UNCLASSIFIED

AD12 6 8 190

ARMED SERVICES TECHNICAL INFOWTION AGENCYALNfN HALL STATOWARLINGN 12, VIRGINIA

UNCLASSIFIED

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.

AD

U -- ()4

€ WATERTOWN ARSENAL LABORATORIES

MECHANICAL AND METALLURGICAL PROPERTIESOF CARBURIZED 8620H STEELFOR MI4 RIFLE COMPONENTS

TECHNICAL REPORT NO. WAL TR 739.1/3

BY

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

Bte1, 8620H

MKWSCAL AND 1'LUGCAL POifUOf CARDUTRIZ 8680K IUFOR 314 RFU 0 0 WK

Tobuloal Report No. AL, TR 759.1/3

By

* JXoseph LD. Sliney

Date of Isom. - November 1963.

OM Code 4010. 25.0005.2.87.6w0 M14 Rifle'

WAIIEW 7 to

WATERTOWN ARSENAL, LABORATORUS

TXTIZ

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.

t tors

ITRO DUCTION

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

-3-

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.

Tbudwese

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

*Private cominication

-4-

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, 5and 6 were mahined, from forged bolts (Figure 1), but their Impact resistanceand fracture appearance were essentially the sam as the specimens machinedfrom the bar stock at the same core hardness level.

The carburlsed later on the Charpy impact specimens Is very hard andbrittle, thus preventing the formation of the shear lips on the edge ofthe specimens (Fire 2).

Standard 0.357-inc.-d1iter tensile and notched tensile specimens wereheat treated with *he first nine groups of Impact specimens, and these dataare also presented In Table IV. fte percent elongation we m aured withpencil scribed marks instead of the conventional prick punch marks becauseof the brittleness and notch sensitivity associated with carburised surfaces.

An evaluation of the notched tensile strength data is obtained by usingthe notched/unnotched tensile ratio. This "notch-strenth ratio" is largerthan unity (approximately 1 5) 'for notch. insensitive materials by an amountwhich depends upon notch contour, and is unity or less for notch.sensitivematerials. The uncrburised specimens exhibited a notch.strength ratio of1.74 and the carburised specimns indicate ratios between 0.86 to 1.11 depend-ing upon core hardness leml. The ater-quenched specimens, having a corehardness level of Rockwell C 45, exhibited the lowest notch.strongth ratio.The theoretical' stress concentration factor (Kt) used on these specimens wasapprox1mtely three, a relatively mild stress concentration factor fordetermining this ratio.

-5-

Core hardness is a major factor affecting the toughness of carburizedparts, since it is related to microstructure in this material and alsobecause it affects energy level per so. Several groups of specimens wereheat treated to obtain high, intermediate, and low core hardness, Bothcarburised and uncarburized specimens were evaluated at the high and inter-mediate hardness, and only carburized specimens were evaluated at the lowesthardness. The core hardness was aried by changing the cooling rate from theaustenitizing temperature, whereas all other procedures, including austeni-tizing and tempering treatments, were held constant.

The impact transition curves of these groups of specimens are plottedin Figure 3, with the associated hardness and metallurgical characteristicspresented in Table IV. Although the transition temperature can be definedin a number of ways, it was decided to select for listing in Table II, thetemperatures at which 10 and 50 percent fibrous fractures occur. 2hesocriteria represent the miniuza temperature at which some energy Is absorbedand the condition at which cracking would not be expected to progress with-out an additional applied load, respectively. It can be observed that car-burization raises the transition temperature and lovers the energy level ata given core hardness.

At a core hardness of Rockwell C 45-47 (Table II), the energy of thecarburized steel is extremely low even at room temperature. At the lowerhardness levels (Rockwell C 31 and 40), the transition temperature is con-siderably lover. The lowest temperature at which some toughness is observ-able, at both these latter hardness levels, is -AF. It is expected thathighly stressed components, possessing sharp notches or cracks, would bevery susceptible to brittle fracture at temperatures at least as high as-4F in this material.

Another method of reducing the hardness of the core is by raising thetempering temperature. Unfortunately, this type of treatment also lowersthe case hardness. Groups of specimens in the carburized and uncarburisedcondition were vat water quenched (to obtain maximum as-quenched corehardness) and tempered at temperatures up to 1000F to assess these treat-ments on toughness., The results of impact tests at -40? are shown In TableVI. There is a significant drop In toughness In the tempering temperaturerange of 4W to TOOF. This condition, which Is observed in msat steelswhen martensite is tempered in this range, Is generally called "blue brittle-ness" and should be avoided in the heat treating of MW& rifle components.The core hardness readings indicate that a manufacturer quenching his co-ponents to martenslte (Rockwell C 45 to 47) might consider tempering In thisembrittling range to meet specified core hardness requirments. Consequently,a maximm teapering tmperature of 50F should be specified to prevent thisembrittlement since an impact test Is not currently required for controllingthe processing of these carburised components.

