Comprehensive Guide to Steels and Metallic Alloys: Classifications, Designations, and Mechanical Properties

Overview of Material Designation Systems

• Every class of material (steels, aluminum alloys, ceramics, polymers, etc.) adopts its own specific designation system.

• Within each single class, there are multiple designation systems that have not yet achieved definitive worldwide unification. These include:

  • American systems (ASTM/AISI).

  • German systems (DIN).

  • European systems (Euronorm - UNI EN).

  • Japanese systems.

  • Commercial systems.

• Steels: This is the most complex category with several coexisting standards:

  • Europe: Euronorms (UNI EN) issued by the European Committee for Standardization (CEN).

  • America: ASTM (American Society for Testing and Materials) in collaboration with AISI (American Iron and Steel Institute).

  • International: ISO (International Standard Institute).

• Aluminum Alloys:

  • For plastic deformation/wrought products: Aluminum Association (AA).

  • For foundry/casting: ISO standards.

• Copper Alloys: Regulated by UNI EN $1982$.

Fundamental Definition and Characteristics of Steels

• Chemical Definition: Steels are $Fe-C$ (Iron-Carbon) alloys where the carbon content, in the absence of other alloying elements, can reach up to approximately $2\%$.

• Cast Irons: If the carbon content exceeds $2\%$ and remains below $5\%$, the material is classified as cast iron (ghise).

• Alloying Elements: Elements are intentionally added in small amounts to meet specific mechanical or physical property requirements or for manufacturing needs.

• Impurities: Elements not intentionally added that originate from the raw materials used in melting or from reactions during manufacturing. Common impurities include:

  • Sulfur ($S$).

  • Phosphorus ($P$).

  • Copper ($Cu$).

  • Tin ($Sn$).

Types of Steels and Industrial Nomenclature

Carbon Steels (Acciai al carbonio)

• They constitute over $90\%$ of all steels produced.

• Content: Variable carbon, generally less than $1.5\%$. Maximums include $1.65\%$ manganese, $0.60\%$ silicon, and $0.60\%$ copper.

• Categorization by Carbon Content:

  1. Extradolci (Extra-soft): Less than $0.15\%$ carbon.

  2. Dolci (Soft): Between $0.15\%$ and $0.25\%$ carbon. (Extra-soft and Soft categories are commonly referred to as "iron").

  3. Semiduri (Semi-hard): Between $0.25\%$ and $0.50\%$ carbon.

  4. Duri (Hard): Over $0.50\%$ carbon.

• Applications: Machine parts, vehicle bodies, building structures, ship hulls, nails, screws, and bolts.

Alloy Steels (Acciai legati)

• Characterized by variable amounts of one or more elements (vanadium, molybdenum, manganese, silicon, copper) in percentages higher than those found in standard carbon steels.

• Applications: Mechanical components such as connecting rods, shafts, pins, steering components, and vehicle axles.

High-Strength Low-Alloy Steels (HSLA)

• Known by the acronym HSLA.

• Contain only small amounts of elements like vanadium, making them more economical than standard alloy steels.

• Produced with special procedures to provide superior mechanical strength (even at low temperatures) and better corrosion resistance compared to carbon steels.

Stainless Steels (Acciai inossidabili)

• Composition: Chromium ($10\%$ to $30\%$), Nickel (up to $25\%$), and other alloying elements.

• Properties: Bright surface finish, protection against atmospheric agents, gases, and corrosive acids. They maintain high mechanical strength for long periods at extremely high or low temperatures.

• Applications: Refinery piping and tanks, chemical plants, jet aircraft, space capsules, surgical instruments, dental/surgical prostheses, cookware, and cutlery.

Tool Steels (Acciai da utensili)

• Used for tools involving chip removal (machining) or plastic deformation.

• Divisions: Cold work, hot work, high-speed (rapidi), and super-high-speed (super-rapidi).

• High-speed steels: Contain tungsten and molybdenum to increase wear resistance during high-speed machining.

• Super-high-speed steels: Often contain cobalt to further enhance performance.

Effects and Percentages of Alloying Elements

Carbon ($C$)

• Fundamental alloying element for iron to become steel.

• Present in the combined form of cementite ($Fe_{3}C$).

• Levels:

  • Max content: $2.06\%$.

  • Case-hardening (carbocementazione): $0.2\%$.

