High-alloy materials possessing good corrosion resistance in hot, sour brines are being seriously considered for offshore applications. These alloys are available in a wide range of strength levels which can be attained by the methods of cold working or precipitation hardening.
Results of testing INCONEL* alloys 625 and 718 and INCOLOY* alloys 825 and 925 for resistance to general corrosion, hydrogen embrittlement, and stress-corrosion cracking (SCC) are presented. This paper also contrasts the range of mechanical properties available in precipitation-hardened materials with properties of coldworkedalloys.
The severe environments presently encountered in intermediate-depth wells coupled with increasing costs of inhibition, particularly in offshore wells, have caused considerable interest in highly alloyed, corrosion-resistant materials1.
To meet the high strength requirements, tubular products in nickel-base alloys can readily be produced with uniform properties via cold working processes. Wellhead and subsurface equipment frequently cannot be uniformly cold worked, thus creating a need for alloys that achieve high strength through a precipitation heat treatment.
This study focuses on two precipitation-hardened alloys, INCOLOY alloy 925 and INCONEL alloy 718, plus two cold worked alloys, INCONEL alloy 625 and INCOLOY alloy 825. Basically, the four alloys represent two different classes of nickel alloys. Alloys 625 and 718 contain more nickel and refractory elements (Mo and Nb) than alloys 825 and 925, whose lower nickel contents place them closer to the iron-base alloys. The alloy classes will behave differently in differing corrosion environments. They represent varying levels of resistance to hydrogen embrittlement, SCC, and general corrosion. Table 1 shows the nominal compositions of the four alloys.
Mechanical properties are shown in Tables 2 (production tubing) and 3 (casing and coupling stock) for cold-worked tubular products. The minimum annealed yield strength of alloy 825 according toASTM B-423 is 30 ksi. Note that moderate cold working raises the yield strength to that of the higher strength API grades. Comparable reductions for alloy 625 produces an even greater yield strength increase owing to its higher work-hardening response. Properties of cold-worked alloy 925 are similar to those of alloy 825.
Much concern2 has been expressed about the effects of " well aging" (under the high downhole temperatures encountered in service) on the stability of highly coldworked alloys. Data in Tables 2 and 3 indicate that the mechanical properties of alloys 825 and 925 are only slightly affected by aging at 6000 F for 1000 hours.
Alloy 925 was recently developed to provide high strength while retaining good ductility in both tubular and solid products. Gamma prime precipitation develops the desired properties in this alloy system. Table 4 compares the yield strengths of alloys 825 and 925. The same thermal treatment was used for both alloys, clearly demonstrating the higher strength capabilities of precipitation-hardened, alloy 925.
In addition to providing an alternative strengthening mechanism to cold-working, precipitation hardening also provides an improved combination of strength and ductility for a given strength level.
