US 2726181 A
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Description (OCR text may contain errors)
Dec. 6, 1955 a. A. LYON 2,726, 8
METHOD OF HEAT TREATING CARTRIDGE CASES Filed Oct. 26, 1951 2 Sheets-Shoatl l I A 14 5 I i: ii 75 I I so .47
ETZETTZTET George A/berf Z9012 E $04. -d/fl flgfzg Dec. 6, 1955 G. A. LYON I 2,726,181
METHOD OF HEAT TREATING CARTRIDGE CASES Filed 06". 26, 1951 2 Sheets-Sheet2 F1 q- P4 i i i Georye Afberzf ZZZ 012 found suitable for processing in accordance with the method of my invention.
It is therefore an important object of this invention to provide a method for the heat treatment of a cartridge case while in an intermediate stage following cold working and rough heading steps, in accordance with which the head end and a contiguous side wall portion of a case are subjected to a heat treatment step in the same heating zone but under such conditions as to impart a differential effect resulting in a relative hardening of the treated side wall portion and either no substantial hardening or even a relative softening of the head end, without, however, substantial coarsening of the grain in either of the regions subjected to the heat treating step.
It is a further important object of this invention to provide a method of heat treating a cold drawn, roughly headed cartridge case shape of between 0.25 and 0.40% carbon content under such controlled and localized conditions of temperature and total time of heat treatment as to obtain the desired degrees of hardness in the metal of the wall portion contiguous to the head end and in the head end, respectively, to facilitate the subsequent machining thereof, without excessive warping of the metal of the case and without substantial coarsening of the grain of the steel undergoing heat treatment.
Other and further important objects of this invention will become apparent from the following description and appended claims.
The method of my invention is illustrated in the accompanying drawings, in which:
Figure l is a longitudinal sectional view of a cartridge case shape in an intermediate stage of processing following a series of cold drawing steps and a rough heading step, illustrating in association therewith a clamping device for aiding in obtaining the differential effect of the heat treatment, as referred to above, the clamping device being partly in section and partly in elevation.
Figure 2 is a fragmentary, sectional view of the lower end of the cartridge case shape, shown with the head end immersed in a fused salt bath to illustrate the first step of the heat treatment.
Figure 3 is a similar fragmentary, sectional view showing the head end of the cartridge case shape immersed more deeply into the fused salt bath.
Figure 4 is a sectional view taken substantially along the line IV-IV of Figure l. t
Figure 5 is a fragmentary, sectional view of the head end of the cartridge case shape illustrating a clamping device similar to that shown in Figures 1 to 4, inclusive, but with an internal plug associated therewith.
Figure 6 is a similar fragmentary, sectional view of the lower end of the cartridge case shape showing a modified form of external clamping device.
Figure 7 is a sectional view taken substantially along the line VII-VII of Figure 6.
Figure 8 is an enlarged, fragmentary, sectional view showing hardness measurements on the Rockwell hardness tester, B and C scale, at different points throughout the head end and contiguous side wall portion of the cartridge case shape, prior to the final cold heading operation but subsequent to the heat treatment of my invention.
Figure 9 is a fragmentary, sectional view of the head end of the cartridge case shape after a final cold headmg operation has been performed thereon;
Figure 10 is a similar fragmentary, sectional view of the head end of the cartridge case shape after the machmmg, tapping and finishing steps have been performed on the head. i
The heat treatment method of my invention is applied to a cartridge case at a point in its manufacture represented by the shape indicated by the reference numeral 10. The shape 10 has been reached by a series of cold workuig steps such as those described in my copending application Serial No. 179,971, filed August 17, 1950,
Preferably, the shape 10 represents the form of the cartridge case after the fourth cold draw and immediately following the rough heading step. At this point, the shape 10 comprises a relatively thin wall 11 of generally circular cross section open at its mouth end, as at 12, and closed at its other end by a roughly formed head 13, with which the thin wall 11 is integrally joined by a tapered portion 14. The entire shape 10 is, of course, integrally formed.
