Metallurgy Fundamentals: Steelmaking and Metal Properties

Crystal Defects and Casting Faults

Crystal defects and imperfections occur during casting, resulting in volume changes and fissures. Rapid cooling produces larger grains that resist traction and increase hardness. Regarding porosity, types such as FN (effervescent), FV (unfit/fit), and calm FF are commonly used.

Physical and Mechanical Characteristics

Metals typically have a density of 7-9 g/cm³. They act as thermal conductors where Q = (t₁ - t₂) S / y and exhibit dilatation defined by ΔL = α · Δt · L₀. They are effective transmitters of sound and are weldable with or without filler material, under pressure, or both. In a plastic state, they are heat resistant and electrically conductive.

Mechanical Properties and Hardness Testing

Properties include bending at 180° for structures and 90° for profiles and sheets. Hardness is the resistance to being scratched or penetrated, measured via:

• Brinell (ball)
• Rockwell (cone)
• Vickers (pyramid)

Elasticity shows a remanence of 0.2% according to Young's Modulus. Plasticity allows metals to resist creep. The maximum load is calculated as max load = max amplitude / section. Malleable and ductile metals show elongation marks every 50 units.

Material Restrictions and Fatigue

Restrictions include the minimum cross-section after rupture. Metals face fatigue from repeated efforts: alternative (traction-compression), intermittent and pulsed (never null), and the endurance limit (infinite repetitions). Tenacity is the energy absorbed before breaking, while resilience is the energy absorbed up to the elastic limit.

Oxidation, Corrosion, and Protection

Oxidation is the loss of electrons, which can be benign. Corrosion is the destruction caused by oxidation. It can be generalized (affecting the lower section), localized due to surface impurities or heterogeneity (e.g., pitting), or intercrystalline (often not visible, caused by low chromium content). Differential aeration contact (with water) corrodes the most electropositive metal. Protection methods include removing impurities, avoiding retention, using paintings, inhibitors (layers), galvanizing, anodic coatings (oxide), or cathodic protection (current).

Metalworking and Steelmaking Processes

Metal forging applies pressure to the structure. Plastic lamination causes brittleness, while other methods include mold casting and machining. In steelmaking, iron phases (alpha, gamma, or delta) function based on temperature (°C). Steel alloys contain 0-2% carbon.

Key Components in Steel Alloys

• Ferrite: Foundries up to 6.67% C.
• Austenite: Low resistance but good hardness.
• Cementite: Resistant to compression and traction, but brittle.
• Pearlite: (0.8% C) Good traction, used for medium elongation.
• Ledeburite: Brittle, resistant to compression.

The Eutectic point is where the alloy blends well at low temperatures without a viscous state. Production involves Coke and Castine (limestone stabilizer); impurities can make the metal unweldable or produce oxidants. Refining is used to remove these impurities.

Special Steels and Standards

Special steels include Manganese and Chromium for resistance, and Nickel for corrosion resistance (e.g., Invar steel). Stainless steel contains Chromium-Nickel at an 18/8 ratio. Standards include UNE 12-22 (AENOR), which is a copy of the American AISI.

Thermal Treatments

Fast cooling is used to increase hardness. Annealing involves heating above the critical point with slow cooling to soften the material. Normalizing (Standard) uses slightly faster cooling. Quenching (Temple) involves fast cooling to create martensite, making the metal hard and resistant. Tempering is performed after quenching to remove tension and fragility. Austempering involves isothermal transformation to condense properties.