  • Quenching and tempering (bonifica): $0.2\%$ to $0.6\%$.

  • Tool steels: $0.5\%$ to $2\%$.

• Effects of increasing $C\%$"

  • Decreased: Machinability, weldability, toughness, cold plasticity.

  • Increased: Strength, hardness, hardenability, wear resistance.

Nickel ($Ni$)

• Levels:

  • Alloy construction steels: $0.7\%$ to $5\%$.

  • Stainless steels: $8\%$ to $25\%$.

• Often paired with chromium and/or molybdenum.

• Effects: Increases strength and hardness in the quenched/tempered state; increases hardenability; improves toughness (resilience) and resistance to corrosion/oxidation even at high temperatures.

Chromium ($Cr$)

• Levels:

  • Alloy construction steels: $0.5\%$ to $3\%$.

  • Tool steels: $0.8\%$ to $13\%$.

  • Stainless steels: $10\%$ to $30\%$.

• Effects: Increases strength, hardness, hardenability, and toughness; improves corrosion and oxidation resistance (similar to Nickel).

Molybdenum ($Mo$)

• Levels:

  • Alloy construction steels: $0.2\%$ to $0.5\%$.

  • Tool steels: up to $9\%$.

  • Stainless steels: up to $6\%$.

• Effects: Increases strength, hardness, tool life, hardenability, and corrosion resistance.

Silicon ($Si$)

• Present in all steels at approximately $0.4\%$ as a deoxidizer.

• In higher amounts (max $2\%$), it increases strength and the elastic limit.

• Effects: Increases wear resistance and quenching capacity; decreases hot and cold deformability.

Manganese ($Mn$)

• Levels:

  • Normal steels: max $0.7\%$.

  • Construction steels: up to $2\%$.

  • Wear-resistant steels: up to $12\%$.

• Effects: Hardens steel without reducing toughness; significantly decreases forgeability and weldability; increases quenching penetration.

Other Elements

• Cobalt ($Co$): Used only in tool steels (super-high-speed) to maintain high hardness and strength at high temperatures.

• Tungsten/Wolfram ($W$): Found only in tool steels; the fundamental element for high-speed steels as it gives high hardness at high temperatures.

• Aluminum ($Al$): Found in nitriding steels to promote nitrogen absorption; acts as a deoxidizer and grain refiner.

• Copper ($Cu$): Found in small quantities in all steels. In common steels, it improves resistance to atmospheric agents and increases the elastic limit.

• Sulfur ($S$): Usually an impurity. Intentionally added (up to $0.32\%$) to improve machinability (free-cutting steels).

• Lead ($Pb$): Insoluble in steel. Added as a fine suspension (up to $0.25\%$) to improve machinability.

• Vanadium ($V$): Construction steels (max $0.2\%$); tool steels (up to $5\%$). Forms carbides that preserve hardness and high-temperature strength.

• Titanium ($Ti$): Found in some austenitic nickel-chromium stainless steels to prevent intercrystalline corrosion; acts as a grain refiner.

Classification according to UNI EN 10020

Classification by Chemical Composition

1. Non-alloy steels: None of the limit values for alloying elements are reached. Limit values ($m/m\%$"**

   • $Mn$: $1.65\%$.

   • $Si$: $0.6\%$.

   • $Cu, Pb$: $0.40\%$.

   • $Al, Co, Cr, Ni, W$: $0.3\%$.

   • $Bi, Se, Te, V$: $0.1\%$.

   • $Mo$: $0.08\%$.

   • $Nb$: $0.06\%$.

   • $Ti, Zr$: $0.05\%$.

   • $B$: $0.0008\%$.

   • Others ($P, S, N$ individually): $0.05\%$.

2. Stainless steels: Minimum $10.5\%$ Chromium and maximum $1.2\%$ Carbon.

3. Alloy steels: Steels that do not fall under stainless and have at least one element exceeding the non-alloy limits.

Classification by Quality Classes

• Non-alloy steels:

  1. Base non-alloy steels: No heat treatment requirements (except annealing), no formal requirements for drawing/formability. Max $P$ and $S$ are each $0.045\%$.

  2. Quality non-alloy steels: Requirements for toughness, grain size, or formability. Generally no uniform heat treatment response or strict inclusion purity required.

  3. Special non-alloy steels: Higher purity than quality steels. Suitable for quenching and tempering or surface hardening.

