US2390417A - Method of manufacturing piston rings - Google Patents

Description

Dec. 4, 1945. H. M. BRAMBERRY 2,390,417

METHOD OF MANUFACTURING PISTON RINGS Filed July 14, 1943 8 Sheets-Sheet l Dec. 4, 1945. H. M. BRAMBERRY 90,417

METHOD OF MANUFACTURING PISTON RINGS Filed July 14, 1943 8 Sheets-Sheet 3 4, 1945. H. M. BRAMBERRY METHOD OF MANUFACTURING PISTON RINGS s Sheeis-Sheet 6 Filed July 14, 1943 Dec. 4, 1945'.

H. M. BRAMB ERRY METHOD OF MANUFACTURING PISTON RINGS Filed July 14, 1943 8 Shee'ts-Sheet 7 4, 1945. H. M. BRAMBERRY 2,390,417

METHOD MANUFACTURING PISTON RINGS Filed July 14, 1943 8 Sheets-Sheet 8 Patented Dec. 4, 1945 OFFICE METHOD OF MANUFACTURING PISTON RINGS Harry M. Bramberry, Oak Park, Ill.

Application July 14, 1943, Serial No. 494,614

21 Claims.

This invention relates to the manufacture of steel articles of small cross-section and particularly to the heat-treating and shaping of such articles composed of special steel.

It is the general object of this invention to provide a novel and improved method of manufacturing steel articles of small section from selected steel alloys possessing special properties and'taking advantage of these properties in such a way as to accomplishv either the shaping of the article; the imparting to the article of the required physical characteristcs; the preserving of the contour of the article against the occurrence of distortion; or selected combinations of these results.

Possibly the most important current application of my invention is in the manufacture of steel piston rings, and, hence, I will give a detailed description of this method of piston ring manufacture as it is actually being successfully practiced in the fabrication of piston rings for military aircraft engines and engines for other equipment. It will be understood, of course, that this is by way of preferred example, it being apparent that a wide variety of other articles may be fabricated in a like or equivalent manner coming within the broad scope of my invention.

I am aware that much previous work has been done in an effort to properly treat, nitride, shape and otherwise process small section steel articles. However, the particular technique or method herein presented is believed to be new and novel over anything heretofore known.

Prior to the present invention, it is not believed that an article of small sectionsuch, for example, as a piston ring--was ever satisfactorily made from steel by subjecting the same to a nitriding treatment without the occurrence of such excessive distortions as to require the removal of practically all of the effective nitrided case in certain areas in order to finish-machine the article to the required dimensions. While I found that nitridng a steel article of small section provided the same with the requisite case.

thickness and wear characteristics-as, for ex-' ample, in providing the wearing surfaces of a piston ring-I also found that this same nitriding process caused so much distortion in the contour of the ring as to necessitate the removal of such a critical proportion of the case thickness as to approach dangerously close to the underlying core structure in certain areas. Prior to the present invention, it has been found that in a high percentage of the resulting rings or other nitrided steel articles of small section, the necessary removal of stock in order to produce uniform dimensions has resulted in actually cutting crltically into the case, which, of course, renders that article or ring completely unsatisfactory.

It is a well-known fact that the aircraft engine builders have been confronted with increased power requirements year after year. Such increased power output has resulted in tremendous piston ring loads, so much so, that the piston ring constitutes the limiting factor with respect to further power increase if the same is to be realized without suffering an appreciable reduction in life and efficiency. To meet the increased power requirement and eliminate the piston ring as being the bottle-neck to further development, it was necessary to adopt new and reduced piston ring proportions, especially a much reduced crosssectional area. Steel, having such high physical characteristics, has been found to meet the requirements, provided the load-carrying surfaces can be so treated as to give a surface, as well as case and core structure that can be loaded to the maximum without the attendant scuffing, scoring and non-compatible condition experienced with the general run of quenched and drawn steels.

It is, of course, known that steel rings as well as cast iron rings may be plated by the chrome process to provide a wear surface with a degree of success. There are, however, a number of disadvantages which render this process incapable of producing rings which will operate under the severe conditions which the rings of the present process are called upon to operate. In the first place, one of the most serious weaknesses in the small section article such as a piston ring treated in accordance with this known chrome process is the resulting hydrogen embrittlement. Also, extreme dimculty is encountered in effecting a uniformly distributed thickness of chrome plate. In addition, when operated under dusty conditions, the plating soon wears away, which immediately results in non-compatible load-carrying surfaces. Therefore, chrome plated rings find limited usefulness where conditions are not as severe as those conditions under which it is contemplated that the present rings must operate.

It is an object of this invention to provide a method of fabricating steel articles of small section-particularly steel piston rings-of a selected steel having the property of growing when heat treated and taking advantage of this growth to produce a more satisfactory structure.

It is another object of this invention to provide an improved method and technique of nitriding steel art cles of small section-particularly steel piston rings of small section-wherein the contour of the structure is preserved or held in such a manner as to prevent the occurrence of distortions normally resulting from such nitriding process.

generally the case and core relationship in each ring resulting from such treatment;

It is a more detailed object of the present inwhich is to be heat treated in accordance with.

the present invention and from which a split piston ring is'subsequently to be made by cutting out a section therefrom in the so-called joint I area:

Fig. 2 is an enlarged broken away section view showing the joint area and th manner of recessing the same prior to heat treatment;

Fig. 3 is an enlarged radial cross-section of a compression ring blank with part of the ring broken away and indicating the uniform composition of the metal and relative size of the section prior to ,being treated in accordance with the present invention;

Fig. 4 is an enlarged radial cross-section view corresponding to Fig. 3 but showing an oil control ring blank and indicating the substantially uniform composition of the metal prior to the ring being treated in accordance with the present invention;

Fig. 5 is an axial section view of my improved male preshaping furnace fixture form, associatedloading ring member, and ring bending tool and guide;

Fig. 6 is a plan view taken substantially on the line 6-6 of Fig. 5 with certain parts removed and others broken away;

Fig. '7 is a plan view taken substantially on the line of Fig. 5 showing an endless ring blank in firmly engaged position about the male preshaping form; I

