US2063325A - Process for minimizing temperature stresses in metallic structures and product thereof - Google Patents

Description

Dec. 8, 1936. N. R. McLEOD PROCESS FOR MINIMIZING TEMPERATURE STRESSES IN METALLIC STRUCTURES AND PRODUCT THEREOF Filed May 10, 1932 IN V EN T 0R. A E/L R M l 1500 ATTORNEY Patented Dec. 8, 1936 UNITED STATES PATENT OFFICE PROCESS FOR MINIMIZING TEMPERATURE STRESSES IN METALLIC STRUCTURES 15 Claims.

The present invention relates to the formation of metallic structures intended for use under such conditions that different portions of the structure are normally subjected to different temperatures,

and the general object of the invention is the formation of such a structure in such manner as to regulate or fix the tendency to relative expansion of the different portions thereof at difierent temperatures, either to the end of minimizing such tendency, with consequent reduction in the socalled temperature stresses to which the structure is subjected, or of insuring a particular distribution of the total'stresses in the structure made desirable by its form and/or conditions of use.

The invention may advantageously be used, for example, in the formation of the tubes of heat transfer apparatus, such as the tube-stills employed in refining petroleum. In accordance with the present invention a tube suitablefor such use is formed of two or more integrally connected concentric layers of different alloy steels, or other suitable metals, having such thermal expansion characteristics that under the normal working temperature and pressure conditions, all, or a large portion, of the temperature stresses in each layer will be confined to that layer and will not be transmitted to any other layer of the tube. This substantially minimizes the effect of temperature stresses upon the effective strength and ultimate safe life of the tube, since the temperature stresses in any tube or tube layer diminish as the thickness'of the tube or tube layer is decreased. While the controlling temperature stress, in general, is not an exact linear function of the ratio of the outer to inner radii of the tube or tube layer, when the tube is formed in accordance with the present invention of three equally thick concentric layers of suitable mate rial, for example, the significant maximum tem perature stress in the tube will be in the neighborhood of one-third of what it would be if the tube were formed wholly of any one of the three different layer materials, assuming the same total wall thickness for both tubes.

In the practical carrying out of the invention it is ordinarily desirable, if not absolutely essential, to employ commercially available and suitable materials. Even when there are no commercial materials available having the precise coeflicients of thermal expansion and other physical characteristics which need to be taken into account to obtain the optimum results possible with the invention, results but little inferior may be obtained by the use of suitable available materials and some small and relatively unimportant departures from the relative proportions of the structure required for an absolutely optimum result. The metallurgical art has already developed many, and is constantly adding to, the number of 5 metallic materials and particularly alloy steels, varying in their characteristics of importance in this connection, and from which those best adapted for any particular use of the present invention maybe selected. Moreover, when occasion 10 requires new materials, the metallurgical art can be expected to develop them in accordance with almost any special specification of characteristics which may be required for the use of the present invention.

In forming a composite structure in accordance with the present invention, the structure will ordinarily be annealed at a suitable annealing temperature safely above, or in some cases below, the maximum working temperature of any por- 20 tion of the structure, and this annealing temperature is a factor to be considered in the design of the structure. For example, if the structure is a tube subjected to working temperatures progressively increasing from the inner to the outer 25 wall surface of the tube, the average contraction or tendency to contract, from normal dimensions at the annealing temperature, of each tube layer is in approximate, though not in exact linear proportion to the product of two factors, namely: the 30 expansion coeflicient of the material, and the difference between the annealing temperature and the average working temperature of the layer.

In consequence, the expansion coefiicient of any outer layer must be suitably greater than the ex- 35 pansion coefficient of any inner layer to obtain the desired result. The actual difierence in such case between the most desirable expansion coefficients for adjacent tube layers is dependent in part upon and ordinarily will increase with the 40' thickness of the layers.

