UNCLASSIFIED

AD 273705

ARMED SERVICES TEHNICAL INFORMATION AGENCYARLNGTON HALL STATIONARLINGIK 12, VIRGINIA

UNCLASSIFIED

BestAvai~lable

Copy

NOTICE: When goverment or other dravings, speci-fications or other data are used for any purposeother than in connection with a definitely relatedgovernment procurement operation, the U. S.Government thereby incurs no responsibility, nor anyobligation vhatsoever; and the fact that the Govern-ment may have fornflated, furnished, or in any waysupplied the said drawings, specifications, or otherdata is not to be regarded by implication or other-vise as in any manner licensing the holder or anyother person or corporation, or conveying any rigbtsor permission to mnfacture, use or sell anypatented invention that my in any vay be relatedthereto.

FILING SUBJECTS: m

C1. Steel, Physical and Mechanical Properties

2. Fracture Toughness3. Rocket Motor Cases

X

z

METALLURGICAL ASPECTS OF FRACTURE AT HIGH STRENGTH LEVEL

Interim Technical Report 0

1V

p

Prepared by

Walter A. Backofen

and

Merrill L. EbnerC Massachusetts Institute of Technology

March 1962

Frank LarsonTechnical Advisor '.

Watertown Arsenal LaboratoriesWatertown, Massachusetts

Contract No. DA-19-020-ORD-5235Ordnance Management Structural Code 5010.11.843Department of the Army Project No. )B93-32-004

FILING SUBJECTS:

I. Steel, Physical and Mechanical Properties2. Fracture Toughness3. Rocket Motor Cases m

METALLURGICAL ASPECTS OF FRACTURE AT HIGH STRENGTH LEVEL

Interim Technical Report o

Prepared by

Welter A. Backofen

and

Merrill L. EbnerMassachusetts Institute of Technology

March 1962

Frank LarsonTechnical Advisor

Watertown Arsenal LaboratoriesWatertown, Massachusetts

Contract No. DA-19-020-ORD-5235Ordnance Management Structural Code 5010o11.843Department of the Army Project No. 5893-32-004

ABSTRACT

Part 1: Fracture Toughness of Hardened and TemperedAISI 4340 Sheet

The fracture toughness, Kc, of quenched and tempered sheet specimens ofair-melted and cross-rolled AISI 4340 was measured by procedures basically inaccordance with those recommended by the special ASTH Comittee on FractureTesting. Fracture toughness, KQ was obtained from the elastic analysis ofIrwin, with and without the plastic-zone correction. and by the elastic-plastic analysis of McClintock. The two values calculated from the Irwinanalysis are essentially the same for specimens tempered below 4000F, butwithout correction values are 15% lower for specimens temppred at 7000F. Kcvalues calculated according to McClintock agree within 20 with the correctedIrwin values for specimens tempered between 500°F and 8000F.

Comparison of the K€ values with those of Raw Indicate that Kc forAISI 430 is very sensitive to processing history. Extensive spl itting, ordelamination, along the plane of the sheet wis observed in all fractures.It Is suggested that this delamination, which is an event related to process-Ing history, might contribute to low plane-strain fracture toughness and hightoughness-transition temperature.

Part 2: Effect of Austenitizing Temperature onFracture Node of AISI 4340

Face-notched specimens of AISI 4340 were austenitized at 1550OF or 230007,quenched, refrigerated, tempered and broken in impact bending. The characterof the fracture surfaces was observed in relationship to austenitizing temper-aturep tempering temperature, and testing temperature. When fracture did notoccur along prior austenite grain boundaries, the surface appeared rougher(presumably associated with greeter fracture toughness) for the larger grainedmaterial austenitized at 23000F regardless of the tempering or testingtemperature. Preliminary fracture-toughness measurements agree with thesefractographic observations.

TABLE OF CONTENTS

ABSTRACT-------------------------------------------------- --------------- I

LIST OF ILLUSTRATIONS---------------------------------------------------LI

LIST OF TABLES---------------------------------------------------------- vi

PART 1: FRACTURE TOUGHNESS OF HARDENED AND TEMPERED AISI 4340 SHEET ---------

I. INTRODUCTION-----------------------------------------------1I

II. EXPERIMENTAL PROCEDURE------------------------------ ------ 4

III. RESULTS --------------------------------------------------- 12

IV. DISCUSSION ------------------------------------------------ 16

V. SUMM4ARY --------------------------------------------------- 26

VI. REFERENCES ------------------------------------------------ 27

PART 2: EFFECT OF AUSTENITIZING TEMPERATURE ON FRACTURE MODE OF AISI 4340 28

I1. INTRODUCTION ---------------------------------------------- 28

I:. EXPERIMENTAL PROCEDURE------------------------------------- 29

Ill. RESULTS --------------------------------------------------- 31

IV. DISCUSSION ------------------------------------------------ 50

V. SUMMARY --------------------------------------------------- 51

VI. REFERENCES ------------------------------------------------ 52

ACKNOWLEDGEMENTS --------------------------------------------------------- 52

LIST OF ILLUSTRATIONS

Figure paes

I-I Test Specimens.---------------------------------------------- 3

1-2 Equipment for Notch Sharpening by Vibrolapping. Pressure onBlade Applied by Weight (not shown) Hung on Neck of Vibrotool --- 5

1-3 Section through a Typical Notch Produced by Vibrolapping.Comparison Wire in Lower Left has 0.0011" Radius, IOOX.------------ 7

1-4 Typical Fractures of Sharply Notched Specimens IllustratingTriangular Areas of Slow Crack Growth. In Specimens Such asThose Tempered at 400OF and 550OF the Rapid Crack Passed fromLeft to Right and Hence the Size of the Slow Crack on the Rightis Measured.------------------------------------------------ 8

1-5 Dimensions Used in Defining Notch Symmetry.---------------------- 9

1-6 Effect of Tempering on Tensile Properties and Hardness ofAISI 4340.------------------------------------------------------- 13

I-7 Effect of Tempering Temperature on Ductility and StrainHardening of AISI 4340 Steel .------------------------------------ 14

1-8 Effect of Tempering Temperature on Net Fracture Stress ofAIS! 4340 Steel .------------------------------------------ ------- 15

1-9 Effect of Tempering Temperature on Fracture Toughness andPercent Shear. Curve Marked "Irwin" is Based on Eqution (3).Curve Marked "Irwin Uncorrected" is Based on Equation (2) .------- 17

1-10 Effect of iempering Temperature on Fracture Toughness asDetermined by the McClintock and Irwin Methods.----------------- 18

I-Il Specimen Tempered at 600°F Loaded to 98 Percent Estimated MaximumLoad, Showing Initiation of Stable Crack Growth Beneath NotchRoot: (a) I Percent Nital Etch, 75X, (b) Unetched, 250X,(c) I Percent Nital Etch, 500X.----------------------------------- 19

1-12 Stable Cracks at Notch Root in Specimen Tempered at 1200°F andLoaded to 95% of Estimated Maximum Load, I Percent Nital Etch:(a) Gross Tearing and Hairli ne Cracks, I OOX, (b) IrregularCrack Growth, O00OX. ----------------------------------------- 20

1-13 Sam Specimen as in Fig. 1-12 showing Onset of Oblique Shear atNotch Root, Tempered at 1200°F: (a) Section Through StableCrack, Showing Shear Bands Growing from Edge of Central Crack,unetched, approx. 30X, (b) Section near the tip of the StableCrack, Showing Pores Linking up Along Oblique Shear Plane,unetched, approx. 35X.------------------------------------------ 21

iii

Figure

1-14 Sam Specimen as Fig. 1-12 Showing Void Formation Along ObliqueShear Plane: (a) Heavily Strained Region Between Voids, SOOX,(b) Agglomeration of Voids,. Heavily Strained Region at VoidEdges, 500X.-------------------------------------------------- 22

1-15 Fracture Surface of Specimen Tempered at 2000F, I Percent l"italEtch: (a) Flat Fracture with Delaminations Along FiberingDirection, 60X, (b) Jagged Secondary Cracks Along FiberingDirection, 500X, (c) Unetched, Showing Jagged, DiscontinuousCrack Growth from Base of Delamination, 250Xo (d) Same as c,,250X* ------------------ ------------------------------------- 23

1-16 Fracture Surface of Specimen Tempered at 7000F, I Percent NitalEtch: (a) 100 Percent Oblique Shear, Approx. L.OX, (b) Delamina-tion Along Fibering Direction, 500X.--------------- q-------------- 24

Ul-1 Face Notched Test Specimen.------------------------------------ 30

11-2 Hardness of Test Specimens after Tempering for One Hour.---------- 32

11-3 Top: Sample H, Austenitized at 15500F, Tempered at 700F,Broken at 700F.

Bottom: Sample MM,, Austenitized at 230007, Tempered at 700F,Broken at 70OF ---------------- ------------------------ 34

zz-4 Top: Sample B, Austenitized at 155007, Tempered at 1500F,Broken at 700F.

Bottom: Samplea LL, Austenitized at 230007, Tamperedat 1500F7,Broken at 7007----------------------------- -35

11-5 Top: Sample E, Austenitized at 15500F, Tampered at 23007,Broken at 700F.