Fig. 8Vis a view similar to Fig. 7 but showing an endless ring blank bent inwardly in the joint area by means of the bending tool and guide;

Fig. 9 is a broken-away plan view partially in section taken substantially on line 9--9 of Fig. 10 showing the complete furnace fixture with the rings in position firmly engaging the inner male preshaping form and with the outer female nitriding form positioned in embraced relation thereabout and spaced slightly therefrom;

Fig. 10 1s an axial sect on view taken substantially on thelirie I 0-H] of Fig. 9;

Fig. 11 is a view indicating schematically the nitriding furnace with the furnace fixture positioned within the furnace;

Fig. 12 is an enlarged broken-away section view corresponding to a portion of Fig. 10 and showing a group of endless compression ring blanks in close engagement with the wall of the inner male preshaping form prior to the heat treatment;

F g. 13 is a broken-away cross-section view corresponding to Fig. 12 but showing the position of the endless ring blanks in .pressure' engagement with the inner peripheral wall of the female nitriding form at the end of the nitriding heat treatment and growth process and indicating Fig. 14 is a perspective-like view of the female furnace fixture nitriding form as the same appears after being removed from the furnace following a heat treatment with the ring blanks in position exerting radial pressure against the peripheral wall of the female form requiring the opening of the fastening latch in order to eflect the release. of the rings;

Fig. 15 is a schematic view of the endless, ring blank holding jig and the grinding wheels for removing the stock at the Joint toprovide the finished split piston rin Fig. 16 is a plan view 01a finished piston rin indicated schematically in closed position in the cylinder of an internal combustion or other compression engine;

Fig. 1'1 is an enlarged cross-section view of a compression piston ring blank, such as shown on a smaller scale in Fig. 13 resulting from treating the blank of Fig. 3, and bringing out to advantage the details of the case and core relationship resulting from the present method of heat treatment;

7 Fig. 17a is a view corresponding to Fig. 17 but showing the completed ring section after the finishing operations have been performed;

Fig. 18 is an enlarged cross-section view of an oil control ring blank bringing out to advantage the case and core relationship resulting from heat treating the ring of Fig. 4 in accordance with the present nitriding method;

Fig. 18a is a view corresponding to Fig. 18 but showing the completed oil ring section after the finishing operations have been performed;

Fig. 19 is a plan view corresponding to Fig. 9 but showing an alternative arrangement for practicing the present invention wherein the ring blank is heat treated in the split condition thereof instead of the endless condition of Fig. 9;

Fig. 20 is an axial cross-section view of an internal combustion engine cylinder barrel or liner blank in position in a nitriding furnace form to be heat treated in accordance with the present invention; and

Fig. 21 is a broken-away section of a gear and nitrlding furnace form arranged for heat treating the gear in accordance with the present invention.

The present method involves essentially taking.

lowing the article to grow and exert pressure engagement against'the confining form, I accomplish the very important result of preserving and setting the contour of the article by preventing the occurrence of the normally present distortions caused by the heat treatment.

My invention will first be described by way of preferred example as applied to the fabrication and heat treatment of a steel piston ring. Referring to Fig. 1, a piston ring blank PRB is shown as comprising an endless circular ring, larger in diameter than the finished ring, to be manufactured, by the amount of the selected length of stock to be subsequently removed and indicated at JSR, (joint stock removal).

The rings. the manufacture of which is herein described, are those that I have built for the Wrl'ght-Aero-R-ISZO cyclone engine. The compression ring blank, cross-section of which is shown in- Fig. 3, has an initial outside diameter IOD in the rough form of 6.494"i.001", requiring a subsequent joint stock removal JSR of 1.242" measured on the chord. The 011 com trol ring blank, cross-section of which is shown in Fig. 4, has an initial outside diameter of 6.589":.001", requiring a subsequent Joint stock removal of 1.565". Referring to Fig. 2,'it will be noted that an arcuate concave section CS is removed from each side of the ring blank within the area. JSR and a tool and guide locating notch N is formed in the face of the ring centrally of the area JSR. The function of the concave section is to provide a space for the formation of up-set protuberances when the ring is subsequently pushed or bent radially inwardly as will 7 appear. Otherwise, the occurrence of such protuberances would prevent orinterfere with the fiat contact of adjacent rings when placed side by side in the nitriding fixture to be described.

The initial ring stock or blank for making a compression ring before the same has been treated to provide the important case, core and other physical characteristics is shown in enlarged cross-section in Fig. 3. The section is indicated as being substantially uniform in structural composition throughout. The compression ring initial radial thickness CIRT is 0.1525: .0005" when made of drawn wire. The compression ring initial width CIW after roughing of the sides to provide for gas circulation is .0715":.0005". In other words, the desired surface roughness is effected by honing or grinding each side to provide for the requisite circulation of gas between adjacent rings while in the furnace fixture as will appear. The corners of the blank are rounded with a radius CR of 0.012" to 0.015".

In Fig. 4 there is shown an enlarged crosssection of an oil control ring blank before the same has been treated to provide the important case, core and other physical characteristics. This section is likewise indicated as being substantially uniform in structural composition throughout. The oil ring initial radial thickness OIRT is 0172510005" while the oil ring initial width OIW is .0'7l5:.0005" after roughing of the sides to provide for gas circulation as above described. The inside corners of the blank are likewise given a radius ORI of .012" to 0.015" while the face portion is initially defined by a pair of converging sides OCS terminating in a a known for arriving at and pr parin this shape, any one of which may be employed.

I have devised a novel method or technique for readily providing the optimum-shape of male preshaping and female forms, this subject matter being incorporated in a separate case.

