Ductile or fibrous fracture
The higher temperature side of the ductile/brittle transition is associated with a much
tougher mode of failure, which absorbs much more energy in the impact test. While
the failure mode is often referred to as ductile fracture, it could be described as
rupture, a slow separation process which, although transgranular, is not markedly
crystallographic in nature.
Scanning electron micrographs of the ductile fracture surface, in striking contrast
to those from the smooth faceted cleavage surface, reveal a heavily dimpled surface,
each depression being associated with a hard particle, either a carbide or non-metallic
It is now well established that ductile failure is initiated by the nucleation of
voids at second phase particles. In steels these particles are either carbides, sulphide
or silicate inclusions. The voids form either by cracking of the particles, or by
decohesion at the particle/matrix interfaces, so it is clear that the volume fractions,
distribution and morphology of both carbides and of inclusions are important in
determining the ductile behavior, not only in the simple tensile test, but in complex
Therefore, significant variables, which determine ductility of steels, are to be found
in the steel-making process, where the nature and distribution of inclusions is partly
determined, and in subsequent solidification and working processes. Likewise, the
carbide distribution will depend on composition and on steel-making practice, and
particularly on the final heat treatment involving the transformation from austenite,
which largely determines the carbide size, shape and distribution.
The formation of voids begins very early in a tensile test, as a result of high stresses
imposed by dislocation arrays on individual hard particles. Depending on the strength
of the particle/matrix bond, the voids occur at varying strains, but for inclusions
in steels the bonding is usually weak so voids are observed at low plastic strains.
Many higher strength steels exhibit lower work hardening capacity as shown by relatively
flat stress-strain curves in tension. As a result, at high strains the flow localizes
in shear bands, where intense deformation leads to decohesion, a type of shear fracture.
While the detailed mechanism of this process is not yet clear, it involves the localized
interaction of high dislocation densities with carbide particles.
Role of inclusions in ductility
It is now generally recognized that the deformability of inclusions is a crucial
factor which plays a major role, not only in service where risk of fracture exists,
but also during hot and cold working operations such as rolling, forging, and
Kiessling has divided the inclusions found in steels into five categories relating to
their deformation behavior.
It is now known that ductile failure can be associated with any of the types of
inclusion listed above, from the brittle alumina type to the much more ductile sulphide
inclusions. However, the inclusions are more effective in initiating ductile cracks
above a critical size range. The coarser particles lead to higher local stress
concentrations, which cause localized rupture and micro crack formation. Some
quantitative work has now been done on model systems, e.g. iron-alumina where the
progressive effect on ductility of increasing volume fraction of alumina is readily
shown. The reduction in yield stress, also observed, arises from stress concentrations
around the inclusions and is already evident at relatively low volume fractions.
- Al2O3 and calcium aluminates. These arise during deoxidation of
molten steels. They are brittle solids, which are in practical terms undependable at all
- Spinel type oxides are undeformable up to 1200°C, but may be
deformed above this temperature.
- Silicates of calcium, manganese, iron and aluminum in various
proportions. These inclusions are brittle at room temperature, but increasingly
deformable at higher temperatures. The formability increases with decreasing melting
point of the silicate, e.g. from aluminum silicate to iron and manganese silicates.
- FeO and (FeMn)O. These are plastic at room temperature, but appear
gradually to become less plastic above 400°C.
- Manganese sulphide MnS. This common inclusion type is deformable,
becoming increasingly so as the temperature falls. There are three main types of
MnS inclusion dependent on their mode of formation, which markedly influences their
The presence of particles in the size range 1-35 μm broadens substantially the
temperature range of the ductile/brittle transition in impact tests and also lowers
the energy absorbed during ductile failure, the shelf energy. A fine dispersion of
non-brittle type inclusions can delay cleavage fracture by localized relaxation of
stresses with a concomitant increase in yield stress.
Regarding cyclic stressing, it appears that inclusions must reach a critical size
before they can nucleate a fatigue crack but the size effect depends also very much on
the particle shape, e.g. whether spherical or angular. It has been found in some steels,
e.g. ball bearing steels, that fatigue cracks originate only at brittle oxide inclusions,
and not at manganese sulphide particles or oxides coated with manganese sulphide. In
such circumstances the stresses, which develop at particle interfaces with the steel
matrix, as a result of differences in thermal expansion, appear to play an important
part. It has been found that the highest stresses arise in calcium aluminates, alumina
and spinel inclusions, which have substantially smaller thermal expansion coefficients
than steel. These inclusions have the most deleterious effects on fatigue life.
The behavior of ductile inclusions such as MnS during fabrication processes involving
deformation has a marked effect on the ductility of the final product. Types I and III
manganese sulphide will be deformed to ellipsoidal shapes, while Type II colonies will
rotate during rolling into the rolling plane, giving rise to very much reduced toughness
and ductility in the transverse direction. This type of sulphide precipitate is the most
harmful so efforts are now made to eliminate it by addition of strong sulphide forming
elements such as Ti, Zr and Ca.
The lack of ductility is undoubtedly encouraged by the formation at the inclusion
interfaces of voids because the MnS contracts more than the iron matrix on cooling,
and the interfacial bond is probably insufficiently strong to suppress void formation.
The variation in ductility with direction in rolled steels can be extreme because of
the directionality of the strings of sulphide inclusions, and this in turn can adversely
affect ductility during many working operations.
Cracking can also occur during welding of steel sheet with low transverse ductility.
This takes place particularly in the parent plate under butt welds, the cracks following
the line of the sulphide inclusion stringers. The phenomenon is referred to as lamellar
Role of carbides in ductility
The ductility of steel is also influenced by the carbide distribution, which can vary
from spheroidal particles to lamellar pearlitic cementite. Comparing spheroidal
cementite with sulphides of similar morphology, the carbide particles are stronger
and do not crack or exhibit decohesion at small strains, with the result that a
spheroidized steel can withstand substantial deformation before voids are nucleated
and so exhibits good ductility. The strain needed for void nucleation decreases with
increasing volume fraction of carbide and so can be linked to the carbon content of
Pearlitic cementite also does not crack at small strains, but the critical strain for
void nucleation is lower than for spheroidized carbides. Another factor, which reduces
the overall ductility of pearlitic steels, is the fact that once a single lamella
cracks, the crack is transmitted over much of a pearlite colony leading to well-defined
cracks in the pearlite regions. The result is that the normal ductile dimpled fractures
are obtained with fractured pearlite at the base of the dimples.