Kazunari Fujiyama

Basic investigation for life assessment technology of modified 9Cr–1Mo steel

Abstract In order to develop life assessment techniques for aged components made of modified 9Cr–1Mo steel, specimens were artificially deteriorated by aging, creep and fatigue tests at elevated temperatures, and associated changes in the microstructure and mechanical properties were examined. It was observed that aging resulted in formation of Laves phase causing a decrease in toughness. The creep damage in base metal could be correlated with decrease in hardness, while creep damage in weldments could be correlated with the area fraction and density of creep voids. Creep rupture in weldments occurred in the fine-grained heat affected zone by the formation and growth of creep voids. The fatigue damage in base metal correlated to the maximum length of a crack among micro-cracks initiated during fatigue cycles.

Risk based engineering for design, material selection and maintenance of power plants

Abstract Risk based engineering (RBE) provides a measure of decision making through risk assessment of design, manufacturing, operation and maintenance. The application study of RBE to fossil power plants is presented here especially focusing on steam turbines. Typical damage modes and mechanisms are presented, and failure scenarios are expressed in terms of event trees. The comprehensive risk assessment approach is expressed in terms of the conventional risk matrix method for prioritizing the important events and the probabilistic approach for more quantitative evaluation. To make more rational decision making, a ‘Bayesian’ network approach is introduced as an effective tool of casual structure determination in the probabilistic risk assessment framework. The Bayesian network approach for determining the casual structure of risk scenario is shown to be a potential tool for constructing the risk scenario automatically in the data-driven manner even in the case of insufficient available data. Finally a material selection and design map is presented for the creep-fatigue damage condition of turbine components using a two-parameter probability distribution function to make a reliable initial design of high temperature structures.

Inelastic stress-strain response for notched specimen of 214Cr-1Mo steel at 600°C

Abstract Following a series of cooperative studies A-I and A-II (phase III) concerning the inelastic behaviour of high temperature materials under uniform state of stress, finite element analyses were carried out on circumferential notched cylinders subjected to plasticity-creep interaction conditions. Using an electric capacitance type extensometer “Strain-Pecker”, which is capable of measuring a local strain response with a gauge length of 0.5 mm under high temperature conditions, stress-strain responses for both global and local regions near the notch root were evaluated. Ten kinds of inelastic constitutive model were introduced into a finite element code, and the responses for four kinds of loading pattern were examined for two types of notch shape.

J-Integral Approach to Creep-Fatigue Crack Propagation in Lead-Free Solder under Various Loading Waveforms

Crack propagation tests of lead-free solder were conducted at room temperature in air using center-notched plates under load-controlled conditions with three waveforms: triangular pp waveform having fast loading and unloading rates, cp-h waveform having a hold time under tension, and cc-h waveform having a hold time under tension and compression. The J integral was evaluated from load-displacement curves. For fatigue loading of pp waveform, the crack propagation rate was expressed as a power function of the fatigue J-integral range. The creep component due to the hold time greatly accelerated the crack propagation rate. The creep crack propagation rate was found to be a power function of the creep J integral range for each case of cp-h and cc-h waveforms. The creep crack propagation rate for cp-h waveform was higher than that for cc-h waveform. Displacement-controlled tests were also performed under four triangular strain waveforms: pp, cp, cc and pc. For the case of pp waveform, the crack propagation rate was also expressed as the same power function of the fatigue J integral range as in the case of load-controlled tests. The creep crack propagation rate was expressed as a power function of the creep J integral range for each case of cp, pc and cc waveforms. Microscopic observations were conducted to clarify micromechanisms of creep-fatigue crack propagation.

The Statistical Material Selection and Structural Design Maps for High Temperature Components

The concept of material selection map proposed by Ashby [1] was modified to statistical maps for reliability design of high temperature components. The proposed procedure for making maps contained following steps; (i)define a performance index such as cost, (ii)define constraint conditions such as material strength, (iii)mapping the performance index under the constraint conditions and statistical distribution of material properties. The statistical distribution of material properties were expressed as the function of normalized parameters to obtain unified regressions for wide variation of material heats. The material properties used were tensile, creep and low cycle fatigue related ones for several ferritic and austenitic heat resistant steels mostly referred to NIMS (National Institute for Materials Science) database. This proposed mapping procedure was applied to the design of pipes and flanges under creep and thermomechanical low cycle fatigue conditions. The maps could show the statistically upper and lower bounds of allowable geometrical dimensions. The maps showed the effectiveness for optimizing the dimensions from the design aspects of creep and fatigue reliability.Copyright

Damage informatics concept for plant life-cycle management

Abstract ‘Damage informatics’ means here a total life-cycle management methodology including event scenario making, statistical damage analyses and risk-based maintenance decision-making. Event scenarios are expressed in terms of event trees for various components in steam turbines and gas turbines. The field damage data of specific machine types are analysed statistically and the probability of damage or failure can be expressed through the bivariate distribution function of total start-up cycles and operation time. For gas turbine nozzle cracking data, a damage evolution law for low cycle fatigue cracking is applied and the nozzle position dependence is clarified as the major cause of data dispersion. The risks are obtained by the product of probability and consequence of damage/failure and then shown to provide the basis of maintenance decision-making.