Next, the ring blank PRB is pressed or bent inwardly in the area opposite the Joint stock removal portion JSR by means of a uitable tool ll. The male preshaping form I2 is provided with a fiat portion I5 and the ring blank; is bent inwardly with reference thereto (Figs. 6 to 8) to provide for resilient action of the ring blank and to eflect conformity of the inner periphery of the ring blank against the preshaping male form. Also of importance is the fact that this prevents fatal distortion thereof which would otherwise result from forcing the same about a male preshaping form not provided with such flat portion. This preshaping male form I2 is given a concentric shape to that of the inner periphery of the female form I! (Fig. 9) but, of course, smaller in order to permit of the introduction of the ring blanks PRB therebetween. With the ring thus in place about the male preshaping form. l2 and the female nitriding form I! placed in embracing relation thereabout (Figs' 9 and 10), there exists a slight clearance between the outer periphery of the ring blank PRB and the inner periphery of the female form l1 around the entire circumference; except in the area JSR opposite the fiat portion ii of the male preshaping form, which is not important since this section of the ring will later be removed. For example, in the present case involving the manufacture of a compression ring having a finished closed ID of 6.125", a final radial thickness of 0.150" and final.

as Nitralloy N because of the unusual properties of this metal and the novel and unexpected manner in which the same responds to the method steps herein disclosed.

Nitralloy N is a commercially available alloy steel. The composition of Nitralloy N comprises- C (Carbon) ..1 per cent" Mn (Manganese) do 040-.70 Si (Silicon) maximum 0.30 A1 (Aluminum) per cent 1.10-1.40 Cr (Chromium) do 1.00-1.30 Mo (Molybdenum) e do 0.20-0.30

. Ni (Nickel) do 3.25-3.75

a ring guide ID by a reciprocable loading tool I and the balance iron, except, of course, :for impurities. The exact extent to which variations in this composition of Nitralloy may be permissible has not been exactly determined. However, the product supplied, for example, by the Allegheny Ludlum Steel Company, or any of the other sources and specified as conforming with the above composition has, in my experience, been found to produce rings of the desired characteristics. I do not claim any invention in this particular metal alloy per'se but do claim the herein-disclosed method of manufacturing articles' from this alloy, as well as from others having the essential common characteristics.

with the furnace fixture containing the rings in position within the furnace 19 .as indicated in- Fig. 11, the same is first subjected to a purging Y which comprises blowing gaseous ammonia therethrough at a relatively high rate and at a soaking temperature of the order of 925:5" F. for a period of about two hours. During this time a heavy ammonia flow is maintained, with the result that ammonia disassociation is prevented as well as preventing the beginning of nitriding action on the ring blank. Of particular importance is the fact that during this soaking period the rings are sufficiently relieved to effect a set in the shape thereof to the shape of the male preshaping form l2 about which the same have been previously introduced. The accomplishment of this shaping and setting is believed to be the result of an increase in the physicals of the blank as evidenced by an appreciable increase in Rockwell hardness, which occurs in the Nitralloy blank as a direct result of heating in the temperature range of from 900 to 1000 F. This increase of the physicals of the Nitralloy N blank under these conditions appears to be a unique characteristic of this Nitralloy "N and is not present in any other known alloy, at least to this marked degree. The heavy ammonia flow serves the purpose of preventing nitriding and its accompanying growth until the Dreshaping or setting of the ring blank to the shape of the male form has been accomplished. This preshaping and setting is'very important because the male furnace form has a concentric shape to that of the female furnace form. The ring blank having thus been first shaped and set about the male form will subsequently grow away from I the male form and into uniform pressure contact with the inner peripheral wall-of the female form.

The actual preshaping temperature as well as soaking time employed is a result of experiment in the manufacture of rings and may be varied somewhat depending upon circumstances. It is to be noted, for example, that a somewhat higher temperature such as a temperature of the order of 975 F. approaching more closely th critical or normalizing temperature of 1300 F., would effect a more complete normalizing and setting to shape. Such higher temperature would be accompanied by the growth phenomenon where subjected to ammonia gas and hence would defeat the preshaping and setting to the shape of the male form. Where we subsequently nitride and grow a ring blank away from the male form and into contact with the female form as herein contemplated, it has been found that the degree of normalizing and setting to shape accomplished at a relatively low temperatur of the order of 925 F. is suflicient,

As will be pointed out in another case directed to the broad aspects of shaping by heating and normalizing, I have found that by carrying this temperature higher in a non-oxidizing atmosphere a somewhat less expensive but less eflicient ring may be manufactured suitable for subsequent use in a plating process such as that commonlyknown as chrome plating. No complete detailed description of the several alternative techniques of preshaping in a non-oxidizing atmosphere without actual growth will be described herein as the present case is primarily directed to the unusual phenomenon of growth in confinement.

Following the above preshaping heat-when the more important growth ln'conflnement portion of my process'is to be practiced as herein contemplated-the rings are'next subjected to a normal nitriding atmosphere for an additional time period of about thirty-five hours at 995:t5' F., within the furnace outside the retort (see Fig. 11) This results in the ring blanks PRB growin away from the male form l2 and into contact with the embracing inner peripheral wall of the concentric female form II, which is a very important aspect of my invention.

While the above temperature ranges and nitriding time are employed because of the particularly satisfactory results obtained therewith. it will be understood that in the broader aspects of my invention I contemplate other temperatures coming within the nitriding range as well as other nitriding time periods so long 'as the same are effective to produce the herein disclosed growth in confinement.

By taking advantage of this growth characteristic of the metal under the nitriding conditions and allowing the ring blanks to grow into contact with the embracing female form, a most important result is obtained. The female form I'l, being made of substantial cross-section, retains its identical shape, and when the relatively small section ring blanks PRB are made to grow into pressure contact therewith, the surface contour of the ring is thoroughly preserved and the overall shape of the female nitriding form attained by the ring blank resulting in a highly satisfactory ultimate ring, notwithstanding the relatively small section of the blank involved. The furnace fixtures are made of 2330 S. A. E. (Society of Automotive Engineers) steel and are so completely stabilized, as well as protected by electroplated surfaces, that they retain their shape over along period of operation and many heat cycles.

Particular attention is drawn to an unusual and phenomenal characteristic of Nitralloy N when treated as described above. From the very beginning of the heat treatment there occurs an appreciable increase in the physicals of the core. This is not true of any other known alloy. In the case of the present small section ring, this increase in physicals of the core is particularly significant in that it makes the core a suitable carrier for the outer hard case.

