REPORT

P A G E 1 O F 2 3

DATE: 4 December 1986

C O N T R A C T R E Q U I R E M E N T S

HARD STEEL SHOT PEENING AND CERAMIC SHOT PEENING EFFECTS ON

FATIGUE OF HIGH STRENGTH STEEL

C O N T R A C T ITEM

C O D E 26512

PREPARED BY: S . Lee v J TECHNICAL APPROVAL:

CHECKED B Y - J. Greenspan BY: C.J. Weizenecker \u

DEPARTMENT: Mfg. & Matls. Engrg. APPROVED BY:

M O D E L C O N T R A C T NO.

G A C 3 2 4 A R E V . 3

SECTION: Metallurgical Engrg. APPROVED BY:

REVIS IONS

D A T E REV.

B Y R E V I S I O N S & A D D E D P A G E S R E M A R K S

GAC 3288 R E V ' 11-83

SUBJECT :

Hard Steel Shot Peening and Ceramic Shot Peening Effects on Fatigue of High Strength Steel

SUMMARY :

The effect on fatigue of high strength steel due to hard steel shot peening and ceramic shot peening was evaluated by fatigue testing. The results indicated that peening with hard (S170) shot at 6A and 10A Almen intensities resulted in 52-77% increase in life above the unpeened control when compared with standard shot (9-12% increase). Similar results were obtained with ceramic shot with S170 at 6A. An increase in shot size to S230 produced approximately equivalent results for standard, hard and ceramic shot at 6A and 10A intensities. Inclusion of hard and ceramic shot into GSS 5310 will be actively pursued.

INTRODUCTION:

During landing, plastic deformation at the F-14 MLG axle radius results in residual tensile stresses, which have caused diminished endurance strength and reduced stress corrosion resistance. Additionally, machining marks on the F-14 stinger I.D. are a source for concern with respect to part endurance. Current peening with standard shot does not produce optimized surfaces since the part which is fabricated from 300M (R 53-56), is shot

C peened with shot softer than the part. The effectiveness of this operation is problematical.

The results from a previous program (Reference 1) show that a higher magni- tude of surface residual compressive stresses can be obtained by peening with hard shot (R 65, S230 size) than with standard shot (R 46). It was c C desired to determme the effect of peening parameters and shot type (includ- ing other sizes of hard shot) on the fatigue life of shot peened steel.

Steel parts heat treated to 260 ksi min. are peened with hard shot (RC 55 to 65) at various intensities, throughout the aircraft industry: (1) McDonnell Douglas (P.S. 14023) - 0.005 to 0.008A, (2) Northrop (MA-57) - 0.008 to 0.014A and (3) Boeing (BAC-5730) - 0.006 to 0.010A. However, no fatigue data on the effectiveness of hard shot (compared with standard shot) is currently avai1abl.e.

Ceramic media also has the ability to develop magnitudes of residual stress similar to hard steel shot. Ceramic shot effects on fatigue of high strength steel were also included for evaluation.

OBJECTIVES:

o To evaluate hard steel shot peening and ceramic shot peening effects on fatigue of high strength steel.

o To include hard shot (steel and ceramic) requirements in the GSS 5310 shot peening specification if hard shot peening provides a significant improvement in fatigue life for high strength steel.

REPORT M&ME-441A-86R-041 D A T E

December 1986 Orurnmen Aerospace & orporat~on

CODE 26512

PAGE

PROCEDURE:

Fatigue specimens of 300M steel (Per 45M PS325 - Figure 1) and tensile specimens (Per TGS 5308 - Figure 2) were machined in the longitudinal grain direction from bar stock and heat treated to 280-300 ksi (R 53-55).

C

Mechanical properties of the material were determined by the Riehle F14-60 testing machine and DAR-20 extensometer. The test results are presented in Table I.

The specimens were peened using different shot, different sizes and different intensities according to the schedules in Table 111, by Metal Improvement Company. Fifteen groups (6 in each group) of peened specimens plus one group in the as-machined condition for control purposes were subjected to fatigue testing.

The specimens were subjected to a two-level block loading spectrum (Reference 2) shown below to simulate the loading on the F-14 arresting hook stinger. The maximum stress applied was 208 ksi.

Two-Level Block Spectrum

Level Maximum Minimum No. of Cycles

1 100.00% 0.0 1

Fatigue testing was conducted on a computer controlled MTS servo-hydraulic testing machine.

