Mechanical and Microstructural Characterization of AF955 (UNS N09955) Nickel-Based Superalloy After Different Heat Treat
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55 Nickel-Based Superalloy
NICKEL-BASED superalloys are commonly used in extraction and the processing of oil and natural gas, owing to their excellent corrosion resistance.[1–3] The environments encountered in oil and gas extraction may contain significant levels of aggressive chemicals (e.g., H2S, CO, Cl, S2), which can be particularly harmful
ANDREA SERAFINI and MARIA FRANCESCA BRUNELLA are with the Dipartimento di Chimica, Materiali e Ingegneria Chimica Giulio Natta, Via L. Mancinelli, 7, 20131 Milano, Italy. GIULIANO ANGELLA is with the CNR-ICMATE Istituto di Chimica della Materia Condensata e di Tecnologie per l’Energia, Via Roberto Cozzi, 53, 20125 Milano, Italy. Contact e-mail: [email protected] CARLO MALARA is with Foroni S. p. A, Via Ambrogio Colombo, 285, 21055 Gorla Minore, VA, Italy. Manuscript submitted January 12, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
for materials, also because of the high pressure and temperature (200 C to 300 C) conditions during extraction.[4] Therefore, owing to their excellent corrosion resistance and strength at room and high temperatures,[5–9] the precipitation-hardened (PH) nickel-based superalloys are pre-eminent candidates for such applications. The AF955 superalloy with a chemical composition similar to IN-718 was developed in response to the industrial need for an alloy capable of preserving the exceptional properties of the PH nickel-based superalloys, of respecting the new and more stringent microstructure requirements expressed in the latest revision of the American Petroleum Institute (API) Specification API 6ACRA,[10] and of providing an adequate resistance to hydrogen embrittlement. The AF955 superalloy was designated as UNS N09955 by the Unified Numbering System (UNS) and is a PH nickel-based superalloy with a minimum 40 Pitting Resistance Equivalent Number, as defined by NACE MR0175/ISO 15156.[11] It is available with two strength
levels obtained through two different heat treatments: 827 MPa (120 ksi) minimum 0.2 pct offset yield strength (MYS) and 965 MPa (140 ksi) MYS. The 827 MPa (120 ksi) MYS grade is designated as Gr. 3, while the 965 MPa (140 ksi) MYS grade is designated as Gr. 3HS. Details of the properties of this alloy are given elsewhere.[12,13] For both grades, standard products are round bars ranging from 50.8 mm (2 in.) to 304.8 mm (12 in.) in diameter by about 6 m (20 ft.) to 10 m (32.8 ft.) in length.
(2)
From the VAR ingot, wrought round bars were obtained through the following main manufacturing steps: (1)
B. AF955 Microstructure Similarly to other PH nickel-based superalloys,[14–17] AF955 contains a considerable amount of alloying elements (such as Ti, Nb and Al) that are expected to contribute to the alloy hardening through the precipitation of the gamma prime (c¢) and the gamma double prime (c¢¢) phases. The c¢ phase Ni3(Al, Ti) is an ordered L12 crystallographic structure that precipitates consistently with the face-centered-cubic (fcc) c matrix and (cube-on-cube relationship (001)c¢||(001)c [100]c¢||[100]c) and exhibit
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