For purposes of illustration, the shape 10 of Figure 1 may be considered as representing the size and dimensions obtained at the intermediate stage indicated in the production of a three-inch/SO mm. cartridge case. The shape has been formed in the manner already referred to from a medium soft steel within the limits for carbon content of 0.25 to 0.40%, and preferably with a carbon content of around 0.35%. The steel may, but need not be, in a spheroidized state at the start of the cold drawing, or other cold working operations.
By way of typical example, the head end 13, prior to the heat treatment steps about to be described, may have a Rockwell hardness of to 92, B scale, while the side wall 11 has a Rockwell hardness of to 102, B scale. The head end 13 is relatively thicker than the side wall 11 and thus represents a greater concentration of mass than the side wall. This difference in mass as between the head end and the contiguous side wall portions is taken advantage of, and may be augmented in the manner about to be described, in heat treating the shape 10 to obtain a differential eflect between the two regions of the case.
In the heat treatment of shape 10, I prefer to use a salt bath, indicated generally by the reference numeral 15. The salt bath 15 is maintained in a fused state within a suitable container 16 and at a temperature depending upon the particular analysis of the steel undergoing heat treatment and the particular results desired. In general, the temperature of the salt bath 15 will be between l250 and 1650 F., and for a steel meeting the specifications of SAE 1035, will preferably be between 1450 and 1650 F.
While a salt bath is illustrated as constituting the heat treating zone, other specific types of heating zones may be employed. Some of the advantages of a fused salt bath for heat treating purposes are the ease with which its temperature can be controlled within the desired narrow limits, the high specific heat of the fused salt and the relatively rapid rate of heat transfer from the salt bath to the article undergoing heat treatment, with consequent shortening of the total time required for the heat treatmentof the article. In addition, a salt bath can be chosen that will be relatively non-corrosive toward the metal of the article and that will protect the article from corrosion during subsequent processing steps, while at the same time being readily removable by the application of water.
As illustrated in Figures 1 to 4, inclusive, the cartridge case shape 10, prior to immersion in the salt bath 15, is mounted in a combination hanger and clamping device, indicated generally by the reference numeral 75. Said device 75 comprises a bale 76 in the form of an inverted U, the bight 17 serving for the suspension of the device from a conveyor. .The lower free ends of the bale 76 are threaded for a portion of their length, as at 18, and a wall-engaging member 19 and a head-engaging member 20 are mounted on said lower threaded ends 18, 18 by means of suitable nuts 21.
The wall-engaging member 19 is in the form of a ring, or annulus, having aligned bores 22 freely receiving the threaded leg portions 18, and having an inner tapered wall portion 23 merging downwardly into a lower substantially cylindrical inner peripheral surface 24. The inside diameter of the peripheral surface 24 is substantially the same, or slightly less than the outside diameter of the generally cylindrical side wall of the cartridge case shape s eamer 10, so, as to effect closezcontact therewith. The tapered surface-23ffacilitatescentering of the cartridge case shape in thetmountingof the clamping device thereon. The member wrisheld in iadjusted position, by means of the nuts; 21, so as to contact the side wall 11 just above the level towhichthe cartridge case shape is to be" finally immersed .in the salt bath l The head-clamping member 21} is also annular in form and comprises a ring-like body portion 25 having an inner surface 26 contoured to fit the-outer annular periphery 27 of the heady end. 13, and having aradially extending annular flange 28 that'isapertured to freely receive the lower end portions 18 of the bale 76. The head-clamping member 20 is positioned and held in place on the lower threaded ends 18 by means of nuts 21 threaded on said endsabove' and below the flange 28. The body portion 25 is provided with an opening-29 which leaves the centralbottomaface-of the head-13exposed directly'to the fused salt bath 15, when; the. cartridge case shape is immersed therein, as shown in Figures 2 and 3.
The distance, represented by the letter A, between the lower surface of the clamping member 19 and the upper surface of the clamping member 24) is the height to which the-side. wall; portion contiguous to the head 13 is to be hardened by the heat treatment method about 'to be described Thatrportion ofthe head 13 that. lies below the-level of the upper surface ofthe clamping member 20. represents the head end portion of thecartridge case shape, that.=is not tobe hardenedv substantially, and preferablyisto-be softened, during the heattreatment operation. Thismeans -that:at-the-level-of theuppert surface of the clamping member 20,'there is a-transition poinn'which is represented-by the letter T.
