Measurement of local creep properties in stainless steel welds

A high temperature measurement system for creep deformation based on the digital image correlation (DIC) technique is described. The new system is applied to study the behaviour of a multi-pass welded joint in a high temperature tensile test and a load controlled creep test at 545°C. Spatially resolved tensile properties and time dependent creep deformation properties across a thick section type 316 stainless steel multi-pass welded joint are presented and discussed. Significantly lower creep strain rates are observed in the HAZ than in the parent material which is attributed to the introduction of substantial plastic strain in the parent material on initial loading. The weld metal shows the fastest creep rates and a variation that appear to correlate with individual weld passes. The visual information provides not only the local creep strain distribution but also the reduction of area and true stress distribution based on strains measured in the transverse direction. The results demonstrate the capability of the DIC technique for full field measurement of displacement and strain at high temperature long term creep tests

Measurement of Creep Deformation across Welds in 316H Stainless Steel Using Digital Image Correlation

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

Measurement of Creep Deformation in Stainless Steel Welded Joints

This article reports early findings of an experimental programme aimed at determining local creep properties of welded joints made from AISI Type 316H austenitic stainless steel. For this purpose, 3 mm thick, flat cross-weld specimens were cut from a pipe and subjected to creep testing at 550°C. In order to determine local creep properties around the weld within the gauge section of the specimens, a full field measurement system based on digital image correlation (DIC) technique has been developed. A purpose built furnace with an optical window was used to allow the gauge section of the specimens to be photographed during testing. The influence of the window opening on the temperature distribution inside the furnace was tested using five thermocouples embedded into a dummy specimen. A digital SLR camera with a 200 mm macro lens and an optical fibre illumination was used to acquire the photos. The gauge section of the specimens was sprayed with a high temperature resistant paint to obtain a speckle pattern, which is required by the DIC. The problems associated with the use of DIC at high temperatures, e.g. image distortion due to convective currents, surface oxidation, etc., and the techniques to overcome these are also discussed in the article. Full field displacement measurements allowed the local creep strain in the weld metal, HAZ and the parent material to be determined