In the practical use of the invention, the steps taken to minimize temperature stresses are dependent not only upon temperature conditions, but upon other conditions subjecting the struc- 45 ture formed to mechanical stress. When the structure is an oil still tube, for example, the maximum significant tube wall stress will comprise a substantial component due to the internal fluid or oil pressure as well as a substantial 50 temperature stress-component. In the case of a homogeneous tube wall with optimum wall thickness, the two components will ordinarily be of about equal value and the effective tube strength will be diminished by either an increase 6 or decrease in the tube wall thickness. This for the reason that on any increase in wall thickness, the disadvantageous increase in the temperature stress component will be more rapid than, and will more than neutralize the advantage from, the corresponding reduction in the stress component due to internal pressure, while on a reduction in wall thickness, the internal pressure stress component will be more rapidly increased than the temperature stress component will be decreased. For optimum results in designing a structure to be constructed in accordance with the invention, account must be taken not only of the thermal expansion characteristics of each material employed, but also of its thermal conductivity and its elasticity characteristics commonly referred to as Youngs modulus and Poissons ratio. It is also necessary, of course, to

take into account the safe working stress limitations of the material at the working temperatures involved, and the materials used in any case should be of a character to suitably resist ,the heating, oxidizing, corrosive or analogous actions to which they may be subjected.

In the accompanying drawing:

Fig. 1 is a transverse section of a composite tube formed in accordance with the present invention; and

Fig. 2 is an elevation of an internal combustion engine piston formed in accordance with the present invention.

The tube A shown in Fig. 1, has its wall formed of integrally connected concentric layers A, A and A formed of materials having difierent thermal coeflicients of expansion, and adapted for use as a heat transfer tube in such apparatus, as the tube stills employed in refining petroleum. To minimize the maximum stress in the wall of the tube A under operating conditions such that the temperature of the outer surface of the layer A is greater than the teniperature of the inner surface of the inner layer A as will normally be the case in tube still use, the thermal coefli cient of expansion for the middle layer A will be smaller than the thermal coefficient of expansion for the outer layer A, and will be greater than the thermal coefllcient of expansion for the inner layer A A better understanding of the nature and principle of the invention, and of its advantages, may be facilitated by an explanation and comparison of the stresses set up intwo oil still tubes subjected to the same operating conditions, and one formed in the usual manner of homogeneous metal while the other is a tube, like the tube A oi. Fig. 1, which is formed in accordance with the present invention.

In an oil still tube the oil or internal fluid pressure is usually substantial and the walls of the tube must be of substantial thickness. An oil still tube when exposed exteriorly to high temperatures necessarily absorbs heat at a rapid rate and its outer wall surface becomes substantially hotter than its inner wall surface. In any steady condition of operation the temperature differential between the inner and outer wall surfaces of the tube constitutes the heat head necessary to compel the flow of heat from the outer to the inner surface of the tube at the same rate at which the outer surface absorbs heat. Whenever the existing heat head becomes temporarily ins'ufllcient to cause heat to flow through the tube wallv as rapidly as heat is absorbed by the outer surface of the wall, the temperature of such outer surface increases until the increased heat flow,

due to the increase in heat head, becomes equal to the rate of heat absorption, which, with constant furnace conditions, diminishes as the temperature of the outer tube surface increases.

While the temperature and pressure conditions to which tubes formed in accordancewith the present invention are subjected may vary widely, it may be helpful to consider the stresses in some specific tube subject to certain conditions which are normal and typical in character for such specific tube and conditions. For example, let

us consider an oil still tube 4 in inside diameter with a wall one-third of an inch thick and containing oil at a pressure of 1500# per square inch, and at an average temperature of 850, and absorbing heat at the rate of 30,000 B. t. u. per hour per square foot of externally exposed tube wall surface area. Under such conditions the inner tube wall surface may be assumed to be at a temperature of 950, and the outer wall surface temperature will be that required for the flow of heat through the tube wall at a rate equal to the specified rate of heat absorption. When the iting design stress, is the so-called internal hoop stress, namely: the circumferential tensile stress at the inner wall tending to form a diametral' plane tube wall splitting crack opening from, or starting at, the inner surface of the wall. This inner hoop stress includes a component due to the internal fluid pressure and a temperature stress component. 0n the assumptions stated in e preceding paragraph,-the internal hoop unit stress component due to internal pressure would be approximately 9800 lbs. per square inch; assuming an elastic material.

