Investigation of Micro-structure and Creep Life Analysis of Centrifugally Cast Fe–Cr–Ni Alloy Reformer Tubes (original) (raw)
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Microstructure Evaluation During Short Term Creep of Cr35Ni45Nb Cast Alloy Reformer Tube
Metals and Materials International, 2020
In this work, a Cr35Ni45Nb alloy tube for use in a reformer furnace is subjected to high temperature (871 °C and 927 °C) creep regimes, and the microstructural changes (e.g. carbide precipitation and phase transformation) and consequent creep damage are investigated. The results of creep tests under closely similar applied stresses give different values of the Larson-Miller parameter (LMP) at different temperatures. As a result, the calculated creep rupture time at 871 °C based on the LMP value measured at 927 °C is approximately twice that which was actually measured at 871 °C. The microstructure of the as-cast tube is found to consist of an austenitic matrix with networks of MC (NbC) and eutectic (M 23 C 6) carbides located both at grain boundaries and between dendrites. It is noted that the M 23 C 6 carbide is not a primary eutectic carbide that can be observed in the as-cast Cr35Ni45Nb alloys. It can be argued that the primary eutectic M 7 C 3 carbides were transformed into M 23 C 6 due to the heat of spiral welding during the tube manufacturing process. After creep, the NbC carbides at both locations were mostly transformed to the G-phase (Nb 3 Ni 2 Si) and all the precipitates formed inside the austenitic matrix were composed of the M 23 C 6 and G-phase. Creep cavities were initiated around the G-phase and grew cracks along the grain boundaries due to the formation of a Cr-depleted zone and the G-phase.
Microstructural characterization of modified 25Cr–35Ni centrifugally cast steel furnace tubes
Materials Characterization, 2002
HP steels modified with Nb and Nb-plus-Ti, produced in the form of centrifugally cast tubes, were characterized by means of light optical microscopy, scanning electron microscopy (SEM) with secondary and backscattered electron imaging, and transmission electron microscopy (TEM) with energy-dispersive spectroscopy (EDS) and selected area diffraction. A complete description of the phases present in the ascast, welded, and aged condition was made as a function of the modifying elements. The chromium carbide stoichiometry, secondary precipitation, NbC instability at elevated temperatures, the G-phase transformation, and the effect of Ti on this reaction are discussed. It is indicated that partial transformation of G-phase in the alloy modified with Ti is one of the explanations for the superior creep properties observed in this material. D
Transactions of the Indian Institute of Metals, 2015
Premature failures of 25Cr35NiNb microalloyed steel reformer tubes are due to progressive microstructural degradation of the material during service exposure at high temperature. Limited information is available in the open literature regarding the influence of temperature on microstructure and mechanical properties of this material. Effect of temperature on the microstructure, carbide content and hardness of the steel across the wall thickness of tube exposed for 11 years is presented in this paper. No evidence of any microscopic defects was observed in specimen exposed at 650°C, whereas micro-voids were observed along grain boundary regions in the tube exposed at 997°C. Grain size increased with increase in service temperature. In the tube exposed at higher temperature, near the outer wall, carbides were distributed uniformly in the matrix, whereas carbides were concentrated at grain boundary near the inner tube wall, resulting in higher hardness near the outer wall of the tube.
High-temperature Tensile Properties and Creep Life Assessment of 25Cr35NiNb Micro-alloyed Steel
Journal of Materials Engineering and Performance, 2016
Reformer tubes in petrochemical industries are exposed to high temperatures and gas pressure for prolonged period. Exposure of these tubes at severe operating conditions results in change in the microstructure and degradation of mechanical properties which may lead to premature failure. The present work highlights the high-temperature tensile properties and remaining creep life prediction using Larson-Miller parametric technique of service exposed 25Cr35NiNb micro-alloyed reformer tube. YoungÕs modulus, yield strength, and ultimate tensile strength of the steel are lower than the virgin material and decreases with the increase in temperature. Ductility continuously increases with the increase in temperature up to 1000°C. Strain hardening exponent increases up to 600°C, beyond which it starts decreasing. The tensile properties are discussed with reference to microstructure and fractographs. Based on Larson-Miller technique, a creep life of at least 8.3 years is predicted for the service exposed material at 800°C and 5 MPa.