Referring further to the furnace fixture (Figs. 5 to 14) the ring blanks PRB are held edgewise in a. flat parallel condition by a flat and parallel shoulder 22 provided therefor on the male form i2 at one extremity and by a cover 23 at the opposite extremity thereof having thereon a parallel surface 24 in contact with the topmost ring blank. This cover is held or clamped in position by the use of Belleville spring washer l8 having a constant spring rate and being adjusted to the desired pressure by a through bolt 25 and nut 26.

The surrounding female form I1 restson and floats upon the base 21 of the fixture being provided with positioning means which may be in the form of a pin 28 carried by female form I! and riding in a radially extending keyway 29. This arrangement allows for the free movement of the female form H to adjust itself with reference to the growth of the ring blanks, thus assisting in providing uniform pressure engagement of the ring blanks with the female form throughout the inner circumference thereof. A locking pin 3| passes through the head of bolt 25 and into base 21 preventing relative rotation upon the turning of nut 26.

The female form I! is preferably composed of a split ring of many times the cross-section of the ring blank or blanks to be grown into radial pressure engagement therewith. I have found that a female form I! composed of 2330 SAE steel, and nickel plated, is satisfactory for manufacturing the present rings when given a radial thickness of the order of from five to ten times that of the rings to be nitrided therein. The

axial width will depend upon the number of rings to be treated in a single heat, the female form 11 shown in Fig. 14 having a width of the order of 2" and having a capacity of twelve rings. 1

For accurately maintaining the inner peripheral circumference, the ends of the female ring form ll are provided with a drilled and reamed hole extending in the axial direction, through which the ring is sut at 32 and into which drilled hole is placed a dowel 33. The female ring ends are arranged to be fastened against this dowel 33 by means of a pivotal latch 34.

When the nitriding treatment has been completed, it will appear that the rings may be readily removed from confinement within the female form 11 by first releasing latch or fastening means 34. This is followed by employing a wrench or wedge inserted in the radially extending slot of the female form i! for slightly opening the same.

It is particularly imp rtant that the nitriding atmosphere be circulated to all of the surfaces of all of the ring blanks substantially uniformly in order that the depth of the resulting nitrided case below the several surfaces may be made substantially uniform. In a piston ring this is important in order to provide the proper physical male form, horizontal circulating channels 33 (see Fig. 12). Male form I2 is likewise provided with axially extending recessed gas channels 42. Base 21 is provided with gas passages 43 communicating with the lower extremities of chan'- nels 31 and 42 as well as with a gas passage 44 communicating with the top extremity of passages 31 and 42 through the interiorof male form l2 by way of opening 46 and space 41 between male member I2 and cover 23. The spacing 41 provides for firm pressure contact between the top ring and the cover member 23. This is very important in accomplishing the requ site substantially perfect flatness of the resulting rings. This cover is also provided with a gas opening 48. A pin 43 passes through cover 23, male member l2 and base 21 functioning to maintain the proper relation therebetween.

When the endless ring blanks are removed from the female form I! following the above nitricling growth treatment it is found that a cross-section therethrough presents a very definite structural picture. Referring to Fig. 17, a cross-section of a compression ring is shown as composed of the inner core, the adjacent transition zone CHTZ, and the prime or hard case CHPC and an outer matted layer CHM. The transition zone CHTZ has a general depth of the order of .0035" to .0065". The prime hard case CHPC thickness is of the order of .008" to .011". Following the removal of the endless rings from the female form I! as noted above, the rings are lapped or finished-honed on each side to remove characteristics including: the proper free joint opening when the section of stock JSR has been removed; the proper radial pressure characteristics including the radial point pressures, that is, radial pressures at the ends of the ring, which pressures must be held within close limits; the

proper depth of case below the cylinder wall engaging face; the proper tensile strength, and hence, the proper fatigue characteristics; the proper conformity with the contour irregularities of the cylinder walls at high temperatures and pressures; and full control of flatness. To provide this uniform exposure to the nitriding gases, the inner peripheral wall of female form I! is fabricated with a plurality of axially extending recessed gas channels 31.. These gas channels are of such small circumferential extent as not to make any impression upon the outer periphery or face of the rings when the same exert radial pressure thereagainst. I have found that about 17 of these gas channels 3'! having a circumferential length each of about 0.062" and a depth of about 0.031" are suflicient to provide ample circulation to the ring blank faces. Particular attention is directed to the fact that the compression ring blanks are formed with a small radius CR of .012" to .015" at each extremity of the ring face with the result that adjacent rings define with the inner peripheral wall of the female form-during the final portion of the heat-forward gas circulating channels 38 (see Fig. 13).

As pointed out above, the side of the ring pin 3| passes through the head of bolt 25 and blanks has been given thedesired surface roughness providing for gas circulation therebetween.

When the heat is firstbegun andthe rings firmly engage the male preshaping form I 2 of the inner radiused corners CR define with the the matted surface CHM and to reach the underlying structure of the prime hard case CHPC removing as little of the latter as is necessary in order to reach a uniform surface. It is found necessary to remove only about .0005" to .001" from each side of the ring blank in order to obtain a uniform hard case CPCS having a depth of the order of .007" to .010" below the surface on each side.

Next the rings are "set up to remove approxiured along the outer chord as indicated on Fig. 15. In accomplishing this the rings are held in a fixture 54 while the grinding wheels 55 cut out the requisite sections. Finally the rings are codirectionally finish-honed on the cylinder wall engaging surface to remove the matted layer and to leave as much as possible of the underlying prime hard case. As a result of my novel process, I am able to leave a prim hard case depth CPCF on the cylinder wall engaging face of the present rings of the order of .0055" to .0085", by honing away CY to a depth of only about 00025.. A case depth of .0040" to .0090" is within acceptable limits to produce a satisfactory ring, when the inaccuracy of potting up and honing is considered.

