R N Ghosh

Modelling high temperature creep of CrMo steel

Abstract A computer based design aid called CRISPEN using established physical models of creep deformation has been developed jointly by NPL Teddington and Cambridge University for creep strain prediction of engineering alloys. This has been used to successfully predict the creep behaviour of a range of superalloys where strain softening is the most dominant mechanism of creep deformation. The present work examines how this approach could be modified to include the effect of softening due to time dependent particle coarsening, which is the most dominant mechanism of creep deformation in CrMo steel. A computer program has been developed to analyse creep curves to extract the relevant constants. A large volume of existing database on 2.25 Cr1Mo steel has been used to validate the approach. The physical significance of the parameters used has been analysed to identify the nature of stress dependence of particle coarsening behaviour in these steels.

Modelling effects of specimen size and shape on creep of metals and alloys

Most of the creep data required for the design of high temperature components are collected from tests conducted in air. Since metals and alloys are prone to oxidation under such conditions, test data are expected to exhibit a significant dependence on section size and shape of the test piece. The present work is an attempt to develop a simple model which could simulate the effect of both section size and shape of the test piece and suggest methods of estimating material constants directly from the creep curve. Specimen shape and size dependence of creep behavior is primarily due to the environment interaction. Synergy between creep and oxidation can be modelled by numerical integration for three different sectional shapes viz. circular, flat and tubular. Creep strain time plots of specimens having different shape but the same perimeter to cross sectional area are coincident.

Creep life extension of high-temperature components under wall thinning conditions

Current methods of life extension of high temperature components have been examined. Importance of changing section size as a result of corrosion and erosion has been emphasized. A simple method based on the influence of service exposure on rupture strength of the material and the data collected on loss of section size during routine shut-down of plants has been suggested. Highlights of the computer software developed to assist such an exercise have been discussed. The outputs have been shown to compare fairly well with the results reported in literature.

Clip: Computer software for creep life prediction of engineering materials

A menu driven user friendly software (CLIP) is developed for Creep Life Prediction of Engineering Materials. This software analyses the creep rupture data using three commonly used parameters, i.e. Larson Miller, Sherby-Dorn and Manson-Hafred parameters. Creep life of the material under user defined stress and temperature conditions is also calculated by this software.

An analysis of tertiary creep in 2(1/4) Cr-1 Mo-steel

Creep strain time plots of Cr-Mo steel exhibit a promin-ent tertiary stage. The primary s and the secondary stages are o small that they might be overlooked(1). These steels are being used extensively as high temperature compon-ents. Therefore, attempts are being made to discover a suitable mathematical formulation of the process so that correct predictions of the remanant life of such parts can be made at any stage during the service.

Creep Behaviour and Stress Analysis of a Seventeen-Year Service-Exposed Primary Super Heater Tube in a Thermal Power Plant

Abstract The elastic, inelastic deformation and stress distribution pattern of a seventeen year service-exposed primary super heater tube in a 120 MW boiler of a thermal power plant have been evaluated by using both analytical and numerical techniques. The commercial finite element computer code ANSYS® was used for stress analysis. The methodology is a valuable design tool for development of new boiler tubes as well as for considering the effects of numerous operating variables on creep life. The health of the tube was also assessed based on microstructure, hardness and a few conventional creep tests carried out at 500 °C at various stress levels (40 to 177 MPa). Results revealed that there was not much variation in the microstructure and hardness of the service-exposed tubes compared to the virgin material. Creep deformation behaviour of the service-exposed and virgin tubes of the same material at 40 MPa revealed that deterioration of the creep properties fell within the 20% scatter band, which is well within the specified limits of ASTM standard. The service-exposed primary super heater tube is thus in a good state of health.