Mechanical properties, corrosion resistance and immunity to environmentally assisted cracking are the key elements in selection of fasteners for sub-sea applications. While corrosion resistance and mechanical properties are attainable, finding materials that offer resistance to cracking in seawater service in areas of intense cathodic polarization or in galvanically coupled systems has been a major challenge.
High strength low alloy steels have had acceptable performance in cathodically protected systems. Their applications have been limited to maximum yield strength of 140 ksi (965 MPa) and a maximum hardness of 34 HRC. Development of deepwater has required larger size and higher strength fasteners than have been typically used for sub-sea applications. Costly interventions have necessitated the need for increased reliability for subsea systems including fasteners.
Laboratory testing to qualify several materials for high strength fastener applications for two projects has shown that, in the presence of cathodic protection, nickel alloys 718 and 725 offer the best resistance to hydrogen embrittlement and are available in the strength/size required for high pressure and moderate temperature applications.
The results of the study demonstrated the susceptibility of high strength alloy steel fasteners to hydrogen embrittlement in cathodically protected systems when the yield strength exceeds 140 ksi (965 MPa).
It is recognized that high strength steels are prone to SCC and HE and their resistance decreases with the increase in strength. The resistance of these materials is generally expressed in terms of a hardness limit, above which the material is not recommended for use in the specific environment (e.g. sea water), or in terms of the SCC or HE stress intensity factor threshold (KISCC, KIHE). Steels with strength levels below 120 ksi (827 MPa) yield strength are resistant to SCC and HE(1,2). Above 120 ksi (827 MPa) yield strength KISCC decreases with the increase in yield strength, with a typical threshold stress intensity value of 50 - 80 ksi.in1/2 (56 - 88 MPa.m1/2) for 145 ksi (1000 MPa) yield strength material(1,2). Typical fracture toughness (KIC) for145 ksi (1000 MPa) yield strength quenched and tempered steels is 200 ksi.in1/2 (220 MPa.m1/2).
In the absence of cathodic protection, high strength steels are typically more resistant to SCC in seawater. NASA has evaluated several aerospace grade high strength low alloy steels and shown that AISI 4340 is resistant to SCC up to a tensile strength of 180 ksi (1241MPa), equivalent to approximately 40 HRC(3). For cathodically protected components, historically alloy steel fasteners were limited to A320 B7M and L7M grades with a maximum hardness of 22 HRC (235 HB), the limit for sour service applications. Recent studies have shown that limiting sub-sea fasteners to the sour service requirements is overly conservative and sub-sea fasteners exposed to cathodic protection can be used to a maximum hardness of 337 HV (34 HRC) in accordance with the ISO/DIS 13628-1 and API 17D recommended practice4-5.