If a tempering temperature above 70?F is employed, the case hardnessis lowered to about 45-47 Rockwell C, which is considered to be too softfor the wear resistance required for the components.

-6-

These results indicate the desirability of investigating the use ofother steels having a maximum "as-quenched" core hardness of 43-44 Rockwell Cand sufficient hardenability to provide a martensitic structure. This Vouldpermit the components to be quenched to mrtensite having a hardness of leossthan Rockvell C 42 after tempering at 350 to 425F.

FatiEue

The second important factor to consider in the service of bolts andreceivers is their short cycle fatigue life (10,000 to 20,000 rounds).After some consideration, it vas decided to concentrate on rotating beautype (B. R. Moore) fatigue tests because of their vide acceptance and theavailability of testing machines in sufficient number at Watertovn Arsenalso that extensive testing could be accomplished quickly.

Both notched and unnotched rotating beau tests were conducted on spc i-mans In the carburied and uncarburized conditions. Specimen sizes andnotch configurations considered are presented in Figure 4. The notchedand unnotched specimens have the same net cross sectional area at mid-span,and the stress levels plotted in the subsequent figures (5 through 8) werecomputed by the simple bean formila. The curves presented are the best fitcurves for the data obtained.

The first series of specimens was tested at 10,000 rpm speed and thesedata are presented in Figure 5. Each curve has been numbered with itsappro~priate group moer In this and subsequent figures. The metallurgicaldata for all sets of fatigue specimens are listed in Table V with its apro-priate group number.

The effect of carburizing Is clearly evident in Figure 5. The carbur-id case has Improved the fatigue properties by a combination of residualcompressive stresses and a strengthening of the outside fibers vhere themximm bending' stress occurs. A group of specimens polished in the longL-tudinal direction before carburising is also presented in Figure 5 to dam-strate the coparison with a machined finish. The machnaing or tool marksare lose detrimental to fatigue life at the higher stress levels (flnitelife region), but this condition did produce a marked effect upon the en-durance limit. The slack-quenched group of specimens (Group 2) was boldIn air 30 seconds before quenching into oil, a procedure which produced45-50 percent free ferrite in the core. These soft ferrite areas appearedto have a marked effect in lowering the cycles to failure at the higb stresslevels.

lhs effect of a 45 degree notch with a notch radius of 0.010 and 0.030inch in both the carburized and uncarburized conditions, tested at a speedof 1800 rpm, is presented in Figure 6. The sharp carburized notch (0.010Inch radius) decreases the fatigue strength at a life of 10,000 cycles toa stress level equivalent to that of uncarburized notched fatigue specimens.Also presented In this figure is a curve obtained with smooth specimens whichhad 0.002 Inch removed from the surface by using number 80 grit paper,

-7-

polishing in the circumferential direction. The reduction in the fatigueproperties is very slight indicating that this amount of material removal,when done with care, is not harmful to the fatigue properties.

The effect of 90 degree notches having radii of 0.002, 0.010 and 0.030inches, is presented graphically in Figure 7. These data indicate thatthis variation in notch severity does not influence the finite fatiguelife significantly. It has been well established3 in the literature thatnotches do not exhibit their full effect in fatigue. The fatigue stressconcentration factor (Kf) is less than the theoretical stress concentrationfactor (Kt). But, as indicated in Figure 7, there is a reduction in strengthby the introduction of a notch in the finite portion and a larger reductionin the endurance limit. Therefore, the largest notch radius allowable shouldbe used.

The effects of various metallurgical conditions upon the fatiguecharacteristics of carburized 8620H, when tested at 1800 RPM, are presentedin Figure 8. The major detriment to fatigue was a slight decarburized layercaused by air cooling from the carburizing temperature followed by reausten-itizing in neutral salt (Group 7 specimens). Therefore, this practics- shouldbe avoided in the manufacture of the N14 rifle components. Long carburiz-ing time increased the fatigue resistance slightly by increasing the carbur-ized depth.

