Moat, In situ observation of strain and phase transformation in plastically deformed 301 austenitic stainless steel, (2016), AbstractTo inform the design of superior transformation-induced plasticity (TRIP) steels, it is important to understand what happens at the microstructural length scales. In this study, strain-induced martensitic transformation is studied by in situ digital image correlation (DIC) in a scanning electron microscope. Digital image correlation at submicron length scales enables mapping of transformation strains with high confidence. These are correlated with electron backscatter diffraction (EBSD) prior to and post the deformation process to get a comprehensive understanding of the strain-induced transformation mechanism. The results are compared with mathematical models for enhanced prediction of strain-induced martensitic phase transformation.

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Spatially resolved measurement of creep deformation across weldments at high temperature cannot be achieved using standard extensometry approaches. In this investigation, a Digital Image Correlation (DIC) based system has been developed for long-term high-temperature creep strain measurement in order to characterise the material deformation behaviour of separate regions of a multi-pass weld. The optical system was sufficiently stable to allow a sequence of photographs to be taken suitable for DIC analysis of creep specimens tested at a temperature of 545°C for over 2000 h. The images were analysed to produce local creep deformation curves from two cross-weld samples cut from contrasting regions of a multi-pass V-groove weld joining thick-section AISI Type 316H austenitic stainless steel. It is shown that for this weld, the root pass is the weakest region of the structure in creep, most likely due to the large number of thermal cycles it has experienced during the fabrication process. The DIC based measurement method offers improved spatial resolution over conventional methods and greatly reduces the amount of material required for creep characterisation of weldments.

This paper is a research output of DMW-Creep project which is part of a national UK programme through the RCUK Energy programme and India's Department of Atomic Energy. The research is focussed on understanding the characteristics of welded joints between austenitic stainless steel and ferritic steel that are widely used in many nuclear power generating plants and petrochemical industries as well as conventional coal and gas-fired power systems. The members of the DMW-Creep project have undertaken parallel round robin activities measuring the residual stresses generated by a dissimilar metal weld (DMW) between AISI 316L(N) austenitic stainless steel and P91 ferritic-martensitic steel. Electron beam (EB) welding was employed to produce a single bead weld on a plate specimen and an additional smoothing pass (known cosmetic pass) was then introduced using a defocused beam. The welding residual stresses have been measured by five experimental methods including (I) neutron diffraction (ND), (II) X-Ray diffraction (XRD), (III) contour method (CM), (IV) incremental deep hole drilling (iDHD) and (V) incremental centre hole drilling (iCHD). The round robin measurements of weld residual stresses are compared in order to characterise surface and sub-surface residual stresses comprehensively

Digital Image Correlation is not the intended use for consumer colour cameras, but with care they can be successfully employed in such a role. The main obstacle is the sparsely sampled colour data caused by the use of a colour filter array (CFA) to separate the colour channels. It is shown that the method used to convert consumer camera raw files into a monochrome image suitable for digital image correlation (DIC) can have a significant effect on the DIC output. A number of widely available software packages and two in-house methods are evaluated in terms of their performance when used with DIC. Using an in-plane rotating disc to produce a highly constrained displacement field, it was found that the bicubic spline based in-house demosaicing method outperformed the other methods in terms of accuracy and aliasing suppression.

Deterioration assessment of materials is essential to the continued effective operation of critical components in thermal power plants. Establishing the degree of creep exhaustion of power engineering alloys operating at high temperatures and stresses guides maintenance strategies to ensure reliable plant operation. Within progressive inspection philosophies, traditional laboratorybased creep testing is often difficult to conduct on ex-service steel due to the limited material availability from which to machine standard specimen geometries. This work investigates a technique for the measurement of creep strain curves at several stresses and at 600 C from a single test using a nontraditional specimen geometry together with full-field strain measurement through digital image correlation (DIC). Of interest is ex-service X20CrMoV12-1 (X20) which is widely used in older, subcritical thermal power plants. The paper aims to show that multiple creep curves can be resolved over a spatially varying stress field using DIC whilst preserving material economy. Differences in creep behaviour between ex-service X20 with varying levels of service exposure are evident from quantitative comparisons of the creep strain and rate curves through threshold stress computation which agrees with hardness measurements and microstructural observation of subgrains and precipitates using electron microscopy. These single-specimen tests yield high densities of creep data which can be used in the calibration of creep damage models for characterisation of ex-service X20.

The Open University's repository of research publications and other research outputs The influence of temperature on deformation-induced martensitic transformation in 301 stainless steel

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