Steel

Steel is an iron-based alloy family in which carbon and other alloying elements control structure and properties. It is not a single compound with one fixed carbon content.

Cooling and heat treatment change the distribution of phases and microstructural features. The same overall elemental composition can therefore give very different strength, hardness, and ductility depending on how the material was processed.

A steel with an austenitic, face-centered cubic matrix in the specified condition. Alloying and temperature determine its stability.

A stainless steel with an austenitic matrix. Some grades are used in nuclear components, where void swelling and helium embrittlement can limit performance.

A chromium–nickel–molybdenum stainless-steel grade. Annealed and cold-worked conditions can have different irradiation microstructures.

A chromium-bearing steel whose protective surface film can confer corrosion resistance. Composition, microstructure and environment determine whether that protection persists.

Steel microstructure describes the spatial arrangement of phases, grains, defects, and precipitates in a steel. Ferrite, austenite, cementite-containing mixtures, and martensitic structures are important examples.

These features influence dislocation motion and fracture, connecting thermal processing with mechanical performance. Overall carbon percentage alone cannot specify a steel's microstructure or uniquely determine its properties.

A nitric-acid/alcohol metallographic etchant. In pearlitic steel it can preferentially remove ferrite, leaving cementite standing higher. In an atomic force microscopy height image, darker then means lower surface height, not an optical reflectivity or atomic-number contrast.

Heating quenched steel below the austenitizing range to adjust its microstructure and relieve stresses. Carbon redistribution and carbide formation can improve toughness at a cost in hardness.

A microstructure in which carbon redistribution and carbide formation reduce the supersaturation of a martensitic matrix. Plate or lath morphology can remain while local strain and thermodynamic phase composition evolve. The term describes a material state, not a universal guarantee of a particular toughness.

A chart of the time needed to reach specified transformation fractions after cooling to, and holding at, a fixed temperature. Its curves are not themselves continuous-cooling histories; estimating a critical cooling rate from their nose is an approximation.

A product of a displacive transformation. In carbon steel, rapid cooling can trap carbon in a distorted iron lattice, producing high hardness and low as-quenched toughness.

A martensitic variant is one of the lattice deformations corresponding to the low-temperature crystal phase. The parent crystal's discrete symmetries generate different variants. In a nonlinear elasticity model each representative $U_i$ supplies a hyperelastic energy well $SO(3)U_i$. A representative need not itself be a symmetric stretch tensor; multiplication by a proper rotation leaves the same well.

Deformation twinning joins differently oriented crystal variants across a coherent interface. In a piecewise affine nonlinear elasticity model the two deformation gradients must differ by $a\otimes N$, where $N$ is the reference interface normal. The Hadamard jump condition gives this requirement from continuity along every interface tangent. Compatible alternating layers can then make a mixture of variants whose macroscopic shape differs from either variant alone.

A twin plane is a compatible planar interface between differently oriented crystal variants. In a piecewise homogeneous elastic deformation its reference normal is the $N$ in a rank-one connection between the two variant deformation gradients. Continuity of the deformation holds along this plane even though its derivative changes. Other material planes crossing the interface can kink after deformation.

Let $B=\mathrm{diag}(b,a,a)$, with $a,b>0$, $a\ne b$, and $A=R\mathrm{diag}(a,b,a)$ be a twin-related gradient with $\cos \theta =2ab/(a^2+b^2)$, $\sin \theta =\kappa (b^2-a^2)/(a^2+b^2)$, $\kappa =\pm 1$. Their jump is $(b^2-a^2)(-b,\kappa a,0)\otimes (1,\kappa ,0)/(a^2+b^2)$.

A plane through the origin with normal $p=(p_1,p_2,0)$ is spanned by $e_3$ and $t=(-p_2,p_1,0)$. Both gradients map $e_3$ to the same direction, so the two image half-planes are coplanar exactly when $At$ and $Bt$ are parallel in their transverse section. Direct calculation givesIts nonzero prefactor makes planarity equivalent to $p\parallel (1,\kappa ,0)$, the twin normal. On that plane the two affine maps agree, proving sufficiency as well. The restriction on the third normal component matters; this statement is not a classification of every possible material plane.

A lamellar two-phase microconstituent of ferrite and cementite, rather than a single thermodynamic phase. Shorter diffusion distances at greater undercooling generally produce finer lamellae before diffusional transformation is bypassed.

The proportions of ferrite and cementite within the two-phase pearlite microstructure. A lever rule on a weight-percent-carbon diagram gives mass fractions; converting those to volume fractions requires the phase densities. Proeutectoid phases outside pearlite must be treated separately.

Cementite is iron carbide, $\mathrm{F}{\mathrm{e}}_3\mathrm{C}$, a hard phase important in many steel microstructures. It can occur with ferrite in organized mixtures such as pearlite.

The amount, size, and distribution of cementite affect mechanical behavior because a carbide particle interacts differently with deformation than the surrounding iron-rich phase. Chemical composition and spatial arrangement must therefore be considered together.

The face-centred cubic iron-rich solid phase of steel. Its transformation products depend on composition and cooling history.

The body-centred cubic iron-rich solid phase of ordinary low-temperature steel, with low equilibrium carbon solubility.