Ductile fracture

At the microscopic level, ductile fracture is characterized by a dimple structure (Fig.). 1a): The micropores caused by inclusions or coarse precipitates are enlarged, and the material between them is necked and sheared during further production. The depth of these depressions can be considered a measure of the ductility of the metal.

a) Two-phase CrNi steel with micro-dual-phase structure: containing nitrogen carbide in the depressions.

When inclusions are large and flat, such as sulfides and oxides in ordinary rolled steel, they are perpendicular to the rolling plane (short transverse) and cause "lamellar tearing". The flat troughs filled with inclusions are separated by cracks and have dimples (figure). 1b).

B) Stratified shear in mild steel: the groove is filled with flat inclusions separated by a dimpled matrix.

As a special case, unstructured regions can be found. Cold-worked austenitic manganese steel for generator end rings (Figure). 1c). The yield strength of this steel is limited, which creates a rough slip zone and provides a crack path.

C) Cold worked austenitic manganese steel: non-structural zones appear due to cracking in the slip zone.

brittle fracture. In general, brittle fracture propagation is transcrystalline. In the case of a quenched and tempered steel having a high metal strength, cracks sometimes propagate along grain boundaries (intergranular fracture). The boundary of the crack is austenite, which forms the high-temperature phase of the steel as described above. During cooling, when these grain boundaries disappear, the austenite transforms into ferrite or martensite.

Fracture

During cracking, separation proceeds along specific crystal planes, and in BCC metals, generally along {100} crystal planes. FCC metals do not undergo cracking under normal conditions.

a) Chromium-containing high alloy steel: with carbide as the crack source.

The local slip process of crack nucleation is hindered, as in the precipitated phase (Figure). 2a) or at the grain boundary. Since the particles are different from each other, the fracture is a step step (Figure). 2b). The new fractures are connected together and a river pattern is formed by further fracture propagation.

B) Cast steel: river grain, grain boundary crack initiation.

The flatness of the fissure surface suggests that only two atomic planes participate in the formation of the fracture surface. During the cracking process, however, a plastic zone is also formed in front of the running crack. This plastic zone consumes a major part of the work of fracture. It depends on the transverse extent of the plastic zone when the crack surface material is plastically deformed.

During the welding process, the austenite near the fusion line of the two-phase austenitic ferrite CrNi steel (25% Cr5% Ni) can be transformed into δ ferrite. During subsequent cooling, the δ-ferrite transformation will occur again, which will lead to the formation of a thin shell along the grain boundaries.

Given the ferritic state, steel embrittlement occurs as the temperature drops, just like other ferritic steels, but not as quickly. In the cracking process, the ferrite grains are fractured along the {100} crystal plane in the usual manner. When the crack stops when it reaches the grain boundary, it is mainly due to the presence of ductile austenite on the grain boundary. New crack nucleation near the grain. Therefore, the cracking between different grains is independent, and only after a certain yield can the grain boundaries be separated. Therefore, there is no river pattern (Fig.). 2C).

C) Two-phase CrNi steel: single grain isolated cracking, no river grain.

In quenched and tempered steels, martensite can also crack and crack along the {100} surface. Due to the significant difference in orientation between the individual martensite blocks, the cracks are blocked by the intersection of grain boundaries, similar to the two-phase steel mentioned earlier, that is, the shearing process must occur and the cleavage plane is difficult to see (Figure 1). 2D).