Bottom: Sample JJ, Austenitized at 23000F7, Tempered at 2300F,Broken at 7007. ------------------- m--------------------- 36

11-6 Top: Sample L, Austenitized at 155007, Tempered at 3000F,Broken at 700F. This Specimen was Etched with 4%f Picral.

Bottom: Samplea CCC, Austenitized at 23000F, Tempered at 30007,Broken at 7007----------------------------------------------- 37

11-7 Top: Samplea K, Austenitized at 15500F, Tempered at 400OFBroken at 700F.

Bottom: Sample FF# Austenitized at 2300%F Tempered at 4000F,Broken at 7007.------- ----------------------- ---------- 38

IV

Figure page

11-8 Top: Sample G. Austenitized at 15500F, Tempered at 4500F,Broken at 700F.

Bottom: Sample BS, Austenitized at 23000F, Tempered at 4500F,Broken at 70OF ---------------------------------------- 39

11-9 Top: Sample C, Austenitized at 15500F., Tempered at 50007,Broken at 700F.

Bottom: Sample HI$, Austenitized at 23000F7, Tempered at 5000F,Broken at 7007.---------------------------------------- 40

11-10 Top: Sample D, Austenitized at 15500F, Tempered at 5500F7,Broken at 7007.

Bottom: Sample DD, Austenitized at 23000F, Tempered at 5500F,Broken at 70OF.---------------------------------------- 41

11-11 Top: Sample M, Austenitized at 15500F, Tempered at 70007,Broken at 700F.

Bottom: Sample EE, Austenitized at 230007, Tempered at 7000F,Broken at 700F.---------------------------------------- 42

11-12 Top: Sample AAA, Austenitized at 155007, Tempered at 40007,Broken at -.3460F.

Bottom: Sample Up Austenitized at 230007, Temered at 4000F7,Broken at -.346OF------------------------------------------------ 44

11-13 Top: Sample A; Austenitized at 155007, Tempered at 40007,Broken at -1090F.

Bottom, Sampl e X, Austenitized at 23000F, Tampered at 4000FpBroken at -109 0F -------------------------------------- 45

11-14 Top: Sample K; Austenitized at 1550o1F. Tempered at 40007,Broken at 7007.

Bottom:- Sample V9 Austenitized at 230007, Tempered at 4000F,Broken at 700F---.------------------------------------ 4

11-15 Top: Sample F7 Austenitized at 15500F, Temered at 4000F,Broken at 21207.

Bottom! Sample BBB, Austenitized at 23000F, Tempered at 40007,Broken at 2120F7.-------------------------- ------------ 47

11-16 Percentage of Fracture along Prior Austenite Grain Boundaries forSpecimens Tempered at Various Temperatures.---------------------- 48

11-17 Effect of Processing History on Kc for AISI 4340. Data of Rawef rom Ref . 5.---------------------------------------- --------- 49

v

LIST OF TABLES

Table Pae

I-1 Effect of Melting Practice on Fracture Toughness ---------------- 2

1-2 Chemical Composition of Sheet Tested- ---------------------------- 4

1-3 Notch Symmetry ------------------------------------------------ 11

11-1 Etchants Employed in Metallography------------------------------- 31

11-2 Fracture Toughness of As-Quenched Edge-Notched Sheet Specimens 43

vi

PART I: FRACTURE TOUGHNESS OF HARDENED AND TEMPEREDAISI 434o SHEET

I. INTRODUCTION

The well-known need in missile technology for the highest practical strength-weight ratio has required the use of high-strength steels in a lightly temperednotch-sensitive condition. One consequence of such applications has been acertain amount of failure in motor casings by rapid propagation of small flawsundetected in final inspection. The mechanics of crack propagation have developedin the last decade into a utful tool for studying notch sensitivity. After thefirst analysis by Griffith, Orowen(2) and Irwin(3.3 suggested that the basicconcept could be applied to describe the propagation of cracks in large steelstructures. The formulation of Irwin, in which the energy term in the Griffithequation is deduced from the elastic stress distribution at instability, hasproven especially valuable.

The fracture toughness measured with notched sheet specimens of high-strength steel is dependent on such testing variables as the thickness of thesoample and on the testing temperature. For a given composition. it is also de-pendent on processing history. Differences in microstructure, for example,resulting from different melting and rolling practices my have important effectson fracture toughness. In this connection, steels produced by consutrode andvacuum melting generally possess higher toughness and higher itch strength thanthe same steels when air melted, as indicated in Table - 15) Unidirectionalsolidification of the starting ingot may also yie!d material with greater frac-ture toughness, since freezing in this fashion has been found to increasemarkedly the reduction of area in tension of AISI 4330 and 4340 steels.) Re-duced microporosity and inclusion content and improved chemical homogeniety areidentified with such improvement through control of melting practice. Stillother evidence establishes the effect of rolling history, in particular thelarge differences betyen longitudinal and transverse properties of unidirec-tionally rolled sheett5) compared to the far more isotropic fracture toughnessin specimens from the plane of sheet that has been produced by cross-rolling.

The work being reported was the initial phase in a study of the effect ofprocessing history on fracture toughness. It was necessary that detailed atten-tion be paid to the problem of measuring the fracture toughness of sheetmaterial, and this is the principal subject discussed here. For determinationof the fracture toughness of sheet materials, test pr cedures have been recom-mended by the ASTM based on the Irwin point of view.)A These specificationsare most helpful in fixing specimen dimensions and notch sharpness and werefollowed in the present work with some few exceptions, which are noted.

'-44

- ~4-

z 41% 0 %.

L- x1- 4J

4cCU) - -

tiJ

I-A

CLIN 0. C

0 ~-0

H 0

4* t% L. 9-- ->

4,0

2

8"

1.52.5 -4---2.575 0 T

0 1.855(a) kb- 1.055

10710

(C).

.. ........

Figre -i Tet Secien

30

1I. EXPERIMENTAL PROCEDURE

I . Specimen Preparation and Heat Treatment: The AISI 4340 shoot from whichall specimens were preparedp was air melted sandwich crosi-rolled and spheroidize-annealed. its composition is given in Table 1-2.

TABLE I- 2

CHEMICAL COMPOSITION OF SHEET TESTED

C Mn P S Ni Cr Si Cu v

Ladle 0.40 0.78 0.010 0.016 1.78 0.85 0.23 0.25 0.06 ---

Sheet 0.40 0.80 0.007 0.017 1.79 0.83 6.22 0.23 0.05 0.025

All specimens were cut parallel to the long direction of a 0.108 in x 61 In. x 1108 in.sheet. The three types of specimen prepared are described In Fig. I-1. The notchroot radii of those of type (b) were 1.125, 0.125, 0.01 or less than 0.001-in., thelast dimension being used with specimens for fracture-toughness determinations.

Specimens of type (a) ware austenitized for one hour in argon at 1525°F andoil quenched. Specimens (b) and (c) were austenitized for twenty minutes at 1525oFIn molten salt and then quenched Into oil. All were refrigerated in liquid nitro-gen within twenty minutes of quenching. air tempered for one hour, and oil quenchedfrom the tampering temperature.

2. Notch Sharpening: The importance of obtaining notches sufficiently sharpto reduce the net fracture 'ess to the level associated with a "natural" crackhas been widely emphasized.95 Notch radii of less than .001 In. are currentlyspecified. Since notching to radii less than about 0.002 in. by conventionalmachining is difficult and time consuming. notches were first milled In the edge-notched specimens and the root radius then reduced by a vibratory lapping opera-tion designed for the purpose. Most consistent results ware obtained by lappingthe notch sharp prior to heat treatmento and then cleaning briefly again afterhet treatment.

The apparatus for vibrolapping as finally evolved, is showm in Fig. 1-2.The lapping compound is lp diamond paste while the lap Is a single-edged razorblade driven with reciprocating movement by a vibrotool of the type used to markmetal lographic specimens. As the blade is vibrated, a small weight (approximately70 grams)holds it in the notch without stopping it, and a smll-motor drive drawsthe blade slowly through the notch to distribute the blade wear more evenly.During one operation, a new blade is drawn back and forth squarely through thenotch once a minute for five minutes, after which a new blade Is inserted. Threesuch operations, with the side of the specimen facing the tool changed after each,

4

0 .

.0 z

4-

1.,

5

generally produced a satisfactory notch. A typical section through a sharpenednotch is shown in Fig. 1-3. Examination of the sharpened notches under thebinocular microscope indicated that the notch was quite uniform along its length.The section shown in Fig. 1-3 and the optical comparator examination of allspecimens indicated that notch radii less than 0.00025-in. were produced by thistechnique.

