Application of small punch testing on the mechanical and microstructural characterizations of P91 steel at room temperature

• Typical elastoplastic regimes were developed for whole small punch tests.
• Top and bottom displacements of specimens were measured during the punch tests.
• Higher microhardness was found at the thinnest region of punched specimen.
• Increase of dislocation density and reduction subgrains was observed by TEM.

The use of small punch test (SPT) has emerged as a potential technique for mechanical characterization using miniaturized specimens. There is a strong interest in applying SPT for life prediction of power plant components operating at high temperatures. Another important application includes implementing surveillance programs for structural materials of nuclear plants where small volumes of irradiated samples are needed. In this work the small punch test was applied to study the mechanical behavior of P91 steel at room temperature. The selection criteria of the characteristic load PY in terms of two methods are discussed. Using this parameter the relationship with the yield stress is studied. The correlation factors were calculated from SPT curves. Microhardness was used to detect the most strained zones in a cross section of a punched specimen when reached its maximum load. The annular zone under the ball contact area, where the plastic thinning occurred up to maximum load, coincided with the highest microhardness values. Finally, transmission electron microscopy was employed to study the final microstructures after the deformation of the tensile and small punch tested samples. Refining of subgrains and considerable increasing of dislocation density was found with both, tensile and punch tested samples. By estimation of local strain and TEM observations it was confirmed the SPT produces higher deformation than that found with tensile test after rupture.