Results and discussion
Failure modes of the original die
After examination of more than 50 receded dies of the
original configuration (code D01) it was found that only
a small percentage, about 4%, were rejected from the extrusion procedure due to wear, causing a product out ofdimensional limits. All other dies presented cracks,Fig. 2a,
and in some cases, small metal pieces were detached from
the surface of the die profile. The cracked dies were examined with optical and SEM microscopy and it was found udies of the fracture surfaces with SEM
revealed a network of a brittle phase and decohesion
between the matrix and this phase. It became evident that
cracks were associated with the microstructure of the Stellite 4B resulting after several extrusions. Cobalt alloys are
known to have complex microstructures with brittle phases
precipitated from the matrix during extensive heating orformed from the transformation of other microconstituents
present in the microstructure [20–25]. Microstructural
examination of the Stellite 4B before and after the extrusion process (end of die life) revealed that the material
exhibits aging transformation,Fig. 3, which leads to brittleness. Because of this transformation, the hardness of
the Stellite 4B increases from 48HRc to 52HRc. Detailed
examination of the microstructural changes occurring during die service and their role in the reduction of die life are
under investigation and will be presented in a future paper.
On the base of the above considerations it was decided
that modifications in the materials, die design and cutting
procedures should be investigated, in order to assess factors able to prolong die life. It is worth noting that the
actual die life for the configuration D01 was about 80
extrusions.