Continuous in situ measurement of quenching distortions using computer vision

A heat treatment vacuum furnace equipped with a CCD camera is used to follow the shape evolution of two axisymmetric steel samples during gas cooling. The experimental results are compared with numerical simulations performed with the finite element technique. A good agreement is obtained between the theoretical predictions and experiments. The shape evolution of metallic samples are measured experimentally and simulated numerically during all the cooling processes. The accuracy of the measurement is 0.01 mm, while the dimensions of the samples are 40 mm diameter and 80 mm height. Stepwise increasing hole diameters are used in the samples to amplify the shape changes during quenching. The first steel sample does not exhibit phase changes during cooling. The shape evolution is only due to thermal shrinking. Since residual distortions are not observed, the thermal gradients generated during these experiments are not large enough to induce plastic deformation. The second steel sample presents a martensitic transformation during cooling. The shape changes are thus due to both thermal shrinking and phase changes. The residual distortions observed can be attributed to transformation plasticity. It turns out that this physical phenomenon can be studied by the experimental set-up described in this paper.