3. Observation and Estimation of Stable Crack Length: Preliminary testingusing ink staining to estimate the extent of slow crack growth indicated that thearea of stable crack growth measured in this way corresponded closely to the tri-angular "porous tongue" on the fracture surface. Typical fractures are shown inFig. 1-4 in which these areas of slow growth are evident; sectioning of suchcracks in an early stage, prior to fracturej (Fig. I-II) showed the porousappearance to result from numerous small cavities developing with the advancingcrack. Accordingly, slow-crack growth was estimated from the extent of "poroustongue" on the fracture surface, not only because it was an easy and reliabletechnique but also because of the possibility that staining might induce stress-corrosion crackingo particularly in low fracture-toughness material. Of the twostable cracks (one at each notch root)p that which did not accelerate to causefinal fracture was assumed to give the length of the slowly growing crack atinstability. The maximum length, h, of the roughly triangular porous area wasmeasured and the crack length a calculated as

h

here ao is the initial, machined notch length.

Stable cracks grew in discontinuous fashion. The clicking sound of theadvance-arrest process could be detected during some tests by use of a ste;tho-scope, which also made audible much slow crack growth not apparent to the earalone.

4. Measurement of Notch Symmetry: The symmetry of the notch tips withrespect to a line through the loading holes mwas determined in all fracture-toughness specimens. For mesurement, a specimen ms claaped on the table ofa milling machine so that it could be trensleted exactly parallel or nerml tothe line through the loading holes. Grinding the specimen edges parallel toend equidistant from the center line through the holes, to within + 0.0005 in.,permitted the edges to be used for reference in lieu of the centerTine. Lineardistances, which were measured with a travel ling microscope, were reproducibleto within + 0.0005 in.

The eccentricity of the notches was defined as iR a fA-y2 + ALx2 Lwith&x and Ay as represented in Fig. 1-5. Ax was measured directly; after

measuring aI and a2, A y was calculated from

A U . - - a aa"a1) - a2 ) a"2 ""(1)

6

Figure 1-3 -Section through a Typical Notch Produced byVibrolapping. Comparison Wire in Lower Lefthas 0.0013" Radius, IOOX.

Fracture Surfaces of Sharply Notched AISI 4340 Stool Sheet

Tempering Temperature 400°F

550 F

700oF

850OF

Figure 1-4 - Typical Fractures of Sharply Notched SpecimensIllustrating Triangular Areas of Slow CrackGrowth. In Specimens Such as Those Temperedat 400OF and 550OF the Rapid Crack Passed fromLeft to Right and Hence the Size of the SlowCrack on the Right is Measured.

IV z

Figure 1-5 - Dimensions Used in Defining Notch Symmuetry.

The values obtained are given in Table 1-3. If the ASTH specification(4 )

is interpreted as requiring that A R be less than 0.001W or, in the Rresentcase & R be less than 0.002 in., such a notch symmetry condition seem overly

restrictive. For although EA was 0.0037 in. including some values of AK as

large as 0.008, it will be shown that the scatter in K is small suggestingthat AR < .003W may be an acceptable symmetry specification.

5. Fracture Examintion: After fracturing some specimens, one half wasphotographed and the other half nickel plated, sectioned and examined mtal lo-

graphically. The nickel was deposited at a current density of 0.1 to 0.15 amp.per sq. in. from a bath at 70°F containing 300 gns of NiCI2 -6 20 and 30 gis ofH3803 crystals in a liter of water. Although the plate tended to peel on theflat areas, adherence was good on the fracture surface.

6. Calculation: Tensile data consisting of 0.2% offset yield strength,ultimate tensile strength and the strain-hardening exponent of the parabolichardening equation ware obtained from strip specimens of type (c) in Fig. 1-I.Assumino strain hardening of the form a - Agn, n was obtained from the slope ofthe log a - log t relation by a least squares analysis. For this analysis therequired true stress was calculated by dividing the load at any time by the areaat the same time. The logarithmic (true) strain up to necking was calculatedfrom the length changes in a I in. gage length obtained by visually aligning anadjustable length block with finely scribed reference marks.

Fracture toughness, Kc, for the edge-notcho,. peciians (type (a))was cal-culated in two ways. First, from the expressionl1 l

K( a Witan (A) + 0. 1 sn(2)

where

a - gross section stress

W = gross section width

a - crack length at instability

The resulting Kc is considered 'uncorrected" and applies to a finite-widthsaqle of low fracture toughness in which plastic deformation at the notchtip is negligible. When plastic stralg)pare significant# the following"corrected" expression is appropriate. ')

K * K C2sa 2K U J IY t, I'll - + '0.1 sin (- + V(3)c ,, 22 2', ! Way

where a is the tensile ytp&d stress. A convenient graphical solution ofEq. 3 his been published."t)

10

TABLE 1-3

NOTCH SYMMETRY

Specimen Ax R2 __k

D7 .3990 .4020 .0020 .0030 .0036

DI0 .3980 .4045 .0027 .0065 .0071

DII .3965 .3960 .0032 .0015 .0035D12 .3970 .4010 .0000 .00.0 .0040015 .3965 .3995 .0010 .0050 .0032D16 .3955 .4015 .0007 .oo6o .0060D17 .3985 .4005 .0010 .0020 .0022018 .4030 .4040 .0020 .000 .0022024 .3995 .4030 .0020 .0035 .000D27 .3960 .4040 .0005 .0060 .0080

E9 .4000 A0 .0000 .0000 .0000E12 .4020 .4030 .0043 .0010 .00E15 .3985 .4010 .0014 .0025 .0029E20 .4045 .4035 .0033 .0010 .0034E24 .3980 .4005 .0000 .0025 .0025

E25 .3990 .4015 .0000 .0045 .0045E26 .3965 .4030 .0000 .0045 .0045E27 .4010 .4015 .0012 .0005 .0013

Ak .001 y A .0028 Ioo - .00 37

11

By angjgy to an exact olastic-plastic solution for crack growth in torsion,Mcclintock o suggests that the radius of the plastic zone in a notched sheetspecimen at instability is

exp-2(;L. (4)

where

a - fractional elongation at necking of an unnotched strip specimen

aTS u tensile strength of an unnotched strip specimen

B - thickness of the specimen

RA - fractional reduction of area of a mildly notched specimen

Equation (4) applies when R is greater than B. The mildly notched specimenspecified by McClintock is 8ne 20 wide containing keyhole slots 50 deep ateither edge with a root radius of B. In the present work, the reduction inarea at fracture of type (b) specimens (Fig. 1-1) with 0.125 in. radius qgtcheswas considered to approximate that of the required specimen. McCl lntock(1)further suggests that R is related to Kc by

Kc - *TS fx~i (5)

Thus, through Eq. (5) the fracture toughness of a mterial mey be estimatedfrom tensile data on an unnotched and a mildly notched sheet specimen.

III. RESULTS

The ultimate strength, yield strength, and hardness of unnotched stripspecimens decreased with increasing tempering temperature (Fjg1 1-6) in afashion consistent with the date of Shih, Averbach and Cohen t1Y for roundAISI 434.0 specimens. The n values tended to decrease with increasing teper-ing temperature (Fig. 1-7). The trend in the elongation at maximum loadinvolved a minimum at tepering temperatures of 550OF and 700OF (Fig. I-7).

The effect of varying notch radius on the net fracture stress in sheetspecimens of AISI 4340 is given in Fig. 1-8. For radii of 0.125 in. orgreater, the net stress at fracture Is the same as that in the unnotchedcase. Upon tepering below 850 0 F, radii of 0.010 in. or less lead to frac-ture stress decreasing progressively with notch radius.

12

00

0;-t

-0-

00 0

C) 0c0

.4-0) /74- 4

00

EE/0 @0 0,

54

* 0)

0. 0

-0-

4-

~01

4)4

SSaUPJDH (!sd 201) ssai~s

13

35 1

30- -0.48025 -- 0.40

20 %R, 0.32 g_

15 e- 0.24 r

10- _ 0.160

5- 0-- o -0.08

0t

0.15

0.05 V-

0400 600 800 1000 1200

Tempering temperature, OFFigure I- - Effect of Tempering Temperature on Ductility and Strain

Hardening of AISI 434b Steel.

R€ = plastic zone radius at instability from eq. (4)%R.A. = area reduction at fracture in .125" root

radius specimenn = strain hardening exponent of unnotched sheet

specimen•u = fractional elongation at maximum load in an

unnotched sheet specimen

14+

Notch radius, R, inches0 R =O0

260- R= 1.25R=0.125

C3 R=0.010+ R =0.001

CL.C220

180'ID

U +

%6--

z R 0.l0"thick sheet

100- 40.It IoM0

400 600 800 1000 1200Tempering temperature, OF

Figure 1-8 - Effect of Tempering Temperature on Net Fracture stressof AISI 4340 Steel .

15

Fracture-toughness values cal cul ated from data obtained with the sharpestnotches are plotted In Figs. 1-9 and 1-10 as a function of tampering tbaer-stare for two width-to-thickness (V/B) ratio ) The percentage of shear ascomuonly estimated from the fracture surface1 is given in Fig. 1-9. Thewidth (W) only was varied in changing W9B. Even though the narr r specimenshad a V/B ratio less than the 16 required by ASTM specification,m agreementin the calculated Kc values with the wider specimens was good. For specimensteapred over 4000F.. the practical lower l imit for AISI 434s0, Eq. 3 gave In-creasingly higher values of Kc than Eq. 2.

