Steel shows different grain structures depending on the alloy composition and temperature. The resulting
unit cells have different sizes due to the arrangements of the iron-atoms. Temperature changes can cause
a transformation in this arrangement, thereby changing the occupied volume. This transformation is based on a folding-
and shifting-process on an atomic level, making steel uniquely versatile.
The transformation on the atomic level is involved in most of the heat treatments of steel. Depending on the
heat treatment, the grain structure can be refined or the steel can be hardened or softened.
Most of the relevant heat treatments for forgeable steels start out from austenite. The grain-structures and formation
described in the following refer to relatively plain carbon steels. Alloying elements in amounts of a few tenth of a percent
can alter the behavior of the transformations significantly.
Perlite is formed by cooling from an austenite grain. With decreasing temperature, the solubility of carbon in the austenite
declines. This leads to carbon segregation in form of cementite growing into the austenite-grain, starting at the
grain-boundary or an impurity, acting as a catalyst. The formation of this cementite extracts carbon from its surrounding.
The carbon deplete austenite can now turn into ferrite. Some of the remaining carbon is driven out of this zone while
transforming, causing the formation of a new cementite lamella. The austenite grain transforms into lamellar perlite in the above described manner.
The lamella-spacing is determined by the diffusion speed of carbon that decreases at lower temperatures. This leads to the
formation of coarsely-spaced perlite at 1300°F, fine-spaced at 1100°F and extremely fine at 900°F. The hardness of the
steel increases with the fineness of the lamella-spacing.
The carbon diffusion in austenite is practically inhibited at temperatures below 750°F, leading to the formation of martensite or bainite, since
the conditions necessary for lamella-formations are no longer present. Due to the slow diffusion speeds, the transformation starts delayed and the
transformation duration increases at lower temperatures (please refer to the time-temperature transformation diagram). In contrast to the formation of perlite, the unit-cells transform into
supersaturated ferrite cells that grow to needle-like shapes, forming the martensitic or bainitic grain structure.
The diffusion speed of carbon in alpha-iron is significantly higher than in gamma-iron, allowing carbon-segregation inside the supersaturated
ferrite needles. The carbon is segregated in form of finest cementite grains. Bainite is formed from a martensite like structure, being unstable
at the bainite-formation temperatures. Again, the size of the cementite-segregations depends on the carbon diffusion speed, lower temperatures
leading to smaller and finer-spread cementite.
Carbides segregated at 750°F are bigger than the lamellar-spacing of perlite formed at 900°F, resulting in a decrease of hardness for
bainite formed at higher temperatures. Bainite is formed only above 356°F, below this temperature the carbon is entrapped on its interstice, the
transformation occurs immediately.
Carbon segregation and the associate formation of cementite is inhibited if carbon steel (>0.35%C) is quenched. The gamma-iron
transforms into tetragonal distorted alpha-iron, the tetragonally martensite. Even if only about every 10th martensitic cell is
distorted, martensite occupies approximately 1% more volume than ferrite or perlite. The enclosed carbon obstructs crystal slip and
produces internal tensions, causing the hardness and brittleness of martensite.
The formation of martensite starts below the martensitic temperature, 356°F. This temperature depends only on the alloying elements
and not on the cooling rate. The initially forming martensitic needles generate, caused by their bigger volume, compressive stresses
in the remaining austenite. This delays its transformation and often causes a certain amount of austenite remaining at room temperature.
If cooled to the range between 356°F and 210°F and kept at this temperature for an appropriately long time, cubic martensite is formed.
At these temperatures the carbon diffusion is still high enough that finest carbides can be formed inside the martensitic needles, causing
transformations in the atomic lattice. The cubic martensite etches dark in contrast to the white-etching tetragonal distorted martensite.
Cubic martensite has an only slightly decreased hardness but a significantly higher toughness.
Remaining austenite is stable for temperatures below 210°F due to the inhibited carbon migration. It can be eliminated by deep-freezing
Hardeness of the Grain Structure
The hardness is equally influenced by internal tensions (like martensite) and hard segregations (like the cementite-lamellae of
perlite). The hardness due to segregations increases with the fineness of these since these are embedded more strongly in the matrix,
hindering crystal slipping more effectively by their finer spread.
© 2005 G.v.Tardy