In simple bending, the maximum tensile and compressive stresses occurat the outside fibers. The carburized case not only increases the strengthof the case material, but also creates a state of residua compression as aresult of temperature gradients and phase transformations" during heattreatment. These residual compressive stresses shift the point of maximumtensile stress in bending from the surface to some point below the surface.Depending upop the strength of the case, the case depth, and the residualcompressive stresses present, failure will initiate at the point where thenet stresses are maximum or at the case-core junction where a discontinuityof both residudl stresses and microstructure occurs. The presence of anotch results in a stress concentration at the root which raises the stressto a level several times greater than the nominal stress on the outer

surface of a smooth specimen. Therefore, failure in a notched fatigue speci-men Initiates at the root surface of the notch.

Typical carburized fatigue specimen fractures are presented in Figures9 and 10 for the smooth and notched specimens, respectively. It should benoted in Figure 9 that the failure initiated at the case-core junction forthe smooth specimen and in Figure 10 the failure initiated evenly at theoutside layer for the notched specimen.

One Charpy impact specimen and one fatigue specimen from each groupwere examined for case depth, retained austenite, core structure, and corehardness. These data are presented in Table V. The smooth B. P. Moore

-8-

fatigue specimens generally had high core hardness and martensitic micro-structure, but the larger cross section, notched fatigue specimens (Fig. 4)and impact specimens which cooled slower during quenching have lover core hard-nesses and higher percentages of free ferrite. In an effort to overcomethe differeaice in core hardness and microstructure, one set of notchedfatigue specimens was warm water quenched. This quenching severity was toogreat, and the specimens were so distorted that they could not be tested.Therefore, a small part of the decrease in fatigue properties of the notchedspecimens is attributed to lower core hardness.

Typical photomicrographs are presented in Figures 3.1 and 12. Thedifferences in microstructure between the Charpy impact specimens and thefatigue specimens heat treated under the same conditions are due to sectionsize and transformation rates. Consequently, it is necessary to comparespecimens possessing the same hardness and microstructure rather than thesame nominal heat treatment.

Ru'fame Hardness C m=arson

Rockwell C, D, A and superficial scale 4&5N, 30N, 15N and 1g Tukonreadings were conducted on the Charpy impact specimens from Group 1 through9 and converted to Rockwell C readings for comparison. These data arepresented in Table VII. One specimen from each group was sectioned so thatthe Tukon hardness readings could be taken 0.002 inch below the surface.The averagesof these micro-hardness readings were converted to Rockwell C,and these data and core hardness data are also presented in Table VII.The major difference between the Tukon and the Rockwell C readings appearin Group 5 which was carburized only one hour. But this difference cannot be associated entirely with case depth since large differencqs of 7.6and 8.2 points Rockwell C were also observed in Groups 1 and 9, respectively.The maximum differences between the Rockwell D and the Tukon hardness read-ings were obtained in Groups 5 and 7 which are 3.1 and 3.7 points Rockwell C,respectively. Group 7 specimens had a slight decarburized loqer due to theair cooling from the carburizing temperature plus austenitlzing in neutralsalt, while Group 5 had a shallow case. The general agreement among thereadings using all the hardness scales on the uncarburized specimens (Group 3)should be noted.

All groups of cirburized speclmns exhibited an Increase in hardnessin going from the heavy 150Kg Rockwell C to the 15Kg-15N loads. It wasindicated by this investigation, as well as others that reliable readingsof surface hardness of thin cases can only be obtained by using "superficial'hardness tests. Variables present such as case depth, decarburization, and corehardness influence the case hardness readings, but the results of the Rock-well C test alone do not provide a reliable means for identifying the causeof the low reading.

-9-

CONCLUSIONS AND RECONMENDATIONS

1. Impact tests of M14 rifle bolt material (AISI 8620H steel) de-monstrate that the current requirements limiting tempering temperature toa maximum of 425F and core hardness to 35-42 Rockwell C are extremely im-portent in order to provide satisfactory toughness In these carburized cowponents. The upper limit on hardness as well as the tempering temperaturelimit will reject components having extremely low toughness at room temper-ature. The lover limit on hardness is required to limit the presence ofnonmartensitic microstructures which raise the transition temperature forbrittle fracture and also to control fatigue which is adversely affectedby low core hardness (low strength).

