Dr Matthew Peel, Mechanical Engineering, University of Bristol, UK

Evolution of stress in martensite during pulsed laser heating
When Nov 26, 2018
from 02:00 PM to 03:00 PM
Where LR1, Thom Building
Contact Name
Contact Phone 01865-273030
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The stress in martensitic steels during rapid thermal cycling, as occurs during laser cladding or welding, is challenging to predict. Thermal gradients are significant and temperatures shift rapidly over short times. Further, volumetric and shear strains during phase transformations superimpose on the thermal and plastic mismatch strains. These phenomena are readily implemented in FEA models but the resulting predictions of room-temperature residual stress are often unsatisfying. We find that fudging is needed and there can be multiple paths to getting similar answers. This talk will cover a recent experiment that should provide more rigorous validation data for developing models.

The ID31 beam line at the ESRF synchrotron facility has a fortuitous combination of extremely high X-ray flux and a new very high-speed (~250Hz) imaging detector. Each of the 2.4 million pixels is a counting detector with a threshold function that excludes photons outside a fixed range. The result is extremely low noise images with counts (and uncertainties) that are proportional to the incoming intensity. This means we have unprecedented capacity to accurately measure strain even during short events. We used a laser pulse to approximate cladding on thin sections of martensite-clad rail steel. Diffraction patterns were obtained from different locations during sequential pulses while the surface temperatures were measured using a thermal camera. Pre- and post-weld stress maps were measured for comparison. This presentation will cover challenges in obtaining the data, fitting the vast quantities of patterns produced, actually determining the temperature of the clad and, naturally,  interpreting the estimates of residual stress.