In carryingout the method of-heat treatment embody; ing m y invention, .a cartridge case shape carried ina hanger and clamping device 75 as shown inrFigur'e- 1, is-lowered,-head end-first,- into the salt bath-m In'the first immersing step-,rthe head endw13 is-lowered to the leyel; indicated in Figure 2'. By way of example, assuming. that thecartridge. case shape -10 isof the dimensions represented'bya three-inch/ 50mm. case, when finished, the head end-of: the shape 10-is immersed in 'the'salt-bath 15-toathe extent: of. about. 2 inches upwardlyof the outer face 30 of said head end. Upon reaching thisdepth of-.-iinrnersion, the shape 10' is :held immersed at that depth-for a'periodtof about 1 /2 minutes. It is-then' lowered approximately. an. additional inch and threeqnarters' into the salt bath 15,-. to= the; level indicated in Figure 3 and heldimmersed atthatdepth for an additionalone-h-alf minute... At the expiration ofvsaidvone half--minute, thexshapef isquickly. withdrawn-from the salt. i5" and, immediately drastically quenched, preferably; inwater ror innan aqueous saltsolution toefiect rapid-coolingto the temperature ofnthe-circumambient air,-or at least toua temperature below*200' F. During the-quenching operatiom: the-shape 10 is not removed from-the combinedsupportingand clamping device 75; but is carried therein from the salt'bathto the quench bath, After reaching the temperature indicated, the shape is removed from-the quenching bath and; after further cooling if necessaryyissubjected to the'remaining processing steps,,.which may be the customary ones in the makingof. cartridge cases, or variations of such steps; If. the immersion:- steps above described are followed; it will be apparent that the head end of the-shape fOr'Z /m inches. upwardly. from' theend face 30 is' heated for a total of two minutes, while the contiguous wall portion for a height of one andrthree-quarterinches is heated for only one-half minute. Under these conditions for a cartridge case shape of the size and type of steel indicated; the head...en.d 13 will. be heated-toe maximumttemperature not ahovev the lower criticalrange of; the particularsteel used, andcpreferably below, such lower criticaL-range, While the side wall portion contiguous to thethead end and withina bandofabout oneandthreequartersinches in length, starting twoand onesixteenth inches: up from.
the end face: 30, will be-heated. through thecriticalrange; or at'least up'to and including the'upper critical point; Acaof the steel'used. The difference'in temperature? reached by the head end 13 and the contiguous side wall portion is made-possible by the greater concentration of mass of metal'inthe head end l3' 'than in the; side wall 11, and by augmenting: this difference by rneansof the clamping members 20 and 19.
As shown in Figure 3, the immersion of the head-end of the shape ltiis carried only tothe level-at which the clamping member-19 makes contact on its lower face with the surface of the fused saltbath 1-5. The substantial mass of metal in the clamping ring19, being for the most part exposed to the atmosphere above the fused salt bath, tends to extract heatthat; would otherwise be-conducted along the wall 11 of the-shape upwardly beyond the clamping member 19. Thus the portion'of the side wall of the shape above the clamping ring. 19 and indicatedby the reference numeral 11a,- remains relatively cool and unafiected by the heat treatment to which'the head end of the shape is subjected.
