Ductility:

Ductility is the plastic response to tensile force. A Plastic response, or plasticity, is particularly important while a material is to be established through causing the material to flow in during the manufacture of elements. It also becomes significant in components which are subject to compression and tension, at each temperature among the lowest service temperature and the highest service temperature.   The Ductility is necessary for steels used in construction of reactor pressure vessels.  Ductility  is  needed  since  the  vessel  is subjected  to  pressure  and  temperature  stresses  which  must  be  carefully  controlled  to preclude brittle fracture.