Referring now to Figs. 18 and 18a, showing cross-sections of an oil control ring, a similar structural arrangement is presented, the prefix letter 0 being used to differentiate the oil control ring from the compression ring C. The principal difference is in the cylinder wall engaging surface.

As will be noted from the following comparative analysis of the resulting compression ring and oil ring, the prime hard case of the oil ring in the nose area is somewhat deeper as a result of the particular converging construction of this nose. This is a distinct advantage since the unit pressure of this reduced surface in engagement with the embracing cylinder wall is greater with a consequent slightly increased rate of wear over that which is present in the wider face of the compression ring- The following gives a comparison of the compression ring blank before heat treatment (Fig. 3), after heat treatment (Fig. 1'7) and in the finished form (Fig. 17a) (1) Compression ring blank initial outside diameter,,CIOD 6.494 .001 Compression ring blank initial outside circumference mounted on male form, CIOC 20.403" Compression ring blank initial radial thickness, CIR.T 0.1525":.0005 Compression ring blank initial width when rough finished on sides to provide for gas circulation b e t w e e n sides, vCIW .0715":.0005 Compression ring blank heat treated outside circumference, CHOC 20.465"

Inner circumference of female form 20.453" CHOC (20.465") minus CIOC (20.403")=0.062" restrained or confined in fixture average growth. Compression ring blank heat t r e a t e d radial thickness, CERT .1530":.0005 CHRT (0.1530") minus CIR'I' (0.1525") =0.0005" average growth. Compression ring blank heat treated width, CHW .072:!:.0005" CHW (0.07175" mean) minus CIW (0.07125" mean) =.0005" a v e r a g e growth. Compression ring blank heat treated prime hard nitrided case, CHPC .008"to .011" Compression r i n g b1 an k heat treated depth of transition zone, CHTZ--- .0035" to .0065" Compression ring finished, cut, closed, outside (1 i a m e t e r, CFOD 6.125":.001 Compression ring finished, cut closed, radial t h i c k n e s s, CFRT .150"- -.0015 CHRT (0.153" mean) minus CFRT (0.150" mean)=.0030" maximum removed in finishing.

Although this difference is .0030", we hone a maximum of only .0025" which still keeps within a finished dimension of .150i.0015".

Thus, in finishing the cylinder wall contacting face of the rings, I remove .002" to .003". This portion that is removed is composed of about 0.0005" of matted surface and a thin portion of the underlying hard nitrided case.

Compression ring finished, cut,

closed, width, CFW .070"- -.0005" CHW; (0.07175" mean) minus CFW (.070")=.00175" mean total removed.

Thus, in finishing the sides of the ring, I remove about .0009" from each side. This is composed of about .0005" of matted surface and underlying hard, nitrided case.

Compression ring finished, prime hard nitrided case depth remaining below the face, CPCF .0055" to .0085" Compression ring finished, prime hard nitrided case depth remaining below each side, CPCS .007"-.010

The difference in the finisheddepth of case between the cylinder wall engaging face and the sides is explained by the fact that it is not practicable to effect the same degree of perfection in finishing the cylinder wall engaging face as in finishing the sides. This is accounted for in part by the fact that the rings after being cut must be assembled in a special fixture and then transferred to an arbor on which they are held while being honed. This cannot be accomplished more perfectly than to within .001" to .002".

The following gives a comparison of the oil ring blank before heat treatment (Fig. 4), after heat treatment (Fig. 18) and in the finished form (Fig. 18a) radial thickness, OHRT .173010005" OHRT (.1730" mean) minus OIRT (.1725 mean) ='.0005" average growth. Oil ring heat treated width,

OHW .0720"- -.0005" OHW (.0720" mean) minus OIW (.0715" m e a n)=.0005" a v e r a g e growth.

responding to the depth of casein the compression ring.

The significance of the above outlined small dimensions will now be apparent. By allowing the ring blanks to grow into pressure engagement with the surrounding nitriding female form, I thus prevent any irregular distortions in the contour of the rings. Since there are no undulations or irregularities in the contour, it thus becomes necessary to remove only the very minimum of the exterior surface of the heat treated rings in order to reach the underlying prime hard case structure required for contact with the piston groove and with the cylinder wall. For the same reason the underlying prime case is left with the maximum depth, and this depth is uniform throughout any particular surface.

A starting blank in th form of an endless ring has been found to have definite advantages, including: (1) it can be semi-finished by passing through double-disc grinders with no free ends requiring control; (2) full control of the radial pressure at the points, which is highly desirable,--

and which cannot be accomplished within the required close limits by any'manufacturing technique heretofore known to the art with a split ring; (3) the Sides of a piston ring with surfaces as hard as that after being subjected to the nitriding process can only be reduced in width by some sort of abrasive operation and during this operation, a solid endless piece can be more accurately finished to dimension than can a split piece; and (4) the last and final operation of removing the section JSR at the joint area can be done in group order which is advantageous to production and the maintenance of close tolerance in the relation of the ring ends.

While, as noted above, an endless ring has been disclosed as the blank to be subjected to the nitriding process, and this is the preferred procedure, it is also contemplated that this invention covers the use of a split ring blank (see Fig. 19). In practicing the invention with a split ring blank, the same may be assembled with the ends engaging the proper dimensioned feather 56 within the fixture so as to provide the proper percentage of growth to take place before the ring completely fills the female form and exerts the required pressure thereagainst.

The same fundamental reaction can be accomplished without the use of a feather by using a split ring of the correct circumference leaving a clearance in the joint area to provide for the proper percentage of growth before the ends come into contact to create the desired'pressure against the restraining female form. In addition, the

ends of such a ring may be so angled as to cause the pressure at the periphery of the ring directly at the joint to become either negative or positive.

