Whether you're cooking dinner or eating it, the knives,
spoons, and forks you're using are probably made of steel. It's easy to take those utensils for granted,
except when they bend, chip, corrode, or lose their sharpness. But how does good cutlery differ
from bad, and what compromises are involved in making practical utensils? In this Quick Study,
we'll take a look at the science of steel.
Iron
Steel begins with iron, which, like
most metals, is a crystalline solid. A macroscopic piece of room-temperature iron contains many
crystalline grains of ferrite—iron atoms arranged in a body-centered cubic lattice. The
vast, uninterrupted atomic planes in those grains make iron soft. When iron is subjected to shear
stress—that is, when nearby layers are pushed in opposite directions—the atomic planes
slide across one another in a process called slip. When slip occurs in an iron crystal, the crystal's
shape changes forever, and the iron bends permanently. That's fine in a twist tie, not so good in
a knife.
Slip is facilitated by
two types of crystal flaws known collectively as dislocations; the figure illustrates both. An
edge dislocation is the edge of an incomplete atomic plane whose abrupt termination midway through
the lattice interrupts the smooth pattern of planes surrounding it. A screw dislocation has a spiral-staircase
character in that a line of atoms circling the dislocation's axis returns to a different layer.
Dislocations reduce the energy barrier that inhibits atomic planes from sliding across one another
by allowing the planes to slide piecemeal.
To make iron harder, something
must impede the migration of dislocations and thereby curtail slip. One effective technique is
to mechanically work the material by pounding it, rolling it, or folding it at modest temperatures
so that it develops huge numbers of dislocations, many of which are entangled. Entanglement makes
the dislocations less mobile, which reduces slip and hardens the iron.
Carbon steel
Another way to harden iron is to add a
small amount of carbon to it. The resulting alloy is steel. When alloyed with a few other elements,
steel can be made to do amazing things.
Carbon is nearly insoluble
in ferrite, so adding it to iron dramatically complicates the structure of the resulting steel.
The carbon can exist dispersed in the ferrite as a hard iron carbide called cementite, or it can remain
in supersaturation within the lattice of a distorted ferrite known as martensite. The more carbon
in the steel (up to about 1% by weight) and the more finely and evenly dispersed the cementite, the
more difficult it is for dislocations to move within a grain to facilitate slip and, thus, the harder
the steel is. Martensite, which forms when carbon atoms occupying interstitial sites in the ferrite
lattice distort the ferrite into a tetragonal crystal, is the hardest steel structure of all and
the key ingredient in the cutting edges of the best knives.
But the ability of carbon
to make steel much harder than pure iron would be irrelevant if no practical way existed to disperse
the cementite in ferrite or to turn ferrite plus carbon into martensite. That's where temperature
enters the picture. When steel containing more than 0.02% carbon by weight is heated above 727 °C,
it undergoes a solid–solid phase transition that transforms its body-centered cubic ferrite
crystals into face-centered cubic austenite crystals. That subtle phase change can be detected
as a slight decrease in volume; austenite is a little denser than ferrite. It's also detectable
magnetically: Ferrite is ferromagnetic below its Curie temperature; austenite never is.
Carbon is far more soluble
in austenite than in ferrite. Below 727 °C, steel is mostly carbon-poor ferrite, and virtually
all of the steel's carbon must be accommodated in separated phases of cementite or martensite.
But above the threshold temperature at which the ferrite begins to transform into austenite, the
excess carbon begins to dissolve into the austenite. Although ferrite, cementite, and austenite
phases can coexist at certain temperatures and carbon contents, at temperatures above 912 °C,
steel with less than 1% carbon is pure austenite. Part of the shaping required to form knives and
utensils is usually done on the hot austenitic steel, which is soft and ductile despite its dissolved
carbon.
To disperse cementite
throughout the steel or to form martensite, metalworkers heat the steel well above the ferrite-to-austenite
transition temperature. They then quench the steel with water, oil, or air so that the austenite
quickly transforms back into ferrite and the cementite begins to precipitate out of solution.
The faster and further the steel is cooled, the more finely dispersed the cementite and the harder
the steel.