In the homogeneous'alloy 0 tube with its inner and outer surfaces at temperatures of 950 and 994, respectively, the expansion, or tendency to expansion, of the portion of the tube wall adjacent its outer surface relative to the inner portion of the wall subjects to the tube wall material to a circumferential temperature stress which varies between a maximum compression stress at the outer surface of the wall and a maximum tensile stress at the inner surface of the wall. The temperature stress at the inner surface of the wall forms one component of the total inner hoop stress. With the assumptions made above, the internal fluid pressure stress component and the temperature stress compopent of the inner hoop stress are of the same general magnitude. In precise figures, I compute the inner hoop temperature stress component of the tube to be 9300 lbs. per square inch. With an internal pressure component of 9800 lbs., this gives a total inner hoop stress of 19,100 lbs. per square inch, which would be a practically possible, though uncomfortably high maximum stressfor the tube conditions assumed.

With the practice now customary in the use of tube oil stills, the normal safe life of such a tube under good conditions would be estimated at about 10,000 working hours at most, 75

and at the end of such period the tube would be replaced as a matter of course. The safe life of such a tube is computed on the assumption that the "creep of the tube material would produce 1% deformation in 10,000 hours, and that when so much deformation due to creep has occurred, further use of the tube in an oil still would not besafe. The creep referred to is the flow of the tube wall material occurring under the stresses and at the temperatures to which the tube is subjected. While the tube material seems elastic, it will actually have a certain plasticity under the assumed stress and temperature conditions of operation.

While it is often the practice to compute the safe life of a tube still in the manner stated above, metallurgists are not in entire agreement as to the actual physical laws determining the extent of creep occurring as the use of the tube continues, and the practice outlined above is considered by some metallurgists to err somewhat on the side of safety.

In the homogeneous tube, the inner hoop stress does not depend significantly on the annealing temperature, but it does depend upon various physical characteristics of the tube material and on the tube proportions. An increase or decrease in Young's Modulus for the tube material tends to respectively increase or decrease the temperature component of the internal hoop stress. An increase or decrease'in Poissons ratio for the tube material also tends to respectively increase or decrease the temperature component of the internal hoop stress. Other things being equal, the significant temperature stress is obviously greater or less of course, accordingly as the temperature difference between the inner and outer tube walls is great or small. Such temperature difference is a. function of, and can be determined from, the quantity of heat absorbed, the thermal conductivity of the material, the wall thickness, and the shape of the tube. I believe the maximum internal unit hoop stress H, of such a homogeneous oil still tube as has been discussed is given with substantial accuracy by the following Equation (1):

1 EaQr 2K Log.

K -i-l (1) EH F J W YET-1) which is vmathematically exact for a perfectly elastic material, and in which:

P=The oil pressure, in pounds per square inch,

K=The ratio of the outer and inner tube wall radii,

Log. K=The natural or hyperbolic logarithm of K,

E=Yo ungs Modulus for the tube material in lbs. per sq. in.

=Poisson's ratio for the tube material.

a.'=The coefflcient of linear thermal expansion of the tube material within the working temperature range.

Q=The B. t. u. of heat absorbed per square foot of outer tube surface area exposed per hour.