In Fig. 1-10 values of Kc according to the, McClintock analysis are plottedon the basis of data from Fig. 1-7. As showi in Fig. I-10, the analysis ofMcClintock and Irwin corrected for the effect of plastic zone agres clesely forspecimens teapered above about 5500F. McCl intock restricts his analysis tocases where, the plastic zone size, Re,, as calculated from Eq. (4) and plottedin Fig. 1-7, Is greater than B, a condition which Is met for specimens temperedabove about 60007. The close correlation between the two analyses suggests thatanalytical methods for correcting K for plastic strain prior to crack In-stability are In relatively good orLr.

Stable (slow) cracks were formed in the testing of all specimens temperedat 400O7 or above. In two instances (after tempering at 60007 and 120007)specimens suspected of containing such cracks,. from the clicks emitted, wereunloaded at 95 to 98% of the estimated fracture load, sectioned and micro-scopicallIy examined. Sect ions were prepared by mil ig, either parallel ornormal to the sheet surface, followed by mael legraphic, pol ishing. Depeninglaon tampering tomperature, two types of crack patterns appeared. One, found ina specimen tempered at 60007, consisted of a connected serie of eavitias asshown In Fig. 1-11. The other, observed in a specimen tompered at 1200F(Figs. 1-11 to 1-14), Involved a network of cracks, which appears to be merenearly typical of highly tampered mg Is)a and my be related to the fine"hair-line cracks noted by Puttick~Iin Ingot Iron#

A common detail in the fracture surfaces was slipping or dolamirationparallel to the plane of the sheet. This occurred over both the flat-fracture areas, as shown in Fig. 1-15, and on the macroscopic shear areaas well, Fig..1-16. in certain cases, Inclusions were observed In thedelaminations or In the paths of cracks extending from them (Fig. 1-15c).

IV. DISCUSSION

The agreement between the Irwin analysis corrected for plastic zones andthe equations suggested by McClintock Is quite good over the range where Rt0 Isgreater than B (Fig. 1-7). If the reduction in area of the mildly notchedspecimen could be Inferred from the reduction In area of the unnotched stripspecimen, the McCl intock approach would provide a technique for deducing K.for moderately notch-sensitive meterial from a simple strip tensile test . InIts present state, however, Mcclintock 'a analysis must be regarded as tentative

16

1501

0

125 - 006

100 tIrwin.,

(n 75-

II Irwin uncorrected

0j0

25 i 9L 3

0~ 075- 0

0)

Oe 25-

00 V 100 250 400 550 TOO

Tempering temperature, OFFigure 1-9 - EFfect of Tempering Temperature on Fracture Toughness

and Percent Shear. Curve Marked "Irwin" is Based on

Equation (3). Curve Marked "Irwin Uncorrected" isBased on Equation (2).

17

ODo

12

0 EE

ED.

0 - 00

0 -0

I L4- Sn

0

c4.

0000 x

Eo L.

18

.i.L

~ "'"

L.S (16

.X.

%W/A.PA

4!<"

Figure I-11 Specimen Tempered at 600OF Loaded to 98 Percent EstimatedMaximum Load, Showing Initiation of Stable Crack GrowthBeneath Notch Root: (a) I Percent Nita] Etch, 75X,(b) Unetched, 250X, (c) I Percent Nita) Etch, 500X.

19

,16,

IV4

I Pecn.it]Ec:*a rs 'Fern an Hir

lin CacsJOX,(b Ireulr rak roth.100 l.

* -. I.~-** .20

LP

, (b)

Figure 1-13 - Same Specimen as in Fig. 1-12 showing Onset of ObliqueShear at Notch Root, Tempered at 1200OF: (a) SectionThrough Stable Crack, Showing Shear Bands Growing fromEdge of Central Crack, unetched, approx. 30X. (b) Sectionnear the tip of the Stable Crack, Showing Pores Linkingup Along Oblique Shear Plane, unetched, approx. 35X.

21

* ;fi

(b)

op •

Figure 1-14 - Same Specimen as Fig. 1-12 Showing Void Formation AlongOblique Shear Plane: (a) Heavily Strained Region Be-tween Voids, 500X, (b) Agglomeration of Voids, HeavilyStrained Region at Void Edges, 500X.

22

- 'S

*4W4

WON I . d .(..)

Figure 1-15 -Fracture Surface of Specimen Tempered at 2000F. 1 PercentNita] Etch: (a) Flat Fracture with Delaminations AlongFibering Direction, 60X, (b) Jagged Secondary Cracks AlongFibering Direction,. 500X, (c) Unetched,. Showing Jagged,Discontinuous Crack Growth from Base of Delamination, 250X,(d) Same as c, 250X.

23

4-.i

e'*( 1'4. 7

1171

43<

Figure 1-16 -Fracture Surface of Specimen Tempered at 7000Fp I PercentNita] Etch: (a) 100 Percent Oblique Shear, Approxc. 40X,(b) Delamination Along Fibering Direction, 500X.

since the method is based on a rather extended analogy involving terms with aphysical relationship to fracturing that is difficult to establish.

In Fig. 1-17 4the fracture toughness data obtained in the present work isplotted together with Rawe's data on AISI 4340 prepared by air melting andunidirectionally rolling. The latter data relate to slightly thinner sheetthan that of the present work (9.080 in. vs. 0.108 in.) and a longer totaltempering time (2 + 2 hrs. vs. r hr.). Both'!ariations should be reflectedprincipally in vertical displacement of the Kc vs. tempering temperaturecurves, without affecting the scatter. As shown, the fracture toughness ob-tained for air-melted and cross-rolled material in the present work is inter-mediate between that for longitudinal and transverse specimens of air-meltedmaterial obtained in the other work. Such a result is reasonable and thebasis for specifying cross rolling. The most striking feature of Fig. 1-17,however, is the extent to which processing history influences the fracturetoughness. Compared to the scatter in the individual determinations, whichis typically no greater than about + 20%, changing the processing historycan introduce variations as great as 300X. The extreme sensitivity of Kc toprocessing history requires discretion in the use of tabulated Kc values indesign and emphasizes the need for better understanding of how fracturetoughness is affected by process variables.

The delamination observed so extensively raises the question of itshaving a possible role in the fracture process. In particular, it hasappeared that one bridge between considerations of material structure andfracture toughness might be found in delamination, which is clearly a struc-tural matter. Although the details of the delamination mechanism may not befully established, it is recognized that the event is a consequence of amechanical fibering and low fracture-resistance through the thickness of asheet,

A potentially important reason for the suggestion relative to the de-lamination process is that fracture toughness, as measured by Gc or Kc, isfrequently anisotropic. Accordingly, it would seem that elements of struc-ture fixing the anisotropy must enter into the process determining the levelof Kc, which means fibering in the broad sense but more specifically themechanical fibering of elongated particles, secondary phases, etc. Also, theimplication of this marked anisotropy is quite strong that the fracturingprocess does not involve crystallographic cleavage.

Furthermore, there are indications that fracture toughness takes on lowvalues as thickness becomes very small, presumably due to localized, full-shear fracture with low energy absorption. Therefore, a possible sequence ofevents in the development of fast, unstable fracture might be:

1. Plastic deformation and transverse stresses at the crack tipacting to encourage delamination.

2. Fracture of the individual lamina in relatively small volumes(shear mode), resulting in low fracture toughness.

3. Over-all fast, relatively low-energy absorbing fracture, keptin a plane by the delaminating action.

25

Extending these speculations still further, the peculiar sharp temperaturedependence of K. could be introduced; it is rather striking that at temperatureswhere the generally recognized material properties are not strongly temperaturedependent, K0 my increase by as much as a factor of 2 over a 100-200OF intervaland fracture mode may go from little to full shear. The necessary additionalsuggestion is that the delamination tendency may be rather sharply temperaturedependent.

Although the discussion relative to delamination is speculative, the outcomeis a point of view with implications for fracture-toughness control. Specifically,higher fracture toughness ought to follow from suppressing delamination and im-proving the full-shear fracture toughness. Control of sheet-processing historyshould be a most effective way of regulating the delamination tendency.

V. SUMWMRY

1. The measured fracture toughness, K., of air-melted and cross-rolled AISI 4340sheet increased with tempering tqerature, especially rapidly near 350 and 7500 F.Comparison with the data of Raweo5) indicates that for this steel the scatter inKc due to variations in the processing history is much greater than the scatterdue to variations in testing procedure.

2. A vibrolapping technique for sharpening of pre-machined notches in an edge-notched fracture-toughness specimen was developed which reduced the notch-rootradius to less than 0.00025 in. This technique would seem to compete favorablywith fatigue cracking of centrally notched specimens as a method for producingthe sharp notches required in testing sheet material with fracture toughness lessthan about 100,000 psi ".V inW.

3. Stable cracks were observed in all specimens tempered at 400OF(Kc - 100,000 psi in_') or above. Single, stable cracks emanating from thenotch tip were characteristic of specimens tampered at 600°Fp whereas multiple,fine hair-line cracks, ware characteristic of stable cracks in specimenstempered at 12000 F.

