. A power plant having a recirculating system where steam or hot water is circulating, said recirculating system comprising a valve according to any one of claims 1 to 4. A nuclear power plant having a recirculating system where the coolant of a nuclear reactor is circulating, said recirculating system comprising a valve according to any one of claims 1 to 4.

Inventors: CHIBA YOSHITERU, CHIGASAKI MITSUO, KATO TAKAHIKO, KIYOTOKI YOSHIHISA, KUNIYA JIRO, MATSUSHITA SHIZUO, CHIBA, YOSHITERU, CHIGASAKI, MITSUO, KATO, TAKAHIKO, KIYOTOKI, YOSHIHISA, KUNIYA, JIRO, MATSUSHITA, SHIZUO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 2001
Main classification: F16K3
IPC codes: F16K3