X=The coeflicient of thermal conductivity of the tube material, and

v rz=the outer tube wall radius in inches. In the foregoing equation (1) the first term,

of the expression at the righthand side of the the tube, and the second term of said expression represents the temperature stress component of the internal hoop stress. Since in Equation (1), H is a function of K, the optimum value of K is that which will make the value of the derivative (5 E dK of the righthand expression in Equation (1) When the derived equation is solved for K, the value of the latter is found to be 1.169, which corresponds approximately to the assumed tube wall thickness of one-third of an inch. In

fact the tube wall thickness of onethird of an inch was not an arbitrary assumption, but was actually arrived at by the use of the foregoing Equation (2) in the manner indicated.

above.

Let us now consider a tube of the small overall dimensions as the above described homogeneous tube, and subject to the same operating conditions, but formed in accordance with the present invention of three concentric layers A, A and A as shown in Fig. 1, and with said layers so proportioned that the ratio of outer to inner diameter is the same for each layer, and with the different layers formed of materials having such suitably diiferent thermal coefficients of expansion that under normal working conditions there will be little or no radial stress due to temperature acting between any two adjacent layers. Then if the outer layer A is assumed to be made of alloy 0, the internal hoop stress component due to temperature in this layer will be given by an expression for that stress differing from the corresponding expression in the foregoing equation (1), only in that the term 2K Log. K

in Equation (1) is replaced by the term 21:. Log. K,

wherein Kc is the ratio of the outer to the inner radii of the outer tube layer. The temperature component of the inner hoop stress in the outer layer A is thus found to be 2850 lbs. per square inch, and the total inner hoop stress for this layer is found to be 11,400 lbs. per square inch, when the component due to oil pressure is computed by Lams formula.

In such use of the composite tube A the component of the inner hoop stress due to the internal pressure is less in each outer layer than in any layer within it. This follows from the fact that with respect to stresses due to internal pressure the composite tube A with its integrally connected layers does not diil'er from a homogeneous tube. The fact that the hoop stress due to internal pressure diminishes in a homogeneous tube from the inner to the outer wall surface of a tube is well known, as are various more or less rigorous mathematical methods or formulae, for example, Lams formula" for computing the stress at any particular point-between said surfaces.

The temperature stress component of the inner hoop stresses for the different layers A, A and A will ordinarily be of about the same magnitude. Difierences in the thermal conductivities of the different layer materials tend to produce difierent temperature stresses in the different layers, as the temperature stress in any layer increases and decreases as the temperature difference between the inner and outer surfaces of the layer increases and decreases. That temperature difference increases and decreases of course, as the thermal conductivity of the layer material decreases and increases.

' While the temperature stress component of the inner hoop stress of any inner layer may thus be greater or less than the corresponding component for any outer layer, ordinarily the difference will not be great. For illustrative purpose we may properly assume therefore, that in none of the layers A, A and A of the composite tube A will the unit temperature stress significantly exceed 2850 lbs. per square inch, or the total unit stress exceed 12,650 lbs. per square inch. The maximum unit stress in the composite tube A is thus only about 65% of the 19,100 lbs. per square inch maximum unit stress in the homogeneous tube discussed above.

A reduction in the limiting hoop stress of 35% means a very substantial increase in the safe life of an oil still tube, as the rate of creep is-much greater with high stresses than with lower stresses. The amount of creep produced, for example, by stressing a material like alloy C at 1000 F. for a period of 10,000 hours to 20,000 lbs. per square inch, might require for its production something like 100,000 hours under otherwise identical conditions, if the stress developed were only 15,000 lbs. per square inch.

In order that no one of the layers A, A A of the composite tube A may impress radialstress due to temperature on any adjacent layer it is necessary merely that the coeificients of thermal expansion of the different layer materials should be so related to one another, to the relative thicknesses of the layers, and to the temperature at which the tube is annealed, that with the different layers at their respective working temperatures, the outer diameter of each inner layer would be exactly equal to the internal diameter of the surrounding layer if the two layers were separated from one another and subjected to no external force.

