Comparative Stress Corrosion Cracking and General Corrosion Resistance of Annealed and ...

36 Pages·2010·2.47 MB·English

https://ntrs.nasa.gov/search.jsp?R=19980053568 2019-04-12T14:27:10+00:00Z
NASA / TM- 1998-207686
Comparative Stress Corrosion Cracking and
General Corrosion Resistance of Annealed
and Hardened 440C Stainless Steel-New
Techniques in Stress Corrosion Testing
M.J. Mendreck, B.E. Hurless, P.D. Torres, and M.D. Danford
Marshall Space Flight Center, Marshall Space Flight Center, Alabama
April 1998
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NASA/TP-1998-207686
Comparative Stress Corrosion Cracking and
General Corrosion Resistance of Annealed
and Hardened 440C Stainless Steel-New
Techniques in Stress Corrosion Testing
M.J. Mendreck, B.E. Hurless, P.O. Torres, and M.D. Danford
Marshall Space Flight Center, Marshall Space Flight Center, Alabama
National Aeronautics and
Space Administration
Marshall Space Flight Center
April 1998
Available from:
NASA Center for AeroSpace Information National Technical Information Service
800 Elkridge Landing Road 5285 Port Royal Road
Linthicum Heights, MD 21090-2934 Springfield, VA 22161
[phone] [phone]
ii
TABLE OF CONTENTS
1. INTRODUCTION ...................................................................................................................... 1
2. OBJECTIVE ............................................................................................................................... 2
3. PROCEDURES .......................................................................................................................... 3
3.1 General Corrosion Testing ................................................................................................. 3
3.2 Electrochemical Corrosion Testing .................................................................................... 3
3.3 Constant Strain Stress Corrosion Testing .......................................................................... 3
3.4 Stress Corrosion Cracking Using Precracked SE(B) Specimens ...................................... 4
3.4.1 Fracture Toughness Testing ............................................................................. 5
3.4.2 The Incremental Loading Technique ............................................................... 5
3.4.3 The Constant Load Rate Technique ................................................................. 6
4. RESULTS ................................................................................................................................... 7
4.1 General Corrosion Testing ... ..... .... .... ... .... ... ........... ...... ...... .............. ..... ... ............... ........... 7
4.2 Electrochemical Corrosion Testing ............................................................ ;....................... 8
4.3 Constant Strain Stress Corrosion Testing .......................................................................... 9
4.4 Stress Corrosion Cracking Using Precracked SE(B) Specimens ...................................... 9
4.4.1 Fracture Toughness Testing .... ... .......... ..... .... ..... ......... ..... ...... ........... ....... ........ 9
4.4.2 The Incremental Loading Technique ............................................................... 9
4.4.3 The Constant Load Rate Technique ................................................................. 15
4.4.4 Scanning Electron Microscopy ........................................................................ 17
5. DISCUSSION ............................................................................................................................. 24
6. CONCLUSIONS ........................................................................................................................ 25
7. RECOMMENDATIONS ............................................................................................................ 25
8. APPENDIX ................................................................................................................................ 26
iii

