There are literally thousands of steels available today, each one characterized by a
particular trade name or alloy composition. Although a quantitative value of fracture
toughness parameters (e.g., NDT temperature and KIC) for each grade would greatly
facilitate the selection of a material for a particular application, these parameters
are available for only a very few of the steels.
There are primarily two reasons for this. First, because a wide range of microstructures
can be obtained in a steel of given alloy composition, simply by variations in
thermomechanical treatment. Secondly, because the concentration of fabrication defects
(i.e., blow holes, inclusions, and so on) is extremely sensitive to mill practice and
can vary between heats of steel of the same composition or even in different parts of
the same billet.
Since it is microstructure and defect concentration that primarily determine toughness,
rather than composition per se, a large variation in toughness can be produced in a given
steel simply by varying the thermomechanical treatment and fabrication practice.
A detailed understanding of the fracture of steel therefore requires an understanding
of both the physical metallurgical aspects of the material (e.g., what microstructure
will result from a given heat treatment) as well as an understanding of how this
particular microstructure affects the toughness of a structure of given geometry.
The Fracture of Ferritic-Pearlitic Steels
Ferritic-pearlitic steels account for most of the steel tonnage produced today. They
are iron-carbon alloys that generally contain 0.05-0.20% carbon and a few per cent of
other alloying elements that are added to increase yield strength and toughness.
In these steels the microstructure consists of BCC iron (ferrite), containing about 0.01%
carbon and soluble alloying elements, and Fe3C (cementite). In very low carbon
steels the cementite particles (carbides) lie in the ferrite grain boundaries and grains,
but when the carbon content is greater than about 0.02%, most of the Fe3C
forms a lamellar structure with some of the ferrite. This lamellar structure is called
pearlite and it tends to exist as "grains" or nodules, dispersed in the ferrite
matrix. In low carbon (0.10-0.20%) steel (i.e., mild steel) the pearlite accounts for
between 10-25% of the microstructure.
Although the pearlite grains are very hard, they are so widely dispersed that the ferrite
matrix can deform around them with little difficulty. It should be noted, however, that
the ferrite grain size generally decreases with increasing pearlite content because the
formation of pearlite nodules during the transformation interferes with ferrite grain
growth. Consequently the pearlite can indirectly raise σy by raising
From the point of view of fracture analysis, two ranges of carbon content are of most
interest in the low carbon steels: (1) steels containing less than 0.03% carbon where
the presence of pearlite nodules has little effect on toughness, and (2) steels
containing higher carbon contents where the pearlite does have a direct effect on
toughness and the shape of the Charpy curve.
The effect of processing variables. It has been pointed out that the
impact properties of water-quenched steels are superior to those of annealed or normalized
steels because the fast cooling rate prevents the formation of grain boundary cementite
and causes a refinement of ferrite grain size.
Many commercial grades of steel are sold in the "hot-rolled" condition and the
rolling treatments have a considerable effect on impact properties. Rolling to a lower
finishing temperature (controlled rolling) lowers the impact-transition temperature.
This results from the increased cooling rate and corresponding reduced ferrite grain
size. Since thick plates cool more slowly than thin ones, thick plates will have a
larger ferrite grain size and hence are more brittle than thin ones after the same
thermomechanical treatment. Therefore, post rolling normalizing treatments are frequently
given in order to improve the properties of rolled plate.
Hot rolling also produces an anisotropic or directional toughness owing to combinations
of texturing, pearlite banding, and the alignment of inclusions and grain boundaries in
the rolling direction. Texturing is not considered to be important in most low carbon
steels. Pearlite bands (due to phosphorous segregation during casting) and elongated
inclusions are dispersed on too coarse a scale to have an appreciable effect on notch
toughness at the low temperature end of the Charpy transition temperature range.
The effect of ferrite-soluble alloying elements. Most alloying elements
that are added to low carbon steel produce some solid solution hardening at ambient
temperature and thereby raise the lattice friction stress σi.
It is important to appreciate that equation cannot be used to predict the lower yield
stress unless the resultant grain size is known. This, of course, depends on factors
such as normalizing temperature and cooling rate. The importance of this type of approach
is that it allows prediction of the extent that individual alloying elements will decrease
toughness by increasing σi, since NDT increases by about 2°C per ksi
increase in σi.
Regression analyses for NDT temperatures or other Charpy transition temperatures have not
been reported at this time and it is only possible to discuss the effects of the individual
alloying additions on a qualitative basis.
