1. API Standard 6A718 Inclusion of Additional Alloy 718 Strength Classes
Acceptance of alloy 718
at 140 ksi min yield strength
Rashmi Bhavsar
Global Materials Métier Manager
Schlumberger
Feb 2012

2. Agenda Background Inclusion of 718 -140
QC, heat treatment and microstructure
Corrosion testing

3. Inclusion of Additional Alloy 718 Strength Classes
Background
API Standard 6A718, 1st and 2nd Editions, have included only one strength class: 120K
A 120 KSI min yield strength is what has been historically required

4. Inclusion of Additional Alloy 718 Strength Classes
Background – Continued
API 6A718 at higher strength level used for other types of oil and gas production equipment, such as down-hole safety valves
The manufacturers of down-hole safety valves specify Alloy 718 with minimum yield strengths of:
120 KSI
130 KSI
135 KSI
140 KSI

5. Inclusion of Additional Alloy 718 Strength Classes
Background – Continued
The 135 and 140 KSI grades typically need to be precipitation hardened at a lower temperature range than permitted by API 6A718, thereby rendering material heat treated in this manner out of API 6A compliance. Or
hand picked to meet minimum yield strength – not practical

6. Inclusion of Additional Alloy 718 Strength Classes
The 130 KSI strength class could simply be added as another strength class without having to make any other changes.
The 135 KSI and 140 KSI strength classes cannot simply be added in this manner.
It will require aging temperature to be changed to 1400 to 1475F instead 1425 to 1475F for 140 grade
The yield strength range will be 140 to 150 ksi. (the current spec limits yield strength to 145 ksi max for 120 grade)

7. Quality Control and Specification Requirements
Early failures were related to
Poor quality - several macros with inclusions
Chemistry control and Melt practices
Lower annealing temperatures
Poor microstructure - secondary phases and
uneven grain size
Cold working for higher strength
Hence API 6A -718 spec was generated

8. Mechanical Properties

9. Performance tied to the Microstructure

10. 500 Psi H2S, 500 psi CO2, 25%NaCl at 500oF
Testing also successful at 350F & 450F in high H2S/CO2 – No cracks
(30 days test)
144 ksi actual yield strength at room temp – 100% stress level for six months
Courtesy: Foroni Metals

11. Suggested Change to Allow Higher Strength Alloy718
Heat Treatment:
Leave the annealing temp and time as is
Change the aging temperature range from 1425 to 1475F to 1400 to 1475F.
Leave the aging time of 6 to 8 Hrs.
The check is in the microstructure examination

12. Heat Treatment – Annealing
Stay with the higher annealing temperature

13. Heat Treatment - Aging

14. Suggested Change to Allow Higher Strength Alloy718
Mechanical Properties:
Allow maximum yield strength to be 150 ksi
Keep the maximum hardness to 40 HRc.
Keep the Charpy toughness values the same for all strength levels.
The check is in Charpy values

16. Initial ballots
A Ballot in 1982 from various suppliers and Shell Development to change hardness to 40HRC max
Hardness up to 45HRc and YS up to 165 ksi
Most data were at 750 and 1670F

17. Validation Testing Approximate cost $90 to 100K
With 40 HRc Max per Current ISO – min 3 heats
Perform 90 days four point bent beam or c-ring Or
Perform 30/90 days creviced bent beam or c-ring
Perform slow strain rate testing in duplicates for information and ranking
Test 130, 140 and 150 ksi actual yield strength
Test environment
NACE level 6 (500 psi H2S, 500 CO2 and 121,300 mg/l cl at ) 350F)
H2S per NACE limits at 350F
Approximate cost $90 to 100K

18. Future Validation Testing
With 42/43 HRc max – three heats
The same annealing temp and lower aging temperature
To raise the limit in ISO to 43 Max HRc
Testing to be conducted after the 40 HRc max hardness testing completed.
The max hardness to be tested with maximum yield strength available and compare the data to 40 HRc max sample used as reference.
Alternately test at 450F similar to alloy 935 and 945 and 945X.
Ballot for ISO / NACE MR0175
Suggest validation testing if funded