Gently quenched steel
yields pearlite, a layered pattern of ferrite and cementite that is strong but not especially hard.
It yields too easily to make a good knife edge, but can absorb lots of energy when it bends; utensils
with pearlite bodies are extremely sturdy. Moderate quenching gives bainite, considerably harder
than pearlite and with a finer, nonlayered pattern. The fastest and deepest quenching produces
martensite, which is extremely hard and unyielding—just what a good
cutting edge needs.
Deeply quenched steel,
however, is brittle. Thermal stresses frozen in the steel, together with the intrinsic brittleness
of martensite itself, can cause a freshly hardened steel blade to crack spontaneously. To reduce
brittleness, metalworkers temper the steel. That is, they reheat it to a few hundred degrees Celsius
to allow some crystalline rearrangements, including the precipitation and coalescence of some
fine iron carbides and the elimination of any remaining small austenite crystals.
Quenched and tempered
carbon steel is well-suited to knives and other utensils. Over the centuries bladesmiths have
learned how to form and heat-treat carbon steel knives whose bodies are almost unbreakably tough
pearlite or bainite and whose martensite cutting edges stay razor sharp for a long time. Interestingly,
some of the greatest steel implements—those of Damascus steel and Toledo steel—involved
fabrication techniques that were forgotten and are only now beginning to be rediscovered.
Alloy and stainless steels
Simple carbon steel suffers from two
limitations: It requires great skill and care to harden appropriately, and it rusts easily. To
make modern alloy steels stronger and easier to harden, they often contain additional elements
such as vanadium, titanium, niobium, molybdenum, and chromium. Chromium and nickel can also help
prevent rust and corrosion in "stainless" steels.
The added elements in alloy
steels play several roles in strengthening and hardening. First, they control grain growth during
heat treatment. Coarse-grained steel is weaker than fine-grained steel because dislocations
are more mobile in large grains. The grains in hot, fully austenitic carbon steel recrystallize
and grow quickly during heat treatment, which weakens the steel. Carbides and nitrides of niobium
and titanium, however, are relatively stable at high temperatures, and their presence in the steel
throughout its heat treatment prevents the grains from growing.
Second, having fine alloy
carbides and nitrides dispersed throughout the ferrite grains of the finished cutlery directly
blocks the movement of dislocations and impedes slip. Vanadium carbides and nitrides precipitate
during quenching. Chromium and molybdenum act primarily during the tempering process, when fine
carbides of those two elements form. The process of tempering simple carbon steel causes it to lose
some of the hardness it developed during quenching, but tempering steels containing chromium
and molybdenum can actually make them harder.
When its chromium content
exceeds about 11.5% by weight, the steel becomes stainless. It develops a thin layer of chromium
oxide on its surface that prevents rusting. Adding nickel further improves its resistance to chemical
attack. The most common stainless alloys are about 18% chromium and 8% nickel.
Unfortunately, most stainless
steels make poor cutlery because they cannot be hardened thermally. Stainless steels with excessive
chromium are ferritic. That is, they remain ferrite all the way up to their melting points. Stainless
steels with excessive nickel, including the common 18–8 recipe, are austenitic; they remain
austenite all the way down to room temperature. Only stainless steels that undergo a ferrite-to-austenite
transition can be hardened thermally. Those martensitic stainless steels have relatively little
nickel and just enough chromium to make them stainless. They aren't as impervious to rust or chemical
attack as 18–8, but they make much better knives and utensils.
Next time you reach for
a knife or fork, think about the material from which it's made. Most likely, you'll be holding a piece
of martensitic stainless steel, with small ferrite crystals and martensite, perhaps intermingled
with various carbides and nitrides, and wrapped in a thin layer of chromium oxide. It's an ancient
technology that gets better every year.
Lou
Bloomfield is a professor of physics at the University of Virginia in Charlottesville and
author of the textbook How Things Work: The Physics of Everyday Life (Wiley, 2006).
Additional resource
G. Krauss, Steels: Processing, Structure, and Performance, ASM International, Materials Park, OH (2005).
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