In metal forming processes in the tool-workpiece contact area occur high pressure, surface expansion and elevated temperature. It makes ideal circumstances for braking of the lubricant film that causes a direct contact between metallic surfaces. Such a contact usually leads to buildups creation on the tool surface. These phenomena mainly result product surface damage. Damages can also refer to tool surface since the hardness of buildups can be comparable with the tool hardness. The cause of this investigation was finding the answer of the question what was the reason of extreme hardness of buildups created during bulk metal forming processes of 1H18N9T austenitic stainless steel. The studies were conducted using transmission electron microscopy (TEM). On the basis of analysis structural and electron diffraction pattern it was suggested that simultaneously to very well known strengthening mechanism like grain refinement, the additional mechanism can not be excluded. Based mostly on the electron diffraction pattern (appearance of forbidden reflexes for FCC) it is suggested that this additional mechanism could be the marthensitic transformation caused by very high plastic deformation.