These are scanning electron microscope (SEM) images of the fracture surface of a pull stud from a CNC Deckel milling machine.  The pull studs are what the drawbar grabs on the toolholder to hold it in the spindle taper.  This stud and another like it broke before the spindle was turned on, so no major damage.  The images (fractographs) show that the fractures were intergranular brittle, which is not right for properly heat treated steel of this sort.  It also appears that the fractures initiated due to some stress-corrosion cracking, but most of the failure was due to embrittlement, most likely due to incorrect heat treatment.