2. AISI 8620H steel has a borderline hardenability for the sectionsize involved in the bolts and receivers of the M14 rifle. The steel alsohas a carbon content range (0.18-0.23 percent) such that an excessivelyhigh core hardness can occur during heat treatment when the carbon is onthe high side of the acceptable carbon range. Consequently, an alternatealloy steel having higher hardenability and lower maxlmm carbon contentshould be established to permit more latitude in heat treatment and. betteruniformity in mechanical properties of the components (the large changesin section size of the receiver result in excessively high hardnesses inthe rail section and low hardnesses in the muzzle end of the same component).An investigation will be required on the change in machinability and distortionwhich may result from the use of alternate steels.

3. Fatigue tests using rotating beam specimens provided quantitativeinformation on the effect of the most Important variables introduced inthe processing of MI14 rifle bolts and receivers made from AISI 8620H steel7he results support and expand previous work on fatigue with regard to thedetrimental effects of sharp notches, decarburized surfaces, quality ofmachined finish, low core hardness, and nonuniform microstructures (patchyferrite In the ,core).

4. A non-resulphurized material should be employed for bolts and re-ceivers because segregations of nonmetallic inclusions of the type observedIn the current steel cause laminations which can result in catastrophicfailure in critically stressed areas.

-10-

TABLE I

CHEMICAL ANALvSIS OF THREE LOTSOF 8620H STEEL BAR STOCK

dI

Description C Nm Si Ni No P S Cr

AISI 86211 0.18 0.70 0.20 0.40 0.15 ..... 0.035 0.40

0.23 0.90 0.35 0.70 0.60 0.050 0.60

Lot 1 0.20 0.83 0.31 0.50 0.22 0.014 0.045 0.52

Lot 2 0.20 0.83 0.32 0.52 0.18 0.014 0.046 0.54

Lot 3 0.21 0.83 0.30 0.51 0.20 0.015 0.041 0.52

-II-

Q-v

0

C! !

go o a°

E. 441

N 1 0 0 0.

a. -4 p

M . .A, A.Al.A..

.. , 4 g, J ,., ,

.- 1-

7A 1 n m il

CAo0 ~ .AE4 A r ;2

ad co u

TABLE III

CHARPY IMPACT DATA ONCARBURIZED 8620H STEEL

ENERGY ABSMED (Fr-I,)Pre- Notched

Temp.ature Cracked af ter('F)' Carburi med Carburlsd Carburized

series A

+32 4.0 4.0 9.23.7 8.6

-4 4.2-40 4.2

Series B

+32 7.5 6.2 15.610.3 16.5

5.0-40 3.7

Series C

32 15.6 14.2 515.6

-4 12.1-40 6.7

- 13-

ma 4cl 40. V M~* (

0,,

V)woo mov ol Cco Me)

.-466L ;41 C4 00 -4C ci

Mel ex- M *#l R

QM V0 .44 M 0 814MU

-'O W~a4 60M4 9 01%

ID Atk s LAW) @41- m..) C4 -win Vm CM)

V) ..4 ..Ir C. -o

cia -c C4C C4

0 4010

-4

Ie fl aN 84

09e' en w

044 v e

TABLE V

METALLURGICAL PROPERTIES OF 8620H MECHANICAL TEST SPECIMENS

OjHigh Low CrbonCase Free Temperature Tempered Retained Core

Specimen Heat Depth ainito Martensit* Ausonite HardnessGroup Treatment (inches_ (_ (= h (%) (%I Rockwell

CHARPY INdPACT SPECINUIqS_____ _____

I Carburize 1-2/3 0.011 6 84 9 ... 32.6hr. oil QuenchTemper

2 Carburize 1-2/3 0.013 6 85 7 ... 35.6hr. Hold 30 se.in air, Oil QuenchTemper

3 Austenitise 8 90 2 32.7(Neutral Salt)hr. Oil QuenchTemper

4 Carburise 1-2/3 hr. 0.013 3 70 27 36.0Oil Quench Temper

5 Carburize I hr. 0.005 15 77 8 34.5Oil Quench Temper

i Carburise 2j hroe 0.016 5 60 35 ... 36.8Oil Quench -Temper

7 Carburise 1-2/3 hr. 0.011 10 50 40 38.6Air cool Austen-itive 1550 F X hr.Oil Quemch Temper

I Carburise 1-2/3 hr. 0.012 5 16 77 45 4arm Water Qench.