Owing to the relative thinness of the layers in comparison with their diameters no significant error is made in assuming that the total unit expansion in layer diameter is obtained by multiplying' the diiference between the annealing temperature and the average working temperature in the layer by the average coefficient-of expansion of the layer material for this temperature range. This assumption will be exactly true for one cylindrical surface lying between bounding surfaces of the layer. Thus on the. reasonable assumption for the conditions previously assumed, that the temperature at which the particular composite tube described above is annealed is 1720, and that the average temperatures for the three layers A A and A are 963 986 2 and 1005 respectively, it follows that for the optimum result, the ratio of the coeflicient oi expansion of the inner layer A to that for the middle layer A material should be .equal to inc-986% or .969, and that the ratio of the thermal coeflicient of the inner layer A to that for the outer layer A should be equal to If the outer layer of the composite tube A is made of the previously mentioned alloy C, which for the temperature range 950-1750, has an average thermal coefficient of expansion of 12.33, the average coefficients of expansion of the middle and outer layer materials for the temperatures between their working and annealing temperatures should be approximately 11.98 and 11.66, respectively.

If the readily available materials do not include those having the coefficients of expansion just computed for the middle and outer layer materials, materials having slightly different coefficients may be employed with little sacrifice of the optimum advantages of the invention. In such case if the layers are of such thickness as to make the ratios of outer to inner diameter for each layer equal there will be some small radial stress imparted by one layer to another, but the effect of such radial stresses will be.small in comparison with the stress advantage over the homogeneous tube obtained by the use of the invention. Furthermore in such case, all radial stresses between layers due to temperature may be avoided by a suitable slight variation in thickness of the difierent layers. For example, I have found'that a composite tube A of the normal dimensions of the homogeneous tube discussed above, and subject to the same conditions, will be theoretically free from radial stress between layers due to temperature, when the inner, middle and outer layers A A and A, respectively, are made with the following thicknesses .09, .115 and .132 inch, respectively, and of the commercial alloys which I designate herein as alloys CA, CB and C, respectively.

The actual calculated maximum unit stress in such a composite tube A, under the assumed normal working conditions, is then 13,300 pounds per square inch, which is less than 70% of the maximum calculated for the homogeneous tube.

Said alloy C, to which repeated references have previously been made, and said alloys CA and CB have compositions and characteristics given in the following table, which reproduces data of the United States Bureau of Standards available to the public.

The foregoing symbols a, X, E and d are used .viously mentioned Equation (1).

head, per inch of length of flow path; and E is Young's Modulus in pounds per square inch, to

be multiplied by 10 Poissons ratio at is a pure dimensional ratio.

It will be noticed that while equal thickness ratios for the layers require coeflicients of expansion of 11.66, 11.98, 12.33, the alloys CA, CB and C do not exactly fulfill these requirements, their actual coefficients being 11.67, 12.00 and 12.33, respectively. To eliminate radial stresses due to temperature between the layers of materials CA, CB and C, it is only necessary, however,

to employ thickness ratios of 1.045, 1.055, and

1.060, instead of the assumed ratios all equal to 1.053. These result in layer thicknesses of .09", .115", and .132 respectively, instead of the ifiiially assumed thicknesses, .107", .113" and As those skilled in the art will readily under stand, when a composite tube A is made up of layers differing in thickness and material, the stresses in each layer may be computed from the pre- It should be borne in mind, however, in so computing the stresses for each layer other than the inner layer, that the factor P of Equation (1) is not the full internal fluid pressure but a smaller factor which may be determined by Lams formula or in some analogous manner. As those skilled in the art will understand, however, other mathematical methods may be employed in computing the stresses and quickly determining the optimum tube dimensions with suitable accuracy in any particular case or condition.

With a composite tube formed in accordance with the present invention and annealed at a temperature above the working temperatures, temperature stresses are created in the tube when the latter cools uniformly. Such stresses will be substantial, of course, when the tube cools uniformly down to ordinary atmospheric tempera.- tures from such an annealing temperature as that of 1720" suggested above.