LIST OF FIGURES
1. Round tensile specimen for constant strain stress corrosion testing . ........... .... .... ......... ......... 4
2. Single edge notched bend specimen SE(B) for crack growth testing..................................... 5
3. Annealed and hardened 440C stainless steel after 30 days exposure to
100-percent R.H. at 95 of ....................................................................................................... 7
4. Annealed and braycote 601 grease plated, and hardened and braycote 601 grease plated
440C stainless steel after 30 days exposure to 100-percent R.H. at 95 of ............................. 8
5. Electrochemical corrosion rates for hardened and annealed 440C... ...... .......... ... ...... ... ..... ..... 9
6. Incremental step load response of specimen HI in 3.5-percent NaCI.................................... 10
7. Incremental step load response of specimen H2 in 3.5-percent NaCI.................................... 11
8. Incremental step load response of specimen H6 in 3.5-percent NaCI.................................... 12
9. Incremental step load response of specimen A5 in 3.5-percent NaCl .................................... 13
10. Incremental loading response of specimenA6 in 3.5-percent NaCI ...................................... 14
11. Constant loading rate response of specimen H7 in 3.5-percent NaCI.................................... 15
12. Constant loading rate response of specimen A2 in 3.5-percent NaCI .................................... 16
13. SEM photograph of specimen HI fracture surface ................................................................ 17
14. SEM photograph of specimen H2 fracture surface ................................................................ 18
15. SEM photograph of specimen H6 fracture surface ....................................... ........ ................. 19
16. SEM photograph of specimen A5 fracture surface ...... ...... .......... .............. ....... ...................... 20
17. SEM photograph of specimen A6 fracture surface ................................................................. 21
18. SEM photograph of specimen H7 fracture surface ................................................................ 22
19. SEM photograph of specimen A2 fracture surface ............... .................... .......... ...... ...... ........ 23
v
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LIST OF TABLES
1. Time to failure of constant strain stress corrosion specimens .. ......... ........... ... ...... .... ..... ..... ...... 26
2. Summary of results from incremental step load and constant load rate testing ............... ......... 26
see
3. Proposed rating system for susceptibility ......................................................................... 26
vii
TECHNICAL PUBLICATION
COMPARATIVE STRESS CORROSION CRACKING AND GENERAL CORROSION RESIS·
TANCE OF ANNEALED AND HARDENED 440C STAINLESS STEEL-NEW TECHNIQUES
IN STRESS CORROSION TESTING
1. INTRODUCTION
Current methods for evaluating the susceptibility of a material to stress corrosion cracking (SCC)
involve loading specimens in tension to varying levels of the material's yield strength, placing the speci
mens in a corrosive environment, and monitoring the time to failure. Specimen configurations range from
smooth bar tensile, to C-ring, to 3-point bend. At the Marshall Space Flight Center, the constant strain,
smooth bar tensile specimen is used almost exclusively. Specimens are typically loaded in quintuplicate to
0, 25, 50, 75 and 90 percent of the material's yield strength and tested in three environments: High humid
ity (IOO-percent relative humidity (R.H.), 95 OF), 5-percent NaCl salt fog and 3.5-percent NaCI alternate
immersion. Evaluation of material susceptibility to SCC is based on time to failure at each stress level.
While these techniques for SCC testing have worked very well over the years, there are some
shortcomings. First, these methods yield only qUalitative data. A true threshold stress for SCC is not ob
tained because this threshold, typically designated KISCO requires consideration of both the bulk stress in
the specimen and the concentration of this stress around defects in the specimen. Crack growth in the
smooth bar specimens occurs when a corrosion pit or other defect grows to a critical size, which decreases
with increasing applied stress. The size of this defect is not readily quantified using smooth bar specimens.
The time to failure at a particular stress level becomes more a question of defect initiation and growth to
critical size rather than initiation of crack growth. In the study of SCC, we are, from a design standpoint,
much more concerned with that combination of flaw size, stress and environment which will cause an
existing defect to grow than with the time required to initiate a flaw. Second, these techniques are long
duration, requiring at least 30 days and sometimes up to 6 months to generate useful data. Finally, these
techniques tend to be very labor intensive and costly (five specimens at five stress levels in three environ
ments yields 75 specimens).
Alternative techniques which involve the use off atigue-precracked single-edge notched bend (SE(B))
specimens, yield quantitative data. Specimen defect size is readily determined from compliance equations.
Examples include the bolt-loaded wedge-opening-Ioading (WOL) and the cantilever beam techniques.
While it is possible with these techniques to obtain a true threshold for SCC (KIscd, the test duration is
prohibitively long, taking up to a year or more. In addition, there is some question of whether a true
threshold is actually obtained, particularly in the WOL technique, since self loading of the crack due to
corrosion product accumulation can yield conservatively low results.