US 3579805 A
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United States Patent 13,579,805
 Inventor Howard Berdolt Kast 3,098,662 7/1963 lversen 285/18 Fairfield, Ohio 3,316,129 4/1967 Token et al..... 148/135 ] Appl. No. 742,882 3,408,178 10/1968 Meyers et a1. 148/142X giled d J y 3 32 FOREIGN PATENTS atente ay Assignee General Electric p y 475,244 1 1/1937 Great Britain ..29/(SHPT)D1g.
 METHOD OF FORMING INTERFERENCE FITS BY (SKFIDigest); 285/18, 381; 148/4, 135, 127, 142
 References Cited UNITED STATES PATENTS 2,647,847 8/1953 Black et al. ..29/.(SI-1FT)Dig.UX
Primary Examiner-Charlie T. Moon AttorneysDerek P. Lawrence, E. S. Lee, 111, Lee H. Sachs,
Frank L. Neuhauser, Oscar B. Waddell and Melvin M. Goldenberg ABSTRACT: An interference fit can be provided between metal components where at least one of the components is formed of a precipitation-hardenable metal alloy which undergoes a substantially permanent dimensional change in addition to a reversible dimensional change upon heat treatment at elevated temperatures. The interference fit itself will be irreversible if both components are formed of the preciptitationhardenable alloy and reversible if only one of the components is formed of the alloy.
METHOD OF FORMING INTERFERENCE FITS B Y HEAT TREATMENT The invention described and claimed in the US. Pat. application herein resulted from work done under US. Govemment contract FASS-66-6. The US. Government has an irrevocable, nonexclusive license under. said application to practice and have practiced the invention claimed herein, including the unlimited right to sublicense others to practice and have practiced the claimed'invention for any purpose whatsoever.
FIELD OF THE INVENTION The present invention relates to a method for forming an interference fit between metal components and more particularly to a method wherein at least one of the components is formed of a metal alloy which undergoes dimensional change upon heat treatment thereof and wherein at least a portion of the dimensional change is substantially permanent.
DESCRIPTION OF THE PRIOR ART The formation of interference fits between metal parts is a technique which has been practiced for many years. There are a number of widely used, relatively standard methods which have come into common acceptance. Primarily these methods entail the machining of the outer diameter of a male or internal member to a slightly greater dimension than the internal diameter of a female or external member and then either heating the external member or cooling the internal member and interfitting the members to effect the interference fit. More specifically in the process wherein the external member is heated, the member expands sufficiently to permit the posi tioning of the internal member therein and then cools and shrinks to tightly engage the internal member. In the other technique, the internal member is cooled sufficiently to cause it to shrink to a size which can be easily positioned within the external member. The subsequent return of the internal member to ambient causes it to expand and tightly engage the external member.
In another similar well known technique the-inner member may be cooled while simultaneously the outer-member is heated. Thus the inner member contracts while the outer member expands. The members are brought together, the inner inserted within the outer, and the assembly permitted to equalize in temperature. Upon equalizing, the inner member expands, the outer member contracts and the interference fit is thereby effected.
The foregoing well known techniques, although widely applicable, have certain serious drawbacks. First, since the members'are interfrtted while at least one of them is in the expanded or contracted condition, a premium is placed upon the rapid relative positioning of the members to assure that they are properly positioned when the expected return to original dimensions occurs. Thus the serious danger exists of seizing expensive parts before they can be positioned relative to each other. Another serious disadvantage of these techniques is that the individual members are each subjected to differing thermal environments and thus residual stresses remain in each member and ultimately appear in the assembly.
In response to these drawbacks, methods were developed whereby both components were simultaneously heated or cooled thus relieving the stress problem, and wherebythe positioning of the components could be effected while each was at room temperature, thus eliminating the seizing problem. These newer techniques relied in large part upon the characteristic of several ferrous alloys that cooling of the alloy to subzero temperatures, particularly in the range of 50 to 100 F, would cause an expansion of the alloy, and that such expansion would remain when the alloy returned to normal temperatures.