4. in all fractures examined# the material tended to split or delaminate inthe rolling plane. Such a development may have a strong effect on the fracturetoughness by reduction of the size of plastic zone accompanying the runningcrack. Further work on the relationship of the delamination tendency toprocessing history appears promising.

26

VI. REFERENCES

1. A.A. Griffith, Phil. Trans. of Royal Soc. (London) 221A (1920), p. 163.

2. E. Orowen, Fatigue and Fracture of Metals, John Wiley, Now York (1952),P. 154.

3. G.R. Irwin, ASH Symposium, Fracturina of Metals, (191e8)p p. 152.

4. ASTM Bulletinp Jan. 1960,9 p. 29.

5. R. Rawe, Aerojet-General Corp., Report M2003,9 Feb. 1960.

6. S.Z. Uram, N.C. Flemings, and H.F. Taylor, Tran. AFS 68 (1960), p. 347.

7. Materials Research and Standards, Nov. (1961),. p. 877.

8. F.A. McClintock, Discussion ASTM Bulletin April (1961),. p. 277.

9. C.H. Shih, B.L. Averbach and M. Cohen, Trans. ASH 48B (1956), p. 86.

10. K.E. Puttick, Phil. Meg. Ser. 8,. 4 (1959),. p. 964.

27

PART 2: EFFECT OF AUSTENITUZNG TEMPERATURE ONFRACTURE MODE OF AISI 4340

1. INTRODUCTION

The problems associated with the low ductility and fracture toughness ofhardened and tempered low-alloy steels are currently of much interest. Thesesteels, of which AISI 4340 is typical, are brittle in the as-quenched conditionand consequently are seldom used without tempering. With AISI 4340, for example,a min nmam practical tempering temperature is about 4000F. "Shih, Averbach, andCohen()ll showed that the ductility of AISI 4340, as measured by the percentreduction-in-area in tension, dropped abruptly for tempering temperatures below4000 F. In Part I it was demonstrated that the fracture toughness of this materialclosely followed the ductility trend, also dropping abruptly when tempering wasdone below 4000 F.

One factor which my be involved in the brittleness of both as-quenched andquenched and tempered steels is separation at prior austenite grain boundariesjfracture along such a path would probably require little plastic wrk for itspropagation. Under at least three conditions of heat trea|ment, fracture in steelmy follow prior austenite grain boundaries: in overheated steel, in steeltempered In the 500OF embrittle,,ent range, and in as-quenched steel. The per-sistence of facets# corresponding to prior austenite grain boundaries# In thefractures of overheated st is the bai of the definition and masuremenkofthe degree of overheating.?!1 Grossmen31 end koyerts, Sumps and Sheehanhave shown that the percentage of fracture along prior austenite grain boundariesincreases markedly for spetens tempered in the 500°,F embrittlament range.Lament, Averbach and Cohen{5J have suggested Iprecipitation l eading to carbidefilms at these sites. Turkalo's observation(') that 50% of the fracture In anas-quenched fine-grained steel (O.55 Co 1.30% Mn) broken in Impact at 1960 Cfollowed prior austenite grain boundaries suggests that cracking here my be acontributing factor In as-quenched brittleness.

The interest In the present work was a correlation between fracture tough-ness and fracture path. Since the amount of austenite grain boundary fractureIs difficult to estimate in fine-grained steel, some specimens with large prioraustenite grain size were examined. Thus, the study consisted of a comparisonof the fracture appearance of singly face-notched specimens austenitized at1550 and 23O0OFp oil quenched, refrigerated, tempered at various temperaturesand broken in impact bending at several temperatures. In additionj, tw con-ventional edge-notched specimens were broken at room temperature after quench-ing from 2300OF and refrigeration.

One shortcoming of any study such as this is the difficulty of relatingductility or fracture toughness to fracture-topography examination of thefracture. in the present works a ductile (non-faceted) appearance was asso-ciated with fracture toughness, the level increasing as topography becamerougher. It was realized that the surface roughness produced by passing thecrack through the thickness was probably greater than that produced by

28

a crack passing through the width. In addition to surface roughness and the amountof grain-boundary fracture the relative amount of delamination, the tendency forsplitting to occur along the rolling plane at its intersection with the fracturesurface was noted.

II. EXPERIMENTAL PROCEDURE

The same sheet of air melted and cross-rolled AISI 4340 steel described inPart I was used here. Specimen dimensions are given in Fig. 11-I. All were cutfrom the sheet with their long axis parallel to the long axis of the specimensin Part 1, so that the plane of fracture had the same orientation in the sheetin both cases.

The specimens were wired together and austenitized within a zircon tubeheated in a silicon-carbide resistance furnace. After one hour at temperaturethe vacuum was broken and followed by quenching in oil. It was estimated bylater metallographic examination that 0.0005-0.001-in. of scale formed duringthe approximately 30-second period that hot specimens were in air. Inmediatelyafter quenching the specimens were refrigerated in liquid nitrogen and stored atthat temperature until tempered for one hour in a circulating air furnace.

Two austenitizing temperatures were used, 1550 or 23000 F. The former ledto a grain size of ASTH 7-8, Aile the latter gave a grain-size number less then 1,according to the ASTH comparative method; the mean grain diameter in the lattercase was about 0.025 in.

The specimens were broken by clamping one half in a vise and striking theprotruding half with a hamer. For testing at other than room temperature aspecimen, mounted In the vise, was placed first in a constant temperature bath.After reaching bath temperature it was removed and broken within 3 seconds. Thetesting temperatures were -346oF (boiling liquid nitrogen), -109OF (acetone anddry ice), 70oF (room temperture), 2120F (boiling water).

Immediately after fracture the surface of one hal f was nickel plated bythe technique described in Part I. Then an arbitrary section through thefracture surface normal to the notch was made with an abrasive wheel and thesection freed by a cross cut. The freed piece was mounted, polished, etchedand photographed. The etchants used are shown in Table 11-I. Etch No. 2revealed prior austenite grain boundaries in specimens austenitized at 2300OFbut not at 15500F. Etch No. I was used to bring out the mertensite structureafter quenching from the lower temperature. The other half of the brokenspecimen was placed in a plastic envelope containing a vapor-phase rust in-hibitor for subsequent observation and hardness determination.

29

2"

Notch radius approximately900 0.005 Is

0.105"

0.075"

Figure II - Face Notctied rest Specimen

30

TABLE 11-1

ETCHANTS EMPLOYED IN NETALLOGRAPHY

Austenit izing

Temperature Etchant

1550°F (I) 1% nital

2300OF (2) 1 gram picric acidI ml HC (36%)100 ml. methyl alcohol

Two edge-notched specimens, I-in. wide with 0.20-in. deep notches, were pre-pared to measure the fracture toughness of coarse-grainedo as-quenched material.After machining and preliminary notch sharpening by vibrolapping, they were cov-ered with a thin refractory coating, austenitized for one hour at 2300°F inmechanical vacuum, oil quenched, and placed in liquid nitrogen. The notches wereagain cleaned and sharpened by a short period of vibrolapping. Surface grinding,with a large excess of coolant, symmetrically reduced the thickness to .063 tn.,giving W/ a 16 and removed any surface pits. The average hardness was 56Rockwell-C on ground surface and 55 on the adjacent unground surface, indicatingthat any decarburization was slight.

Slow crack propagation In one of the edge-notched saoles was measured bystaining with recorder inkj In the other the total was estimated from thefracture surface. In both cases the progress of the stable crack could befollowed with a stethescope.

III. RESULTS

The Rockwell-C hardness of the face-notched specimens austenitized at1550°F and 23000 F, and specimens austenitized at 15250 F from Part I are plottedin Fig. 11-2 as a function of tempering temerature. The lowsr hardness ofthose austenitized at 2300OF and temered below 400 0F is probably due to de-carburization. It was later found that the refractory coating led to higheras-quenched hardness (about 55 RC), indicative of less carbon loss duringaustenitizing. The surfaces of the as-quenched and broken refractory-coatedspecimens contained about 20% intercrystal line fracture, in contrast to aboutIOX for those heat treated without coating. Thus fracture mode is somaetatsensitive to carbon level in this carbon range. The amount of intercrystallinefracture in as-quenched coarse-grained samples was not altered by eliminatingthe liquid-nitrogen quench or introducing the notch after heat treatment.

31

0

0

0

00 107 0

a) 0Lou-40 ~a) C0

14. k, 4-

/ CA

a O) 0 CL13 E -c NCP L

N- -% 4~ C:

4.- L O' - 0

4F- T.c

(n' 0

00

00 \0

I)0 it) 0

@IOS 9 1 PM>POH0 'SS@UPAIOH

32

As Quenched Observations: As-quenched fracture surfaces appear tougher forthe material of larger austenite grain size, as comparison of Figs. 11-3 and 11-4shows. In support of this qualitative indication, KC values for the as-quenchedcoarse-grained specimens measured with edge-notched strips are given in Table 11-2along with the values for as-quenched fine-grained specimens from Part I. SinceKc for specimen 812 is probably low owing to corrosion by the staining ink, dataon unstained specimen 821 indicates an increase in Kc of 3-4 times with the higheraustenitizing temperature. The stress relief effected by refrigeration in liquidnitrogen, the delamination tendency of the steel. the relative thinness of thespecimen and the loading rate may all influence the observed toughness of un-tempered mrtensite formed from coarse-grained austenite. In this tougher, as-quenched conditionp slow intergranular cracking was a prominent feature of thefracture. As heard in the stethescope, slow crack-growth started at about 60Xof the fracture load and continued to fracture. There were two distinct partsto the fracture, a portion identified with the slow intergranular cracking, whichwas quite extensive, and the reminder of essentially 10% shear associated withrapid fracturing. Ink staining established that the intergranular crackingoccurred slowly. Interestingly, the ink also increased the rate of crack ex-tension at constant load; the cracking to be heard with the stethescope increasedmarkedly when more ink was added to the notch, even at constant load greater thanthat to start the slow crack. Such a stress-corrosion effect undoubtedly accountsfor the lower K. of the stained specimen.

