G-Phase and ?’ Phase embrittlement in Heat Resistant alloy

Introduction:

Niobium-modified heat-resistant alloys (HP-Mod HRA), specifically those classified as ASTM A297 HP15Nb and CT15C, are known to experience a loss of creep strength and creep ductility when exposed to operating temperatures between 1300 °F (700 °C) to 1650 °F(900 °C). This decline in performance is attributed to the formation of embrittled niobium-nickel-silicide phases, the G-phase (Ni16Nb6Si7)?[1]?[2].

Similar embrittle often encountered in the centrifugally cast heater tubes and static casting (also known as return bend) inside the radiant box, operated between 1560 °F (850 °C) to 1900 °F (1035°C) due to the formation of η'-phase (Nb3Ni2Si)?[2].

G phase embrittlement:

G-phases typically exhibit formulas of the structure A16B6C7, wherein A and B denote transition elements, while C signifies a Group IV element, such as silicon or germanium. These phases are characterized by a face-centered cubic unit cell comprising 116 atoms. In the case of HP-Mod HRA, component A is nickel, component B is primarily niobium (Nb), with minor quantities of iron and chromium present, and component C is silicon. Energy Dispersive Spectroscopy (EDS) generally identifies these phases based on the stoichiometric composition, which comprises 55.8 wt% Ni, 32.9 wt% Nb, and 11.6 wt% Si.?[2]?[1]

The G-phase is a diffusion-controlled process that occurs within a temperature range of 1300 °F (700 °C) to 1650 °F (900 °C)?[3]. It nucleates at grain boundaries where silicon (Si) segregates alongside niobium carbide (NbC). As the G-phase grows with continued exposure to elevated temperatures, the niobium carbide (NbC) is gradually depleted. This explains the common observation of the G-phase along the boundaries of dendrites. Additionally, the G-phase has been observed as interdendritic precipitates after extended aging, typically after several thousand hours (approximately 15000 hours).?[2]?[4].

The impact of G-phase embrittlement is characterized by a significant reduction in both creep ductility and creep strength. The degradation of mechanical properties is more pronounced in static casting processes, where the average grain size is larger. For instance, observations have indicated that static casting can exhibit nearly a 50% reduction in ductility, whereas centrifugal casting demonstrates a less severe impact from the G-phase, resulting in approximately a 10% reduction in ductility.?[2]. This variation is primarily attributed to differences in grain size. In static casting, the larger grain size leads to the formation of considerably larger and blocky precipitates that are more widely spaced. This configuration facilitates an increase in dislocation movement. In contrast, in centrifugal casting, the smaller dendrite size yields smaller precipitates positioned in closer proximity, thereby impeding some dislocation movement. Thus, the morphological and chemical transformations of precipitates during aging can substantially influence the oxidation, carburization, and creep resistance of the metallurgical properties.

The cracks are nucleated by the initiation of the creep voids at the interfaces between coarse G-phase precipitates and the surrounding matrix. This is one reason why metallurgical analysis of many failures exhibits cracks that are often observed near the G-phase with distinct characteristics, the areas enriched with silicon (Si) and niobium (Nb)?[4]?[2].

?’ phase embrittlement:

The ?’ phase precipitates have a face-centered cubic structure, similar to the G-phase, but they form at higher temperatures and after prolonged exposure. These precipitates have formulas of the type A3B2C, where A and B represent transition elements (mainly Cr, Mn, Nb, and Ni), and C is a Group IV element, such as silicon. The stoichiometric composition is approximately 65.7 wt% Nb, 27.7 wt% Ni, and 6.6 wt% Si?[5]?[2], though variation in composition, especially measured by EDS is expected. Research conducted by Hoffman identifies a precipitate with a slightly different composition: 51.8 wt% Nb, 36 wt% Ni, and 6.9 wt% Si, along with minor concentrations of Fe and Cr. This deviation from stoichiometry is most likely due to the alteration of overall composition by the presence of Cr and Fe. In HP-modified micro-alloy (25Cr-35Ni-Nb with rare earth microalloying element), the ?’ phase formed after prolonged exposure of approximately 10,000 hours of aging at 1922°F (1050°C)?[2]. The precipitates are primarily found along the boundaries of the dendrites, forming a network of M23C6-type secondary carbides. ?’ phase shows a similar effect as the G-phase, reducing ductility and creep strength. These precipitates are coherent with the matrix, softer than the matrix, and comparatively smaller in size (compared to M23C6 precipitates and blocky MC carbides), which means they should not significantly impact mechanical properties or weldability. However, this eutectic lamellar structure can become liquid at elevated temperatures, which may promote hot-tearing cracks during or shortly after welding and often while in service?[6].