RESULTS AND DISCUSSION:

The results of these tests are tabulated in Tables I1 & I11 and presented graphically in Figures 3 through 6.

o The fatigue testing results showed that a significant increase in fatigue life (50-77%) can be obtained at a 6A peening intensity for both hard shot (Rc 65) and ceramic shot (Moh 6-7) at sizes S170 and S230 and for standard shot (Rc 46) at S230. The next significant increases (40-44%) were obtained at a 10A intensity for standard shot and hard shot with S230. The increases for standard shot with S170 at 6A and 10A intensities were only 12% and 9% respectively, and for S230 at 16A, 14%.

o In the program on X-ray measurement of residual stresses on shot peened low alloy steel (Reference 1) it was found that the surface compressive stress was maximized at the lowest peening intensity (6A) within the intensity range explored (6A-16A) for 300M steel. Boeing showed similar results (Reference 3). The fatigue results of this test seem to bear that out (Table I11 and Figure 6).

GAC 3288 R E V I 1-83

R E P O R T M&ME-441A-86R-041 D A T E &December 1986

Grumman Aerospace orporation CODE 26512

PAGE

RESULTS AND DISCUSSION: - (Continued)

Hard shot (Rc 65) peening resulted in a significant increase (44-77%) in cycles to failure for 300M steel under the various conditions (S170/6~, S170/10A, S230/6A and S230/10A) except for (S230/16A) under which only an 18% increase was obtained. The results indicated that the compressive stress was maximized at the surface for ~170/6A and became less effective as size lint ens it^ increased. The distribution of compressive residual stresses varies with peening condition.

Ceramic shot (Moh 6-7) peening also resulted in a substantial increase in cycles to failure, 62% for ~170/6A and 59% for ~23016A; a moderate increase, 23% for S170/10A and 25% for S230/10A and 10% for S230/16A. It indicated that the same results as hard shot could be obtained with ceramic shot at an intensity of 6A for both sizes (S170 and S230).

Examination of the fracture surfaces from specimens with the lowest fatigue life from each of the groups revealed that the failure origin was at the surface near the edge and that no subsurface failure origins were encountered. The other higher fatigue cycle specimens showed similar conditions. Metallurgical examination of cross-sections near the failure origins showed the presence of surface irregularities and laps in various degrees in all sixteen groups including the control. Probably they were partly due to fabrication and partly due to peening (except on the controls). A typical example is presented in Figure 7.

The effect of peening on the specimen was evaluated by mounting the observed edge of the peened surface and polishing superficially. After nital etch, strain lines were observed as semi-circles of various sizes and quantity (Figure 8). The strain lines at higher magnification showed texturing (preferred orientation) Figures 9 and 10. Examination of the low fatigue life specimens from the fifteen groups (one from each group) showed that the strain lines appeared to be in direct proportion with peening intensity, shot hardness and size. Significant strain lines were found for peening at an intensity of 16A with S230 hard shot or ceramic shot and much less for peening at an intensity of 6A with standard 5170 shot (Figures 11 and 12). The strain lines were apparently the result of plastic deformation of the material. Optimum amount of plastic deforma- tion as obtained at an intensity of 6A enhanced the maximum compressive stress at the surface and in turn, the increase in fatigue life. Significant plastic deformation as from 16A intensity, caused strain- induced damage to the part and reduced the fatigue life.

CONCLUSIONS:

1. The results of this test indicated that the fatigue life of 300M (Rc 55) steel can be increased slightly (below 50%) with standard shot peening; a significant increase (50-77%), however, was shown after hard shot peening to an intensity of 6A with S170 or S230 shot size. A similar increase was accomplished for ceramic shot peening.

GAC 328R R E V 1 1 1-83

D A T E 4 December -1986 Grumman Aerospace Corporat~on

CODE 2651 2

PAGE

2. High peening intensities (16A) showed very slight improvement in fatigue life for either standard (14%), hard (18%) or ceramic (10%) shot. Apparently, the compressive stress was not maximized at the surface and strain induced damage was intensified.

3 . 6A intensity peening resulted in longer life than that achieved by peening at higher intensities (10A or 16A) and was consistent with prior X-ray residual stress results which showed a higher surface compressive stress from 6A intensity peening.

4. The results were in agreement with the finding of Boeing (Report BMT-SR-846) and McDonnell (Report No. TR 514-434/TM256-4016).

RECOMMENDATIONS:

o Hard steel shot peening and ceramic shot peening should be included in the GSS 5310 shot peening specification.

o The process should be recommended for the F-14 axle radius, landing gear and I.D. of the stinger.

References:

1. MP-MWENG-IM-78-135 2. NOTE (10-18-84), R. E. Hooson, GAC Structural Analysis 3. Boeing Company Report No. 6-8856-425-C5, 5-27-69)

GAC 328R R E V I 11-83

REPORT M&ME-441A-86R-041 D A T E 4 December -1986

Grumman Aerospace Corporat~on CODE 26512

P a g e 6

TABLE 1

TENSILE TEST RESULTS

Specimen 'TU %Y Percent Percent

('KS 1 1 (KS 1 ) Elongat Ion Reduc t l o n

of Area

1

2

3

4

Average

Shot Peen

Parmeterr

Control

Dn-pe ened

TABLE II:

'IATICUE TEST RESULTS

. I ' , P a g e 7

Specimen

1

2

3

4

L - Lapping

R - Notch

6 - Stra in Line

A - Normalized data due t o

high fatigue data r c a t t t r .

