Grain structure

Austenite Transformation

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

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 1300F, fine-spaced at 1100F and extremely fine at 900F. The hardness of the steel increases with the fineness of the lamella-spacing.

Bainite

The carbon diffusion in austenite is practically inhibited at temperatures below 750F, 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 750F are bigger than the lamellar-spacing of perlite formed at 900F, resulting in a decrease of hardness for bainite formed at higher temperatures. Bainite is formed only above 356F, below this temperature the carbon is entrapped on its interstice, the transformation occurs immediately.

Martensite

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, 356F. 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 356F and 210F 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 210F due to the inhibited carbon migration. It can be eliminated by deep-freezing or tempering.

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