Tomper

9 Carburise 1-2/3 hr. 0.011 6 70 24 .. 33.7Oil QMunh. NoTemper

fR. R. MOORE FATIGUE SPECIMENS

I Carburise 1-2/3 hr.Oil Quench Temper 0.015 0 10 62 2 43.0

2 Carburize 1-2/3 hr. 0.013 45 -- 55 5 3..7Hld 30 sec. in air.Oil Quench Temper

3 Automitive . 1 99 - 44.5(Neutral Salt)hr. Oil QaohTemper

4 Carburls 1-2/3 hr, 0.018 2 26 70 35 41.1Oil Quench Temper

S Carburise 1 hr. 0.014 2 76 20 5 4Oil Quench Temper

6 Carburlse 7A hr. 0.021 0 1 82 20 45.2O11 Quech Temper

7 Carburls 1-2/3 hr. 0.017 1 3 96 1 44.6Air cool Austen.itise 1550 F X hrOil Quench Teper .

(Continued)- 15--

TABLE V (Continued)

METALLURGICAL PROPERTIES OF 8620H MECHANICAL TEST SPICIMENS

HighTempera- Low CarbonSpeci- Case Free ture Tempered Retained Coremen Heat Depth Ferrite Bainite Martensite Austenite HardnessGroup Treatment (inches) ( (%) () Rockwell C

R. R. MOORE FATIGUE SPECIMENS (Continued)

8 Carburize 1-2/3 hr. 0.018 0 1 99 10 46.4Warm Water Quench.Temper

9 Carburize 1-2/3 hr, 0.019 2 92 6 0 44.4Oil Quench. NoTemper

Polished1800RPM 10 Carburize 1-2/3 hr. 0.018 1 15 84 1 43.6Test Speed Oil Quench TemperPolished 11 Carburize 1- 2/3 hr. 0.014 3 20 77 2 43.110.000RPM Oil Quench Temper450 Notch 12 Austenitize ----- 2 18 s0 41.70.0100 Rod. (Neutral Salt)Uncarburize hr oil QuenchMeper450 Notch 13 Austenitize 12 12 76 -- 42.80.0309 Rod. (Neutral Salt)Uncarburize hr. Oil Quench#e..per

45 ° Notch 14 Carburize 1-2/3 hr. 0.014 15 50 35 1 36.50.0160 Rod. Oil Quench TemperCarburize450 Notch 15 Carburize 1-2/3 hr, 0.011 15 40 45 0 36.30.030" POd. Oil Quench TemperCOrburize

90 Notch 16 Carburize 1-2/3 hr. 0.012 18 15 67 1 36.70.030' Rd. Oil Quench Temper900 Notch 17 Carburize 1-2/3 hr. 0.017 15 60 25 2 32.50.010' Rod. Oil Quench Temper450 Notch 18 Carburize 1-2/3 hr. 0.015 a 67 25 5 33.90.002" Rod. Oil Quench Temper900 Notch 19 Carburize 1-2/3 hr. 0.015 8 70 25 5 31.00.002' Rod. Oil Quench TemperPolished 20 Carburize 1-2/3 hr, 0.014 2 13 5 3 44.10.002" Oil Quench TemperReoyed anRod.

Notes: 1. All specimens were carburised or austenitized at 1560 F.2. All tempering was performed at 375 F for one hour.

3. Percent retained austenite was not determined on Charpy impact specimens in Groups I to 9.4. Case depth was determined by Tukon hardness surveys on Groups I to 9.

-16-

TABLE VI

TEMPERING CHARACTERISTICS

OF 8620H STEEL

Uncarburised CarburLxodTemerg E Fracture Care Boom Fracturo HardnessT(ore For~r Hardness d kw o11ll C

(ft-lb) Fibrous lockwell C (ft-lb) Fibrous Core Came

As-quench 17.5 20 47.0 2.5 0 46.6 64.0200 15.1 15 47.4 2.5 0 45.6 64.4300 18.1 25 47.0 3.4 0 45.6 61.7400 18.3 35 45.0 3.4 0 45.9 5.8500 15.5 t0 43. 1.O 0 4t.6 5.6600 9.5 10 42.0 1.8 0 41.0 53.4700 15.3 15 40.7 2.3 0 41.3 50.6S0 31.4 75 38.2 9.7 0 33.8 46.7900 47.8 100 33.8 30.2 100 34.6 43.91000 67.9 100 30.4 37.7 100 30.6 33.7

Notes: 1. All specimens quenched from 1580 into warm water: 157h. 12 11TH. 3FrF carburled;LOwW. uncarburized.