Specifically US. Pat. No. 2,647,847 teaches that the inter-- nal member may be formed of this ferrous alloy while the external member is formed of a metal having more conventional expansion properties. The internal diameter of the outer member is machined to a dimension larger than the outer diameter of the internal member and the components are positioned relative to one another in the desired configuration. Both the internal and external members are then cooled to subzero temperatures whereupon the internal member permanently expands while the external member temporarily contracts. Upon returning to normal temperatures, the external member returns to its original dimension however the permanent expansion of the internal member causes the interference fit.
Still another technique utilizing the freezing" principle is disclosed in'U.S.v Pat. No. 3,098,662. This technique builds upon the advantages inherent in prior methods but additionally permitsthe use of similar alloys for both the intemaland external members. According to this method both'members are formed of an alloy which expands upon cooling and remains at the expanded dimensions upon return to normal temperatures. The external member is preexpanded by exposing it to subzero temperatures to increase the dimensions size of the bore. However, the internal member will expand at the low temperatures to form the interference fit.
Although, apparently many of the prior art defects were overcome by these. latter techniques, it has been found that the formation of interference fits at the low temperatures required by thesetechniques is undesirable. This is particularly so because treatment at low temperature is likely to introduce stress crackingdue to the low ductility of the components. In additionassemblies treated at these low temperatures generally have to be subsequently heat treated atelevated temperatures prior to use in order to relieve stresses and to precipitation-harden;
SUMMARY OF THE INVENTION It is, therefore, an object of the present invention to provide an improved method for the formation of interference fits which does not require that-the components be exposed to subzero temperatures.
It is another object of this invention to provide an improved method whereby the drawbacks of prior 'art techniques, such as seizing prior to proper positioning and the existence of hamrful stresses in the components can be eliminated.
It is still another object of this invention to provide an improved technique wherein the requirement for stress relieving and precipitation hardening at elevated temperatures subsequent to the formation "of the interference fit can be eliminated and these steps integrally included into the method of forming the interference fit.
Other objects and advantages will become apparent from the following description and appended claims.
In accordance with the objects of this invention a method for forming interference fits between interfitting components is provided wherein dimensional changes in the components.
are brought about by heat treatment at elevated temperatures. At least one of the components is formed of a precipitation hardenable alloy which undergoes a substantially irreversible dimensional change as well as a reversible dimensionalchange upon heat treatment. By substantially irreversible dimensionaldimensions, assembling the components and then subjecting the assembly to a similar precipitation-hardening heat treatment to alter the dimensions of the previously untreated component and to form the interference fit. The previously treated component will remain unchanged during the assembly heat treatment. Since the components are constructed of the same alloy the interference fit is irreversible because each component will expand or contract in precisely the same manner as its mate regardless of the thermal conditions to which it may be exposed.
In another embodiment of this invention a reversible interference fit is provided by forming just one of the components from the precipitation-hardenable alloy and forming the other froma conventional alloy which undergoes only reversible expansion or contraction upon heat treatment. The components are assembled and the assembly is subjected to a precipitationhardening heat treatment to cause a substantially irreversible dimensional change in the precipitation-hardenable alloy and a corresponding expansion in the conventional alloy. Upon cooling the precipitation-hardenable alloy retains the substantially irreversible portion of its dimensional change and the conventional alloy returns to its original size. The interference fit has been effected by the substantially irreversible dimensional change of the precipitation-hardenable alloy. However, unlike the irreversible interference fit, upon heating to a temperature above the solutioning temperature of the precipitation-hardenable alloy, the dimensional change therein can be reversed and the components separated.