Teioerina: Effects of tempering after the different austenitizing treatmentsare brought out in Figs. 11-3 to ZI-11. In considering only the 15500 F ousten-itizing treatment, surface roughness is small for tempering at 70 and 150°F(Figs. 11-3 and 11-4 top), increases rapidly with tempering between 300 and 500OF(Figs. 11-6 through 11-9 top) and then increases slowly with further increases intempering temperature (Figs. II-10 and 11-11 top); qualitatively. one mightanticipate a paralle! trend in fracture toughnessp as was in fact demonstratedin Fig. 1-9;

In the case of 2300OF austentiizing, surface roughness is slightly greaterafter tempering at 400-450OF (Figs. 11-7 and I-8 bottom) than in the as-quenchedcondition. The amount of grain boundary fracture upon tampering at 150°F and 230OFis somewhat greater, in comparison to the as-quenched and 400OF observations.Thus one might infer that the fracture toughness of coarse-grained material de-creases a little as tempering temperature is raised to about 200OF and thenincreases with further increase in tempering temperature to the range 400-4500F.

All comparisons (Figs. 11-5 to 11-8) show greater surface roughness in thecoarse-grained fractures for tempering temperatures to 4500F. Thus a furtherimplication is greater fracture toughness for the coarse-grained materialtempered between 70-4500F.

Fracture after austenitizing at 2300°F and tempering at 550°F and 700OFfollows prior austanite grain boundaries almost exclusively (Figs. II-10 andII-I bottom) as indicated in Fig. 11-17. The observation agrees with thatof Baeyertz, Bumps and Sheehan and Grossman, whose data are included inFig. 11-17. All such observations establish the intercrystalline nature offracture in coarsep and probably fine, grained specimens tempered in the500OF "embrittlement range".

33

I 7~j'

700F, Brke at 70O

43

IK.

Figure 11-4 -Top: Sample B, Austenitized at 15500F, Tempered atP50 0F, Broken at 70OF

Bottom: Sample LL, Austenitized at 23000F, Tempered at1500F, Broken at 70OF

Magnifications: Sections, 50X; Fracture Surfaces, 7X

35

Figure HI-5 Top: Sample E, Austenitized at 15500F. Tempered at2300F, Broken at 70OF

Bottom: Sample'JJ, Austenitized at 23000F, Tempered at2300F, Broken at 700 F

Magnifications: Sections, S~OX; Fracture Surfaces, 7X

36

Figure 11-6 -Top: Sample L, Austenitized at 15500 F, Tempered at3000 F, Broken at 700 F. This Specimen wasEtched with 4%X Picral.

Bottom: Sample CCC., Austenitized at 23000 F, Temperedat 3000 F, Broken at 70OF

Magnifications: Sections, :)'X; Fracture Surfaces, 7X

37

.7,7

Figure 11-7 Top: Sample K, Austenitized at 15500Ft Tempered at

4000F, Broken at 70OF

Bottom: Sample FF, Austenitized at 23000F, Tempered at

4000F, Broken at 70OF

Magnifications: Sections, 5OX; Fracture Surfaces, 7X

38

46

40 B

• 2'I t•

Figure II-8 - Top: Sample G, Austenitized at 1550F, Tempered at'+50°F, Broken at 70°F

Bottom: Sample BB, Austenitized at 23000 F. Tempered at450 0F, Broken at 70OF

Magnifications: Sections, 50X; Fracture Surfaces. 7X

39

Figure 11-9 Top: Sample C, Austenitized at 15500 F,. Tempered at5000F, Broken at 70OF

Bottom: Sample HH, Austenitized at 23000F, Tempered at5000F, Broken at 70OF

Magnifications: Sections, 5OX; Fracture Surfaces, 7X

40

Figure II-10 -Top: Sample D,. Austenitized at 15500F. Tempered at550 0F, Broken at 70OF

Bottom: Sample DD, Austenitized at 23000F, Tempered at5500F, Broken at 70OF

Magnifications: Sections, 50X; Fracture Surfaces, 7X

41

Figure II-11 Top: Sample M, Austenitized at 15500F, Tempered at7000F, Broken at 70OF

Bottom: Sample EE, Austenitized at 23000F, Tempered at7000F, Broken at 70IF

Magnifications: Sections, 5OX; Fracture Surfaces, 7X

42

TABLE 11-2

FRACTURE TOUGHNESS OF AS-(QIENCNED EDGE-NOTCHED

$HEET SPECIMENS

Austenitizing KC

Specimen Hardness# R c Temperature OF psi JIM. Reinrks

E 15 56 1525 26,000---

E 20 55.5 1525 26,600 --

E 24 56 1525 26,000---

B 12 55 2300 85,000 stained

8 21 55 2300 90..000 unstainled# 0opcrack taken asintergramil arportion offracture

43

Figure 11-12 -Top: Sample AAA, Austenitized at 1550VF, Tempered at4000F, Broken at -346 0 F

Bottom: Sample U, Austenitized at 23000F, Tempered at4000F, Broken at -346 0 F

Magnifications: Sections, 50X; Fracture Surfaces, 7X

44

Figure 11-13 -Top: Sample A, Austenitized at 15500F, Tempered at

4000F, Broken at -109OF

Magnifications: Sections, 50X; Fracture Surfaces, 7X

~45

4

Figure 11-14 -Top: Sample K, Austenitized at 15500F, Tempered at~40OF2 Broken at 70OF

Bottom: Sample V, Austenitized at 23000F, Tempered at4000F) Broken at 70OF

Magnifications: Sections, 50X; Fracture Surfaces, 7X

46

A .

Figure 11-15 -Top. Sample F, Austenitized at 15500F, Tempered at~400 0F, Broken at 212OF

Bottom: Sample BBB, Austenitized at 23000F, Tempered at4000F, Broken at 212OF

Magnifications: Sections, 50X; Fracture Surfaces, 7X

47

e U-

00I

4-~ r() c

.4)

T) 4)0

o 0N (a

CL 4

I0 rIi I I I I IIM"

saLI~~punoqI UIu $IBSJOI~~~~~~d~ MUI ~lD4 O8OU3

4- 04-t

O0) 0 0 0 4C0 0

CI~jC00 0

000~ -6~ CL

C , o

> LO'4-

U4000

4-,0~0

to %.

8 80i~

~±A Sd~l'3~Isseu~6no e~frDD.

49

Tl~tns L~rstre:The surfaca roughness of specimens hardened f rom 1550OFand tomeed at 400CWdrops markedly as the testing temperature is lowered from70 to -346OF (Figs. 11-12 to 11-15 tqp. The impli ed drop in fracture toughnessis reasonable. Srawtey and loachemp7 for exaple, have shown that Kc of0.063 In. thick air-malted ADS 6434 steel quenched from 16000F anf~ amerod onehour at 4.00OF drops from 94 kpsI (in.)1/2 at OOF to 62 kpsi.(in.) at -1100F.

The fracture of all specimens quenched from 2300OF and teapered at 400OFware ductile when tested at down to -34e6OF (Figs. 11-12 to 11-16 bottom). Theamount of grain-boundary fracture was small (less than lOXJ and independent oftesting temperature. No cleavage could be observed, even at the lowest testtemperature (-3460F# Fig. 11-12 bottom). As Judged from the mote extensive anduniform surface roughness, It might be expected that fracture toug hness ofcoarse-grained materiel would be greater at all testing teoaperatures and thetoughness transition temperaturefwould be lower.

Recent work by Katz (8) gives some Indication that such predictions made onthe basis of fracture appearance may be confirmed with p1lane-st rain toughnessvalues (GIC) Of AISI 430i rod. Katz compared GzC as measured i.th circomfer-ential ly notched specimens, (0.001 In. radius) eustenitized at 1550 and 23000F#refrigerated# and tapred at 4s000F. Values after austenitizing at 2300PF were251 and 114 in lb./in.' at 70 and -236SF, respectively, while the comparatle,values for the lower austenitizing temperature were 79 and 47 in.-lb./in.'