Risk Analysis of Failure Under Combined Damage Modes of Steam Turbine Components

The Risk Based Maintenance (RBM) procedure was applied to steam turbine casing as a typical example of components suffering from combined damage modes such as creep-fatigue cracking. Risk analysis was conducted through the field inspection database for cracks at the portion under creep-fatigue conditions. The primary stage of RBM is semi-quantitative risk analysis for risk prioritization of events using risk matrixes coupled with parts breakdown trees and event trees. The secondary stage is quantitative probabilistic risk assessment (PRA) to optimize maintenance intervals for the prioritized issues. The unreliability functions were expressed in two-dimensional log-normal type probability functions of operation time and start-up cycles. As the field data include censored or sustained ones, the precise correlation factors are not always obtained from the data sets. To estimate the most likely correlation factor, the Bayesian inference was introduced to the analysis of two-dimensional probability functions. The risk functions of operation periods were obtained using the assumed operation pattern, that is, the ratio of start-up cycles to operation time by substituting this relation into the two-dimensional probability distribution functions. Total expected cost function was defined as the sum of periodical repair cost rate and the total risk cost of subject event and component. The cost function has usually the optimum cost point and it can be used as the basis of the decision making of maintenance intervals.Copyright

Crystallographic assessment of creep damage in high chromium steel weld joints using EBSD observation

Abstract Microstructural change during creep damage process was investigated on the fine grained heat affected zone (HAZ) and the coarse grained HAZ of 12Cr steel casting base metal welded by 9Cr weld steel, using a scanning electron microscope with electron backscattering diffraction pattern (EBSD) equipment. Creep rupture and interrupted creep test samples were cut at mid-section along longitudinal direction and inverse pole figure, image quality and kernel average misorientation maps were obtained through EBSD observation. The measured quantities were creep void size, grain size, precipitation size, area averaged image quality value IQ ave and area averaged kernel average misorientation KAM ave. Micro Vickers hardness test was also conducted to investigate the hardness distribution across the weld joint. Among various creep damage evaluation parameters, hardness, area fraction of voids, KAM ave and area fraction of precipitates at each HAZ showed apparent changes with creep damage time fraction at HAZ portions of subject material. The hardness was the most useful parameter for both HAZ portions and showed the unique correlation with KAM ave for creep voided portions. It was also demonstrated that EBSD observation was an effective tool for characterising crystallographic change inside grains during creep damage process.

Creep Damage Assessment Through EBSD Method and Hardness Measurement for a High Chromium Turbine Rotor Steel Forging

10Cr-1Mo-1W-VNbN steel forging for steam turbine rotors was investigated using TEM (Transmission Electron Microscope), SEM (Scanning Electron Microscope) with EBSD (Electron BackScattering Diffraction pattern) method and nano-indentation tester for the assessment of microstructural change during creep damage process. Long term creep rupture tests and interrupted creep tests were conducted for the subject material and then effective damage parameters were assessed and compared with each other. Dislocation substructure which was observed through TEM thin foil method showed increasing lath/block width according to creep damage accumulation and the same feature was observed through EBSD IPF mapping more clearly and easily. EBSD KAM mapping was also conducted and averaged KAM was shown as an effective index for measuring dislocation microstructural changes during creep. Nano-indentation tests were conducted at the same position of EBSD measurement, which revealed that there was a good correlation between hardness value and the square root of averaged KAM. The differential equation of dislocation density with creep time was arranged to estimate the relationship between averaged KAM and time through the relationship between hardness and dislocation density. The creep damage estimation curves were obtained successfully by the equation expressed with stress and temperature term. The dislocation density based creep damage assessment curves could be effective for creep life assessment of high temperature components.Copyright

Analysis of Multifactor Damage for Heat Resistant Alloy. II. Multifactor Damage Simulation Analysis of High Temperature Low Cycle Fatigue for Superalloy Based on the Statistical Information of Crack Distribution.

Actual components used under severe conditions often suffer from the complicated material damage affected by multiple influencing factors. “Multifactor damage simulation analysis” was proposed to solve such complex damage problems. In this paper, a discrete cluster model of material and the associated damage state matrix [DN] are introduced to solve a high temperature low cycle fatigue damage problem of Co-base superalloy FSX414.The damage state at Nth cycle is determined by the damage state at (N-1)th cycle expressed as follows.[DN]=[Λ][DN-1]As this equation is similar to the discrete dynamical system and operator matrix [Λ] is non-linear, the chaotic behavior, of damage evolution may occur. Instead of solving the matrix equation directly, simulation was conducted using 2-dimensional model of 2500 clusters at 5×5mm area for the dendrite and grain boundary structure of FSX414. Damage of crack initiation and growth was calculated deterministically and the randomness was only introduced at the initial condition of the material.High temperature low cycle fatigue test was conducted to follow up the damage process at the total strain range of 1% and at the temperature of 1123K. Surface crack morphologies were investigated by the replication technique at 20% and 40% of the failure life.The trends of crack numbers, maximum crack length and mean crack length against fatigue cycles were obtained by the simulation. These trends were complicated like chaos due to the interaction of cracks and material structure. The crack length distribution by the simulations agreed well with the experimental results and better agreement was obtained by using intermittent inspection informations. These results suggested that the method would be effective for damage prediction of actual components based on inspection informations.