Critical Characteristics of Failure Analysis and Microstructure Assessment:

From a failure analysis perspective, G-phase and η’ phase embrittlement may frequently be misidentified as Stress Relaxation Cracking (SRC) due to their similar characteristics. Metallographic analyses at lower magnifications, including optical and Scanning Electron Microscopy (SEM), show intergranular cracks filled with niobium (Nb) and silicon (Si) complexes, along with voids at the crack tips. These features may suggest SRC, identified by the presence of creep voids located ahead of the crack tips, where cracks are often filled with oxides or softer phases.

The metallurgical rationale behind this potential misidentification lies in the fact that both G-phases and η’ phases are generally softer than the surrounding matrix. These phases tend to precipitate at grain boundaries, which weakens these boundaries, leading to the rapid coalescence of creep voids. Thus, the phenomenon produces characteristic metallographic features: cracks formed from coalesces of these voids appear like creep voids in front of the crack tip, which is an iconic feature of SRC.

Impacted Equipment:

In the petrochemical industry, the G-phase is frequently observed in components made from HP-mod HRA when operating within a temperature range of 1300 °F (700 °C) to 1650 °F (900 °C).

·?In a Steam Methane Reformer (SMR), this phenomenon typically occurs in the furnace radiant tube outlet section (outlet manifold) and in the transfer lines generally made of CT15C.

·?In ethylene production units (steam crackers), the furnace's radiant outlet transition piece is usually constructed using static or centrifugal casting methods and made of HP15Nb, 25-35LC (a proprietary alloy), or CT15C.

How to minimize the embrittlement:

Silicon (Si) and niobium (Nb) are the key elements that affect the formation of the G-phase and the η' phase. By carefully managing the concentrations of these elements and controlling the grain size, the material's embrittlement can be significantly reduced.

?

References:

[1] R. C. Ecob, R. C. Lobb and V. L. Kohler , "The formation of G-phase in 20/25 Nb stainless steel AGR fuel cladding alloy and its effect on creep properties," JOURNAL OF MATERIALS SCIENCE, vol. 22, p. 2867 2880, November 1987.

[2] J. J. Hoffman, "High Temperature Aging Characteristics of 20Cr32Ni1Nb Casting," in NACE, Houston, TX, 2000.

[3] G. D. Barbabela, L. Henrique de Almeida, T. Luiz da Silveira and I. L. May, "Phase Characterization in Two Centrifugally Cast HK Stainless Steel Tubes," Materials Characterization, vol. 26, no. 1, pp. 1-7, 1991.

[4] M. Abbasi, I. Park, Y. Ro, Y. Ji, R. Ayer and J.-H. Shim, "G-phase formation in twenty-years aged heat-resistant cast austenitic steel reformer tube," Materials Characterization, vol. 148, pp. 297-306, 2019.

[5] E. I. Gladyshevskii, Y. B. Kuz'ma and P. I. Kripyakevich, "The crystal structure of Mn3Ni2Si, V3Ni2Si, Nb3Ni2Si and related Cr AND Ta compound," Journal of Structural Chemistry(Translated from Zhurnal Strukturnoi Khimii), vol. 4, no. 8, pp. 372-379, 1963.

[6] K. Hasegawa, "REPAIR WELDING AND METALLURGY OF HP-MODIFIED ALLOY AFTER LONG TERM OPERATION," Idemitsu Engineering Co. Ltd 37-24 Shinden-cho, Chuo-ku, Chiba 260-0027, Japan, Chiba, Japan, 2001.

[7] F. G. Caballero, P. Imizcoz, V. Lopez, L. F. Alvarez and C. Garc?′a de Andre′s, "Use of titanium and zirconium in centrifugally cast heat resistant steel," Materials Science and Technology, vol. 23, no. 5, pp. 528-534, 2007.

[8] S. Venkataraman and D. Jakobi, "Paper #9351: Review on the Heat Resistant Stainless Steel Alloys Used for the Steam Methane Reformer Outlet System," in National Association of Corrosion Engineer, Houston, TX, 2017.

[9] R. Kirchheiner and P. Woelpert, "Niobium in centrifugally cast tubes for petrochemical applications," in International Symposium Niobium, 2001.

?