The temperature stresses at temperatures less than the working temperatures are ordinarily unimportant, however, particularly as the creep rate is entirely negligible arthe lower temperatures, and the allowable unit stress in the metal is very much higher at atmospheric than at working temperatures.

Composite tubes like the tube A embodying the present invention may be manufactured in various ways now known to the practical art. For example, such a tube may be a seamless tube formed in known ways from a solid or pierced steel billet formed with concentric shell portions of the desired materials, or the tubes may be welded tubes formed from strips cut from rolled composite sheets. It would be possible also, though not economically desirable in ordinary cases, to separately form the different layers, 'assemble them one within another and then weld the layers together to diminish joint resistance to heat flow between adjacent layers.

Composite tubes formed in accordance with the present invention are obviously not restricted in their field of use to oil stills. On the contrary they may be used as boiler tubes, as water wall tubes, as radiant heat superheater tubes, and in fact as heat transfer tubes in any form of heat transfer apparatus ,where the tube wall thickness and temperature conditions are such as to make the temperature stress in a homogeneous tube a matter of significant practical importance. The invention is obviously useful, also, in forming the cylinders of an internal combustion and other engines, and tanks or vessels in which fluids are stored or held at temperatures and pressures difierent from those external to said tanks or vessels.

The invention, moreover, is not restricted to use in forming tubes, cylinders or analogous hollow bodies, but may advantageously be used in making any metallic structure having different portions subjected to difierent temperatures which create objectionable temperature stresses when the structure is'formed of a single material or of different materials not advantageously combined with respect to variation in coeflicients of expansion. For example, in an internal combustion engine piston B such as is shown in Fig. 2, the temperature of the intermediate and central piston portions 13 and B are substantially higher than the temperature of the peripheral piston portion B. In consequence, such a piston may advantageously be made with its outer'portion B of a material having a coefiicient of thermal expansion suitably differing from the coefficient of expansion of the material of the intermediate portion B of the piston, and with a suitable difference between the coefficients of thermal expansion of the intermediate and central portions B and B In a structure in which the working temperature of a portion of the structure is above the annealing temperature of the structure, as is necessarily the case with the inner portions of a piston of the form shown in Fig. 2, when used in some internal combustion engines, the working temperature of the hotter portion is an annealing temperature, and there will be no significant stresses under working conditions between portions of the structure at different temperatures. such as exist in the case of a heat transfer tube, for example, in which the'working temperatures of all portions of the tube are below the annealing temperature. While in the case of a piston or other structure operating with one portion thereof at a temperature above the annealing temperature, the use of the invention is not necessary for the purpose of avoiding significant temperature stresses between different portions of spective working temperatures, the use of the invention is of substantial value, nevertheless, because it eliminates or minimizes tendencies to distortion and cracking which would otherwise exist during the periods in which the structure is changing in either direction between its cold or non-working temperature, and its working temperatures.

The thermal coefficients of different portions of a structure may be varied to control stress conditions in the structure for other purposes than to minimize the maximum stresses in the structure. In particular, they may sometimes be varied to advantage to increase the maximum stress in some portion or portions of a structure in order that a reduction may be made in the maximum stress in some other portion or portions of the the structure when the latter are at their restructure in which stress reduction is of especial importance. For instance, in tubes having walls formed of integrally connected concentric layers of different materials like the tube A shown in Fig. 1, it is desirable sometimes to make the inner layer, and sometimes the outer layer of a material not strong enough to withstand as great a maximum stress as can be safely withstood by the materials forming the other layers of the tube.