I In manufacturing a ring in accordance with the latter practice, it would be necessary to remove the ends in a similar manner to the removal of the joint areajrom the first and preferred endless blank. a

Different percentages of the total growth for pressure engagement with the female member may be used following actual contact of the outer periphery of the ring with the female member, ranging anywhere from 20% to 40% of the total growth. It has been found, for example, that a particularly satisfactory Nitrallo N steel piston ring of 6.125" O. D., 0.150" final radial thickness and 0.070" final width is obtained where seventy per cent (70%) of the possible growth of the ring is allowed to take place before the outer periphery or face of the ring exerts an appreciable pressure against the surrounding wall of the female member. Thirt per cent of the total growth is thus employed after actual pressure engagement with the female member for preserving the contour and flatness of the final ring and imparting to the ring other improved properties flowing from this process. a

By following the above step of first forming the ring over the concentric male preshaping member [2, there results the very novel advantage of having the ring grow substantially uniformly and identicall into engagement with the female member I! throughout the circumference, with the exception, of course, of the joint area JSR. This effects a substantially uniform pressure between the ring and the inner peripheral wall of the female member and substantially complete contact therewith. This is clearly distinguishable from taking a circular ring, compressing the same along one diameter to form aparabolic-like shape and inserting the same into the female member and following this with the is practically that of the engine cylinder diameter for which size the ring is intended. A further and ver marked defect in this irregular growth into contact with the female-form would be the fact that non-uniform radial pressures would be produced between the ring face and the female member, some of these pressures being so high that the nitriding atmosphere would be unable to gain access to the ring while in other places the atmosphere would gain access, thus resulting in a non-uniform nitrided case. It is essential that the small section steel ring involved in the nitriding operation absorb the nitriding atmosphere uniformly over all surfaces so as to prevent bi-metal effect which would result in not only a distorted contour but an out of flat condition which cannot be corrected.

If the case is appreciably thicker on the inner periphery of the ring than on the outer, when the joint pieceJSR is cut out, the unstable condition will immediately evidence itself by the fact that the ring opens because of the high stress on the inner periphery; conversely, the opposite result takes place if the case on the outer periphery is of greater thickness than on the inner.

The same result as that obtained by the use of the male preshaping member can be obtained to lesser degree by pre-bending the ring blank in the area. of from three to nine o'clock to a small radius in the areas of two and ten oclock before placing the same in the female member.

The relation between the total nitrided case cross-sectional area and the total core cross-sectional area is critical in articles of small sections and has a definite relation to the total length of the piece, such, for example, as the herein-disclosed piston ring. This relation determines the amount of growth possible per unit of circumference during the nitriding furnace time cycle selected thus to obtain the requisite pressure against the female form and effect the desired physical characteristics. A satisfactory prime hard case thickness, for example, in an aircraft nitrided steel piston ring of an 0. D. 6.125" has been found to be .0055 to .0085", this being arrived at bymanufacturing and engine operation requirements and conditions made necessary in order to obtain the desired high fatigue value and resiliency for high output, high-speed internal combustion engines.

It is believed that no one has heretofore appreciated the significance of the optimum relationship between the case and core area of a nitrided steel piston ring and how to secure the same. From said relationship an accurate value can be established from which the percentage factor for determining the percentage of the total growth can be selected for predetermining 'ing furnace fixture growth of a steel article or steel ring blank may be determined, the necessary specimens are subjected to a nitriding heat, being measured accurately before and after the heat and from their individual growth an average value of growth is established per unit of length. With the.,length of the female interior circumference known, the required length or diameter of a ring blank of the same steel as the specimens can be easily determined from the value previously established by specimen, in order to employ the proper percentage of the growth of the ring to exert the required pressure on the interior of the female form necessary to preserve the shape of the finished ring without adversely affecting the penetration of the gases to the face thereof.

My invention has been described above in considerable detail inconnection with the specific manufacture of piston rings in order to teach,

with the requisite completeness, how to practice this important process. From this teaching it is believed that the application of my invention to the manufacture of other articles involving like problems will be apparent.

Referring to Fig. 20, one example of an alternative application of my invention is indicated schematically. A Nitralloy N steel cylinder liner blank 51 is shown in position in spaced relation to the embracing furnace fixture 58 prior to heat treatment. As in the case of the abovedescribed piston ring manufacture, the assembly will be placed in a nitriding furnace, such as l9 shown in Fig. 11, subjected to a nitriding atmosphere and selected temperature cycle for a selected time. The cylinder blank 51 will .be caused to grow into radial pressure engagement with the surrounding wall of fixture 58 and this pressure engagement will be utilized to preserve the contour of the relatively thin sectioned cylinder blank during the formation thereon of the requisite nitrided case. Subsequently, the cylinder 51 is removed from the fixture, honed and otherwise finished for installation in an engine.

As above emphasized, the fact that the cylinder blank is preserved or held against distortion during nitriding it is only necessary to machine deep enough'into the working surface of the cylinder to remove the matted surface therefrom, thus leaving a uniform depth of prime hard nitrided case of the order of .006" to .009". As in the case of a piston ring, the prevention of the formation of distortions in the contour makes this result possible.

While furnace fixture 58 is not disclosed in detail, it will be apparent that the same can be built in an equivalent manner to that of female member I! of furnace fixture l3. This may include splitting the form as indicated at 58a, as

well as providing a cover member 58b, Belleville spring washers 58c and fastening cap screws 58d. Referring to Fig. 21, there is indicated schematically the manner of practicing my invention in nitriding a steel Nitralloy N gear 6|. Gear BI is indicated in spaced relation to the embrac- 62 prior to the growth of the gear into pressure engagement with the" form in a nitriding atmosphere. While the furnace fixture for receivin ear 6| has not been shown in detail, it will be apparent that the same may be constructed in a similar manner to the above furnace fixture. This may include a plurality of separable segments H and an. embracing split ring Ila. This ring may be provided with a fastoning means, such as that shown in Fig. 14.

Although I have described my invention in usually novel properties in connection with Nitralloym because of its unpracticing this method, it is to be understood that in the broader aspects of my invention, 1 also contemplate the use of any metal or alloy having the requisite properties. I

While my invention has otherwise been described in connection with certain specific embodiments thereof, it is to be understood that in its broader aspects I consider this invention to be basic and not limited to the specific examples given, the same being defined by the appended claims.

