Ductile fracture
At the micro level, ductile fracture is characterized by the dimple structure (Fig. 1a): micropores caused by inclusions or coarse precipitates are expanded and the materials between them are necked and sheared during further yield. The depth of these dimples can be considered as a measure of metal ductility.
a) Two phase CrNi steel with micro biphase structure: nitrogen carbide is contained in the dimples.
When inclusions are large and flat, such as sulphides and oxides in common rolled steel, they are perpendicular to the rolling plane (short transverse) and cause "lamellar tearing". The inclusion filled flat grooves are separated by the fracture with dimples（ Fig. 1b).
b) Layered shear in mild steel: the grooves are filled with flat inclusions separated by a matrix with dimples.
As a special case, unstructured areas can be found. In Cold Worked Austenitic Manganese Steel for generator end rings (Fig. 1c). The yield strength of this steel has limitations, which forms a coarse slip zone and provides a crack path.
c) Cold working austenitic manganese steel: the non structural zone appears due to the cracking of slip zone.
Brittle fracture. In general, brittle fracture propagation is transgranular. In the case of high metal strength, quenching and tempering steel, cracks sometimes propagate along the grain boundaries (intergranular fracture). The boundary of the crack is austenite, forming the high temperature phase of the steel as described above. During cooling, austenite is transformed into ferrite or martensite when these grain boundaries disappear.
Cleavage fracture
During the cleavage, the separation is carried out along a specific crystal surface, and in BCC METALS, it is generally along a {100} crystal surface. FCC metal does not undergo cleavage under normal conditions.
a) Chromium containing high alloy steel: carbide as the source of crack.
The local slip process of the crack nucleation is hindered, such as in the precipitate phase (Fig. 2a) or at the grain boundary. Because the particles are different from each other, the cracks are terraced steps (Fig. 2b). The new cracks are connected together, and the river pattern is formed by the further crack propagation.
b) Cast steel: river pattern, crack initiation occurs at grain boundary.
The flatness of cleavage surface shows that only two atomic planes are involved in the formation of fracture surface. But during the cracking process, the plastic zone also formed in front of the running crack. This plastic zone consumes the main part of the fracture work. It depends on the transverse range of plastic zone when the material of crack surface is plastic deformation.
During welding, austenite near the fusion line of two-phase austenite ferrite CrNi steel (25% Cr 5% nickel) can be converted into δ Ferrite. During the subsequent cooling process, δ- The ferrite transformation will occur again, which will lead to the formation of thin shell along the grain boundary.
In view of the ferrite state, the steel embrittlement occurs with the temperature drop, just like other ferritic steel, but not so fast. During the cracking process, the ferrite grains fracture along the {100} crystal surface in the usual way. When the crack stops when it reaches the grain boundary, it is mainly due to the existence of ductile austenite on the grain boundary. The new crack nucleation near the grain. Therefore, the cracking between different grains is independent, and only after a certain yield can the grain boundary be separated. Therefore, there is no river pattern (Fig. 2C).
c) Two phase CrNi steel: single grain is isolated and cracked, without river pattern.
In quenched and tempered steel, martensite will also cleavage and crack along the {100} surface. Because of the significant difference in orientation between individual martensite blocks, cracks are blocked by the cross of grain boundaries, similar to the two-phase steel mentioned earlier, that is, shear process must occur, and the cleavage surface is difficult to be seen (Fig. 2D).
d) Hardened Low Alloy Steel: a fine cleavage surface.
