3D creep cavitation characteristics and residual life assessment in high temperature steels: A critical review

Abstract

The paper seeks to assess critically the need for a new paradigm in creep life estimation of service exposed steels and provide possible new directions of residual life assessment, in the light of a decade of explosive application of micro-tomography on structural materials to characterise the 3D nature of cavitation damage. The phenomenon of creep failure using synchrotron micro-tomography (SR-µCT) has been studied at large synchrotron sources where imaging on conventional structural materials such as steels with high absorption to X-rays has been realized. The unique feature of the SR-µCT studies is the direct imaging in 3D of cavitation present in the bulk (hundreds of micrometers in size), revealing the spatial characteristics and morphology of the creep voids. Further the quantitative analyses of cavitation characteristics contained in the 3D datasets, which when scaled with time, stress and temperature provides functional information amenable for developing constitutive equations for creep fracture. These being based on SR-µCT, which due to its non-destructive feature provides high fidelity data that significantly reduces the ambiguity in the development of the functional relationships related to creep failure. Thus the use of such constitutive equations in an explicit manner to estimate the residual life of the components affected by creep and the consequent assessment of structural integrity would prove invaluable. This paper summarizes the studies related to creep in materials using micro-tomography with special emphasis on a 10.86%Cr heat resistant steel to show the type of data available which could be relevant for life assessment and design against creep failure. A brief discussion of the existing methods of residual life estimations in the light of the availability of the 3D micro-tomography data available in recent literature follows. Finally the possible new directions of using the microtomography data to formulate advanced residual life estimation methods are briefly touched.

The development of creep damage constitutive equations for high Cr steel

“…Figure 6. The probability density function of cavity equivalent R for E911, dots: experimental data from ref [12,25], curve by Equation (3.5.b). The experimental data based on the NIMS creep rupture time under different stress levels and temperature on typical ASME Grade 92 (9Cr-1.8W-0.5Mo-V-Nb) steels [11], and the value of U' was calculated as shown in Tables 6, 7 and 8.…”

“…The number density of cavities at failure for CB8 under a range of stress level has been researched by Xu et al [1], which the set of 3D cavity data referenced by X-ray micro-tomography. Fortunately, there is another set of 3D cavity data that the number density of cavities at failure for MARBN-heat 1 crosswelds under a range of stress levels from 70 MPa to 130 MPa are available [13,25]. The relevant data were extracted by reading the published graph and table.…”

“…No noticed difference of the predicted probability density function by Equation (3.1) (with α = 0.9999) and Equation (3.4) dots: experimental data from ref[12,25] curves.…”

In Situ 3D-µ-Tomography on Particle-Reinforced Light Metal Matrix Composite Materials under Creep Conditions

“…In situ micro-tomography focusing on the investigation of the phenomenon of creep damage in MMCs provide a new insight into the internal material processes under mechanical and/or thermal loading [26][27][28][29][30][31].…”

“…In this study, time-resolved in situ 3D-analyses by synchrotron X-ray µ-tomography were carried out to analyze the formation and growth of the cavitation based on the number and the volume as well as the morphology of the voids during creep to approach the creep mechanisms behind in the early and late creep stages. In situ micro-tomography focusing on the investigation of the phenomenon of creep damage in MMCs provide a new insight into the internal material processes under mechanical and/or thermal loading [26][27][28][29][30][31].…”