I claim:

1. The method of manufacturing articles having at least one relatively small cross-sectional dimension and composed of a steel alloy having the property of growing when subjected to a nitriding heat treating process. which method comprises confining a blank from which said article is tobe made in a furnace fixture having an internal contour corresponding to the external contour of the final article and being slightly larger than the blank, subjecting said blank to a nitriding atmosphere fora selected temperature and time cycle, employing the growth of said blank to exert pressure against said fixture, said pressure being effective to hold the contour of said blank and prevent the occurrence therein of distortion, and removing said blank from said fixture.

2. The method of manufacturing an article of small section of a steel having the property of growing or elongating when nitrided, which comprises subjecting a substantially circular metal blank, from which said article is to be made, to a nitriding atmosphere within a confining form or fixture which is larger than the blank and using the growth property of the-metal, due to the nitriding thereof, to effect radial pressure engagement with the form for preventing the inherent distortion tendency of said article and subsequently removing the article from the form and employing the same for some function outside of said form.

3. The method of manufacturing a steel article having at least one relatively small cross-sectional dimension of the order of 0.030" to 0.l30", said steel being an alloy having the property of growing or becoming elongated when subjected to a nitriding treatment, said method comprising confining a blank from which said article is to be made in a furnace fixture having an internal contour corresponding to the external contour of the final article and being slightly larger than the blank, subjecting said blank to a nitriding atmosphere for a selected temperature and time cycle, employing the growth or enlargement of said blank to contact and exert pressure against said fixture, thus to prevent the inherent distortion tendency of the contourof said blank during the nitriding operation and subsequently removing the article from the form and employing the same for some function outside the form.

4. The method of heat treating a substantially circular Nitralloy N steel article having such a small section as to normally result in the contour thereof becoming distorted when subjected to a nitriding process, which method comprises confining said article in a furnace fixture having a formed interior corresponding to that of said article but being slightly larger and permitting of the ready insertion of said article therein, said fixture being of a sufllciently rigid construction as notto be distorted by a ni-tridins process or the pressure of the growth of said article, sub-- jecting said assembly to a nitriding atmosphere for a predetermined period of time and at a predetermined temperature and employing the radial growth characteristic of said metal article during a predetermined percentage of the total growth thereof when subjected to said nitriding atmosphere to effect radial contact of said article with said formed interior and the exertion of sufiicient pressure thereagainst to cause said article to maintain conformity with said formed interior andprevent the occurrence of irregularities in the contour of" said article, and subsequently removing said article from said formed fixture and adapting the same for performing some function exterior to said fixture.

5. The method of heat treating'a nitridable steel piston ring blank which includes: placing said steel blank in a confining fixture of larger internal dimension than the external dimension of the blank, subjecting the same to a nitriding atmosphere for a selected temperature and time cycle, employing the growth characteristic of the nitridable steel while being nitrided to effect contact with and to exert pressure against said fixture for preventing the inherent distortion tendency of the contour of said ring and subsequently removing the ring from the fixture and adapting the same for use in a cylinder.

6. A method of heat treating a Nitralloy N steel piston ring blank which includes placing a ring blank of relatively small section in a furnace fixture arranged to confine the external radial periphery of the ring against growth beyond a predetermined amount thus to employ the growth characteristics of the nitridable ring blank while being nitrided for exerting pressure against said fixture for holding the same against the inherent distortion tendency of the contour thereof, subjecting the ring to a nitriding atmosphere for a predetermined temperature and time cycle and subsequently removing the ring blank from the fixture and performing the necessary finishing operations thereon to produce a completed piston ring.

.7. The method of manufacturing a split piston ring which includes placing an endless Nitralloy "N, steel ring blank of relatively small crosssection in a confining furnace fixture, subjecting the same to a nitriding atmosphere for a selected temperature and time cycle, employing the radial growth characteristic of the Nitralloy while being nitrided to effect pressure against said fixture for preventing the inherent distortion tendency of the contour of said ring, subsequently removing the ring blank from the fixture, finishing the sides of said ring blank, then cutting out a predetermined section of stock to provide a selected free joint opening, closing the ring on a holding mandrel and honing the face of said ring to remove the matted surface therefrom and provide a prime hard nitrided case having a uniform depth below said working surface of the order of .0040" to .0090".

circumference of saidring blank, subjecting the ring to a nitriding atmosphere for a selected temperature and time cycle, employing thegrowtn or elongation characteristic of the Nitralloy N when subjected to the nitriding treatment. for exerting radial pressure against the paraboliclike inner peripheral wall of said furnace fixture for holding said ring against the inherent contour distortion normally present therein when subjected to such nitriding process, subsequently removing the ring from the fixture, finishing the ring blank on both sides to remove the matted surface and provide a prime hard nitridedcase of uniform depth, cutting out a selected, length of stock from the joint area, closing the ring on a holding arbor and honing the cylinder wall engaging surface to remove the matted surface and provide a hard nitrided working face with a uniform hard prime case depth of the order of .0040" to .0090".

9. The method of manufacturing a finished steel piston ringhaving an outside diameter when installed in a cylinder of the order of 6.125", a radial thickness of the order of 0.150", a width between sides of the order of 0.070", which method comprises employing an endless Nitralloy "N steel ring blank of an outside circumference in rough of the order of 20.403", a radial thickness of the order of 0.1525 plus or minus .0005", a width between sides of the order of 0.0715": .0005", placing said endless blank in a nitriding female furnace fixture form having an inner peripheral contour parabolic-like in shape and having a circumference greater than the external circumference of said endless ring blank by a predetermined amount, subjecting the ring blank to a nitriding atmosphere for a predetermined temperature and time cycle, employing the growth of elongation characteristic of the Nitralloy N for exerting radial pressure against said parabolic-like .inner periphery of said female niltriding form during a selected percentage of the growth of said ring blank for the purpose of preventing the occurrence of irregular distortions in the contour when subjected to such nitriding treatment, subsequently removing the ring blank from the fixture, finishing the sides thereof to remove the matted surfaces and provide a uniform predetermined depth of hard prime case below said surfaces, cutting out a selected length of stock from the area. at twelve oclock or the commonly referred to joint area, closing said ring about a honing mandrel and honing the cylinder engaging surface thereof to remove the matted surface and provide a uniform depth of prime hard nitrided case of the order of .0055" to .0085".