In the quenching operation, the cooling of the head 13 proceeds at a slower rate than thecooling'of the contiguousportion A of the side wall, both becauseof' the greater mass of thehead and also because of the contact between the periphery of the head andthe clamping member 20. Thus, while the wall portion A is:- being rapidly cooled from its AC3 point to a temperature of around 200 F., or lower, the'metal of the head 13 is beingcooled more slowly from a temperature well below its A c3 point. Consequently, the metal of the head 13 is nothardened, and may evenbecome somewhat softer asa resultof the heat treatment if carried out under optimum conditions, while the metal of the contiguous wall portion A develops itsmaximum hardness even though subjected to the same fused salt bath and the same quenching step.- Y
After the quenching operation; if the heat treatmenthas been carried out in the manner justdescribed, the head end 13 will have an average Rockwell, B scale; hardness of about with a-maximum value up-to about 98 near the transition line T, between the head and 1 side wall, while the contiguous portion Aofthe side wall-'will have imparted to it an average Rockwell hardness, C scale;
Y of about 49, with a'minimum value of, about 43 adjacent the same transitionv line T. Actual values forthe Rock well hardness determinations are shown in'Fig; 8. The dots shown on the section of the head end of the heat treated cartridge case shape in Figure 8 represent points where actual hardness measurements were taken. These hardness values have been averaged for the head 13 below the transition line T and for the contiguous'wallportion A above the transition line- T. Theaveragehardness values,.betore and after the heat treatment, may be-summarized as follows:
In order to achieve the desired difierentialhai dness as between the head and the contiguous wall portion,"it may be necessary to further augment'the mass of-metal?in' contact with the head 13- and incontact with the upper sidewall. portion 110. A modificationiof the clamping members forthisflpurpose isillus'trated in Figure 5,. wherein, in addition" toth'e'clamping' assembly .75, there is used an inner plug, represented generally by the refercnce'numeral -35. Said inner plug'35 comprises'upper' and-lower heads- 36 and 37, 'joined by a stem' 38. The. plug 35 may suitably be formed" as a casting, preferably ofa metal or alloy having; good heat transfer character istics; such as copper. The-upperhead- 36is of'cylin drical shape with an outside diameter substantially the same as the inside-diameter of the cartridge case shape so as to be in sliding contact therewith. The lower head 37 is contoured on its lower face 39 to conform with the upper face 40 of the head 13. Consequently, when the plug 35 is in place, the lower surface of the head 37 is in full surface contact with the inner face 40 of the head to a point adjacent the transition line T. A rod 41 is threaded into the upper head 36 of the plug 35 to facilitate its insertion and removal into and out of the cartridge case shape 10. The plug 35 thus serves to augment the mass of metal at the head end and above the contiguous wall portion A and thereby prevent the head end and the upper wall portion 11a from reaching the temperature of the fused salt bath during the immersion steps, while permitting the contiguous side wall portion A to reach the fused salt bath temperature.
The temperature of the fused salt bath will vary in accordance with the carbon content of the shape to be heat treated so that for any given specification of steel, the temperature of the fused salt bath will be at least equal to the A03 point of that steel, and preferably slightly higher. Thus, as previously stated the temperature of the salt bath will be between l250 and 1650 F., and preferably between 1450 and 1650 F. This preferred higher range includes the A03 point of any of the medium soft steels for which the heat treatment method of my invention is particularly suited.
By way of illustration, if the cartridge case shape is formed of a steel meeting the specifications of SAE 1035, and if the salt bath 15 is maintained at a temperature of about 1600 F., then under the conditions of immersion specified above and using the form of clamping device and inner plug shown in Figure 5, the maximum temperature to which the metal of the head 13 will be raised during immersion is about 1250 to 1275 F., while the contiguous wall portion A will, of course, reach the temperature of the fused salt bath, namely, about 1600 F. Since the upper critical point, Aca, for SAE 1035 steel is about 1430 F., this means that the contiguous wall portion A will be heated through its Acs point, while the metal of the head portion 13 will never reach its AC3 point. Consequently, the metal of the head 13 will be little affected by the heat treatment, but will tend to be softened rather than hardened, while the metal of the contiguous wall portion A will be hardened to the maximum extent possible as a result of the heat treatment.
A further modification of a clamping device is illustrated in Figures 6 and 7, wherein the upper clamping member 19a is formed in a plurality of parts, including a main ring 45 having an inner peripheral groove 46, in which are positioned arcuate segments 47 that are urged by means of springs 48 into pressure contact with the outer surface of the side wall 11 of the cartridge case shape 10. The inner surface of the main ring 45 is tapered, as at 2311, while the inner surface of each of the segments 47 is cylindrical, as at 24a to effect close surface contact with the outer surface of the side wall 11. The clamping device 19a shown in Figures 6 and 7 may, of course, be used in conjunction with the inner plug 35, or may be used separately in the heat treatment step.