For example, the inner tube layer A may be formed of some acid resisting material incapable of supporting more than the normal temperature stresses set up in that layer at its working temperatures. In such case the outer layers A and A may advantageously be so formed that under normal working conditions they will produce a radial stress acting on the outside of the inner layer A of the same, or approximately the same magnitude as the internal fluid pressure. In that case the maximum stresses in the outer layers are advantageously increased so that the reduced stresses upon the acid resisting inner layer will not exceed a safe limit. In the case, for example, of a tube absorbing radiant heat under such conditions that the outer tube surface temperature becomes unusually high, the outer layer A of a tube formed of concentric layers, as is the tube A of Fig. 1, may well consist of material selected for its high heat resistant properties and not strong enough to carry any, or but a small portion, of the stress due to the internal fluid pressure. In such case the thermal coefiicient of expansion of the outer layer material may well be made sufliciently high, relative to those of the inner and intermediate materials, so that the layers A and A will transmit little or no radial stress to the outer layer A under normal working conditions. In this case the middle and inner layers must be formed to withstand all or all but little of the stresses due to the internal fluid pressure in addition to the temperature stresses to which those layers are subjected.

As those skilled in the art will understand, some features of the present invention may sometimes be used without making use of other features, and in particular, a great practical advantage from the invention may be obtained without such precision in design as is required for the attainment of the maximum possible advantage from the invention.

Having now described my invention, what I claim as new and desire to secure by Letters Patent, is

1. In forming a structure comprising rigidly connected portions adapted for use with different normal working temperatures, the method of regulating the stress distribution in said structure which consists in forming said portions of materials having such different coefficients of thermal expansion and so annealing said structure that when the temperature of each of said portions changes from its annealing temperature to its normal working temperature the tendency to expansion of said portions relative to one another will subject some portion of said structure to less stress than it would be subjected to if all portions of said structure were formed of materials having the same coefiicients of thermal expansion.

2. The method of forming a structure com-' prising rigidly connected portions adapted for use with different normal working temperatures which consists in forming said portions of materials having such different coeflicients of thermal expansion and so annealing said structure that when the temperature of each of said portions changes from its annealing temperature to its normal working temperatures there will be relatively little or no tendency for expansion of any one of said portions relative to any other of said portions.

3. The method of forming a metallic structure comprising integrally connected portions normally subject to different working temperatures which consists in forming said portions of such diiferent materials and annealing the structure at such an annealing temperature different from any of said working temperatures, that the difference in coefficients of thermal expansion of the different portions will substantially minimize the tendency of any one of said portions to expand relative to any other of said portions when said portions are brought from the annealing temperature ,to their respective working temperatures.

4. The method of forming a metallic structure adapted for use under conditions subjecting different portions of said structure to substantially different working temperatures, which consists in forming the said portions of materials having such diiferent coefiicients of thermal expansion and so annealing said structure that when said portions are thereafter subjected to said working temperatures, the-last mentioned temperatures will produce relatively little or no stress action between said difierent portions.

5. The method of forming a heat transfer tube adapted for operation with working temperatures varying progressively between the inner and outer surfaces of the tube wall, which consists in forming the tube of concentric integrally connected layers of materials having such coefflcients of thermal expansion for the different layers varying progressively in the same order as do the working temperatures of the layers, and so annealing said tube that when the diflerent layers change in temperature from their annealing to their working temperatures, the relative expansion of the diflerent layers is minimized.

6. The method of forming a heat transfer tube adapted for operation with working temperatures varying progressively between the inner and outer surfaces of the tube wall, which consists in forming the tube of concentric integrally connected layers of such materials that the coeflicients of thermal expansion for the difierent layers vary progressively in the same order as do the working temperatures of the layers, and annealing the tube at a temperature above said work temperatures.

7. In forming a heat transfer tube adapted for use with normal working temperatures progressively varying between the inner and outer surfaces'of the tube wall, the method which consists in forming the tube of integrally connected concentric layers of such materials that the 00cmcients of thermal expansion of the different layer materials so progressively increase from the cooler to the hotter side of the tube wall, and ofisuch relative thicknesses and annealing the tube at such a temperature different from said working temperatures that when the tube is thereafter subjected to the latter temperatures, the differences between the temperatures of different portions of the tube wall will create little or no radial stress between adjacent layers.