Manganese. Most commercial steels contain about 0.5% manganese to serve
as a deoxidizer and to tie up sulfur as manganese sulfide, thereby preventing the
occurrence of hot-cracking. In low carbon steels this effect is outweighed by the
ability of manganese:
- to decrease the tendency for the formation of films of grain boundary cementite in
air-cooled or furnace-cooled specimens containing 0.05% carbon, thereby lowering the
value of γm;
- to cause a slight reduction in ferrite grain size;
- to produce a much finer pearlite structure.
The first two of these effects account for the lowering of the NDT temperature with
additions. The third effect as well as the first cause the
Charpy curves to become sharper.
In steels containing higher carbon contents the effect of manganese on the 50% transition
temperature is less pronounced, probably because the amount of pearlite rather than the
distribution of grain boundary cementite is the most important factor in determining this
transition temperature when the pearlite content is high. It should also be noted that if
the carbon content is relatively high (greater than 0.15%) a high manganese content may
have a detrimental effect on the impact properties of normalized steels because the high
hardenability of the steel causes the austenite to transform to the brittle upper bainite
structure rather than ferrite or pearlite.
Nickel. Nickel, like manganese, is able to improve the toughness of
iron carbon alloys. The magnitude of the effort is dependent on carbon content and heat
treatment. In very low (about 0.02%) carbon steels, nickel additions up to 2% are able
to prevent the formation of grain boundary cementite in hot-rolled and normalized alloys
and cause a substantial decrease in the initiation-transition temperature TS(N), and a
sharpening of the Charpy curves.
Further additions of nickel produce substantially smaller improvements in impact
properties. In alloys containing carbon contents lower than this, such that carbides
are not present after normalizing, nickel has a smaller effect on the transition
temperature. The principal beneficial effect of nickel additions to commercial steels
containing about 0.1% carbon results from the substantial grain-size refinement and
reduction of free nitrogen content after normalizing. The reasons for this behavior are
not clear at present; it may be related to the fact that nickel is an austenite stabilizer
and consequently lowers the temperature at which the austenite decomposition will take
Phosphorous. In pure iron-phosphorus alloys, intergranular embrittlement
can occur from the segregation of phosphorous at ferrite grain boundaries, which lowers
the value of γm. Also, phosphorus additions produce a significant increase
in σi and a coarsening of ferrite grain size since phosphorus is a
ferrite stabilizer. These effects combine to make phosphorus an extremely effective
embrittling agent, even when fracture occurs transgranularly.
Silicon. Silicon is added to some commercial steels to deoxidize or
"kill" them, and in this respect the silicon produces beneficial effects on
impact properties. When manganese and aluminum are present, a large fraction of the
silicon is dissolved in the ferrite and this raises σi by solid solution hardening.
This effect, coupled with the fact that silicon additions raise ky, causes the
50% transition temperature to increase by about 44°C per wt per cent silicon in
iron-carbon alloys of constant grain size. In addition, silicon, like phosphorus, is a
ferrite stabilizer and hence promotes ferrite grain growth. The net effect of silicon
additions in normalized alloys is to raise the average energy-transition temperature by
about 60°C per wt per cent silicon added.
Aluminum. The effect of alloying or killing a steel with aluminum is
twofold. First, the aluminum combines with some of the nitrogen in solution to form AlN.
The removal of this free nitrogen leads to a decrease in transition temperature because
σi is decreased and γm/ky is increased, as described
above. Second, the AlN particles that form interfere with ferrite grain growth and
consequently refine the ferrite grain size. These combined effects cause the transition
temperature to decrease about 40°C per 0.1% aluminum added. However, additions of
aluminum greater than that required to tie up the nitrogen have little effect.
Oxygen. Oxygen additions promote intergranular fracture in iron alloys.
These fractures are thought to result from the segregation of oxygen to ferrite grain
boundaries. In alloys that contain a high oxygen content (greater than 0.01 %), fracture
occurs along the continuous path provided by the embrittled grain boundary.
In alloys of lower oxygen content, cracks are nucleated at the grain boundary and then
propagate transgranularly. The problem of oxygen embrittlement can be solved by the
addition of deoxidizing elements such as carbon, manganese, silicon, aluminum, and
zirconium, which react with the oxygen to form oxide particles, thereby removing the
oxygen from the boundary region. These oxide particles are beneficial in their own right
because they retard the growth of the ferrite grains, thereby increasing d-1/2.