Cyclrs To

I a i lure Remr rks

14024

25132

17520

15160

14392 .

12564

(16465)

Page, 8 ; ;

Cyclrr To

Shot Peen

P l r m e terr

Q c l e r To

?&ilurt Remarks -.

18210

26 160

25920

23978

14840

29332

( 2 3 0 7 3 )

20044

11076

Shot Peen

Specimens

1

2

3

4

5

6

Wean)

1

2

3

4

5

6

Orean)

1

2

3

4

5

6

(Mean)

Cycler To

Fai lure Remarb --

27356 L/N/S

18 29 2

19636

2 1034

26944

37251

( 2 5 0 8 6 )

22045

28479

24354

26116

25282

28132

( 2 5 7 3 5 )

22966 ' .

25176

28466

19274

28884

17268

( 2 3 6 7 2 )

P a g e 10

Shot Peen

Parme tcrr

UOH 6-7

S 1 7 0

6A

Specimens

1

CHe an)

, L

P a g e ' I I

P a g e 1 2 Shot Peen

Parame ttrr

Cycler To

?a1 lure Remarks -.

19412

27712

20182

28258

26 170

35528

( 2 6 2 1 0 )

21022

19204

23632

22466

22328

15134

(20631)

18656 .

21470

16188

P a g e 13

"

TAaLL I11 - 300H H U T TREATED K ) (RCSS 1- 200% C O V e M C E

Spec faen Shot Shot teenin8 ?&tifie* Wycle** Rr&idual* * *

Croup No. Bardnemm S ire Inten8i ty y e Inereaie S t r r 8 r e 8 m 1 ) - - - Control -

No Shot Peening

STEEL SROT -

XI I Mob 6-7 4 a

XI11 Hoh 6-7

XIV noh 6-7

) XV Uoh 6-7

Average va lue of the aroup.

*a Z h t i g u e Q e l e Increase - Specimen - Control x LOO

Control

I Residual S t r e s s e l Previoutly Determined.

P a g e 14

P a g e 15

. ,

I

P a g e 1 6

STEEL SPECIMENS (Fk ss) P€ENED WITH STEEL M S OF HARDNESS m 6 FCUmffi SUES & INTENSITES

CONTROL - UNPEENED

F I G U R E 3

P a g e 1 7

-- UNPEENEC

F I G U R E 4

SOM STEEL 8PECIMFNS (Fk 55)

CONTROL - UNPEENED

P a g e 18

F I G U R E 5

F I G U R E 6

I F I G U R E 7 ETCHANT = 2% NITAL MAG: 500X

Photomicrograph of fatigue test (27356 cycles) specimen peened with hard shot (Rc 65) (S170; 10A intensity) showing the initiation site at the edge surface on the fracture surface lapping, and strain Lines near the initiation site (arrows).

D A T E 4 December. 1986 Grumman Aerospace Corporat~on

CODE 76512

PAGE 2 1

FIGURE 8 ETCHANT = 2% NITAL MAG: lOOX

Photomicrographs of specimen peened with standard shot (Rc 46) under condition S170/16A resulting in low fatigue life (11076,-9 showing the strain line size variation as the grinding and polishing progressed.

D A T E 4 December .I986 Grumman Aerospace Corporet~on

PAGE 2 2

FIGURE 9 ETCHANT = 2% NITAL

Enlarged photo of strain line (Figure 8 arrows).

MAG :

FIGURE 10 MAG :

Photomicrograph of strain line at high magnification showing texturing (preferred orientation) resulting from strain.

DATE 4 December 1 9 8 6 Grumman Aerospace Corporatton

CODE >E.:-'2

FIGURE 11 ETCHANG = 2% NITAL MAG: 500X

Photomicrograph of specimen peened w i t h hard s h o t (Rc 65) under c o n d i t i o n S230116A r e s u l t i n g i n f a t i g u e l i f e (18842m) showing a n abundance of s t r a i n l i n e s p r e s e n t .

FIGURE 12 ETCHANT = 2% NITAL MAG: 500X

Photomicrograph of specimen peened w i t h ceramic s h o t (Moh 6-7) under c o n d i t i o n S230116A r e s u l t i n g i n f a t i g u e l i f e (16278+) showing a n abundance of s t r a i n l i n e s .

R E P O R T M&ME-441A-86R-041 D A T E 4 December 1986

Grumman Aerospace Corporation CODE 76512