Technical Field

This invention has to do with nuclear reactor heat exchangers, particularly of the liquid sodium type, and more particularly with improvements in such heat exchangers by reducing friction, and concomitant wear, at the locus of tube support in such heat exchangers, during relative movement of the tubes and support structure during thermal cycling of the reactor. In a broader aspect, the invention is concerned with definition of bearing surfaces on iron base alloy structures and articles of manufacture, by a diffusion coating effected in situ in the structure or article surface, developing a nickel aluminide surface layer, which has been found to be an usuable bearing material in extreme corrosion, very high temperature use applications, including the liquid sodium/steam heat exchangers commonly used in nuclear power plants.

Background Art

The heat generated by a nuclear reactor is typically withdrawn in one class of such reactors by heating sodium in the reactor and heat exchanging the sodium outside the reactor with with steam. This exchange is commonly effected in a shell and tube .type heat exchanger in which a plurality of tubes are arrayed on parallel paths, supported periodically by transverse plates which are apertured to receive in supporting relation individual ones of the tubes. The liquified

sodium is passed through the tank-like shell and the steam through the tubes.

Nuclear reactors used for electrical power generation typically are cycled between an upper and lower temperature over a 24 hour period in a manner keyed to cyclic power demand. This thermal cycling may carry the temperatures within the heat exchanger between 550°C. and 750°C. over the course of each day of operation. The effect on the shell and tube heat exc anger is to have relative movement between the tubes and the surrounding aperturtes of the supporting plates. This πove ent generates friction, causes wear and ultimately the weakening and failure of the piping walls. Such failures can shut down reactors. Repair of the heat exchangers is costly and hazardous.

There is a need therefore to increase the life of shell and tube heat exchangers such as those used in nuclear reactor power plants, and to do this there is a need to develop an improved heat exchanger not prone to tubing wear on thermal cycling. In particular there is a need to provide a means such as a reduced friction bearing, a bearing particularly which will not seize even in the extremes of operating conditions found in the sodium heat exchanger. Description of the Invention

It is therefore an object of the invention to provide an improved heat exchanger, useful in nuclear power generating plants. Other objects are to provide improvements in the shell and tube type heat exchanger whereby friction and wear are reduced despite thermal

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cycling, to provide an anti-seizure bearing enabling thermal cycling in heat exchangers without undue strain on the exchanger components, to provide a low friction bearing for various applications, and to provide improvements in bearing and other properties of iron base alloys and articles of manufacture exhibiting such improved properties, for divers use applications.

These and other objects of the invention to become apparent hereinafter are realized in an anti-seizure bearing adapted for use in the high temperature, chemically aggressive environment of a sodium heat exchanger for nuclear reactors, the bearing comprising an iron based structure formed into a first part, and a second part cooperating therewith, the first part having in contact with the second part in bearing defining relation a surface layer consisting essentially of nickel aluminide. Typically, the surface layer contains at least 30% nickel by weight, the balance consists essentially of iron and aluminum, and preferably the surface layer contains at from 30 to 70% nickel, and the balance consists essentially of iron and aluminum, e.g. where the surface layer contains from 30 to 70% nickel, there can also be present up to 40% iron and up to 30% aluminum.

In a typical embodiment of the invention, the first part comprises a heat exchange tubing support, the second part comprises heat exchange tubing supported by the tubing support, the support and tubing being subject to relative movement responsive to thermal cycling in the surrounding ambient, and at least one of the first and second parts defines a bearing comprising the surface

layer at the locus of tubing support against frictional wearing of the tubing or support during thermal cycling induced movement. More particularly, the first part comprises a tubing support plate having a plurality of second part tubing receiving supports, the surfaces of the supports in contact with the tubing second part defining said bearing surface layer. E.g., the tubing receiving supports comprise apertures in the support plate adapted and arranged to support the tubing, the apertures defining the surface layer in contact with the tubing as the bearing.

The invention contemplates, therefore, in a shell and tube heat exchanger, in which the tubes are supported within the shell by an iron based structure, the improvement comprising a bearing surface at the locus of tube support within the shell, the bearing surface consisting essentially of nickel aluminide formed on the iron based structure.

More broadly, the invention provides in an article of manufacture comprising iron base alloy, such as a shell and tube heat exchanger, the improvement comprising an erosion and corrosion resistant surface layer on the article surface, the layer consisting essentially of nickel aluminide.

