Abstract:
A method is provided for enhancing heat transfer within an X-ray vacuum tube, from a hot component such as the rotating anode assembly to a cooler component such as the metal tube housing, by increasing surface emissivity of respective components. The method comprises the steps of fabricating each component from an alloy containing a specified minimum amount of chromium, and then implementing a first heating operation, wherein a fabricated component is heated in a dry hydrogen atmosphere for a first specified time period. Thereafter, a second heating operation is implemented, wherein the fabricated component is heated in a wet hydrogen atmosphere for a second specified time period. This procedure forms a refractory chromium oxide coating on the component that exhibits high absorption in the NIR region of the electromagnetic spectrum.

Description:
BACKGROUND OF THE INVENTION 
     The invention disclosed and claimed herein generally pertains to a method for improving or enhancing thermal radiation transfer between selected X-ray tube components. More particularly, the invention pertains to a method for substantially increasing the ability of X-ray tube components to either emit or absorb thermal radiation in the Near Infrared Radiation (NIR) region, in order to enhance X-ray tube cooling. Even more particularly, the invention pertains to a method for forming a chromium oxide coating on components fabricated from high chromium content alloys specifically to increase the absorption of thermal radiation. 
     In a rotating anode X-ray tube a beam of electrons is directed through a vacuum and across very high voltage, such as 120 kilovolts, from a cathode to a focal spot position on a tungsten alloy anode target. X-rays are produced as electrons strike the tungsten target track, which is rotated at high speed, and are directed toward an X-ray transmissive window or port plate, provided in the tube housing. However, the conversion efficiency of X-ray tubes is quite low. More specifically, the total fraction of X-ray power emitted from the X-ray tube is typically less than 1% of the total power input. Thus, the remainder, in excess of 99% of the input electron beam power, is converted to thermal energy and contributes solely to heating the rotating anode assembly. Such energy must be dissipated in the forms of both thermal radiation and thermal conduction. Hot anodes in X-ray tubes emit thermal radiation with wavelengths of about 0.4 to about 25 microns, depending on temperature. This range is mainly contained in a region of the electromagnetic spectrum called the Near Infrared Radiation (NIR) region which covers wavelengths from about 0.7 to 25 microns. Failure to effectively remove or otherwise manage this fraction of non-productive energy limits tube performance, both by limiting continuous output power and by reducing the duration of transient, high power cycles. For rotating anode X-ray tubes, the added complexity of accelerated bearing wear is usually associated with a lack of effective cooling. 
     In a common arrangement, the X-ray producing components of a tube are contained within a tube housing, formed of stainless steel or other metal. Much of the excess heat is directed to the inner surface of the tube housing by means of thermal radiation. That is, a hot surface within the tube vacuum, such as the hot anode surface, will dissipate power to a cooler surface within the same vacuum space (e.g., the inner surface of the vacuum housing) by the emission of electromagnetic radiation. Since the radiation strikes the inner surface of the vacuum housing, it is very desirable to enhance the absorption of radiation at that location and minimize the amount of heat reflected back to the rotary anode and other internal tube components. The heat transferred to the housing may then be readily removed from the X-ray tube by means of a cooling fluid (usually, but not limited to, a dielectric mineral oil) which is circulated around the outer surface of the tube housing. Typically, the heat is carried by the cooling oil to a heat exchanger and dissipated thereby. 
     Generally, the efficiency of the thermal radiation transfer process can be engineered and exploited by adjusting the emissivity of X-ray tube component surfaces, such as the anode and housing inner surfaces, which are emitters and absorbers respectively, of thermal radiation. Herein, “emissivity” is defined as a measure of the efficiency of NIR absorption relative to the theoretically ideal “black body” absorber. The emissivity will be expressed as a fraction of the theoretical ideal. For example, at a given wavelength, a surface with an emissivity of 0.5 will absorb 50% of the radiant power that a theoretically ideal black body is capable of absorbing. Accordingly, increasing the inner surface emissivity of the vacuum housing reduces the fraction of radiation power reflected thereby back toward the hot anode. 
     In metals, surface techniques that roughen the surface tend to improve the emissivity of the surface, especially in the critical NIR region of the electromagnetic spectrum of a hot rotating anode X-ray tube. In the past, methods such as grit-blasting, acid etching and plasma etching have been routinely used to increase surface emissivity. High emissivity coatings consisting of oxides, nitrides or carbides, have also been used and have been deposited by a number of methods, including plasma spray, chemical vapor deposition and physical vapor deposition. The type of process utilized and the materials selected are dictated by the application, the temperature range of interest and the environment to which the coating is exposed. However, prior art oxide coatings generally comprise nickel or iron oxides. It is very common for these oxides to reduce or evaporate when subjected to intense heat, that is, to give up oxygen and go back to base metal. Moreover, it has been found that coatings applied by plasma spray techniques tend to flake or crack off. It has also been found that efforts to increase emissivity by roughening a surface, such as by grit-blasting or acid etching, may leave an undesirable residue or may have non-uniform results over a surface. 