8. In forming a heat transfer tube adapted for use with normal working temperatures-progressively varying between the inner and outer surfaces of the tube wall, the method which consists in forming the tube with integrally connected concentric layers of such materials that the coeflicients of thermal expansion of the different layer materials progressively increase from the cooler to the hotter side of the tube wall, annealing the tube at a predetermined annealing temperature difierent from said working temperatures, and so proportioning the thicknesses of the layers relative to said coefficients and temperatures that when the annealed tube is subjected to said working temperatures, the differences between the last mentioned temperatures will create little or no radial stress between adjacent layers.

9. A heat transfer tube adapted for use under normal Working temperatures progressively varying between the inner and outer sides of the tube wall, and composed of integrally connected concentric layers of materials having such different coefl'icients of thermal expansion that when the temperature of said layers changes froma common annealing temperature to their respective normal working temperatures there will be little or no tendency to thermal expansion of one layer relative to another.

10. In the manufacture of a tube for use under working conditions subjecting the tube wall to a fluid internal pressure substantially higher than the external radial pressure on the tube and subjecting the tube wall to difierent temperatures at its inner'and outer sides, the method which consists in forming the tube wall of concentric rigidly connected layers of such thicknesses and of materials having such different coeflicients of thermal expansion and so annealing the tube that the unit hoop stresses in the different layers resulting from the difierences in said pressures and temperatures will be substantially equalized.

11. In the manufacture of a tube for use under working conditions subjecting the tube wall to predetermined internal and external radial pressures and to predetermined difierent temperatures at its inner and outer sides, the method which consists in forming the tube wall of concentric rigidly connected layers of such thicknesses and of materials having such diflerent coefiicients of thermal expansion and so annealing the tube that the resultant stresses impressed upon the different layers will be different from those to which they would be subjected if all of said layers were formed of the same material and will be in predetermined proportion to the respective capacities of the different layers to safely withstand the stresses impressed upon them.

12. In the manufactureol. a tube for use under working conditions subjecting the tube wall to predetermined internal and external radial pressures and to predetermined different temperatures at its inner and outer sides, the method which consists in forming the tube wall of concentric rigidly connected layers of such thicknesses and of materials having such difierent coeificients of thermal expansion and so annealing the tube that the resultant stresses impressed upon the diiferent layers will be better proportione'd to their respective capacities to withstand said stresses than they would be if all of said layers were formed of the same material.

13. In the manufacture of a structure comprising rigidly connected portions for use under working conditions subjecting said portions to difierent temperatures and to stresses other than temperature stresses, the method which consists in forming said portions of materials having such different coefiicients of thermal expansion and so annealing the structure that in said use the resultant stresses impressed upon the different portions will be better proportioned to the respective capacities of said portions tosafely withstand the stresses impressed upon them than would be the case if all of the said portions were formed of the same material.

14. In the manufacture of the wall of a heat transfer element for use under working conditions subjecting the wall of the element to substantially different fluid pressures on its two surfaces and to the flow of heat through the Wall, the method which consists in forming the wall of rigidly connected portions of such thicknesses and of materials having such different coeflicients of thermal expansion, and so annealing the element that the maximum significant stress in each difierent portion throughout any range of temperature and pressure variation occurring in the normal use of the wall will have a predetermined value within the safe allowable stress range of the material of that portion.

15. A heat transfer tube adapted for use under working temperatures progressively varying between the inner and outer sides of the tube wall, and comprising integrally connected concentric layers of materials having difierent thermal expansion characteristics so related to one another and to other physical properties of the materials, to the proportions of the layers, and to said working temperatures that the distribution of stresses in the tube under working conditions will difier in a desirable and predetermined manner from those which would exist under similar working conditions in a tube of similar dimensions and formed of a single one of the said materials.

NEIL R. MCLEOD.

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