Patent Number: 053176103
Section: description

Metals and ceramics that can be used in the present invention will now be described. A) A lower layer coating of Ni--Cr and an upper layer coating of WC and Ni--Cr [WC+(Ni--Cr)] (atmospheric plasma thermal spraying). For the lower layer coating of Ni--Cr, a coating with a composition of Ni:Cr=80:20 is preferable as an under layer because it is easy to melt upon heating during the thermal spraying and with which sufficient bonding strength is obtained through fusion with the parent metal. For the upper layer coating of WC+(Ni--Cr), WC is used because of its excellent corrosion-resistant and erosion-resistant properties and Ni--Cr is added as an intergranular bonding material to help bonding because WC is hard to melt. Generally, a composition in the vicinity of WC:Ni--Cr=1:2 is preferable because of its high strength. Also, when providing the lower and upper layer coatings, it is preferable to place a Ni--Cr lower layer coating whose thickness is about a third of the total thickness of the entire coating on the surface of the parent metal and a Ni--Cr coating which contains WC on the lower layer coating, so that sufficient bonding strength to the parent metal can be obtained. According to this, strong bonding is obtained between the parent metal and the lower layer coating and between the lower and upper layer coatings, and in particular, the corrosion and erosion resistant property which WC in the upper layer coating has against fluid can be utilized. Namely, the optimum thickness ratio of the lower layer coating to the upper layer coating is approximately 1:2. B) A single layer coating of WC and Co [WC+Co] or WC and Ni--Cr [WC+(Ni--Cr)1 (jet kote thermal spraying). Because WC does not easily melt upon heating during thermal spraying, Co or Ni--Cr is added as an intergranular bonding material. Because thermal spraying is performed by jet kote thermal spraying which can remarkably improve melting rate of WC as compared with atmospheric plasma thermal spraying, only a small amount of Co or Ni--Cr is needed to mix into WC and a composition of 88WC+12Co or 88WC+12(Ni--Cr) is preferred. The jet kote thermal spraying belongs to the high energy gas thermal spray coating method and is powder thermal spraying which can produce a very sharp and high density coating by making use of an ultrasupersonic jet of combustion gas having a velocity of about Mach 5. C) Austenitic stainless steel (diamond jet thermal spraying). In this case, the spraying of an austenitic stainless steel having a composition by weight percent of 10 to 14% Ni, 16 to 18% Cr, 2 to 3% Mo, and balance Fe, which corresponds to SUS316, is performed by means of so-called diamond jet thermal spraying which is one of the high energy gas thermal spraying methods. The diamond jet thermal spraying is a spraying method for producing a coating which is not porous and has a high density using high kinetic energy, and this method also allows control over the amount of heat, providing high bonding strength and a superb finished surface. According to the present invention, reductions of wall thickness due to erosion-corrosion occurring in devices and parts which are made of carbon steel, such as piping and various kinds of valves, and which are used in the wet steam system, the feedwater and condensate system and the drain system of thermal or nuclear power plants can be prevented at low costs. Diagrams of the embodiments are shown respectively for the case in which the present invention is applied to an elbow in FIG. 1, a branch pipe in FIG. 2 and a pipe disposed on the downstream side of a control valve in FIG. 3. In these drawings the hatched portions inside the pipes are where coating is applied according to the present invention. Reference numeral 6 indicates a control valve and 7 indicates piping. The reduction of wall thickness due to erosion-corrosion in the piping made of carbon steel used in the wet steam system, the feedwater and condensate system, and the drain system of a thermal or nuclear power plant is significant in parts of the piping where a curved flow is formed, such as an elbow, a bend, a branch pipe, a junction pipe, and also in portions of straight pipes installed on the downstream side of a curved pipe and located within a length which is approximately twice the pipe caliber from the pipe, and further in pipes provided on the downstream side of the parts forming a restricted flow, such as a control valve and an orifice. Therefore, to control development of thickness reductions due to erosion-corrosion in the piping for a new system to be built, it is effective and economical to form the coating of metal or ceramic of the present invention in advance only on the inner surface of various piping parts, such as those mentioned above, which are known to often undergo thickness reductions due to erosion-corrosion. Also, as for parts which are already in an existing system, it is possible to control the further progress of thickness reduction by forming the coating of metal or ceramic of the present invention on the surface of such parts where the thickness reduction has already occured to some extent. This can be done at a low cost because the existing parts are used further and not replaced in their entirety. According to the present invention, to the portions hatched inside the pipe walls in FIGS. 1 through 3, the following coatings, for example, are applied. a) A lower layer coating of Ni--Cr and an upper layer coating of WC+(Ni--Cr) can be applied by means of atmospheric plasma thermal spraying. The lower layer coating of Ni--Cr has a thickness of 0.2 mm with a composition of Ni:Cr=80:20, and the upper layer coating of WC+(Ni--Cr) has a thickness of 0.4 mm with a composition of WC:Ni--Cr=1:2 (Ni:Cr=80:20). b) A single layer coating of WC+Co or WC+(Ni--Cr) can be applied by means of jet kote thermal spraying. The single layer coating of WC+Co has a thickness of 0.15 mm with a composition of WC:Co 88:12. The single layer coating of WC+(Ni--Cr) has a thickness of 0.15 mm with a composition of WC:Ni--Cr=88:12 (Ni:Cr=80:20). c) A coating of SUS 316 can be applied with a thickness of 0.4 mm. Thickness reductions of the piping due to erosion-corrosion can be effectively prevented by forming any of the above-mentioned coatings. Next, other embodiments in which the present invention is applied to various kinds of valves for a thermal or nuclear power plant will be described. Diagrams of the embodiments are shown respectively for the case in which the present invention is applied to a gate valve in FIG. 4, a globe valve in FIG. 5, and a check valve in FIG. 6. In these drawings, the portions hatched inside the valve walls are where the coating of the present invention is applied. In FIGS. 4 through 6, reference numeral 1 denotes a valve casing, 2 a valve seat, 3 a valving element, 4 a connecting pipe and 5 an operation handle of the valving element. Additionally, when a coating is formed by thermal spraying according to the present invention, it is preferable to have the thermal spraying applied after finishing the assembly of parts by welding. For example, the welding of the valve casing 1 with the valve seat 2 and the welding of the valve casing 1 with the connecting pipe 5 correspond to this welding which should be done before spraying. This is to protect the coatings of metal or ceramic formed by spraying from being damaged by the heat during the welding. On the portions of the valve casing 1 and the valving element 3 shown in FIGS. 4 to 6 marked with hatching, any coating of the embodiments described in connection with FIGS. 1 through 3 can be formed to obtain similar effects as those of the described embodiments.