     SUMMARY OF THE INVENTION 
     The invention is directed to a comparatively simple technique for enhancing thermal radiation heat transfer between components within an X-ray vacuum tube, that is, from a hot component such as the rotating anode assembly to a cooler component such as the metal tube housing. These results are achieved by increasing the surface emissivity of the components, and more particularly by forming a chromium oxide coating thereon. By selective oxidation of the chromium alloying agent in a high chromium content alloy, in accordance with the inventive method described herein, it is possible to form refractive, oxide coatings that exhibit high absorption in the NIR region of the electromagnetic spectrum. This coating is tenaciously bonded to the base metal and does not evaporate or reduce at very high temperatures, such as 1000° C., in vacuum. By oxidizing the surface of the vacuum housing, target cooling is enhanced significantly, as a greater fraction of the NIR power radiated thereto is absorbed rather than reflected back to the hot target. The vacuum housing temperature increases as it absorbs NIR, and is subsequently cooled by the lower temperature dielectric oil flowing over its external surface. 
     The invention is usefully embodied as a method for providing a selected X-ray tube component which has a desired thermal radiation transfer characteristic. The method comprises the steps of fabricating the component from an alloy containing a specified minimum amount of chromium, and then implementing a first heating operation which comprises heating the fabricated component in a dry hydrogen atmosphere, for a first specified time period, at a temperature selected from the range 1100°-1150° C. Thereafter, a second heating operation is implemented, wherein the fabricated component is heated in a wet hydrogen atmosphere for a second specified time period at a temperature selected from the same range. Preferably, the method also includes the step of purging the fabricated component with a selected inert gas or nitrogen, between the first and second heating operations. This invention will solution anneal and transform alloys that respond to heat treating and age hardening (examples include martensitic stainless steels and superalloys). Subsequent thermal processing after coating may be necessary for alloys that fall under these categories. For example, precipitation aging of a superalloy could be accommodated in the same furnace during the cool-down step immediately after the wet hydrogen fire. 
     In a preferred embodiment, the component is fabricated from an alloy which is at least 12% chromium by weight. Higher chromium content alloys will yield higher emissivity values and form coatings that have greater thermal stability. Alloys that have chromium contents &gt;18% (i.e. 300 series stainless steels) are considered the ideal embodiment of this invention. The dry hydrogen atmosphere of the first heating operation has a dew point which is less than −5° C., and the wet hydrogen atmosphere of the second heating operation has a dew point which is on the order of 18° C. or greater. Preferably also, the fabricated component is selectively cooled between the first and second heating operations. Usefully, components selected for the method include all of the subassemblies that constitute the X-ray tube vacuum housing. 
     In another embodiment, the invention comprises a product formed by the method described above. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view with a section broken away, showing an X-ray tube provided with an embodiment of the invention. 
     FIG. 2 is a sectional view taken along lines  2 — 2  of FIG. 1, showing the tube housing provided with a coating formed in accordance with an embodiment of the invention. 
     FIG. 3 is a table showing emissivity measurements of sample components provided with the coating described herein, as a function of wet hydrogen atmosphere dew point and over a specific range of wavelengths in the NIR spectrum. 
     FIG. 4 is a graph depicting effects of very high temperatures on the emissivity of a component coated in accordance with an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there is shown an X-ray tube  10 . In accordance with conventional practice, tube  10  generally includes a metal housing  12 , which supports other X-ray tube components of tube  10  including a cathode  16 , and also provides a protective vacuum enclosure  14  therefor. Housing  12  comprises an alloy such as stainless steel which, for reasons set forth hereinafter, has a high chromium content. More particularly, to achieve maximum emissivity values and the highest resistance to thermal degradation, the chromium content of housing  12  must be &gt;18% by weight. Cathode  16  directs a high energy stream of electrons  24  onto a target track  18  of an anode  20 , which consists of a disk composed of a low expansivity refractory metal, for example, a molybdenum based alloy. Anode  20  is continually rotated by means of an anode mounting and drive mechanism  22 , described hereinafter. Target track  18  has an annular or ring-shaped configuration and typically comprises a tungsten based alloy integrally bonded to the molybdenum based anode disk  20 . As anode  20  rotates, the stream of electrons from cathode  16  impinges upon a continually changing portion of track  18  to generate X-rays. Electrons strike the target  18  at a focal spot which generally remains at a position  26  as the anode target rotates. A beam of X-rays  28  is thereby generated, which is projected from the anode focal spot through an X-ray transmissive window  30 , provided in the side of housing  12 . 