Metals
This study presents a failure analysis in two reformer tubes used for hydrogen production in a petrochemical industry. These tubes (Tube A and Tube B) were made by the centrifugal casting of HP-Nb alloy in such a way that one contained titanium as a micro-element, and the other was free from titanium in its chemical composition. Although the two tubes were subjected to similar creep conditions, Tube A failed after only 46,000 h of operation against the design life of 100,000 h. SEM images showed initiation and growth of creep pores next to chromium carbide particles, as well as the formation of microcracks in Cr23C6 carbides. Pore initiation occurs as a result of grain boundary sliding and is strongly dependent on structural morphology. The tube containing titanium (Tube B) showed higher thermal stability and higher creep resistance than the tube without titanium (Tube A), which was due to the formation of finer and more discrete carbide particles. The final fracture of the tube wit...
Condition Assessment Strategies in Centrifugally Cast HP Steam Reformer Tube Alloys
In steam methane reforming one of the key reliability aspects relates to ensuring the integrity and reliable operation of the catalyst filled radiant tubes. The tubes experience temperatures beyond 900°C for a considerable length of time, which makes creep the prime mode of failure. Predicting and assessing creep damage rates is challenging for a number of reasons: the alloys are not subject to any international specifications, the tubes are cast and their microstructures evolve substantially in service. These factors are known to lead to considerable scatter in creep life, strain and ductility response. This paper describes the initial steps of a current approach to managing tube life and outlines the early stages of a study aimed at development of a methodology to utilise appropriate inspection in combination with judicious creep strain/rupture testing to provide an effective strategy for longterm assessment and management of reformer tube integrity. The work involves creep testin...
Microstructural evaluation of welded fresh-to-aged reformer tubes used in hydrogen production plants
Engineering Failure Analysis, 2018
Heat resistant reformer tubes comprise a significant fraction of petrochemical reforming plants cost considering their high alloy content (i.e. 25Cr-35Ni-1Nb-0.1Ti). The bottom portion of tubes experiences the highest temperatures in the furnaces leading to microstructural changes, creep damage, and loss of elongation over their service life which in this case is twenty years. There is a cost-and time-driven motivation to only replace this portion of tubes by welding in contrast with replacing entire set of tubes which is the common industrial practice. However, welding new to aged tubes may lead to reliability issues due to difference in mechanical properties as a result of microstructural differences. In the current study, the microstructure and tensile properties of aged and new tubes have been evaluated in an effort to qualify the mechanical integrity of weldments. Welding trials are carried out to investigate the microstructure of the aged-to-new weldments and correlate it with the tensile properties (particularly elongation). Findings reveal that the heat affected zone of aged tubes is prone to micro-cracking of bulky primary carbides and incipient melting particularly at the inner surface where the root pass is applied. Adopting preheating for the root pass is effective in reducing carbide micro-cracking by decreasing cooling rate which assists in the accommodation of stresses generated by thermal contraction. Despite presence of carbide micro-cracks, tensile elongation is not severely affected as aged-to-new welds exhibit comparable and slightly higher elongation than aged base metals (above 4%). It is proposed that this is partially due to the orientation of micro-cracks in carbides. Further microstructural and tensile property results are presented and discussed.
Carburisation of Fe-Ni-Cr alloys at high temperatures
A large number of radiant tubes belonging to an ethylene furnace of a petrochemical plant failed dur- ing service. All tubes exhibited severe carburisation, while some of them lost their structural integrity and sagged. The tube material was based on a Fe-Ni-Cr alloy system with three varying compositions. Scan- ning electron microscopy and energy dispersive X-ray spectroscopy were used to characterize the micro- structure and elemental composition of the tube material. Microhardness was tested to determine their mechanical strength. Experimental results indicated that the sagged tubes exhibited a higher degree of carburisation as compared to other tubes. The microstructure of these tubes also revealed coarser Cr-carbide precipitation and a continuous carbide lattice at austenite grain boundaries. It was concluded that exposure to excessive temperature during service was responsible for the degradation of all tube materials. Based on the above results, it is recommended that better ...