AD12 6 8 190
ARMED SERVICES TECHNICAL INFOWTION AGENCYALNfN HALL STATOWARLINGN 12, VIRGINIA
NOTICE: Mken goveinnt or other dravinps, speci-fications or other data are used for amy purposeother than in connection with a definitely relatedoverment procument operation, the U. S.
Oovwermnent thereby incurs no responsibility, nor anyobligation whatsoever; and the fact that the Govern-ment my have fozuated, furnished, or in any waysupplied the said drawings, specifications, or otherdata is not to be egarded by implication or other-vise as in any mnner licensing the holder or anyother person or corporation, or conveying any rlhtsor peruission to ,factre use or sell anypatented invention that my in any way be relatedthereto.
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WATERTOWN ARSENAL LABORATORIES
MECHANICAL AND METALLURGICAL PROPERTIESOF CARBURIZED 8620H STEELFOR MI4 RIFLE COMPONENTS
TECHNICAL REPORT NO. WAL TR 739.1/3
JOSEPH L. SLINEY
DATE OF ISSUE - NOVEMBER Il
Code qOO. 21.Oos. 2. n7.6b NI RIFLE
WATERTOWN ARSENALVATEITOWN 72, MASS.
Sn1l arm, rifle 1114
Ibterials syaluat ion,oarbur ized
MKWSCAL AND 1'LUGCAL POifUOf CARDUTRIZ 8680K IUFOR 314 RFU 0 0 WK
Tobuloal Report No. AL, TR 759.1/3
* JXoseph LD. Sliney
Date of Isom. - November 1963.
OM Code 4010. 25.0005.2.87.6w0 M14 Rifle'
WAIIEW 7 to
WATERTOWN ARSENAL, LABORATORUS
MUCHANICAL AND METALWROICAL PROPETINSOF CARBUZiD 862O1 STEELFOR N114 RIFIZ CONPONENTS
Mmhanical and metallurgical teats have been conducted to aid in thedavelolmnt of improved processes and materials for NI1 rifle bolts andreceivers which are currently being manufactured from carburized 8620Hresulphurlsed steel. The investigation revealed the extreme brittlenesswhich can occur In carburized components, especially those having smallfillet radii. This brittleness is associated particularly vith bigh orehardness and nonmartntlc miorostructures vhich can occur In AIS 86M0steel over the allovable rage of composition. The stu d ontras theImports=* of close control over the composition and beat treatment of thismaterial and the advantages from a performance standpoint of employing asteel having higher hardenablilty and lover carbon content. The sin4y alsoprov1des an evaluation of several Important variables upon fatigue propertiesof 86M steel.
Metallurgical studies are being conducted to aid in the development ofimproved processes and materials for the M1 bolts and receivers vhich arecurrently being manufactured from carburized 8620H resulphurized steel.This study has been aimed at evaluating the mechanical and metallurgicalproperties of the 8620H remalphurized steel as related to Its use in theM31 rifle bolts with particular emphasis on the problem of cracking in thelocking lug. The data and conclusions contained here are also applicableto the receiver on the same weapon. The high surface hardness has beenspecified to provide wear resistance and minimize deformation under con-centrated loads at the locking lug surfaces.
During service testing, and also experimental proof testing, severalbolts failed by fracturing in the locking lug fillet1 after firing a rela-tively small number of rounds. Also, a large number of bolts which didnot fail during proof firing exhibited cracks in this critically stressedarea. In addition to the bolt problem several receivers manufactured froma maverick steel (SAN 1330) exhibited brittle fracture after a small numberof rounds were fired in the weapons.
Upon consideration of the types of failures which occurred, It vasdecided that both impact properties (or notch sensitivity) and fatigue wereInvolved. Consequently, the effects of heat treating variables and notchacuity on notched bar impact properties and rotating bean fatigue propertieswere assessed. Since carburised steel Is a complex structure Involving alarge umber of varlables, the study has been limited mainly to the variablesobserved in the bolts and receivers made in the current production of M14rifles.
MATERIAL AND PROCESO
All materials used in this study were obtained from Springfield Armoryeither in the form of 15/16-Inch. diameter bar stock or closed die bolt forg-igs (Figure 1). A chemical analysis was conducted on each lot of materialprocured from the Armory and these analyses re presented in Table I. Thematerials employed are about In the middle of the composition range for the86203 steel.
