Abstract
The crystallographic packet or microstructural unit was determined with 15° and 5° boundary grain misorientations in a quenched and tempered X38CrMoV5-1 steel, yielding results of 0.68 and 0.59 μm, respectively. Though both values are similar, the latter has been taken as the best to calculate the effective surface energy of cleavage fracture, γ_p, leading to a value of 15.8 J m^–2. Thus it is demonstrated that the microstructural unit controlling crack propagation is that determined by EBSD with a 5° misorientation angle. Other structural parameters, like the parent austenite grain size and the morphological martensitic packet, have been ruled out.

Key words
EBSD, crack growth, fracture toughness, precipitation, steel

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