Introduction to Metallurgy: Steel, Casting, and Alloys
What is Steel? What is Casting?
Steel is an alloy of iron and carbon, where the carbon content does not exceed 2.1% by weight. Typically, carbon content ranges between 0.2% and 0.3%.
Casting is a forming process that involves pouring molten metal into a mold with the desired shape and allowing it to solidify.
Classes of Casting:
- Atruchada Casting
- White Iron
- Gray Iron
- Malleable Iron
- Ductile Iron
Key Concepts in Materials Science
Yield Strength
When a material is subjected to external forces, there is initially a linear relationship between stress and strain. This relationship holds until a certain point (yield strength) is reached, beyond which the deformations become irreversible (permanent).
Aging
Aging is a process where castings are held at room temperature for a specific duration to enhance dimensional stability or improve mechanical properties like hardness and yield strength. This process, also known as maturation, involves structural changes within the material.
Inoculation
Inoculation involves adding core products to the liquid metal, which act as nucleation sites for crystallization or grain refinement.
Pig Iron
Pig iron is the initial product of iron smelting, obtained in a blast furnace from iron ores.
Bronze
Bronze refers to a range of metal alloys primarily composed of copper (Cu) with 3% to 20% tin (Sn).
System
A system is a collection of chemical substances isolated for study. It can be a mixture of simple and composite substances in various proportions, exhibiting physical, chemical, or physicochemical interactions.
Phases
A phase is any part of a physical system with distinct properties compared to other parts.
Component
A component is a simple or compound substance initially present in the system.
Constituents
Constituents are the distinguishable parts of a system in equilibrium.
Sintering
Sintering is the process of agglomerating fine particles by molding and heating them below the melting point of the primary component. This process relies on atomic diffusion.
Peritectic Reaction
A peritectic reaction occurs when a liquid and a solid phase combine to form a new solid phase.
Segregation
Segregation refers to chemical heterogeneities that arise during solidification.
Alatropia
Alatropia is the excess magnesium that remains around the grain boundaries in aluminum alloys.
Alloy
An alloy is a system composed of multiple elements, with at least one being a metal at room temperature, that retains metallic properties.
Ultralight Displacement
Ultralight displacement refers to lightweight alloys, often used in aerospace applications.
Diffusion
Diffusion is the movement of atoms from their equilibrium positions in a crystal lattice to neighboring positions.
Bainite
Bainite is a fine mixture of ferrite and cementite formed through the isothermal transformation of austenite, initiated by ferrite formation.
Resilience
Resilience quantifies the energy absorbed by a material before fracturing under impact, per unit fracture surface area.
Steadite
Steadite is a hard and brittle eutectic with a melting point of 960°C, often appearing gray in castings.
Anisotropy
Anisotropy is the property of matter where certain physical properties (e.g., elasticity, temperature, conductivity, light propagation speed) vary depending on the direction of measurement.
Blowholes
Blowholes are cavities formed by gases dissolved in an alloy that become trapped during solidification.
Hardenability
Hardenability is the capacity of a material to be hardened through heat treatment.
Carburization
Carburization is the process of combining carbon and iron to produce steel.
Graphite
Graphite is a compound of iron (Fe) and carbon (C) that crystallizes in a hexagonal structure. It is soft, gray, brittle, and does not form chips easily.
Fatigue
Fatigue is the tendency of a material to fracture under repeated cyclic loading below its ultimate tensile strength.
Dendrites
When a pure metal is cooled uniformly, nucleation sites form at the solidification temperature. From these sites, crystal growth occurs along specific crystallographic axes, leading to tree-like structures called dendrites.
Grain Refining
Grain refining is a heat treatment process, typically annealing, used to reduce grain size in metals, resulting in improved mechanical properties.
Composites
Composite materials, also known as hybrid materials, are engineered materials formed by combining two or more distinct materials with different properties to achieve enhanced overall performance.
Chemical Affinity
Electron affinity (A) is the energy change associated with a neutral atom (X) in the gaseous state gaining an electron to become a negatively charged ion (X–), also in the gaseous state.
Recrystallization
Recrystallization is a process where a cold-worked metal is heated to a specific temperature for a sufficient time to form a new, distortion-free grain structure. This process reduces dislocation density and restores ductility.
Grain Growth
After crystallization, if the temperature is further increased above the recrystallization temperature, grain growth occurs, leading to larger grain sizes.
Critical Temperatures in Steel
- Ac0 (210°C): Curie temperature for magnetic transformation in cementite.
- Ac1 (723°C): Lower critical temperature, marking the start of austenite formation from pearlite.
- Ac2 (768°C): Curie temperature for magnetic transformation in ferrite.
- Ac3 (723-910°C): Upper critical temperature, marking the end of ferrite transformation to austenite.
- Ac4 (1492°C): No upper limit for austenite.