Preferably in such embodiments, the iron base alloy is nickel and aluminum free; the iron base alloy is coated with nickel, and the nickel coating has been aluminized to form the nickel aluminide surface layer; the nickel aluminide surface layer is from 0.025 to 0.1 millimeters in depth; a layer of iron aluminide underlays

the nickel aluminide surface layer; the iron aluminide underlayer is 0.025 to 0.05 millimeters in depth; and a nickel iron aluminum ternary alloy layer exists between the surface nickel aluminide layer and the iron aluminide underlayer.

In general, the iron base alloy comprises at least 60% iron by weight, most preferably comprises by weight about 2% chromium, 1% molybdenum, less than 1% carbon and the balance iron. Such and similar iron base alloys are nickel and aluminum free; coated with nickel, and the nickel coating has been aluminized to form the nickel aluminide surface layer; the nickel aluminide layer is about 0.05 millimeter in depth; a layer of iron aluminide underlays the nickel aluminide surface layer; the iron aluminide underlayer is about 0.05 millimeter in depth; there is further present a nickel iron aluminum ternary alloy layer between the surface nickel aluminide layer and the iron aluminide underlayer, about 0.3 millimeter in depth, the article having a surface consisting essentially of 30 to 70 weight per cent nickel, up to 40 weight per cent iron and up to 30 weight per cent aluminum; and wherein the surface layer defines a bearing surface.

Accordingly, the invention provides in a shell and tube heat exchanger, in which the tubes are supported within the shell by an iron base alloy structure, the alloy comprising at least 60% iron by weight, the improvement comprising a bearing surface at the locus of tube support within the shell, the bearing surface consisting essentially of nickel aluminide formed on the iron base alloy structure.

There is further contemplated, in accordance with the invention, in an article of manufacture comprising iron base alloy, the improvement comprising an erosion and corrosion resistant surface layer on the article surface comprising by weight from 30 to 70% nickel, up to 40% iron and up to 30% aluminum, the layer consisting essentially of nickel aluminide and iron aluminide. As in previous embodiments, preferably the iron base alloy is nickel and aluminum free; the iron base alloy is coated with nickel, and the nickel coating has been aluminized to form the nickel aluminide surface layer; the nickel aluminide surface layer is from 0.025 to 0.1 millimeters in depth; the aluminides define distinct layers, and the layer of iron aluminide underlays the nickel aluminide layer; the iron aluminide underlayer is 0.025 to 0.05 millimeters in depth; there is also present a nickel iron aluminum ternary alloy layer between the nickel aluminide layer and the iron aluminide underlayer; the iron base alloy comprises at least 60% iron by weight and preferably: comprises by weight about 2% chromium, 1% molybdenum, less than 1% carbon and the balance iron; is nickel and aluminum free; is coated with nickel, and the nickel coating has been aluminized to form the nickel aluminide surface layer; the nickel aluminide surface layer is about 0.05 millimeter in depth; the layer of iron aluminide and layer of nickel aluminide define a surface diffusion alloy layer about 0.013 millimeter in depth; the iron aluminide underlayer is about 0.05 millimeter in depth; there is present a nickel iron aluminum ternary alloy layer between the surface nickel aluminide layer and the iron aluminide

unde layer about 0.3 millimeter in depth; and the surface layer defines a bearing surface.

The invention further contemplates the method of forming a bearing surface on an iron bas ^" e article of manufacture, including in the region of desired bearing properties diffusing aluminum into the article surface through a layer of nickel in nickel aluminide forming relation. Preferably, the layer of nickel on the article in the region is formed in advance of the diffusion, e.g. by electroplating, electroless plating, painting, plasma, or chemical decomposition methods, as from nickel carbonyl. Typically, the method includes plating nickel onto the article region to a depth of about 0.005 to 0.012 millimeter, and followed by immersing the article in a diffusion pack comprising powdered aluminum, a halogen activator and a refractory adapted to aluminum diffusion into the article through the nickel layer in nickel aluminide forming relation, under diffusion coating layer forming conditions including a temperature from about 800°C. to 1000°C. or more for from about 5 to 10 hours.

The Drawings

The invention will be further described as to an illustrative embodiment in conjunction with the attached drawings in which:

Fig. 1 is a fragmentary, perspective view of a shell and tube heat exchanger in accordance with the invention;

Fig. 2 is a view taken on line 2-2 in Fig. 1; and

Fig. 3 is a view taken on line 3-3 in Fig. 2.

