The excellent high strength and corrosion resistance of age-hardenable nickel-alloys have been put to use in marine engineering for many years. Alloy 725 exhibits resistance to hydrogen embrittlement and seawater corrosion along with excellent strength, ductility and toughness in both the annealed and age-hardened condition and the direct age-hardened condition. In these age-hardened conditions, alloy 725 exhibits excellent potential for marine springs, fasteners and hardware.
The U.S. Navy and their contractors often use corrosion-resistant fasteners with corrosion-sensitive materials such as steel, which require cathodic protection. For example, MONEL alloy K-500 (UNS N05500) fasteners are used with alloy steel in a seawater environment. The steel receives cathodic protection from sacrificial anodes. The protection is extended to the alloy K-500 fasteners. Failures of the alloy K-500 have occurred due to hydrogen embrittlement problems and also due to corrosion resulting from galvanic interaction with more noble materials. The U.S. Navy currently has a need to upgrade some alloy K-500 fasteners, which can suffer hydrogen embrittlement (HE), with a high-strength, more corrosion-resistant alloy. INCONEL alloys 725 (UNSN07725) is a highly corrosion-resistant, nickel-based alloys, which exhibit high strength, toughness and superior corrosion resistance and therefore are excellent candidate materials to as upgrades to alloy K-500 as fastener materials for the Navy in marine vessel construction. Direct aged alloy 725 exhibits the required properties for military spring applications. Direct aged alloy 725 refers to material in the solution annealed, cold worked and age-hardened condition.
Alloy 725 is resistant to hydrogen embrittlement in the NACE International TM01771 sulfide stress cracking test and are listed in the NACE MR01752 document "Standard Material Requirements ? Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment" and to chloride stress corrosion cracking in severe sour brine environments. Sulfide stress cracking is considered in the oilfield to be the most severe for hydrogen embrittlement. Depending on the alloy, applications include use in chemical and food processing, marine and offshore platform equipment, oilfield wellhead and subsurface equipment and tubular goods for severe sour service, salt plant evaporators, air pollution control systems, condenser tubing, service water piping and feed water heaters in the power industry.
DISCUSSION
Testing and Compostion
Unless otherwise specified, duplicate corrosion specimens were tested, and alloy 725 was tested in the solution annealed plus age-hardened condition. The limiting chemical compositions of the alloys are listed in Table 1. The alloys are generally compared with materials that are commonly used as marine fasteners.
Mechanical Properties
Table 2 displays the average room temperature tensile and Charpy-V-Notch (CVN) impact properties for alloy 725, 0.625 to 8.0 in. (16 to 203 mm) diameter hot finished, solution annealed plus age-hardened bar. Excellent strength, ductility and toughness are observed. These are average properties and do not represent specification minimums, which are a function of hot finish technique and bar diameter. Longitudinal and transverse orientations exhibit similar properties. Depending on the application, a minimum yield strength of 170 ksi (1,172 MPa) to 210 ksi (1,448 MPa) is required for direct aged alloy 725.