2. All specimens tooted at -40 F

TABLE VII

HARDNESS SURVEYS ON 8620H

CARBURIZED CHARPY IMPACT SPECIMENS

Coro300CeaMe Heat ? )D Ak 4W 30- 15N-CC TaleG

Group Treatment 1LAG IN low I dud 0f- 159GR Dike.G

Carburive 1530 F 32.6 51.7 56.5 61.5 62.3 62.7 61.9 59.31-2/3 hr. O11

07fihr2 Crburlwe 1530 F 35.6 53.3 56.8 61.2 62.3 61.7 32.4 57.3

1-2/3 hr. boldIn air 30 soconds

OilF hrnc

3 AstitLse X hr 32.7 35.7 3B.6 39.3 36.8 37.1 37.0 34.0O11 QuemokTemper 375 F I hr

4 Carburlme 1580 F 36.0 53.9 56.2 61.4 61.0 62.1 62.8 53.01-2/3 hr. 011aenahTemper

5 Crburlse 1530 F 34.5 45.5 51.2 56.3 56.7 59.5 60.5 54.31 hr. O11 Quench.Temper 375 Fl hr

6 Carburlse 1500 F 36.8 57.3 60.1 61.2 60.7 60.7 62.1 59.32-%~ hr, 011Qwunb. Tempr375 F I hr"

(Continued)

-17-

TABLE VII (Continued)

HARDNESS SURVEYS ON 8620H

CARBURIZED CHARPY IMPACT SPECIMENS

CaseCore

Grop Tatmnt II& %5~ a8 5i 4 _ _

7 Carburise 1560 F 36.6 55.6 57.3 57.9 55.7 57.0 55.3 54.01-2/$ hrAir 1oAnstentivu 1550 F

It r Oi QanchTemper 3-75 F I hr

S Carburise 1680 F 45.4 55.2 58.1 60.6 61.2 61.3 62.0 56.01-2/3 hrIao Water amashTemper 375 Flhbr

* Carburiso 1560 F 33.7 55.3 62.6 65.8 66.0 55.2 66.3 63.51-2/3 hrOil QiaRSONo Temper

Notes: 1. Tho above hardness surveys are all Rockwell C readings converted from the indlostedscale.

2. The Tukon readings are an average of two readings tab=e 0.002 1ash below surfeaoon opposite fames.

3. Eamh roading (eoepL Tuken) r epreests as average of surveys (4 or more readlng)tabs. on two or nor, samples ron each group.

-18-

MACROPUOTOGRAPH OF Nll

CLOSED DIE BOLT FORGING

-19- FIGURE I

TEST TEMPERATURE

-40 F+76 F

CARSURIZED 1580 FI HR 40OMINOIL QUENCHED

TEMPERED 375 F I NI

6.2 ft-lb 25.4 ft-lb

CARERIZED 1580 FIlNl 40NIN

NELD 11 AIR 80 SECOIL QUENED

TEMPERED 875 F I II

3.0 ft-lb 6.2 ft-lb

AUSTENITIZED 16O FIs MNNOI1L QUENCHED

TEMPERED 275 F I HR

8.8 ft-lb 31.8 ft-lb

COMPARISON OF CARBURIZED AND UNCARBURIZED8620H STEEL CHARPY IMPACT SPECIMENS

-20- F IGURE 2

% FIBROSS FRACIURE

0 00

1-7-7

-SS

%%m~ FRACTUR SMlV

.!3 32 23!0

gel0 -44

It

$I- IIM A

'itFIGURE 3

SPECIMEN A

R.s*.002SPECIMEN 8 ft .010R. u.030

NOTCHDEPTH w .079

SPECIMEN C &4*022O

ROTATING BEAN FATIGUE SPECIMENS

-22- FIGURE 4

1F!

~~I T-1~'

WUIOER~~~~~ L4SRS EERAS(YLS

ROAIGSA AIUPROPETIESOF 820N tEL

Testing Spee1 100 P

-2- IUR

upop

... .. ...NUBE .7 STES .E L (CYCLES)..

ROAIGSE4FTWPROPRTIE OF 620HSTEE

Tetn S.e 180 .RPM...

-4-IUR

:21

Testig sped 80 Ri

NOTATINFGUR 7ENFAIU

4IIci i i i ---.....

--- ~~~~~ - -..... . . . . . . . . . .. . . . . .

........ ...I

. . . . . . . . . . .

-26 FYUR a- rF- 1

CU.IIo')

ej

0

LA. (J

C

.U-

-27- FIGURE 9

Y.,~ -r.