I 10. In a furnace fixture for nitriding articles composed of a nitridable steel alloy having the property of growing or becoming elongated when subjected to a nitriding heat treatment, a nitriding female furnace form having an inner peri- 8. The method of manufacturing a split piston phery corresponding in contour to the external periphery of the article to be nitrided therein, but being of such size as to permit of the introduction of the article therein prior to nitriding treatment, said fixture being eifective to resist the pressure exerted by growth of said article thereagainst thus to preserve the contour of the ar-' ticle against the formation of irregular distortions in the contour thereof, said fixture having" means providing for the release of said article fol-- lowing the nitriding treatment. I I 11. In a furnace fixture ar an me ts a posed of anitridable steel alloy having 'the ith t male furnace form having a predetermined shaped inner periphery corresponding in contour to the external periphery of the completed article to be nitrided, but beingof a larger size to permit the purpose of removing said ring following the heat treatment.

12. The structure defined in claim 11, wherein said fastening and releasing means comprises a latch.

13. In a, furnace fixture assembly for nitriding nitridable steel piston rings having the property of growing or enlarging when subjected to a nitriding heat treatment, a nitriding female furbolic-like in cross-section and having aninner circumference slightly larger than the external circumference of an endless piston ring blank of nitridable steel to be treated therein, a male prev nace form having a formed inner periphery parashaping furnace form having a'formed external periphery concentric in shape to the paraboliclike inner periphery of said female member but spaced therefrom by a predetermined amount.

means for introducing endless ring blanks over said male member including a guide member leading thereto and effective to change the circular ring blank to the parabolic-like shape of the male member, said male member being effective to pre-shape the ring blank during the heating operation following which said ring is subsequently grown away from said male memher and into pressure contact with said female member inner periphery for preserving the contour of said ring against the formationtherein of irregular distortions.

14. The furnace fixture assembly defined in claim 13 being further characterized by the provision of axially extending gas communicating channels formed in the peripheral wall of said female form and said male form.

15. The furnace fixture assembly defined in claim 13 being further characterized by the provision of axially extending gas communicating channels formed in the peripheral wall of said female form and said male form, means defining a base plate supporting said male and female forms, said base plate being provided with a ra dially extending guide channel, an axially extending pin protruding from said female form and arranged to be guided in said radially extendin channel, a cover for said male form having a peripheral shoulder arranged to engage one side extremity of an outside ring, a bolt received centrally of said assembly, a Belleville type spring washer and a fastening means therefor for holding said assembly together and maintaining a predetermined end pressure on said rings.

16. In a furnace fixture assembly for nitriding split nitrldable steel piston rings having the property of growing or becoming elongated when subjected to a nitriding heat treatment, a nitriding female furnace form having a formed inner periphery corresponding to the external periphery of a split piston ring when free, and having an inner circumference slightly larger than the external circumference of said split piston ring when free plus the length of the free Joint opening, means defining a feather or filler arranged to be interposed between the ends'of the split rings to be treated in said female form, whereby the introduction of nitriding atmosphere about rings so confined and at nitriding temperature for a selected period of time is effective to cause said rings to grow into radial pressure engagement with saidfemale form and thus hold the contour of said rings in such a manner as to prevent the occurrence of distortions therein.

1'7. The method of manufacturing a cylinder which includes placing a tubular cylinder blank of a nitridable steel having the property of growing when nitrided in a nitriding female furnace form having a formed circular inner periphery slightly larger than the external circumference of said cylinder blank, nitriding said blank in a nitriding atmosphere for a selected period of time and at a selected temperature to effect the growth of said blank to contact with and exert radial pressure against said furnace form, said radial pressure being effective to hold the contour of said blank and prevent the occurrence therein of distortions, and removing said blank from said form.

' 18. The method of manufacturing a Nitralloy N piston ring from a split, normally circular piston ring blank, which comprises spreading the free ends of said blank 9. selecteddistance apart, confining the blank within a furnace fixture, unlformly subjecting all surfaces of said blank to heat and a nitriding atmosphere to a'temperature below the critical temperature of the Nitralloy N, for a selected period of time, and using the growth property of the metal due to nitriding thereof to effect radial pressure engagement with the interior of the fixture.

19. The method of manufacturing a Nitralloy N piston ring from a split, normally circular piston ring blank, which comprises placing said blank in a confining fixture of larger internal dimension than the external dimension of the.

blank, spreading the free ends of the blank a selected distance apart, subjecting all surfaces of said blank to a nitriding atmosphere for a selected temperature and time period, and employing the growth of the blank while being nitrided, to contact with and exert pressure against said fixture to prevent distortion of the blank.

20. The method of heat treating a plurality of nitridable steel piston ring blanks, which comprises roughin the sides of said blanks to a desired degree of roughness, stacking said blanks with the roughened sides of adjacent blanks in contact in a confining fixture of larger internal dimension than the external dimensions of the blanks, subjecting the blanks to a nitriding atmosphere for a selected temperature and time cycle, the roughened sides of said blanks insuring ,nitriding' of both sides of the blanks,and employ. ing the growth of the blanks while being nitrided, to contact with and exert pressure against said fixture to prevent distortion of the blanks.

21. The method of heat treating a. plurality of nitridable steel, split piston ring blanks, which desired degree of roughness, stacking said blanks 2,sao,417

with the roughened sides or adjacent blanks in contact within a confining fixture of larger internal dimension than the external dimensions of the blanks, spreading the free ends or said blanks a predetermined distance apart, applying a selected yieldable load to the sides oi said blanks to maintain the same in normal flat condition, uniformlysubiecting all surfaces or said blanks to a nitridingatmosphere for a selected temperature and time cycle, the roughened sides of said blanks insuring nitriding 01 both sida of all of the blanks. and employing the growth of the blanks while being nitrided, to contact with and exert pressure against said fixture to prevent d18- tortion oi the blanks.

, HARRY M. Y.

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