Figure 5 - Cleavage observed in fracture surface of impact specimen (a) and hydrogen 
induced cleavage (b). Ferritic ductile irom. 
At the mixed fracture region, some characteristic aspects can be verified. The gra- 
phite nodules are shown separated from the matrix, indicating dimples initiation a- 
round the nodules. The cleavage facets are localized predominantly around the cavi- 
ties developed over the graphite nodules (figure 4-b). The river patterns of the cle- 
avage facets show that the fracture propagates from the ferrite/graphite interface 
to the matrix. At these cleavage facets on blackheart malleable iron one can observe 
the presence of sulfides separated from the matrix (figure 4-b), indicating the oc- 
currence of previous development of dimples at these places. In the areas further 
from the graphite nodules the fracture occurs by dimples, of small size, associated 
mainly to MnS inclusions. 
Utilizing specimens with incremental loading (figure 1), it was possible to identify 
the sequence of steps which occurs in the fracture process, In samples without hydro- 
gen the fracture is initiated by the graphite cracking (in blackheart and ductile 
irons) or by the graphite/ferrite interface decohesion (in ductile irons) (figure 6- —_ 
a), with small plastic deformation of the matrix. It is followed by the growth of the 
dimples around the graphite, with severe plastic deformation of ferrite (figure 6-b). Po 
In blackheart malleable iron the nucleation of dimples also occurs around MnS inclu- . 
sions. Microcracks propagate from the dimples, perpendicular to the tensile stress 
direction, followed by large plastic deformation (figure 6-c). The growth of these pn 
cracks leads to dimple link-up (figure 6-d) and to the final fracture. This sequence Tr 
of events is similar to that registered by Voigt et all (19, 20). 
In samples with hydrogen, the fracture also initiate by the graphite cracking or by = 
the graphite/ferrite interface decohesion. It is followed by the growth of the dim- HR 
ples around the graphite, with plastic deformation of the matrix and, in blackheart 
malleable iron, the development of dimples over MnS inclusions also occurs. However, N 
even with relatively small sized dimples, cleavage crack development occurs from the Te 
dimples around the graphite (figure 7-a and 7-b), perpendicularly to the tensile 
Prakt. Met. Sonderbd. 21 (1990) 
126