09

L.

ow

-28- FGURE 1

FATIGUE SPECIMEN

50$ Free Ferrite5%LwCarbon Tempered Mrtens It.Rockwell C 31.7

CHARPY IMPACT SPECIMEN10% Free Ferrite251 Low Carbon Tempered Martens gte2kWh~ Temperature Bainite

CORE STRUCTURES OF DELAYED OIL QUENCHED SPECIMENS(PIcral - NCI Etch - 1Mag. XIOOO)

-29- FIGURE 11

FATIGUE SPEC lIEN100% Low Carbon Tewered MortensitsRockwell C rn.e

CAIPY IMPACT SPEC lIEN15% Free Ferrite

aIo Carbon Tempered Ihrtensito79 Nigh Toeportvre linitelockwo I C S3k 6

CORE STRUCTURES OF OIL QUENCHED SPECIMENS(Picral - HCI Etch - Nag. XIOOO)

-30- FIGURE 12

REFERENCES

1. INGRAHAM, J. M., REED, E. L., RODGERS, E. H., and GALLIVAN, J. P.,Preliminary Materials Study on 7.62mm N14 Rifle Components (Receiversand Bolts), Watertown Arsenal Laboratories, WAL R 739.1/1, September

2. SACKS, G., Survey of Low-Alloy Aircraft Steels Heat Treated to High-Strength Levels - Part 5, Syracuse University, WADC Technical Report53-254., January 1954.

3. HYLAND, F. B., On the Determination of the Fatigue Factor, Kf,Utilizing a "Critical Volume" Concept and the Elastic Stress Solution,Watertown Arsenal Laboratories, WAL TR 893/201, June 1958.

4. BIDWELL, J. B., et al, Fatigue and Durability of Carburized Steel,American Society for Metals, 1957.

-31-

WATERTOWN ARSENAL

WATERTOWN 72, MASS.

TECHNICAL REPORT DISTRIBUTION

Report No.: WAL TR 739.1/3 Title: Mechanical and MetallurgicalNovember 1961 Properties of Carburized 8620H

Steel for M14 Rifle Components

Distribution List approved by 1st Indorsement from Ordnance Weapons Coimand,ORDOW-IM, dated 15 September 1959.

No. ofCopies TO

Commander, Armed Services Technical Information Agency,Arlington Hall Station, Arlington 12, Virginia

10 ATTN: TIPDR

1 U. S. Army Research Office (Durham), Box CM, Duke Station, Durham,North Carolina

Chief of Ordnance, Department of the Army, Washington 25, D. C.1 ATTN: 0RDIX2 ORDIR, Weapons & Fire Control Branch2 ORDTB, Research & Materials

Cobminding General, Aberdeen Proving Ground, Maryland2 ATTN: 0DBG

2 Coinnding General, Army Ballistic Missile Agency,Redstone Arsenal, Alabama

2 Comanding General, Ordnance Tank-Automotive Command,1501 Beard Street, Detroit 9, Michigan

Commanding General, Ordnance Weapons Command,Rook Island, Illinois

1 ATTN: ORDON-IX, Industrial Division1 ORDOW-IM, Ind. Mob. Branch1 CSDOW-TX, Research Division2 ORDOff-GU, Security Office

2 Commanding General, U.S. Army Ordnance Missile Command,Redstone Arsenal, Alabama

2 Co.'anding General, U.S. Army Ordnance Special WeaponsAmmunition Command, Dover, New Jersey

Commanding General, U.S. Army Rocket and Guided Missile Agency,Redstone Arsenal, Alabama

2 ATTN: ORDXR-OTL, Technical Library

2 Commanding Officer, Detroit Arsenal, Center Line, Michigan

2 Commanding Officer, Frankford Arsenal, Philadelphia 37, Pa.

No. ofCopies TO

2 Commanding Officer, Ordnance Ammunition Command, Joliet, Illinois

Commanding Officer, Ordnance Materials Research Office,Watertown Arsenal, Watertown 72, Mass.

1 ATTN: RPD

2 Commanding Officer, Picatinny Arsenal, Dover, New Jersey

Commanding Officer, Rock Island Arsenal, Rock Island, Illinois1 ATTN: 9320, Research & Development Division1 5100, Industrial Engineering Division

Commanding Officer, Springfield Armory, Springfield 1, Mass.1 ATTN: ORDBD-TX, Research & Development Division

1 ORDBD-EG, Engineering Division

2 Commanding Officer, Watervliet Arsenal, Watervliet, New York

1 Chief, Bureau of Naval Weapons, Department of the Navy,Washington 25, D. C.

1 Chief, Bureau of Ships, Department of the Navy,Washington 25, D. C.

1 Chief, Office of Naval Research, Department of the Navy,Washington 25, D. C.

1 Director, Naval Research Laboratory, Anacostia Station,Washington 25, D. C.

1 Superintendent, Naval Weapons Plant, Department of the Navy,Washington 25, D. C.

Commanding General, Wright Air Development Division,Wright-Patterson Air Force Base, Ohio

