Metals, Alloys, and Steels: Properties and Applications

Posted on Apr 6, 2025 in Electromechanical Engineering

Metals and Alloys: Properties and Types

General Properties of Metallic Materials

Physical Properties

Metallic materials typically have high melting points (Tm), are good heat and electrical conductors, and possess medium thermal expansion coefficients.

Mechanical Properties

Metallic materials are ductile (though generally less so than polymers), exhibit high tensile strength, and possess high toughness.

Chemical Properties

Metallic materials are often highly reactive, particularly susceptible to oxidation (rusting).

Steels: Iron-Carbon Alloys

Steels are alloys primarily composed of iron and carbon. They often contain other elements like silicon, manganese, sulphur, etc. These alloying elements are either intentionally added or retained during the refining process.

Low Carbon Steels (C < 0.25%)

  • Moderately priced.
  • Soft, with low resistance (strength) and high ductility.
  • Can be hardened by cold working but not typically hardened by heat treatment.
  • Exhibit good weldability and high formability.
  • Microstructure: Ferrite + Pearlite.
  • Typical applications: Car body components, structural beams, pipelines.

Medium Carbon Steels (0.25% < C < 0.6%)

  • Stronger than low carbon steels, but less ductile.
  • Mechanical properties depend significantly on carbon content.
  • Alloying elements such as Cr, Ni, Mo can be added to improve heat treatment capacity (hardenability).
  • Heat treatment is necessary to achieve a hardened material.
  • Typical Microstructure after heat treatment: Tempered Martensite.
  • Typical applications: Train wheels, train rails, gears.

High Carbon Steels (0.6% < C < 1.4%)

  • Possess high strength, high hardness, and low ductility.
  • Exhibit high wear resistance.
  • Alloying elements such as V, W, Mo can be added to form very hard carbides, further enhancing properties.
  • Typical applications: Cutting tools, punches, knives.

Tool Steels

Tool steels are essentially high-carbon alloy steels. They are characterized by properties such as good toughness, good wear resistance, very good machinability, and resistance to softening upon heating. Applications include tools used to cut, press, extrude, or coin materials, as well as components for injection molding processes.

Stainless Steels

Steels with greater than 4% Chromium (Cr) addition exhibit good rust resistance. Key types include:

  • Austenitic Stainless Steel: Offers high corrosion resistance but is relatively expensive. Commonly used in the food industry, aircraft components, and food processing equipment. Typically contains 16-26% Cr and up to 35% Nickel (Ni). They are generally non-magnetic and cannot be hardened by heat treatment.
  • Ferritic Stainless Steel: Used in applications like exhaust pipes and valves. Typically contains 10-27% Cr and up to 0.2% Carbon (C). They offer good corrosion resistance and strength. Used in household items and the automobile industry.
  • Martensitic Stainless Steel: Used for surgical instruments, turbine springs, and turbine blades. Typically contains 11.5-18% Cr, up to 1.2% C, and 1-1.2% Ni. These steels are hardenable and have moderate corrosion resistance.

Alloy Steels

Alloying elements are added to steels to achieve one or more specific properties:

  • Increase resistance to corrosion.
  • Improve machinability.
  • Increase resistance to abrasion.
  • Enhance high-temperature properties.

Classification based on alloying content:

  • Low Alloy Steel: Total alloying elements less than 8%.
  • High Alloy Steel: Total alloying elements more than 8%.

High-Speed Steel (HSS)

HSS contains large amounts of tungsten, chromium, vanadium, and sometimes cobalt. It can withstand high temperatures without losing hardness (hot hardness). Used for manufacturing drills, milling cutters, and other cutting tools.

Tungsten High-Speed Steel

Tungsten is commonly used as an alloying element in high-speed steels, typically varying from 1% to 20%. It exists in the steel in the form of ferrite and carbides. Used in high-speed machining operations.

Effects of Alloying Elements in Steel

Alloying elements are added to influence steel properties:

  • Less than 5%: Primarily to improve strength or hardenability.
  • Up to 20% (or more): To impart corrosion resistance or stability at high or low temperatures.

Specific Element Effects:

  • Manganese (Mn): Improves hot ductility. Acts as an austenite stabilizer at low temperatures and stabilizes ferrite at high temperatures. Increases nitrogen solubility, useful for high-nitrogen duplex and austenitic stainless steels.
    • 0.05-0.85% Mn: Improves strength and ductility.
    • 1.5-2% Mn: Further increases strength in the heat-treated condition.
    • 2-10% Mn: Along with carbon, can contribute to brittleness.
    • 11-16% Mn (with 1-1.5% C): Produces a hard and wear-resistant alloy (e.g., Hadfield steel).
  • Nickel (Ni): Improves toughness and impact resistance, even in small quantities.
    • 1.5%-6% Ni: Increases the elastic limit, hardness, and tensile strength.
    • 8-22% Ni (and higher): Improves corrosion resistance (key in austenitic stainless steels) and provides additional strength and hardness.
  • Chromium (Cr): Provides hardness, increased elastic limit, and tensile strength, often without significantly affecting ductility. Improves hardenability, strength, wear resistance, and corrosion resistance (especially above 4%). Key element in all stainless steels. Increases resistance to oxidation at high temperatures and promotes a ferritic microstructure.
  • Molybdenum (Mo): Increases hardness penetration (hardenability), slows the critical quenching speed, and increases high-temperature tensile strength.
  • Silicon (Si): Increases resistance to oxidation at high temperatures, promotes a ferritic microstructure, and increases strength.
  • Vanadium (V): Helps control grain growth during heat treatment, increasing the toughness and strength of the steel. Forms hard carbides, enhancing wear resistance.
  • Titanium (Ti): When used with boron, increases the effectiveness of boron in enhancing the hardenability of steel. Can also form stable carbides.
  • Aluminum (Al): Improves oxidation resistance. In precipitation hardening steels, aluminum is used to form intermetallic compounds that increase strength in the aged condition. Also used as a deoxidizer and for grain size control.
  • Copper (Cu): Enhances atmospheric corrosion resistance. Can decrease work hardening, improve machinability, and improve formability in certain steels.
  • Tungsten (W): Added to improve pitting corrosion resistance and significantly increases hot hardness and wear resistance, especially in tool steels.