The differential effect in hardness as between the head 13 and the contiguous wall portion A is obtained without any significant coarsening of the grain of the steel and without any detrimental warping of the portion of the shape that has been subjected to heat treatment. The advantages of the differential hardening effect are realized in the subsequent cold heading and finishing operations, the results of which are illustrated in Figures 9 and 10, respectively.
In Figure 9, the head end of the shape is shown after a suitable cold heading or coining step has been performed to provide a head end 13a more nearly approaching the shape of the head after the machining and other finishing operations'have been performed. As a result of the heading operation that results in the formation of the head 13a, the average hardness of the steel in the head increases, but only to about 90, B scale, whereas if it were not for the previous heat treatment step, the heading operation would bring the hardness up to about 102, B scale. This lower hardness that results from the heat treatment step facilitates the machining of the head 13!: to give the head 13b (Fig. 10). In addition to being machined, the head 13b is provided with a coaxial tapped bore 50, which is countersunk, as at 51.
The further processing of the cartridge case includes a stress-relief anneal, which involves heating to about 700 F. for about one-half hour. This brings the hardness of the metal of the final contiguous side wall A of the case down to an average hardness of around 41 to 42, C scale. A subsequent full length tapering operation gives the desired taper to the case and also serves to bring the side wall of the case to a substantially true circular cross section. For instance, if the side wall gets out of round as much as 0.013 inch in the heat treatment, the tapering step restores the side wall to truly circular cross section, plus or minus 0.003 inch. This is within the permissible tolerance.
There may optionally be a stress relief heat treatment after tapering.
As a result of the successive cold drawing steps, the grain of the steel after, say, the fourth draw, which is represented by the shape 10 of Figure 1, is greatly elongated, as might be expected. During the stress relieving, after heading and before tapering, the grain structure is considerably refined, although still elongated, and this makes for a better drawing during the full length tapering operation. The stress relief treatment also, as stated, brings the hardness of the metal of the side wall down to an average hardness of about 41 to 42, C scale, which is within the specifications required.
It will be understood that the heat treatment operations referred to may be carried out in a more or less automatically controlled system. For instance, in carrying out the heat treating process, a conveyor will be used which will operate in properly timed cycles for progressively lowering the case by proper increments into the fused salt bath, lifting the case out of the bath and immersing it in the quenching bath.
In summary, the heat treatment step of my invention includes heating the head end of the partially processed cartridge case, or shape, up to or within the lower part of the critical range of the particular steel used, say up to 1250 F., but preferably not in excess of 1275 F., while at the same time bringing the thinner contiguous wall portion of the shape to its upper critical point, or Acs, point, generally in the neighborhood of 1650 F. This heating step in combination with the quenching, results in a softening of the steel of the head end, say from an average of between and 92 down to around 80, and in a hardening of the metal of the side wall, say from an average of between and 102, B scale, up to about 49, C scale, all measured on the Rockwell hardness tester.
After the heading operation to produce the shape 10a illustrated in Figure 4, the average hardness of the head end 13a increases to about 90, B scale, but is still not so hard as it normally would be without the special heat treatment step. This facilitates machining the head end.
In the treatment for stress relief prior to full length tapering of the side wall, the hardness of the side wall is reduced to between 41 and 42, C scale, which is within the specified requirements.
It will thus be appreciated that the heat treatment method of my invention makes possible the use of medium soft steels having a carbon content of between 0.25 and 0.40, whether spheroidized or not, to produce, by a series of cold drawing, heading, annealing and tapering steps, a cartridge case that meets specifications. At the same time, my method eliminates the use of the more expensive and less easily controlled step of hardening the side. wall contiguous to the head'end of. the case by. induction-heating, and eliminates. any excessivewarpage that. might otherwise occur if. the induction. heating step were employed. In addition, the combinationof'processing, steps,-. including the anneal before tapering, results in, afine grain structure that makes for better drawing during. the tapering of the full length otv the case side wait.