IV. DISCUSSION

The results obtained thus for in the program load to a conclusion not Inaccord with more traditional view: If intergranular fracture can be avoided#austenitizing at higher temperatures may prove to be a method for substantial lyIncreasing fracture toughness. Nvesor, the basis for disagreement with such afinding appears largely In earl work on essentially plain-carbon stl * Ithas been doeonstrated by Scott, Nf Schone, (10) Rol f(1 I) and Zackay,(12J forexample, that medium-carbon steals, such as SAE 1 040p do in fact have muchpoorer mehncal properties, the coarser the prior austenite grain size.Bul Ien(3 hs pointed out that the tendency for the formation of a connectedproeutectoid ferrite network Is greeter( cearse-grained as contrasted to fine-grained medium-carbon stools. Brsmen (')has made clear the adverse effect ofsuch a ferrite network on match properties of quenched and tempeired stool. Thit seems that In steal s of SAE 10110 type the morphology of proeutectoid ferritecontributes importantly to the adverse effect of larger prior eustenite grainsize on mechanical properties.

Davenport and Bain( 41~) haealso shown that Increasing prior austenitegrain size has an adverse effect on mechanical properties in an approximatelyeutectoid carbon steel after water quenching to mertensite and tempering toRc 50. Since network formation is unlikely in this comoosition, theY resultsestablish an effect of prior austenite grain size pgr so on properties.Although no data were available at the time relating to the effect of austenite

50

grain size on the mechanical properties of low alloy steels, Davenport and Baindid point out that "inherently fine-grained" and "inherently coarse-grained"steels may differ in their response to grain coarsening. In particular, theysuggested that an inherently fine-grained steel, when coarsened, might lead toa finer omrtnsites as martensite is nucleated within the 4ustenite grain. Thisbehavior would not be expected in carbon steels, as Zackay(12) has shown that inthis material the maertensite plate size closely follows that of the prior aus-tenite grain-diamter. While such an observation may or may not be relevant toincreased toughness with higher austenitizing temperature, It does suggest thatthe mechanical properties of carbon steels and low alloy steels my differ Intheir relationship to grain size.

The question also remains as to whether the thermal treatment or the grainsize it produces elevates toughness. The observation that the fracture afterhardening from 2300°F evidences little delamination, whereas that after harden-Ing from 1550OF shows marked delamination, suggests that considerable reshapingof the inclusion and segregation structure occurs at the higher austenitizingtemperature. These structural changes could have marked effects on ductility,independent of the austenite grain size.

Avoiding intergranular fracture when heating to high austenitizing temper-atures can be difficult in AISI 4340. Heating this steel above about 2200°Fintroduces the possibility of "overheating", a phenomenon characterized by thetendency to fracture along the austenite boundaries produced in the overheatingrange.M) Such a structural deterioratto) hps long been recognized, but It isnot yet understood. Nawrth and Chriatian'1 5Jpoint out that the actual over-heating temperture for AISI 434O is quite variable, and considerable differenceoccurs from heat to heat even though produced according to the sam practice.The relationship of austenItizing temperature, 1oerheating", and resultingmicrostructures is a subject of direct interest In the continuing program.

V. SUMMtARY

1. The appearance of fractured face-notched specimens and two preliminary Kcmesurements at room temperature indicate that the AISI 4340 under studyis tougher when austenitized at 2300OF than at 15500 F.

2. Based on fracture appearance, the fracture toughness of coarse-grainedAISI 4340 sheet initially remains constant or drops slightly and thenincreases with increasing tempering temperature.

3. The appearance of fracture in face-notched specimens suggests that thecoughness transition temperature for hardened AISI 4340 sheet temperedat 400eF is lomr for coerse-grained then for fine-grained material.

51

VI. REFERENCES

1.* C. H. Sho B. L. Averbach and H. Cohen, Trans. ASK AlB (1956)p P. 86.2. D. KC. Sullens, Steel and Its Heat Treatment, Vol. It John Wiley

Hew York (1948), P. 3Lg9.

3. M. S. Grossman, Trans. ADMOI, 167, (1946), p. 39.

4. M. Baeyerts, E. i. Bumps and J. P. Sheehan, Armour Research FoundationTR No. 36 on ONR Contract N6 onr 274TOIl, Project HR 031-115p June 16, 1952.

5. 0. S. Lament, B. L. Averbach and M. Cohen, Trans. ASKH46) (1954), p. 851.

6. A. M. Turkalo, Trans. AME 218 (1960), p. 24.

7. J. E. Srawley and C. D. Beachem# HAL Report 5507, Jan. 10, 1959.

8. R. N. Katz, WA TN 320.1/Bg Dec. 1961.

9. N. Scott, Trans. ASH -4-, (19486)p P. 775.

10. P. Schane, Jr., Trans. ASH, 2t (93Li), p. 1038.

I). R. L. Rolf, Neat Treating Forginnt (La) p. ?j1, "I3.

12. V. F. Zackay, ASH Seminar, Strengthening liechanism of Sol.ds, October(1960), in press.

13. Bullens, op. cit., p. 34.7.

14. E. S. Davenport and E. C. Samp Trans. ASH Up (1934.), p. 879.

Ib-. R. D. Haworthp Jr. and A. F. Christian, Proc. ASTH 4I (1945), P. 407.

ACKNOIEDGEMENTS

The bulk of the data in Part I was submitted at M.I.T. by Richard W.Hertzberg as a thesis in partial fulfillment of the degroo of Master of Scienceand that in Part 2 by Robert N. Katz in partial fulfillment of the degree ofBachelor of Science. The authors are grateful for their efforts on this project.

52

Nussachstts Institute of TechnologyDA- 19-020-ORD-5235

DISTRIBUTION LIST

(Tasks A, B, C &. Do 5010.11.843) No. of Copie

A. Department of Defense

Office of the Director of Defense Research a.EngineeringATTN: Mr. J. C. BarrettRoom 30-1067p The PentagonWashington 25, D. C.I

Advanced Research Project AgencyATTh: Or.- G. MackThe PentagonWashington 25, D. C.I

CommunderArmed Services Technical Information AgencyATTN: TIPORArlington Hall StationArlington 12, Virginia 10

Defense hoels Information CenterBattellal Mmorial InstituteColuus, OhioI

Solid Propellant Information AgencyApplied Physics LaboratoryThe Johns Hopkins UniversitySilver Spring, Maryland 3

B. Oeoartment of the Army - Technical Services

Office Chief of OrdnanceATTN: ORDT0-Material sDepartment of the ArmVWashington 25, D. C.

Commanding GeneralAberdeen Proving groundATTN: Dr. C. Pickett# C&CLAberdeen Proving Ground, Maryl and

Commanding GeneralOrdnance Tank-Automotive Co mmandATTN: Mr. S. Sobak, ORDMC-IF-2Detroit 9p Michigan

Massachusetts Institute of TechnologyOA-1 9-020-ORD-5235

DISTRIBUTION LIST

(Tasks A, B, C t D,. 5010.11.843) No. ef Copies

Commanding Generalordnance Weapons CoommandATTN: Mr. B. Gerke, OROOW-IARock Island, IllinoisI

Co mmanding GeneralU.S. Army Ballistic Missile AgencyATTN: Dr. 6. H. ReisigI

Mr. P. B. Wallace, ORDAB-RPEMIDocumentaion &. Technical Informastion Branch 2ORDA1- IEEI

Redstone Arsenal, Al abamI

Coimmending GeneralU.S. Army Rocket & Guided Missile AgencyATTN: Mr.* Robert Fink, ORDXR-RGSS

Mr. W. H. Thomes, ORDXR-IQJRedstone Arsenal, Alabme

Comanding OfficerFrankford ArsenalATTN: Dr. H. Gisser, ORNBA-1330I

Mr. H. Markus, ORDSA-1320IPhiladelphia 37, Pa.

Cominnding OfficerOrdnance Material s Research OfficeWatertown ArsenalATTN: RPOWatertown 72, Mass.I

Cmmading OfficerPicatinny ArsenalATTN: Mr. J. J. Scavuuzop Plastics 6.Packaging Lab. 3

Mr. D. Stein, OADBS-DE3IDover, N. J.

Commeanding OfficerPLASTECPicatinny ArsenalDover,, N. J.I

Commending OfficerRock island ArsenalATTN: Material s Section, LaboratoryRock Island, IllinoisI

Massachusetts Institute of Technology

DA- 19-020-ORO-5235

DISTRIBUTION LIST

(Tasks A, 3, C & Cj, 5010.11.843) No. of Copies

Commanding OfficerSpringfiel d ArmoryATTN: Mr. R. Korytoski, Research Materials Lab.Springfield I, Mass.

Commanding OfficerWatertown ArsenalATTN: OROSE-LXWatertown 72, Moss. 3

Commanding OfficerWatervi iet ArsenalATTN: Mr. F.. Deshnaw, ORNF-RRWetervl iet, New York

HeadquartersU.S. Army Signal R&O LaboratoryATTN: Mr. H. H. Kedsky# SIGRA/SL-XEFort Monmouth. N. J.

Omeartment of the Arm - Other Arew Ancies

CommanderArmy Research OfficeArlington Nall StationArlington "12, Virginia

Chief of Research and DevelopmentU.S. Army Research and Development Liaison GroupATTN, Dr. B. SteinAPO 757, New York, N. Y.

