Microstructure and properties of alloy HP50–Nb: comparison of as cast and service exposed materials

Abstract

Spun cast alloy HP50-Nb tubing retired from service in a steam reformer after approximately 8 years of service had stress rupture properties suggesting very short remaining life. Microstructural examination, however, indicated little or no creep damage (cavitation) but did reveal extensive microstructural modification. The microstructure, mechanical properties, and stress rupture properties of the ex service material were determined and compared with virgin as cast material. The microstructural changes that occurred during service included the formation of phases rich in alloying elements such as chromium and niobium that would otherwise be expected to contribute to high creep strength. The creep life of the service exposed material was correspondingly short. The creep ductility of the service exposed material, however, was significantly higher than that of the as cast material. The implications of these findings for remaining life assessments of reformer furnaces using these materials are discussed.MST/3207

Microstructure and Carburization Detection in HP Alloy Pyrolysis Tubes

“…Another phase, the diamond cubic g-carbide, has also been identified in ex-service HP-Nb reformer tubes and aged HP-Nb alloy [10,22,[25][26][27]. The typical composition of the transformed phase in the ex-service pyrolysis tube closely resembled the reported compositions.…”

“…The Cr-Ni-Nb-Si rich phase appeared to be transformed from NbC, and the area covered by this phase appeared to increase as distance from the inner diameter increased. The instability of NbC precipitates at temperatures of 700-1050°C has been well reported in the literature [1,[22][23][24][25][26][27]. The product of the transformation is generally identified as G-phase, which has a commonly reported composition of Ni 16 Nb 6 Si 7 , and an FCC structure with a lattice parameter of 1.12 nm [1,23,24].…”

“…The current literature on the NbC-to-G-phase and NbCto-g-carbide transformations in HP-Nb alloys is constrained to ex-service reformer tubes [8,22,[25][26][27] and a limited number of laboratory-aged studies [1,[23][24][25]. Based on the available literature and data from laboratoryaged HP-Nb alloys, Buchanan [10] concluded that the NbC-to-G-phase and NbC-to-g-carbide transformations in aged HP-Nb alloys are dictated by the aging temperature.…”

Failure Analysis of HP40 Steel Tubes utilized in Steam Reformer Units of Petroleum Refinery Plants

“…The combination of high temperature, carbon oxide atmosphere and moderate tensile stresses causes that the damage in these components occurs mainly by the presence of creep, microstructural degradation and metal dusting 1 . These components operate at temperatures of approximately 850 °C and internal pressures between 1 and 3.5 MPa, and its service life is expected to be about 100,000 h [2][3][4][5][6] . This type of components is usually fabricated with heat resistant austenitic stainless steel, like the HP40 steel (25Cr-35Ni steel), which has excellent creep strength, and good resistance to carburization and oxidation [1][2][3][4][5][6] .…”

“…These components operate at temperatures of approximately 850 °C and internal pressures between 1 and 3.5 MPa, and its service life is expected to be about 100,000 h [2][3][4][5][6] . This type of components is usually fabricated with heat resistant austenitic stainless steel, like the HP40 steel (25Cr-35Ni steel), which has excellent creep strength, and good resistance to carburization and oxidation [1][2][3][4][5][6] .…”