It will: be understood that modifications and variationsmay be elfectedwithoutdeparting from the scope of tlie novel concepts of the present invention.
I claim as my invention:
1. In a method of heat treatinga case having a relativelythic'k head end and a relatively thin side Wall, botli tliehead end and Wall having been integrally formed from a-cold'worked blank of steel of 0.25 to 0.40% carbon content, the steps which comprise immersing said case head endfir'st into a heat treating atmosphere having a temperature between 1250 and 1650 F., augmenting the effective mass of the head end and controlling the time and conditions of immersion of said head end and portions of said side wall contiguous therefot'o' bring the head end to a temperature within the'lower portion only of the. critical range of said steel while bringingithe-tliin side wall portions to a temperature of aridttnd thet upper critical point of said ste'el, and quenching the heated head end and wall portions, whereby a partial annealing of the steel of said head end and a hardening of the steel of said side wall portions are effected.
2. In a method of heat treating a case having a relatively thick head end and a relatively thin side wall, both the head end and wall having been integrally formed from a cold worked blank of steel of 0.25 to 0.40% carbon content, the steps which comprise immersing said case head end first into a heat treating atmosphere hav ing a temperature between 1250 and 1650 F., augmenting the effective mass of the head end and controlling the time and conditions of immersion of .said head end and portions of said side wall contiguous thereto to bring the head end to a temperature within the lower portion only of the critical range of said steel while bringing the thin side wall portions to a temperature of around the upper critical point of said steel, and quenching the heated head end and wall portions, whereby a partial annealing of the steel of said head end and a hardening of the steel of said side wall portions are effected, the head end having a Rockwell hardness of about 90 to 92, B scale, before and of about 80, B scale, after said heat treating steps, and said Wall portions having a Rockwell hardness of about 100 to 102, B scale, before and of about 49, C scale, after said heat treating steps.
3. In a method of heat treating a cold work-hardened cartridge case of 0.25 to 0.40% carbon steel to provide a thickened head end of less hardness than the contiguous thin side wall portions, the steps which comprise incrementally advancing said case with its head end first into a heated zone having a temperature between 1250 and 1650 F., augmenting the effective mass of the head end and controlling the time of such incremental advances and the total time of dwell within said zone to bring said head end and contiguous wall portions to such temperatures, respectively, that said head end will have after quenching a lower and said contiguous wall portions will have a higher degree of hardness than their respective initial hardnesses, and quenching the same.
4. In a method of heat treating a cold work-hardened cartridge case of 0.25 to 0.40% carbon steel to provide a thickened head end of less hardness than the contiguous thin side Wall portions, the steps which comprise augmenting the effective mass of the head end of said case, incrementally advancing said case with its head end first into a heated salt bath having a temperature between 1250" and 1650 F. until said head end and a few inches of said contiguous side wall portions are immersed in 10 said. salt. bath controllingg the time ofsuch incremental advancesaand thegtotat t-ime. of; dwell within; said: salt bath to bring saidhead. end and contiguous wall portions'to such temperatures,respectively, that said head end will" have after quenching alower and said contiguous wall. portions. will: have ahigher degree ofhardness than their respective initial hardnesses, and quenching the same.
5. In a methodof heat treating. an intermediate shape of cartridge case of a steel-,eontainingabout 0.30% of carbon thathas been coldworkedto form a thickened head end'andan integraligenerally circular. side: Wall, the. steps which; comprise immersing first-said head endand thenag portion. of said sidewallicontiguous to saidrhead endina salt bath maintained at a temperature of between 1450 and l650 R, augment-ing the effective mass of the head end and controlling; the totaltime and. effect of such immersion of saidhead endand of'said' side wall portions so. as, to raise said head end to a maximum temperature of. only about: 1250 F. whileraising said contiguousside wall portions to substantially the temperature of.said-bath thetotal time of immersion being not over. about" two minutes: for. the equivalent of a 3- inch/50 mm. cartridge case, andimmediately there= after quenching, said case from said. salt bath, whereby a slightv softening of the steel-of said head end and' a; hardening of said-contiguoussidewall portions are obtained as compared with the respective hardness values of the head end and wall portions prior to such heat treating steps without substantially enlarging the grain structure from that of said cold worked steel.