C. Deartment of the Navy

Chief, Bureau of Naval WeaponsDepartment of the NavyATTN: RNWRoom 2225, Munitions huil dingWashington 25, D. C.

Department of the NavyOffice of Naval ResearchATTN: Code 423Washington 25, D. C.

Massachusetts Institute of TechnologyDA- 19-020-ORD-5235

DISTRIBUTION LIST

(Tasks, Ap BP C & D. 5010.11.843) Ng. of CoOies

Department of the NavySpecial Projects OfficeATTN: SP 271Washington 25, D. C.

ComanderU.S. Naval Ordnance LaboratoryATTN: Code WMWhite Oak, Silver Spring, aryl and

ComaenderU.S. Naval Ordnance Test StationATTN: Technical Library BranchChina Lake, California

CommanderU.S. Navel Research LaboratoryATTN: Mr. J. E. SrewleyAnecostia StationWashington 25, D. C.

D. D.nartment of the Air Force

U.S. Air Force Directorate of Research & DevelopmentATTN: Lt. Col. J. B. Shipp. Jr.Room 4-313, The PentagonWashington 25, D. C.

Wright Air Development DivisionATTN: N. Zoeller, ASRCEE-lWright-Pattepson Air Force Besep Ohio 2

ARDC Flight Test CenterATTN: Sol id Systems Division, FTRSCEdwards Air Force Bases Cal ifornia 5

AMC Aeronautical System CenterATTN: Mnufcturing & Materials Technology Div, LNBNDWright Patterson Air Force Basep Ohio 2

Massachusetts Institute of Technology

DA-1 9-020-ORD-5235

DISTRIBUTION LIST

(Tasks, A, B9 C & Di 5010.11.843) NO. of Cosies

E. other Government Agaenclies

National Aeronautics and Space AdministrationATTN: Mr. R. V. Rhode I

Mr. G. C. DoutchWashington D. C.

Dr. W. LucasGeorge C. Marshal I Space Fl ight CenterNational Aeronautics and Space AdministrationATTN: -S&M-Muntsvillep Alabama

Dr. L. JaffeJet Propul sion LaboratoryCalifornia Institute of Technology4800 Oak Grove DrivePasadena, California

Mr. William A. WilsonGeorge C. Marshal I Space Fl ight CenterATTN: M-F&AE-MHuntsville, Alabama

F. Defense Contractors

Aeroj et-General CorporatioeATTN: LibrarianPost Office Box 1166Sacramento, Cal ifornia

Aeroj et-General CorporationATTN: Librarian

Mr. C. A. FournierPost Office Box 296Azusa., California

Allison DivisionGeneral Motors CorporationATTN: Mr. D. K. HaninkIndianapolis 6, Indiana

Massachusetts Institute of TechnologyDA-1I9-020-ORD-5 235

DISTRISTION LIST

(Tasks, A,. S, C &D, 5010.11.843) No. of Copies

ARDE-Portland, Inc.ATTN: Mr. R. Alper100 Century RoadParamus., N. J.

Atlantic Research CorporationATTN: Mr. E. A. OlcottShirley Highway and Edsall RoadAl exandria, Virginia

Curtiss-Wright CorporationWright Aeronautical DivisionATTN: Mr. Rt. S. Shunts

Mr. A. 14. Kettle# Technical LibraryIWood-Ridgep N. J.

Hercules Powder CopanyAllegheny Ballistics LaboratoryATTN: Dr. R. SteiribergerPost Office fox 210Cunberl and, MarylIand

Hughes Aircraft CompanyATTN: LibrarianCulver City, Cal ifornia

Tapco GroupATTN: Mr. W. J. Piper23555 Euclid AvenueCleveland 17, Ohio

Massachusetts Institute of Technology

DA- 19-020 -ORD- 5235

SUPPLE1MENTAL DISTRIBUTION LIST

(Task A, Case Matis. & Fabrication, 5010.11.843) No. of Copies

A. Department of the Navy

Chief, Bureau of Naval WeaponsDepartment of the NavyATTN: Mr. P. GoodwinWashington 25, D. C.I

S. Department of the Air Force

HeadquartersAeronautical Systems DivisionATTN: Dr. Teaborskip ASRCNPWright-Patterson Air Force Base, Ohio

Wright Air Development DivisionATTN: Mr. G. Peterson, ASRCNC-IWright-Patterson Air Force Base, OhioI

C. Defense Contractors

Allegheny Ludlum Steel CorporationResearch CenterATTN: Mr. Rt. A. LulaBreckenridge# Pennsyl vania

Al loyd Electronics-CorporationATTN: Dr. S. S. White35 Caridge ParkwayCambridge, Mass. I

Armco Steel CorporationGeneral OfficesATTN: Mr. J1. BarnettHiddl'etowns OhioI

Battelle Memorial IfistituteATTN: Mr. ft. MonroeI

Mr. G. Faulkner505 King AvenueColumbus It Ohio

Massachusetts Institute of TechnologyDA- 19-020-OftD-5235

SUPPLENTAL DISTRIBUTION LIST

(Task A, Case Mstls. S.Fabrication,. 5010.11.843) No. of Copies

The Boeing CompanyAero Space Division'P. 0. Box 3707Seattle 24, Washington1

Borg-Warner CorporationIngersoll Kailmezoo, DivisionATTN: Mr. L. E. Hershey1810 N. Pitcher St.Kailamezoo, Michigan 1

The Budd CompanyDefense DivisionATTN: Mr. R. C. Dethioff 1

Mr. Ohmen1Philadelphia 32, Pennsyl vania

CI Immix Mol ybdenum CompanyATTN: Mr. ft. ft. Freeman1270 Avenue of the AmericasNew York 20, N. Y.1

Douglas Aircraft Company Inc.Santa Monica DivisionATTN: Mr. J. L.- WalimenSanta Monica., Cal ifornia I

General Electric Campanftocitet Engine SectionFl ight Propulsion Laboratory DepartmentCincinnati 15, Ohio 1

Jones and Loughl in Steel CorporationATTN: Mr. N. Steinbrenner45 South Montgomery AvenueYoungstown I, Ohio1

Arthur 0. Little, Inc.ATTN: Dr. Rt. DavisAcorn ParkCabridge 40, Mass. I

Massachusetts Institute of Technology

DA- 19-020-ORD-5235

SUPPLEMENTAL DISTRIBUTION LIST

(Task A, Case atls. Fabrication, 5010.11.843) No. of Copies

Lyon, Inc.ATTi: Mr. V. Martin13881 V. Chicago BoulevardDetroit, Michigan

Manufacturing LaboratoriesATTN: Dr. P. Fopiano

Dr. V. Radcliffe 121-35 Erie StreetCabridge .2, Mass.

inoeepolis-Honenwel1 Regulator Coaany1230 Soldiers Fie d RoadBoston 35, Mass.

Norris-Thermador CorporationATTN: Mr. L. Shiller5215 South Boyle AvenueLos Angeles 58, California

The Perkin-Elmr CorporationATTN: Mr. H. L. SachsMain AvenueNoral kp Connecticut

Pratt & Whitney AircraftATTN: Mr. F. A. CrosbyEast Hartford, Connecticut

Reactive Metal s CorporationATTN: Mr. H. LundstremNiles, Ohio

Republ ic Steel CorporationResearch CenterATTN: 'Mr. H. P. hMangerIndependence, Ohio

Rohm & Haas CompanyRedstone Arsenal Research DivisionATTN: LibraryHuntsvilles Alabama

Massachusetts Institute of Technology

DA- 1 9-020-ORD-5235

SUPPLE1MENTAL DISTRI3UTION LIST

(Task A, Case Matls. & Fabrication, 5010.11.843) No. of Cooles

Space Technology Laboratories,# Inc*ATTN: Technical Information Center Document procurementPost Office Box 95001Los Angeles 45, CaliforniaI

Thiokol Chemical CorporationUtah DivisionBrigham City, UtahI

Titanium Metals. CorporationATTN: Mr. G. Erbin233 BromadiyNew York, N. Y.

UniversaL-Cyclops Steel Corp.Stewart StreetBridgeville, Pennsylvania

U.S. Borax Research Corp.ATTN: Mr. Rt. J. Brotherton£412 Crescent WayAnaheim,. California

United States Rubber CompanyResearch CenterATTN: Dr. E. J. JossWayne, N. J.

D. Educational Institutions

Mellon InstituteATTN: Dr. N. L. AnthonyI

Mr.- C. J.- Owien4400 Fifth AvenuePittsburgh 13, Pa.

Michigan State UniversityATTN: Mr. R. N. lNamrDepartment of ChemistryEast Lansing, MichiganI

Massachusetts Institute of Technology

OA- I 9-020-ORO-5235

SUPPLEMENTAL DISTRIBUTION LIST

(Task A, Case uats. & Fabrication 5010.11.843) No. of Copie

Ohio State UniversityResearch FoundationATTN: Dr. t. McMasterColumbus, Ohio

E. Coimanding OfficerSeston Ordnance DistrictATTIis Contracting Officer's Rep.p ORDES LDSoston Army BseBoston 10, Massachusetts