BRIEF DESCRIPTION OF THE DRAWINGS The invention will be better understood from the following description taken in conjunction with the accompanying drawings:
FIG. 1 is a fragmentary sectional view illustrating the relative positioning of the internal and external members prior to interfitting to form an irreversible interference fit;
FIG. 2 illustrates the members of FIG. I interfitted to form an assembly;
FIG. 3 illustrates a portion of FIG. 2 greatly exaggerated to show the clearance between members;
FIG. 4 is a fragmentary sectional view illustrating the relative positioning of the internal and external members prior to interfitting to form a reversible interference fit utilizing a precipitation-hardenable alloy which contracts upon heat treatment; and
FIG. 5 is a fragmentary sectional view illustrating the relative positioning of the internal and external members prior to interfitting to form a reversible interference fit utilizing a precipitation-hardenable alloy which expands upon heat treatment.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In providing the interference fits of the present invention at least one of the components, and in some instances both, are formed of a precipitation-hardenable alloy which retains a dimensional change when subjected to heat treatment at elevated temperatures. Such dimensional change is uniform along all axes of the alloy and is unrelated to and independent of the thermal coefiicient of expansion. It is a known property of most precipitation-hardenable alloys that they will undergo such a dimensional change, either expansion or contraction in going from the solution-annealed and cooled condition to the precipitation-hardened condition. The extent and nature of that dimensional change varies from alloy to alloy and is de pendent upon the conditions of the heat treatment. The most important feature in connection with these types of dimensional changes is that the changes are substantially irreversible or substantially permanent. Because the alloy is dimensionally stable after heat treatment an article made from the alloy will not return to its original dimensions upon cooling or other heat treatment below the solutioning temperature of the alloy. This behavior is crucially different from that of conventional alloys which expand upon heating or contract upon cooling but which will return to their original dimensions when returned to normal temperatures.
The term precipitation-hardening heat treatment, as used herein, means a conventional heat treatment by well known techniques at temperatures and for times which will bring about the desired precipitation of hardening constituents but below the solutioning temperature. This temperature will vary from alloy to alloy but in all cases is a value which is easily ascertainable by one skilled in the art. Heat treatment in the range above the minimum temperature necessary to initiate such precipitation will generally increase the extent of the dimensional change. However, in many cases, with such as a stainless steel sometimes identified as I7-4PH, beyond a particular temperature where austenite forms, the increased dimensional change will be achieved at the expense of hardness and strength. Thus the choice of a precipitation-hardening heat treatment temperature will require consideration of desired ultimate properties as well as the desired dimensional change.
Examples of alloys well known in the art and which are known to undergo substantially pennanent dimensional change upon heat treatment include l7-7 PH, l7-4PH, PH 15-7 M, l55 PH and the nickel base super alloys lnconel 718, lnconel X and lnconel W.
In order to utilize the advantageous characteristic of these alloys that they experience a substantially permanent dimensional change, it is preferable first to solution anneal components constructed of these alloys. An example of a typical alloy is 17-4 PH stainless steel consisting nominally by weight of 0.8 Si, 16 Cr, 4 Ni, 0.3 Cb Ta, 3 Cu with the balance Fe and impurities. The solution anneal for such alloy should be accomplished at temperature of from about l,875 -1,925 F. for about 1 hour. The structure of the alloy of this temperature is predominantly austenite with any hardening elements, e.g., copper, dissolved therein. Generally close control of the solution anneal is desirable because temperatures which are too low result in reduced tensile and yield strengths and incomplete solution of the hardening elements. Excessively high temperatures cause excessive grain growth and result in lower tensile ductility and reduced impact strength. The solutioned alloy is then air cooled or oil quenched, in accordance with known techniques, to to transform the austenite to martensite. The martensite may then be aged or precipitation-hardened at temperatures in the range of from about 900 to l,150 F. for at least 1 hour and preferably from l,0501,l00 F. l7-4 PH steel contracts markedly in these temperature ranges, the substantially irreversible contractions ranging from about 0.0004 to 0.0006 inches per inch depending upon the temperature. The higher the temperature the greater such contraction. Also longer exposures at hardening temperatures will increase such contraction. I
The method of forming the interference fit can be better understood by reference to FIG. 1 wherein a body member 10 is shown having a bore 13 therein said bore having an inner diameter A. The body member may alternatively be referred to as an outer member, female member, or sleeve member. An inner member 14, also referred to as a male member or spool member, is shown having an outer diameter B. The members are adapted to form an assembly 15 by interfitting inner member 14 within the bore 13 of outer member 10, as can be seen more clearly in FIG. 2.
In one embodiment of the present invention, an irreversible interference fit is provided. Both members are formed of an alloy which is precipitation-hardenable and which undergoes substantially irreversible dimensional change. These members are solution annealed at a temperature above the solutioning temperature of the alloy and cooled to normal ambient temperatures. Both members are, at this point, prepared to undergo a precipitation-hardening heat treatment in order to take advantage of the accompanying dimensional change.