6. In a method of heat treating an intermediate shape formed of 0.25 to 0.40% carbon steel that has been cold worked to form a closed thickened head and an integral thinner side wall of generally circular section open at its other end, the steps which comprise immersing first said 7 head end and then a contiguous portion of said side wall up from said head end into a fused salt bath maintained at a temperature between 1450 and 1650 F., augmenting the effective mass of the head end and controlling the depth of each immersion, the effective rate of heat transfer from said bath to said head end and to that part of said side wall lying above the point of final immersion, and the total'time of immersion of said head end and of said contiguous side wall portions so that said head end will be raised to a maximum temperature of not over 1275 F., while said side wall portions will be raised to a temperature at least equal to the upper critical point of the steel, removing said shape from said fused salt bath within about two minutes after its initial contact therewith, and immediately thereafter drastically quenching the heat treated portion of said shape to a temperature below about 200 F., whereby a reduction in the hardness of the steel in said head end and an increase in the hardness of the heat treated portion of said side wall are obtained as compared with the hardness values for those respective parts of the shape prior to such heat treatment.
7. In a method of heat treating an intermediate shape formed of 0.25 to 0.40% carbon steel that has been cold Worked to form a closed thickened head end and an integral thinner side Wall of generally circular section open at its other end, the steps which comprise immersing first said head end and then a contiguous portion of said side wall up from said head end into a fused salt bath maintained at a temperature between 1450 to 1650" F., controlling the depth of the immersion, controlling the effective metal mass of said head end and of that portion ofsaid side wall lying above the point of final immersion by supplementing the same'with additional metal in heat conducting contact therewith, and controlling the total time of immersion of saidhead end and of said side wall portion so that said head end will be raised to a maximum temperature of not over 1275 F., while said side wall portion will be raised to a temperature at least equal to the upper critical point of the steel, remov ing said shape from said fused salt bath within about two minutes after its initial contact therewith, and immediately thereafter drastically quenching the heat treated portion of said shape to a temperature below about 200 F., whereby a reduction in the hardness of the steel in said head end and an increase in the hardness of the heat treated portion are effected of said side wall as compared with the hardness values for those respective parts of the shape prior to such heat treatment.
8. In a method of heat treating a steel shape susceptible of being hardened by heating through the Ace point and quenching, the steps of immersing said shape in a fused salt bath having a temperature at least equal to the A03 point, prior to such immersion augmenting the mass of a first portion of said shape by placing said portion in heat conducting relation with an additional mass of metal while leaving a contiguous portion of said shape unaugmented in mass, and controlling the amount of said additional mass and the time of immersion of said first portion and said contiguous portion in said salt bath to prevent the temperature of said first portion from reaching its Ac: point while bringing said contiguous portion to its AC3 point, and quenching said shape, whereby a relative softening of said first portion while at the same time effecting a hardening of said contiguous portion is efiected.
9, In a method of heat treating a cold work-hardened 7 12 cartridge case of 0.25 to 0.40% carbon steel to provide a thickened head end of lower degree of hardness than the contiguous thin side wall portion, the steps of immersing said case, head end first, in a fused salt bath main-. tained at a temperature of from 1450 to 1650 F., prior to such immersion augmenting the mass of said head end while leaving the contiguous side wall portion unaugmented in mass, incrementally advancing said case into said salt bath, holding said head end in said bath for a longer total time than said contiguous side wall portion but selecting the amount by which the mass of said head end is augmented so that said head end never reaches a temperature of over 1275 F. while said contiguous side wall portion reaches the temperature of said bath, removing said case from said bath at the end of said holding period, and quenching said case, whereby a relative softening of said head portion and a relative hardening of said side wall portion are simultaneously effected.
References Cited in the file of this patent UNITED STATES PATENTS 18,327 Darling Oct. 6, 1857 1,296,842 Oflut et al. Mar. 11, 1919 1,333,767 Napier Mar. 16, 1920 1,526,894 Bellis Mar. 17, 1925