GATE MT key terms

A plot of standard Gibbs free energy of oxide formation per mole of oxygen against temperature for various metals. It predicts which metal reduces another's oxide and the equilibrium oxygen partial pressure at a given temperature.

The change in free energy when one mole of a compound forms from its elements in their standard states. A negative value indicates thermodynamic stability; its temperature dependence determines whether a reduction reaction is feasible.

Reduction of a metal oxide or sulfide using carbon or carbon monoxide as the reductant, producing metal plus CO or CO2. Widely used for iron, zinc, and ferroalloys; limited by the temperature at which carbon can reduce the oxide.

The tendency of a species to gain electrons and be reduced, expressed as a standard electrode potential in volts. In extractive metallurgy it ranks metals by their affinity for oxygen or their nobility in aqueous systems.

A ratio expressing the relative amount of basic to acidic oxides in a slag, commonly CaO/SiO2 by mass. It controls slag melting point, viscosity, sulfur and phosphorus capacity, and refractory attack in steelmaking.

A measure of a molten slag's resistance to flow, strongly dependent on temperature and silica content. Low viscosity aids slag-metal reaction kinetics and separation; high viscosity hinders refining and tapping.

Removal of sulfur from hot metal or steel into a basic slag or by injection of reagents such as lime, calcium carbide, or magnesium. Requires low oxygen potential and high slag sulfide capacity.

Removal of phosphorus from steel into an oxidizing basic slag, since phosphorus is more stable as phosphate under oxidizing conditions. Practiced in basic oxygen steelmaking rather than in the reducing blast furnace.

A countercurrent shaft furnace in which iron ore, coke, and limestone descend while hot air ascends, producing liquid pig iron and slag. Reducing gases and heat are generated by coke combustion at the tuyeres.

Production of solid metallic iron from ore below the melting point of iron, typically using natural gas-derived syngas. The product, direct reduced iron, is a feedstock for electric arc furnaces.

A steelmaking converter in which high-purity oxygen is blown onto molten pig iron to oxidize carbon, silicon, manganese, and phosphorus. Fluxes form a basic slag that absorbs phosphorus and sulfur.

A furnace that melts scrap or direct reduced iron using heat from electric arcs struck between graphite electrodes and the charge. It enables flexible, high-recycled-content steelmaking with controlled atmosphere.

Ladle-based refining steps after primary steelmaking, including degassing, deoxidation, alloying, and inclusion modification. Vacuum and inert-gas stirring enable precise composition and cleanliness control.

Removal of dissolved oxygen from liquid steel by adding elements such as aluminum, silicon, or manganese that form oxide inclusions. The choice of deoxidizer controls inclusion type, size, and steel cleanliness.

A non-metallic particle such as an oxide, sulfide, or nitride suspended in solidified steel. Inclusions degrade toughness and fatigue life; their composition and morphology depend on deoxidation and solidification.

A process in which molten steel is poured through a water-cooled mold and withdrawn as a solidifying strand, then cut to length. It improves yield and uniformity compared with ingot casting.

The transformation of a liquid into a crystalline solid by nucleation and growth, releasing latent heat. Cooling rate and temperature gradient determine dendrite morphology, segregation, and casting defects.

A tree-like crystal that grows preferentially along thermal gradients during solidification of an alloy. Primary and secondary arm spacing reflects local cooling rate and influences microsegregation and mechanical properties.

Non-uniform distribution of solute in a solidified alloy, caused by solute rejection at the advancing solid-liquid interface. Microsegregation between dendrite arms can be reduced by homogenization; macrosegregation persists on a larger scale.

A high-temperature heat treatment that allows diffusion to eliminate microsegregation in cast alloys. It reduces chemical banding and improves subsequent hot workability and property uniformity.

Formation and growth of new strain-free grains in a deformed metal during annealing, replacing the deformed microstructure. It requires a critical prior strain and lowers strength while restoring ductility.

Plastic deformation below the recrystallization temperature, which increases dislocation density and strength while reducing ductility. The stored strain energy drives recovery and recrystallization on subsequent annealing.

Increase in yield strength resulting from plastic deformation as dislocations multiply and interact, becoming harder to move. It is described by the flow curve and is removed by annealing.

Increase in average grain size during annealing after recrystallization, driven by reduction of grain-boundary area. Growth kinetics follow a power law; second-phase particles can pin boundaries and inhibit it.

An empirical relation stating that yield strength increases with the inverse square root of grain size. It explains why grain refinement strengthens metals by impeding dislocation pile-ups at boundaries.

A line defect in a crystal lattice marking the boundary between slipped and unslipped regions. Dislocation motion produces plastic deformation; obstacles to motion such as precipitates and boundaries raise strength.

Strengthening of an alloy by solution treatment and aging to form fine coherent precipitates that impede dislocation motion. Strength peaks at a critical precipitate size and overaging softens the alloy.

A heat treatment in which a supersaturated solid solution is held at moderate temperature so that solute atoms cluster and precipitate. Natural aging occurs at room temperature, artificial aging at elevated temperature.

A map of equilibrium phases as a function of temperature, pressure, and composition. It identifies phase fields, eutectics, and solubility limits, guiding alloy design and heat-treatment selection.

A mass-balance method to find the fraction of each phase in a two-phase region of a phase diagram. The fraction of a phase equals the opposite segment length divided by the tie-line length.

A solid-state reaction in which one solid phase transforms into two different solid phases on cooling at a fixed temperature. In steel, austenite transforms to ferrite and cementite at the eutectoid point.

A hard, supersaturated phase formed by diffusionless shear transformation when austenite is quenched rapidly. Its tetragonally distorted lattice arises from trapped carbon and causes high strength with low toughness.

Reheating quenched martensite to allow carbon to precipitate as carbides, trading hardness for toughness. Higher tempering temperatures soften more but improve ductility and relieve residual stresses.

The ability of a steel to form martensite at depth during quenching, measured by the Jominy end-quench test. It depends mainly on alloying elements and prior austenite grain size, not on carbon content alone.

The minimum cooling rate that suppresses diffusional transformation and produces a fully martensitic structure. It decreases as hardenability increases, allowing slower quenching media.

Accelerated corrosion of a metal when it is electrically coupled to a more noble metal in an electrolyte. The less noble metal becomes the anode and dissolves while the nobler metal is protected.

Formation of a thin, adherent, protective oxide film on a metal surface that greatly reduces corrosion rate. Stainless steel relies on a chromium-rich passive film; chloride ions can locally break it down.

Localized breakdown of a passive film leading to deep cavities, often triggered by chloride ions. Pits concentrate anodic dissolution and can cause unexpected perforation with little overall mass loss.