Preventing Failure of High-strength Fasteners Used in Offshore and Subsea Applications
Table 2: Susceptibility to Hydrogen Embrittlement10
Alloy/Condition
AISI 4340 and 4130 >120ksi yield strength (ASTM A320 Gr L43)10,16 AISI 4140
(ASTM A-320 Gr L7 or ASTM A193 Gr B7)10,16 Duplex SS 250710,20,22
254 SMO10,22
Ferralium 25520 Alloy 286 (solution treated and aged)20 Marinel20,22 Beryllium copper20 K-500 all conditions10
Alloy X-75010,21
Alloy 925 (solution treated and aged)10,20
Alloy 62510,22
Alloy 718 (solution treated and single-aged per API 6A 718)10,15 Alloy 725 (solution treated and aged)10,15 Ti-6Al-4V (solution treated and annealed)10,20 Ti-5Al-1Sn-1Zr1V-0.8Mo (as forged)10,20
Hydrogen Embrittlement Susceptibility
Susceptible for hardness >34 HRC
at potential of -950mV or more negative Susceptible for hardness >34 HRC
at potential of -950mVSCE or more negative Slight or some effect of cathodic protection; hardness limit to 28 HRC
No or very little effect of cathodic protection Susceptible at potential of -1,000mVSCE Resistant
No or very little effect of cathodic protection Resistant
Susceptible at potential of -850mV or more negative
Susceptible at potential of -1,000mV or more negative and hardness >35 HRC Susceptible at potential of -1,250mV
Slight or some effect of cathodic protection; hardness limit is 35 HRC
• Susceptible for hardness >40 HRC
at potential of -850mV or more negative • Resistant
Variable susceptibility Resistant
Comments
Based on lab testing and field experience Based on lab testing and field experience Crack growth tests Crack growth tests Based on slow strain rate tests
Based on slow strain rate, C-ring, tensile and fracture mechanics tests Crack growth tests
Based on slow strain rate tests Based on field experience and slow strain rate tests
Based on field experience and slow strain rate tests
Based on slow strain rate tests Crack growth tests
Based on slow strain rate tests and filed experience Based on CT tests
• Susceptible at potential of -850mV or more negative Based on slow strain rate tests, • Resistant
based on CT tests
Based on slow strain rate tests, notched bars, U-bends.
Based on slow strain rate tests and fracture toughness
Table 3: Materials for 150ksi Yield Strength Fasteners
Alloy Group
Steels Stainless steels
Material
AISI 4130 AISI 4340 A286
Custom 455 PH 13-8 Mo S17400 S17700 S15700
Ni-based alloys Co–Ni alloys Be–Cu alloys Ti alloys
Alloy 718 Alloy 725
Alloy X-750 MP35N MP159
MP159 Modified C17200 C17300 C17000
Ti-6Al-6V-2Sn
Ti-6Al-6V-2Sn-4Zr-6Mo Ti-7Al-4Mo
Ti-4.5Al-5Mo-1.5Cr Ti-10V-2Fe-3Al
Ti-8Mo-8V-2Fe-3Al Ti-11.5Mo-6Zr-4.5Sn Ti-3Al-8V-6Cr-4Mo-4Zr Ti-13V-11Cr-3Al
Ti-15V-3Al-3Cr-3Sn Ti-15Mo-2.7Nb-3Al-0.2Si
EAC = environmentally assisted cracking. *Requires further qualification testing.
drilling risers and blowout preventers (BOPs). Polarised potential of -1,050 to -1,100mV (Ag/AgCl/seawater) can be produced from aluminum or zinc anodes in some areas of subsea structures, which represents a substantial level of overprotection for non-ferrous alloys, particularly CRAs that could potentially induce HE.7
HE
becomes more pronounced when the CP level is more negative than -900mV (Ag/AgCl/seawater).7
78
Qualification Testing
In view of the limited data, selecting subsea fastener applications still relies on qualification testing for the specific application. One qualification test reported that resistance to hydrogen cracking in slow strain rate testing for eight high-
EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 1
Localised and galvanic corrosion and EAC resistance data for high-strength materials are limited and somewhat contradictory.11–13
Yield Strength
>150 >150 150 230
190–205 145–170 140–230 170–230 130–176 165 130 230 276 150
145–190 145–190 125–185 145–160 150–160 150–160 120–153 150 170 175
180 (aged) 160 190 205
Size Limit
Very low Very low
Questionable Good Good
Very low Very low Very low Good Good Good
<2 inches Good Good
Low toughness Low toughness Low toughness <3 inches <4 inches <2 inches
Questionable* Good potential <4 inches <3 inches <4 inches <4 inches
Good potential Good potential
Good
EAC Resistance
Questionable* Questionable* Questionable*
Questionable* Questionable* Questionable* Questionable*
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