     FIG. 1 further shows anode mounting and drive mechanism  22  provided with a bearing support member  32  carrying a front set of rotary bearings  34   a  and a rear set of rotary bearings  34   b.  Anode  20  is provided with a shaft  36  having a recess  38  sized to receive member  32  and bearings  34   a  and  34   b,  so that anode shaft  36  and anode  20  are rotatably supported thereby. To rotatably drive the anode, stator windings  40  of an induction motor  42  are mounted on a frame  44 , supported by the housing  12 , and the rotor  46  of the motor is mounted on anode shaft  36 . Thus, when electrical power is applied to the stator windings  40  through a suitable power transmission path (not shown), motor  42  operates, in conventional manner, to rotatably drive rotor  46  and thereby anode shaft  36  and anode  20 . 
     As stated above, a substantial amount of heat is generated during the production of X-rays. This non-productive energy must be substantially removed from the anode  20  and regions proximate thereto. Otherwise, this energy over time may damage components supporting the anode, particularly the front bearings  34   a.  Accordingly, FIG. 1 further shows cooling fluid  48 , typically comprising a dielectric oil, passed across the outer surface  12   a  of housing  12  during operation of tube  10 . Cooling oil  48  is directed through a conduit  50  or the like, which is in close, abutting relationship with outer surface  12   a.  It is to be understood that a number of other conduits are positioned around the circumference of outer surface  12   a.  The oil that passes through respective conduits  50  is pumped through a heat exchanger, which cools the oil and returns it back to the tube. 
     In order for heat to reach cooling oil  48 , it must first be transferred from the anode  20 , or other hot components within enclosure  14 , to the housing  12 . Referring further to FIG. 1, there is shown a thermal radiation component  52 , comprising radiation in the NIR region defined above, which is directed from target track  18  of anode  20  to the inner surface  12   b  of housing  12 . It is very desirable to absorb as much of this heat as possible into the housing  12 , so that it can pass therethrough to cooling oil  48  by thermal conduction. The amount of NIR thermal energy which is reflected back into enclosure  14  is thereby minimized. In accordance with the invention, it has been recognized that absorption of NIR energy can be significantly improved by forming a native oxide coating  54 , composed mainly of chromium oxide, on the inner surface  12   b  of housing  12 . This substantially enhances the cooling of hot, internal tube components that radiate NIR energy to the housing. The temperature of vacuum housing  12  increases as it absorbs NIR energy from the hot components, and the housing is subsequently cooled by the lower temperature dielectric oil  48  flowing over the external surfaces  12   a.  Moreover, anode and other hot internal tube components can be coated as described herein, to enhance emission of NIR power therefrom to the housing  12 . 
     Referring to FIG. 2, there is shown thermal energy components  56 , comprising a substantial portion of the thermal energy of radiation component  52 , being absorbed into housing  12  due to the high level of emissivity provided by coating  54  formed on inner surface  12   b.  Thermal energy components  56  flow through housing  12  to cooling oil  48 , and are removed thereby to cool the X-ray tube  10 . FIG. 2 further shows thermal energy component  58 , comprising a lesser portion of the energy of radiation component  52 , which is reflected back into enclosure  14 . 
     A chromium oxide coating procedure, comprising an illustrative embodiment of the invention, is usefully implemented in connection with an X-ray tube housing  12  which is formed of 304 series stainless steel, and which contains 18%-20% chromium by weight. This requirement is very convenient, since many components fabricated for X-ray tube devices are commonly manufactured from alloys that possess considerable weight fractions of the alloying agent chromium. These alloys include 300 and 400 series stainless steels, nickel-chromium alloys and superalloys including iron, nickel and cobalt-based types. However, in the past these alloys have included chromium primarily to impart corrosion resistance, especially where high service temperatures and oxidizing atmospheres are encountered. 
     It is anticipated that the procedure of the invention will work on any alloy system that contains a sufficient quantity of chromium, that is, which is at least 12% chromium by weight. The procedure requires a furnace capable of operating at temperatures of 1100° C., in both dry hydrogen and wet hydrogen gas atmospheres. The procedure also requires the ability to measure the dew point (d.p.) of the wet hydrogen gas atmosphere, for reasons set forth hereinafter. As is well known, dew point is the temperature at which water vapor, purposely entrained in the hydrogen gas flow, condenses. 