The method of forming the interference fit will depend upon whether the alloy being employed contracts or expands upon heat treatment. If it is a contracting alloy, for example 17-4h steel, the inner or male member can be precipitationharde'ned in accordance with well known techniques to alter its dimensions. It is particularly desirable that all heat treating be accomplished in a vacuum or inert atmosphere to prevent oxidation or corrosion. The internal diameter A of the outer member is adjusted as necessary to be just slightly larger than outer diameter B of the already dimensionally altered inner member. When components and 14 are interfitted to form an assembly 15, as shown in FIG. 3, the clearance a between the members should be smaller than the expected substantially irreversible contraction of the outer member.
The precise clearance is a matter of choice. It is clear that the larger the clearance the easier it will be to interfit the members. On the other hand the smaller the clearance the tighter the resulting interference fit will'be. The assembly 15, consisting of contracted inner member 14 housed within the bore 13 of outer member I0 is then subjected to a precipitation-hardening heat treatment in a manner similar to the heat treatment employed for the inner member alone. Since the inner member is precontracted, its dimensions will not change substantially irreversibly during this second hardening treatment. However, outer member 10 will contract substantially irreversibly to form the interference tit and a tightly interfitting assembly.
In a similar manner an irreversible fit can be formed if the alloy chosen is one which expands substantially irreversibly when subjected to a precipitation-hardening heat treatment, e.g., 17-7 PH steel. In this case, after solution annealing, the outer member is precipitation-hardened to expand and permanently alter its dimensions. The diameters A and B are adjusted to permit the members to be interfitted with a small clearance, as hereinbefore discussed. The interfitted assembly 15 consisting of expanded outer member 14 containing inner member 10 within bore 13 thereof is then subjected to a precipitation-hardening heat treatment. The preexpanded outer member will not change irreversibly its dimensions during this treatment but the inner member will undergo a substantially irreversible expansion to form the interference fit and a tightly interfitting assembly.
The tightly interfitting assemblies thus far described, wherein the components thereof are each fonned of the precipitation-hardenable alloy, are irreversible: the members cannot be separated because both the inner and outer members will undergo precisely the same reversible expansion or contraction in response to any thermal conditions. Even should the assembly be heated above the solutioning temperature of the alloys to retrieve the substantially permanent dimensional change, both alloys would tend to return to their original dimensions in precisely the same manner.
In another embodiment of the present invention, as can be seen in FIGS. 4 and 5, a reversible interference fit can be provided by forming only one of the members of the precipitation-hardenable alloy which undergoes substantially permanent dimensional change. The other component, in this embodiment, is formed of a conventional alloy which expands on heating but returns to its original dimension on return to normal ambient temperatures. FIG. 4 shows the members forming a reversible interference fit with a contractingalloy. The outer member 17 is formed of the precipitation-hardenable alloy and inner member 19 is formed of a conventional alloy. To form the interference fit, the outer member is first solution-annealed and cooled, the members are interfitted with inner member 19 housed within the bore 18 of outer member 17. A slight clearance is provided between inner diameter C of outer member 17 and outer diameter D of inner member 19, and the assembly is subjected to a precipitationhardening heat treatment, preferably in an inert atmosphere. During the high-temperature treatment outermember 17 substantially permanently contracts due to metallurgical changes and expands due to thermal expansion and inner member 19 made of the conventional alloy only expands. Upon cooling,
outer member I7 retains a portion of its dimensional change but inner member 19 returns to its pre-heat-treatment dimension. If the clearances were originally chosen correctly, the substantially permanent contraction will be sufficient to effect the interference fit.
This interference fit, in contrast to those previously discussed is reversible. This is true because the members are formed of dissimilar alloys and each will respond differently to heat treatment. For example if the assembly is annealed above the solutioning temperature the substantially permanent dimensional change in the outer member will be relieved, the contraction will be eliminated and the members will he become separable. Since separation requires such extreme temperatures, the assembly is useful in applications over a very wide range of temperatures without any real danger of the members accidentally or inadvertently separating.
