Structure and phase stability in a cast modified-HP austenite after long-term ageing

Kenik, E.A.; Maziasz, P.J.; Swindeman, R.W.; Cervenka, J.; May, D.

doi:10.1016/s1359-6462(03)00238-0

Cited by 85 publications

(16 citation statements)

References 7 publications

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“…These precipitates are rich in Ni, Nb and Si as shown by the EDS spectrum in Fig. 4, c. As quoted in literature [19,20], Nb carbides are not stable at temperatures between 700 and 1000°C, being transformed into nickel niobium silicides, identified as the G-phase. According to the results of the full area x-ray diffractions, there are four reflections on the diffraction pattern, which correspond to the G-phase.…”

Section: Metallographic Observationssupporting

confidence: 63%

Metallographic and Mechanical Studies of a Cast Heat-Resisting Alloy

Aichaoui¹,

Hadji²

2017

Metallofiz. Noveishie Tekhnol.

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A heat-resistant steel tube from an ammonia plant made of modified HP40 steel that failed after short-term service is studied for damage mechanism. The assessment of material degradation is carried out using optical microscopy, scanning electron microscopy in combination with energy dispersive spectroscopy analysis, x-ray diffraction analysis, and mechanical tests. Results show that precipitation of the chromium-rich carbides induces the coalescence of grain boundaries. Significant growth and poor distribution of secondary carbides are also noticed through the matrix, which lead to a reduction of material ductility even after a short-term service. The main cause of failure appears to be damaged catalyst. Such a problem can cause a rise in temperature leading to localized overheating in the lower part of the tube. Overheating is primarily responsible for significant degradation in microstructure, creep strength, and mechanical properties of the tube.Key words: heat-resistant steel, carbide, damage mechanism, overheating, creep.Для встановлення механізму руйнування було вивчено виготовлену з мо-дифікованої жароміцної сталі НР40 трубу з аміячного заводу, що вийшла з ладу після короткотермінового використання. Вивчення деґрадації ма-теріялу було проведено за допомогою оптичної та сканівної електронної мікроскопій у комбінації з енергодисперсійною спектроскопічною аналі-зою, рентґенодифракційними дослідженнями та механічними випробу-ваннями. Встановлено, що виділення насичених Хромом карбідів викли-кає коалесценцію меж зерен. Також встановлено істотне зростання та ро- Ключові слова: жароміцна сталü, карбід, механізм пошкодження, пере-грів, плазучістü.Для установления механизма разрушения бûла изучена изготовленная из модифицированной жаропрочной стали НР40 труба с аммиачного завода, вûшедшая из строя после кратковременного исполüзования. Èзучение деградации материала бûло проведено с помощüю оптической и сканиру-ющей электронной микроскопий в комбинации с энергодисперсионнûм спектроскопическим анализом, рентгенодифракционнûми исследовани-ями и механическими испûтаниями. Óстановлено, что вûделение насû-щеннûх хромом карбидов вûзûвает коалесценцию границ зёрен. Также установлено существенное увеличение и разрежённое распределение вто-ричнûх карбидов в матрице, что приводит к снижению пластичности ма-териала даже после кратковременного исполüзования. Îсновной причи-ной вûхода труб из строя является повреждённûй катализатор. Эта про-блема может вûзватü повûшение температурû, что приводит к локалüно-му перегреву в нижней части трубû. Èменно перегрев, в первую очередü, ответственен за существеннûе деградацию микроструктурû, ухудшение границû ползучести и механических свойств трубки.Ключевые слова: жаропрочная сталü, карбид, механизм повреждения, перегрев, ползучестü.

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Section: Metallographic Observationssupporting

confidence: 63%

Metallographic and Mechanical Studies of a Cast Heat-Resisting Alloy

Aichaoui¹,

Hadji²

2017

Metallofiz. Noveishie Tekhnol.

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“…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.…”

Section: Cr-ni-nb-si Rich Phasementioning

confidence: 59%

“…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].…”

Section: Cr-ni-nb-si Rich Phasementioning

confidence: 95%

“…In the as-cast condition, M 7 C 3 does not have an orientation relationship with the austenitic matrix [19]. The M 23 C 6 carbides are typically observed to have a cube-cube orientation relationship with the matrix [19,22], the same crystal structure and a lattice parameter almost exactly three times that of the matrix. No orientation relationship between the M 7 C 3 carbides that transform from M 23 C 6 as a result of carburization and the matrix has been reported.…”

Section: Chromium Carbidesmentioning

confidence: 99%

“…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.…”

Section: Cr-ni-nb-si Rich Phasementioning

confidence: 99%

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Microstructure and Carburization Detection in HP Alloy Pyrolysis Tubes

McLeod

Bishop

Stevens

et al. 2015

Metallogr. Microstruct. Anal.

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A highly carburized HP40-Mod alloy ethylene pyrolysis tube was characterized by means of scanning electron microscopy with backscatter electron imaging, electron back-scattered diffraction, energy dispersive spectroscopy, and etching using the NACE International Standard Test Method for evaluation of carburization of ethylene pyrolysis tubes. The response of the tube to eddy current non-destructive testing was measured using the carburization crawler under development at Quest Integrity NZL Ltd. The matrix was significantly depleted of chromium, as low as 4 wt% Cr at the inner wall. M 23 C 6 carbides transformed to M 7 C 3 at the inner wall region and NbC carbides partially transformed to the chromium-rich g-carbide at the outer wall region, both of which likely contributed to the chromium depletion of the matrix. The present results indicate that the NACE etchant attacks the austenitic matrix where chromium content is below approximately 12wt%. A comparison to other ex-service tubes indicates that matrix chromium content and the location of the M 23 C 6 /M 7 C 3 transformation front are useful microstructural characteristics for interpreting eddy current NDT response.

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Failure cause assessment of steam reformer radiant tube after long‐term service

Dobrotvorskaia¹,

Dobrotvorskii²,

Shevyakova³

et al. 2022

Materials & Corrosion

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This study investigates the damage, microstructure, and mechanical evolution of service-exposed HP40NbTi radiant tubes of a steam reformer furnace. Tube failure was reported after 6-year of working at 525°C-830°C in the form of visible longitudinal and transverse cracks. The microstructure was evaluated using optical and scanning electron microscopy equipped with energydispersive X-ray spectrum analysis. The carbide phase transformation was determined by the method based on the matrix dissolution technique and Xray diffraction. The results of this study showed that the primary cause of the longitudinal crack formation is the local overheating and the oxidation of the dendritic grain boundaries. The M 7 C 3 carbide transforms completely into the metastable M 23 C 6 phase during high-temperature service. Metallographic examinations revealed the shrinkage cavities and creep voids that are nucleated on both Cr-and Nb-carbides. The continuous oxide layers of Cr 2 O 3 and SiO 2 formed near the inner and outer tube surfaces and the matrix's chemical composition near the outer tube surface was found to be depleted of Cr and C. Mechanical properties are irregular across the samples. A little margin of safety is noted, while the complete ductility dip is detected at room temperature. The root cause analysis of the failure under investigation showed that the tube failed due to the creep.

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Structure and phase stability in a cast modified-HP austenite after long-term ageing

Cited by 85 publications

References 7 publications

Metallographic and Mechanical Studies of a Cast Heat-Resisting Alloy

Metallographic and Mechanical Studies of a Cast Heat-Resisting Alloy

Microstructure and Carburization Detection in HP Alloy Pyrolysis Tubes

Failure cause assessment of steam reformer radiant tube after long‐term service

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