     In accordance with the coating procedure, the housing  12  (or other component to be coated) is initially fabricated to the shape and design specifications required therefor. In this example, the housing  12  is fabricated from 4 mm thick sheet and the hold times are appropriate for this material form; the time being a function of the thickness, total mass of the component and furnace power available. The fabricated component is then cleaned, prior to further processing, to ensure that it is free of surface contaminants. Thereupon, a first heating operation is implemented, wherein the part or component is placed in the furnace and an 1100° C., 60 minute furnace fire is applied thereto, in a dry hydrogen gas atmosphere. That is, the atmosphere contains hydrogen gas and has a dew point of less than −5° C. The first heating operation is followed by a second heating operation, wherein the component is placed in the furnace and an 1100° C., ninety minute furnace fire is applied thereto, in a wet hydrogen atmosphere. The wet hydrogen atmosphere has a high water content and a dew point on the order of 18° C. 
     It is generally necessary to cool down the component between the dry and wet furnace operations. However, it is critical that the component remain in the furnace during the cool down period, and be purged with either an inert gas or nitrogen. Failure to do this will result in the formation of oxides on the component surfaces that do not have certain characteristics required for the chromium oxide coating, i.e., high emissivity and high temperature stability. 
     The heating operations described above draw chromium to the surface of the component, to form a chromium oxide coating thereon. This procedure will normally produce a uniform, dark green to gray chromium oxide coating, over the entire surface, which exhibits a surface emissivity of about 0.90 at a wavelength of 2 microns NIR-Higher dew points, i.e., dew points in excess of 18° C., and furnace firing times in excess of 90 minutes will produce thicker oxide coatings. Thus, surface emissivity is a function of the dew point value, particularly at low dew point values, whereby the ability to measure dew point is required for the coating procedure. The rate of chromium oxide formation is governed by the diffusion of chromium through the forming oxide layer. Hence, higher temperatures during the wet hydrogen step will increase the rate of chromium oxide formation by increasing the rate of chromium self-diffusion through the coating. However, temperatures greater than 1100° C. may result in deleterious deformation of components due to creep effects. 
     Referring to FIG. 3, there is shown a table illustrating emissivity as a function of dew point of the wet hydrogen atmosphere, i.e., during the second heating operation. More particularly, the table of FIG. 3 sets forth emissivity measurements obtained from several tests, set forth in the table as Tests 1, 2 and 3, respectively. Each test was directed to three component samples, referred to respectively as Samples 1, 2, and 3. Each sample was coated in accordance with the procedure described above, with the dew point of the wet hydrogen atmosphere being different for each coating procedure. Thus, the three samples used for Test 1 were coated at a dew point of 25.2° C., the three samples of Test 2 were coated at a dew point of 18° C., and the three samples of Test 3 were coated at a dew point of 5.8° C. Emissivity of the resulting coatings was measured for the samples prepared for each dew point, at varying levels of NIR. The table of FIG. 3 clearly shows that emissivity is increased by processing components at higher dew point levels. 
     The chromium oxide coating formed by the process described herein must be stable at elevated temperatures, especially in a vacuum environment. This is particularly important for parts or components that are subjected to further high temperature processing after the coating has been formed thereon, in the course of putting components together to form the X-ray tube. For example, coated components could be subjected to brazing or furnace firing operations, wherein reduction and/or evaporation of the chromium oxide coating would significantly reduce the effective NIR absorbtivity of the component surface. Accordingly, to determine the possible loss of emissivity when coated components are exposed to high temperature thermal cycles, e.g., furnace brazing cycles, precipitation aging or tempering, a sample of 1 mm thick  304  stainless steel was provided with the chromium oxide coating as described herein. The sample was then furnace fired in a vacuum over a range of temperatures typically employed in brazing. FIG. 4 shows the emissivity of the sample, measured after such furnace firing (i.e. residual emissivity) and indicates that the emissivity of the sample does not decrease until the firing temperature exceeds 1000° C. for an eight minute exposure in vacuum. Thus, the maximum exposure temperature is found to be at or near 1000° C. It has been discovered that the maximum exposure temperature limit is directly related to the temperature of the wet hydrogen heating operation, described above, and will increase or decrease proportionately as the wet hydrogen operation temperature is increased or decreased, from the 1100° C. value disclosed above. 
     While it has been found that 1100° C. is a preferred furnace firing temperature for both the first and second heating operations, it is anticipated that other embodiments of the invention could use other temperatures therefor. Generally, it is anticipated that any temperature selected from the range 1100°-1250° C. could be used for the first heating operation, and any temperature selected from the same range could be used for the second heating operation. 
     Obviously, many other modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the disclosed concept, the invention may be practiced otherwise than as has been specifically described.