Preferred Modes

Turning now the drawings in detail, in Fig. 1 a section of a shell and tube heat exchanger in accordance with the invention is shown at 10. The exchanger 10 comprises the cylindrical shell 12, a plurality of parallel, longitudinal tubes 14, and a series of transverse support plates 16, set apart along the length of the tubes. The plates 16 are apertured at 18 in distributed relation to receive the tubes 14 snugly in mounting relation. ^' A generally circular series of ports 20 surround the apertures 18 and define there flow passages arranged to carry liquid moving through the shell 12, e.g. sodium, in close proximity to the tubes 14, thus to more effctively transfer heat from the shell liquid to the fluid in the tubes, e.g. steam, all as indicated in the drawing .

Thusfar described the shell and tube exchanger 10 is generally conventional. With reference to Figs. 2 and 3, the bearings of the present invention denoted at 22 in Figs. 1-3, will be described. Initially, however, it will be understood, that in the intended use application of the present bearings, nuclear reactor heat exchangers, there is a diurnal thermal cycle, ranging e.g. from 550°C. to 750°C. This subjects the shell and tube exchanger 10 components to expansion and contraction over this range. This expansion and contraction results in movement relatively between the tubes 14 and the plates 16. In this high temperature, chemically exotic environment of liquid sodium, the normal consequences of such movement can not be abated by usual lubricants, or metallic compositions.

The result of the movement without lubrication is friction and then wear on the tubes at their locus of support, and ultimately seizure or galling and failure. This event is necessarily avoided in good design, and recommended specifications suggest a 40 year life for the tubes, a lifetime that has not been achieved thusfar.

The present invention, however, in accelerated, simulated evaluation indicates it will provide the desired lifetime. This remarkable result is achieved by forming a bearing at the locus of tube support, a bearing which is resistant to the harsh chemical environment, impervious to the extreme temperature conditions, and above all resistant to seizure of the relative parts, all to the end of increasing dramatically the expected life of nuclear reactor heat exchangers.

In Fig. 1 the locus of tube 14 support on plate 16 is indicated at 26. As best seen in Fig. 2, the locus 26 extends radially from the aperture 18 per se defined by the bearing coating of the invention. With reference to Fig. 3, tube 14 bears against a multiple layer diffusion coating shown with exaggerated distinctness in the layers. This bearing coating 28 is formed by diffusion as explained below, and represents a relative interdiffusion of the aperture wall 30 and a diffusion coating composition identified below. The result of the outward migration of the aperture wall 30 and the inward migration of the coating elements, nickel and aluminum, is the bearing coating 28. The coating broadly comprises: an outermost layer 32 of nickel aluminide, approximately 0.025 to 0.1 millimeter in depth,

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and preferably about 0.5 millimeter in depth; an intermediate layer 34 of a ternary alloy system comprising nickel, aluminum and iron, approximately 0.3 millimeter in depth; and an underlayer of iron aluminide approximately 0.025 to 0.05 millimeter in depth, and preferably about 0.05. The diffused coating comprises typically not less than about 30 weight per cent nickel, up to about 40 weight per cent iron and up to about 30 weight per cent aluminum, with the inner layer predominating in iron aluminide, the outer layer predominating in nickel aluminide, and the intermediate layer being in between. These coatings are preferably formed on an iron base alloy structure in which the alloy contains above about 60% iron.

The diffusion is carried out by conventional pack processes. The plates 16, with their apertures 30 in registry are stacked and a diffusion pack added in the hole defined by the apertures. The apertures 30 are initially sized to acommodate the decrease in diameter which accompanies the diffusion. Typical pack composition is:

PACK A (By Weight) 5% Aluminum 95% Aluminum Oxide 0.1% Ammonium Chloride

PACK B (By Weight) 8% Aluminum

22% Chromium 70% Aluminum Oxide _

0.5% Ammonium Chloride

Typical diffusion times are 5 to 10 hours, while temperatures typically range from 800°C. to 1000°C. An inert atmosphere is maintained during diffusion.

The resultant coating lines the aperture as indicated schematically in Fig. 2, dotted line. The coating acts as a bearing for the tubes 14 in the sodium environment of the shell and tube exchanger 10.

While the invention has been particularly described as to an illustrative embodiment in connection with the shell and tube heat exchanger application, it is evident from a consideration of the bearing properties realized, that a novel bearing coating for any iron base alloy article of manufacture has been acheived, regardless of use application, and that a novel diffusion coating of nickel aluminide of such alloys has been discovered with highly useful properties suggesting its use in other places where the non-galling, non-seizing bearing, or other quality of the coating is desired.