In the previous papers, we revealed the mechanism of spinodal decomposition in the aged material of hot-forged and hot-rolled duplex stainless steel using Mossbauer spectroscopy and FE-TEM. Mechanical properties of micro-Vickers hardness, yield stress and Charpy impact energy for the aged material were also studied. It was found that sound velocity in duplex stainless steel increases with aging time, that is progress of thermal aging.

In this paper, some specimens of duplex stainless steel casting aged at 723K up to 300h were prepared. To evaluate the degree of thermal aging, sound velocity of longitudinal wave and surface acoustic wave (SAW) was measured. As the values of sound velocity in the casting tend to scatter due to coarse grain and anisotropy of crystal, the following techniques were applied to minimize the effect of scattering. First, the data obtained were statistically analyzed. As the number of measuring points increases more and more, averaged value of sound velocity has higher reliability and can be used for quantitative evaluation. The number of measuring points required was determined in this study. Second, to measure the sound velocity more precisely, leaky surface acoustic wave (LSAW) was generated in very small region of the material using line-focus large-aperture transducer, and the phase interference technique using burst wave was applied. As a result, it was clear that averaged value of the sound velocity increases with aging time and has correlation with fracture toughness as well.