All carburised mechanical test specimens used In this study were machinedat Watertown Arsenal and heat treated at Springfield Armory. These specimenswere hast treated In groups which consisted of Charpy impact specimens.notched and unnotched tensile specimens and R. R. Moore fatigue spec i .,,swith soe groups consisting of Charpy impact or fatigue specimens separately.The standard carburising cycle as obtained from Springfield Armory consistedof:
Carburlse 1580F - 1-2/3 hours, Oil QuenchTemper 375? - 1 hour
This treatment vas used on the majority of the fatigue specimen groups todetermine the effect of notch configurations upon the fatigue properties ofthis carburized steel. Other heat treatments vere employed to determine theeffect of quenching severity and case depth.
R LTS AND DISCUICHS
Although the various types of specimens were grouped for heat treat-ment, the subsequent results will be presented according to the type oftest conducted. This will provide an opportunity to explain the factors in-fluencing toughness and fatigue In separate discussions.
The toughness of steel In the carbursed and heat-treated conditions,as well as after a sock carburising heat t~atment, vas measured by CharpyInpat tests and, to a limited extent, by notched tensil tests. Charpyinpact tests have been widely used and accepted a a masure of toughnessin homogeneous steel but only limited information is available on the in-pact properties of carburised steel.
Recent studies* by C. Wells have been made using nitrlded Charpy In.pact specimens to separate the crack initiation energy from the crack propa-gation energy. It was suggested by these results that the energy measuredin a carburised Charpy specimen is essentially crack propagation energy.In order to verify this concept, three series of Charpy impct specimens(Series A, B and C, heat treated as Indicated In Table II) vere tested inthe carburized condition, in the pre-cracked-carburised condition, andnotched- after-c arburizing conditior.
The pre-cracking v accomplished by standard fatigue procedures pre-sently being used at Watertown Arsenal Laboratories. In each test series,the energy absorbed from the pre-cracked specimens vas essentially the aeas the uncracked, carburised specimens. These data indicate that the energto Initiate the crack in the carburised Charpy inpact specimen is very lowand uneasurable.. The Charpy impact specimens tat vere notched after car.burnsing (thus eliminating the carburlsed notch) Indicated a general increasein energy absorbed. This energy Is associated vith crack initiation of theuncarburised notches. These data are presented in Table Il.
A variety of carburlsing and/or heat-treating cycles were employed onnine groups of Charpy Inpat specimens. Nalf of the speclams in each groupvere notched prior to heat treatment and the other half after beat treatment.The results of these tests along with eore hardness and tensile tests aresummarised in Table IV. ThO - c date pertinent to these specimensare presented in Table V. A comparlson of Charpy Inpat values obtained at-40? Indicated that eimens notched before carburising absorbed only 2.1to 6 ft-lb except for those susteitised n neutral salt (mock carburised)vhich absorbed 'approximately 20 ft-lb. The specimens notched after
carburizing absorbed somewhat more energy than those mentioned previously.For the carburized specimens (notched before carburizing), there vas verylittle variation in absorbed energy over a range of core hardnesses (Rock-vell C 32 to 38), carburizing cycles (1 hour to 2-1/2 hours), and as a resultof delayed quenching (about 45 percent free ferrite in the core). The useof a vater quench to obtain a high core hardness (Rockwell C 45) vas as-sociated with a significantly lover absorbed energy than that of the othertreatments when specimens were notched before or after carburizing. Therevas considerable variation between the energies absorbed by the duplicatespecimens in the room temperature tests because of the presence of lamina-tione which have a pronounced effect o the energy in these longitudinalspecimens. This variability also tends to obscure the effect of heat treat-ing variables in tests conducted at room temperature. However, the roomtemperature impact values of the water-quenched specimens (Rockwell C 45)vere very low In comparison with the specimens quenched In oil to a hardnessof less than Rockwell C 0.
It should be noted that almost all specimens which exhibited fibrousfractures posbessed laminations. These laminations are attributed, at leastin part, to the presence of sulphide inclusions in this resulphurised steel.yrpical fracture surfaces of Charpy Impact specimens are shown in Figure 2.
It should be emphasized that% if Charpy Impact specimens could be obtainedtransverse to the major vorking direction In this material, even lower Impactresistance would result because the fracture would be progpessLng parallel tothe laminations and not across them. The Charpy impact specimens, Flgr 2,do not display the ludnatons at -4O because the materiel deform so littleduring crack propagation at this test temperature. The specimen groups 4, 5an