Non-Ferrous Metals and Alloys

All metallic elements other than iron are referred to as non-ferrous metals and their alloys.

General Properties of Non-Ferrous Materials

Compared to steels, non-ferrous materials often offer:

  • Lighter weight (lower density)
  • Higher electrical and thermal conductivity
  • Better resistance to corrosion
  • Ease of fabrication
  • Distinctive colors

Specific Non-Ferrous Metals and Alloys

Aluminum Alloys

  • Low density and high corrosion resistance.
  • Good electrical conductivity.
  • Ease of fabrication.
  • Easily recycled.
  • Applications: Aircraft structures, automotive components, outer foil, electrical wiring, packaging.

Titanium Alloys

  • Low density and excellent corrosion resistance.
  • High strength-to-weight ratio and good ductility.
  • Ease of fabrication and machining (though can be challenging).
  • High reactivity at elevated temperatures.
  • Stabilizing elements influence microstructure:
    • α (alpha) stabilizing elements: Al, O, N, C.
    • β (beta) stabilizing elements: Mo, V, Nb, Ta.
  • Applications: Turbojet engines, airframes, biomedical implants, heat exchangers.

Copper (Cu)

  • Pure copper is reddish in color.
  • Highly malleable, ductile, and an excellent conductor of heat and electricity.
  • Used extensively in electrical wiring and various industrial applications like heat exchangers and bearings.

Brass (Copper-Zinc Alloy)

  • An alloy of copper and zinc, containing at least 50% copper.
  • Color ranges from bright yellow to golden.
  • Soft and ductile, yet stronger than pure copper.
  • Possesses good casting properties and is resistant to corrosion.
  • Used for making bearings, pumps, valves, musical instruments, decorative items.

Bronzes (Copper Alloys)

  • Typically alloys of copper, often with tin as the main additive (but can include aluminum, silicon, manganese, etc.).
  • Generally exhibit higher strength than copper, are non-corrosive, and wear-resistant.
  • Applications: Bearings, bushings, marine hardware, sculptures.

Silver (Ag)

  • A very soft, white, and lustrous transition metal.
  • Possesses the highest electrical and thermal conductivity and reflectivity of any metal.
  • Utilities include photography, dentistry, high-capacity zinc long-life batteries, electrical/electronic contacts, printed circuits, and computer keyboards.

Gold (Au)

  • A bright, yellow, and soft metal.
  • Highly malleable and ductile, with good reflectance and high electrical and thermal conductivity.
  • Used in electronic devices (printed circuit boards, connectors, keyboard contacts), jewelry, dentistry, and as a reflector of infrared radiation in heating/drying devices.

Platinum (Pt)

  • A soft, lustrous, silver-white metal.
  • Highly dense, malleable, and ductile.
  • Exhibits high corrosion resistance, a high boiling point, and is considered a noble metal due to its high stability.
  • Used in catalytic converters, laboratory equipment, electrical contacts, dentistry, and jewelry. Various platinum alloys exist for specific applications.

Refractory Metals

A group of metallic elements highly resistant to heat and wear. Tungsten (W), Molybdenum (Mo), Niobium (Nb), Tantalum (Ta), and Rhenium (Re) are key refractory materials. They generally have high densities and hardness at room temperature.

Metals considered refractory typically meet these thresholds:

  • Melting point above 2200°C (approx. 4000°F).
  • Creep resistance above 1500°C (approx. 2700°F).

Niobium (Nb)

  • Is the least dense of the common refractory metals and the only one that can realistically be annealed at lower temperatures.
  • Found in aerospace applications (superalloys) and nuclear reactors.

Molybdenum (Mo)

  • Mainly used over tungsten when cost is a factor; it is cheaper but has comparable properties at moderately high temperatures.
  • Commonly used as a strengthening alloy in steel and in high-temperature applications like furnace parts and missile components.

Tantalum (Ta)

  • Has superior corrosion resistance, particularly against acids.
  • Found most often in the medical field as an alloying element in surgical tooling and implants. Also used in capacitors for electronics like cell phones.

Rhenium (Re)

  • Commonly used as an alloying element in other refractory metal alloys (like tungsten and molybdenum) and nickel-based superalloys, adding ductility and tensile strength, especially at high temperatures.
  • Used in jet engine turbine blades and rocket nozzles.