2 ATTN: WWRCEE

2 Defense Metals Information Center, Battelle Memorial Institute,Columbus 1, Ohio

Commanding Officer, Watertown Arsenal, Watertown 72, Mass.5 ATTN: Tech. Info. Sec.1 Author1 WAL Coordinator, IPM

64 --- TOTAL COPIES DISTRIBUTED

* *~4) .4"4

S fP 0 4. "44?6 "4 t.0

0. -a:d 1. 4, WW 400

~~~"4 4.' 1.4 4) 4 U *a6 -* Iwo64

A 1.- 04) 0,364j. A a 0 .l 0- a0 4M 4 Aa* b 4 -0 41 a ai ,J 'A V *0 .4. 441.46..6 im d -0 0 414 AU 6& 0. h4 of Iw

u 1. *a 0 o . A . 6.5 .6 we mU 60&0 o -W4 so . A .9 %a* .0 U 0 .4 0. . * 4A .4 3 s#. 'D. 1B 0 .

a, I~ so -0.4".4.4 *& 3B .1o. Owl A 00

0 .6 d W%4 0%0 A. or 6of .466 nO60 0A 4161 C. 00 a" .406 966 0 'S O41 *a 4.V

on u4 w 01. 6a4 4q .14 o~ ,0 w 6 #! a atca n 0U"4"466.W"d 6.a * 4~" : 1. 0.a" IIu4 * As 'a , .6a

Ol 16 *0 b. 0U.a .'4~ 0-4 M . 0 . 6 6.~

0.0 woA S.- "4a 65 * - 6~ o0 4~ lb6 6.d 0 v4" s 44144~ ~ ~ ~~1 -4 A-w .0~9.6661U

1.3 1. 4 ug 004 1 ,e i 's *g4. 0 t* 6..* L, .4g a~4.4 .404.641MIgs~~~- 44 f*64 S : V ~

Si~0 .r 40.6loo w.4h4ja60 so I I6 I44 4 4 -A46 0 0. a6 "4

a 00 O ."4 44 00 0.4 go .

he .4US .$A -0 1."4 06 .c b* 241 0 . J-: u 44 4 so 4~ M 6 103 .-066436 4"4 101 aJ .44 9029. g .44. "4 "4

*se .4 *A6G "4s. A.44U * 'I 6. . 6 6 .

.I- "46 .0 .0 1a R66E0.2!

0 6

ad* p 64 .11f *0 4 n - " 1- 1'

14. 1 I a a 40 0 a 4 0 04OA 046 6... 40A "4

* . Ow .46 410*" .40 -4= 4C.) u r-6 "4

:: *e * I a4~ 41.0 41 e 6aa0 "

-a a 1 .0 0 .4 0.0 e4Da .6od b .1 4

00 J 4 ON .4 A bA ,. 2 4 0040 qu.00-4a "4 d a-41.

* ma a6. 0 A400 0 0 "4 .6.1 0 1 "4 us64 [A 01 "

.0* 1 *" @" SUM 4 1. 0 oo @. AS .4 166

04) a 1 .63.*3 a0 0. %4"4 :0 u : .e A -Aal

d. a X a -

-"A 33, - j.4 .36 uo ft

Id6~ 0 0 4 5 1 i .41 a .I ;

a - 44

s-iN 441.404 0 0_ 26

A4 5 4 S. I

M: S036 Z L. .g"4I0. a V A6 U044.44 .4: ~

0.4: 60.0 v . dg

41, Io I.14. 6M0 aU,..,: 684.a -0 0 Ott)1I)qq 0 V 6.4.6.4644

ft 0001 -01 A"AU 4 046806 10lo

.0 44 to 4a -! 06.

"4~I. a. Is .4..6 . : s.46a W x1 .4 14 "4.6. 4 . its6* So

* ~ i 6-W*1f 4 9,4 9.64 0. "4 is;" .1.01.UU.~ U4.

* ~ ~ ~ 1 1 So .

* 64.6 6065060

4 -,a a

U0.6 60 4)00* .4n 6. .4.1w60 0 4 16 * 6" to6 V"I .44% (Mu464

o .04 40 *a *u An J 1 ,

o4*E a .4 66 000. 0 00 0 . 4