Application of nanoindentation technique for structural characterisation of weld materials

Nanoindentation, a modern technique for investigation of mechanical properties of materials using a modified scanning force microscope (SEM), has been employed to examine two different types of boiler shell weld materials from nuclear power plants. These weld materials differ mainly in their manganese content and temperature treatment. The nanoindentation technique allows very small regions in grains to be investigated and different phase structures are distinguished using this technique. It is shown that a difference exists between the microhardness values, which integrate over several grains compared to the nanohardness measurements for weld materials made over smaller regions. The difference between the phase structures of these small regions is determined by a combination of both SFM imaging and the characteristic properties of force–depth curves which allow specific values of hardness and Youngʹs modulus to be determined. Detailed measurements of the nanohardness and Youngʹs modulus for various weld materials are given and these values are related to the material properties. It has been found that the hardness of the grains is always less in the centre and increases towards the grain boundary.