Still another technique for forming a reversible interference fit can be explained with reference to FIG. 5 wherein the inner member 22 is formed of a precipitation-hardenable alloy' which expands upon heat treatment and the outer member 20 is made of a conventional alloy. The interference fit is formed by solution-annealing and cooling inner member 22, interfitting that inner member within the bore 21 of outer member 20 with a slight clearance provided between inner diameter E and outer Diameter F. Upon subjecting the interfit assembly to a precipitation-hardening heat treatment, inner member 22 will substantially permanently expand due to thermal expansion plus metallurgical changeswhile outer member 20 will also expand due to thermal expansion. Upon cooling outer member 20 will return to its pre-heat-treatment dimensions and the interference fit will have been effected in a manner similar to that described in connection with FIG. 4.
In all of the techniques described for forming the interference fits of this invention, a primary advantage is that the members were interfitted to form the assembly while they were at normal ambient temperatures. This eliminates the problems associated with seizing and the pressures of rapid positioning, In each case, however, the step of interfitting the components carries the limitation that the outer diameter of the inner member and the inner diameter of the outer member must have a slight clearance there between. Under no circumstances should the clearance exceed the expected net dimensional change since it should be clear that in such a case no intel-ference fit will be formed.
The following examples are illustrative of the present invention.
Example I An irreversible interference fit was formed by providing inner and outer members of 17 -4PI-I stainless steel and subjecting them to a solution-anneal for about 1 hour at about l,900 F. and then air cooling the members to room temperature. The inner member was subjected to a precipitationhardening heat treatment at about 1,075" F. for about 1 hour to cause the member to contract. The inner member was then interfitted within the bore of the outer member and a small clearance of about 0.0003 inches was provided. The entire assembly was subjected to a precipitation-hardening heat treatment at about l,075 F. for about 1 hour. All heat treatments were accomplished in an argon atmosphere.
The resulting tightly interfitted assembly was tested by applying 4,400 lbs. of force in an attempt to separate the members. The interference fit withstood the applied force with no resulting motion.
Example 2 A reversible interference fit was formed by providing an outer member of l7-4PH stainless steel and an inner member of M2 tool steel. The outer member was solution-annealed at 1,900 F. for 1 hour and air cooled to room temperature. The inner member was interfitted within the bore of the annealed outer member and the interfitted assembly was heat treated at about l,075 F. for about 1 hour. All heat treatments were accomplished in an argon atmosphere.
The tightly interfitted assembly was tested at 4,400 lbs. force and no movement was noted indicating that the interference fit had been formed.
To demonstrate the reversible nature of the fit, the assembly was solution-annealed at about l,900 F. for about 1 hour. A force of only 695 lbs. was needed to start separation and, thereafter, the parts separated easily indicating that the anneal had relieved the dimensional contraction of the outer member.
While the present invention has been described with reference to particular embodiments thereof, it will be understood that numerous modifications can be made by those skilled in the art without actually departing from the scope of the invention.
l. A method for forming an interference fit between inner and outer members adapted to be interfitted, comprising the steps of:
a. providing at least one of said members of a precipitationhardenable alloy which undergoes substantially irreversible dimensional change upon heat treatment thereof,
b. solution annealing said precipitation-hardenable alloy at a temperature in excess of the solutioning temperature of said alloy,
0. interfitting said members to form an assembly with a clearance between said members smaller than the substantially irreversible dimensional change, and
d. subjecting said assembly to a precipitation-hardening heat treatment which causes said substantially irreversible dimensional change.
2. A method, as claimed in claim 1, wherein said precipitation-hardening heat treatment is accomplished at a temperature of from about 900? to 1,150 F. I
3. A method, as claimed in claim 1, wherein said members are both formed of said precipitation-hardenable alloy and step of subjecting said inner member to a precipitationhardening heat treatment after solution-annealing thereof.
6. A method, as claimed in claim 4, wherein said dimensional change is an expansion and including the additional step of subjecting said outer member to a precipitation-hardening heat treatment after solution annealing thereof.
7. A method, as claimed in claim 1 wherein one of said members is formed of said precipitation-hardenable alloy, the other of said members is formed of a conventional alloy, and said resulting interference fit is reversible.
8. A method, as claimed in claim7, including the additional step after solution-annealing of said precipitation-hardenable alloy of cooling said alloy prior to interfitting of said members.
9. A method, as claimed in claim 8, wherein said dimensional change is a contraction and said outer member is formed of said alloy.
10. A method, as claimed in claim 8, wherein said dimensional change is an expansion and said inner member is formed of said alloy.