A Raj

Creep and Fatigue Behavior in Micro-alloyed Steels – A Review

Abstract This is a study of microalloyed steels for power plants and reactors. Components operate at coal dust fire temperature or thermal states of reactors, prone to creep during its service. This is to assess remaining life after passage of valuable life by variation in microstructure, e.g. cavity formation. Precipitation at the sub-grain boundaries and grain interior has increased high temperature strength. Coarsening of these appears at the end of life. Variation of heat treatment like spheroidising in place of solutionizing has been responsive to deteriorate performance. Dislocation interplay with precipitate has been acceptable while interaction among dislocations to forest dislocation has been unacceptable. Dislocation assisted nucleation of precipitates of fine size has been found to strengthen steel by thermo-mechanical control process with in greater heating temperature and lower finish rolling temperature. High temperature performance of materials has been assessed by creep, accelerated creep, creep-fatigue and fatigue performances. Increasing temperature for increasing efficiency has correlated the phase transformation of steel. Fatigue performances have been included in creep properties of materials when intermittent shut down–shut up schedules are operated, e.g. peaking power plants.

Damage Analysis of Service Exposed Reformer Tubes in Petrochemical Industries

Abstract Accelerated creep or stress rupture data is used for remaining life assessment for life management studies of elevated temperature components, e.g. for reformer tubes where packed nickel catalysts are used for synthesis of hydrogen, ammonia etc. This research has become a regular task because of large range of time for failure (3 to 15 years) compared to designed life (11.4 years or 100,000 hours) and huge loss associated to damage, production and safety hazards. Utilization of appropriate inspection during plant shut down has been strategic short term life assessment. Tests have been typically done by high temperature mechanical properties, microstructure analysis and accelerated creep. Inspection of micro-cracks, hot spot formation, carburization/metal dusting for inner wall and oxidation, tube diameter increment for outer wall inspection have been traditional symptoms of expiry of tubes after service exposure. Aim of this review has been to study damage analysis of reformer tube in response to so wide time frame for failures and accidents involved, even after stipulation of designed time schedules.

Creep deformation and damage evaluation of service exposed reformer tube

Abstract This paper aims at studying the creep deformation behaviour and quantifying creep damage of ∼11 years service exposed primary hydrogen reformer tube made of HP40 grade of steel in a petrochemical industry, in terms of Kachanov’s continuum damage mechanics model (K model) and Bogdanoff model (B model) based on Markov process. Hot tensile, conventional creep deformation under identical test conditions, optical microscopy and fractography were extensively carried out. The as received tubes did not possess any sign of degradation including voids or creep cavitations and decarburisation. There was indeed loss of tensile strength from room temperature to 870°C for the bottom portion of the tube due to aging and overheating. Accumulation of damage due to creep has been quantified through microstructural studies in terms of two damage parameters A and A*. Experimental scatter observed in creep deformation and creep strain rate curves of the material at 870°C and at various stress conditions, is probably due to scatter in creep cavitations/voids and also due to variation in the mode of fracture in top as well as bottom portion of the tube. From statistical point of view, Weibull distribution pattern for analysing probability of rupture due to void area, shifts with increase in true strain towards the higher population of void. The estimation of average time to reach a specific damage state from K model and B model is in close agreement with that of experimental data and can describe the sudden changes of the creep damage in the tertiary region as well.

Life Estimation and Creep Damage Quantification of Service Exposed Reformer Tube

Abstract This paper deals with evaluation of creep damage of ~11 years service exposed primary hydrogen reformer tube made of HP-40 grade of steel in a petrochemical industry, which has been carried out in terms of Kachanav’s continuum damage mechanics (CDM) model (K-model) and Bogdanoff model (B-model) based on Markov process. Residual life of the tubes was estimated based on hot tensile, conventional creep deformation under identical test conditions, optical microscopy and fractography. Accumulation of damage due to creep has been quantified through microstructural studies. The as received tubes did not reveal any degradation in the material like creep cavitation or voids, but there was indeed loss of tensile strength from room temperature to 870°C for the bottom portion of the tube due to ageing and overheating. Scatter in creep deformation behaviour of the material is probably due to variation in mode of fracture and scatter in voids. From statistical point of view, Weibull distribution pattern for analysing probability of rupture due to void area shifts with increase in true strain towards the higher population of void. The estimation of mean time to reach a specific damage state from K- model and B-model is in close agreement with that of experimental data and can describe the sudden changes of the creep damage in the tertiary region as well. A remnant life of >10 years is estimated at the operating stress–temperature conditions of the top as well as bottom portion of the tube.

Performance and Life Assessment of Reformer Tubes in Petrochemical Industries

Abstract High temperature mechanical properties of service exposed reformer tubes are compared and revalidated with those of virgin material for the sake of health assessment studies. Technical interpretations of creep and stress rupture (accelerated creep) data usually are in close proximity in absence of defects. Results of extrapolation of accelerated creep data were optimized. This review article is aimed at studying life prediction methodology of reformer tubes operating at high temperature and aggressive chemical environment, in response to the designed time frame for preventing failures and accidents, even before stipulated design time schedules in some cases.