• Stainless steels: Divided based on Nickel content (less than or greater than $2.5\%$) and corrosion resistance.

• Alloy steels:

  1. Quality alloy steels: Specific requirements for toughness, grain size, and formability. Generally not intended for heat treatment. Includes weldable fine-grain structural steels.

  2. Special alloy steels: Precise chemical composition for specific uses (bearings, mechanical construction, tools, pressure vessels).

Steel Designation Systems (UNI EN 10027)

UNI EN 10027-1 (Alphanumeric)

• Group 1: Based on application and mechanical/physical characteristics.

  • $S$: Structural uses (followed by minimum yield strength $R_{e}$ in $N/mm^{2}$").

  • $P$: Pressure uses.

  • $L$: Tubes/Pipes.

  • $E$: Mechanical engineering.

  • $B$: Reinforced concrete.

  • $Y$: Pre-stressed concrete.

  • $R$: Rails.

  • $H$: Cold-rolled flat products.

  • Additional Symbols: $G$ (Castings), $JR, J0, J2$ (Resilience levels), $Q$ (Quenched/Tempered), $N$ (Normalized), $C$ (Cold forming).

• Group 2: Based on chemical composition.

  • Non-alloy with Mn < 1\%" Symbol "C" followed by $100 \times C\%$". Example: $C40$.

  • Low-alloy (Elements < 5\%" Carbon content ($100 \times C\%$") followed by chemical symbols and numbers multiplied by factors.

  • High-alloy (At least one element $\ge 5\%$" Symbol "X" followed by Carbon content, symbols, and actual percentages.

  • High-speed steels: Symbols "HS" followed by percentages of $W, Mo, V, Co$. Example: $HS 6-5-2-5$.

Numerical Multiplication Factors (for Group 2 low-alloys)

• Factor 4: $Co, Cr, Mn, Ni, Si, W$.

• Factor 10: $Al, Be, Cu, Mo, Nb, Pb, Ta, Ti, V, Zr$.

• Factor 100: $N, P, S$.

• Factor 1000: $B$.

UNI EN 10027-2 (Numerical)

• Fixed number of 7 digits: $A.BCDE.XX$.

• $1.xxxx$ denotes Steel.

• $2.xxxx$ denotes Heavy Alloys ($Cu, Pb$).

• $3.xxxx$ denotes Light Alloys ($Al, Ti$).

Classification by Use/Application

Construction Steels

• Case-hardening (Cementazione): Carbon < 0.2\%. Surface enrichment of carbon followed by quenching. Hard exterior, tough core. Uses: gears, shafts, pins. Examples: $18NiCrMo5$, $C10$.

• Quenching and Tempering (Bonifica): Carbon $0.2\%-0.6\%$. Treated at approx $600\,^{\circ}C$. Supports shocks and vibrations. Uses: crankshafts, connecting rods. Examples: $C40$, $38NiCrMo4$.

• Nitriding Steels: Carbon $0.29\%-0.43\%$. Surface hardening for extreme wear/corrosion. Examples: $38CrAlMo7$.

• Spring Steels: Carbon > 0.4\%. High elastic limit. Example: $52SiCr5$.

• Bearing Steels: Carbon $\approx 1\%$. High wear and compression resistance. Example: $100Cr6$ (used for balls, rollers).

Stainless Steels

• Martensitic: Carbon $0.2\%-1\%$, Chromium $11\%-18\%$. Quenchable/Magnetic. AISI 410, 420. Excellent for mechanical parts under load.

• Ferritic: Chromium $12\%-27\%$. Low carbon. Not quenchable/Magnetic. AISI 430. Used for sinks, appliances.

• Austenitic: Chromium $18\%-25\%$, Nickel $8\%-20\%$. Carbon < 0.08\%. Non-magnetic/Non-quenchable. 18/8 steels. Excellent corrosion resistance and weldability. AISI 304.

• Duplex (Austeno-ferritic): Chromium > 16\%, Nickel $5\%-6\%$. Mixed structure. High mechanical strength and resistance to pitting.

Tool Steels

• Cold Work: Operating below $200\,^{\circ}C$. High wear resistance.

• Hot Work: Operating between $450\,^{\circ}C-600\,^{\circ}C$. Stamped dies, extrusion tools. AISI H-13.

• High-Speed (HS): No Cobalt, higher toughness.

• Super-High-Speed (HSS): Includes Cobalt for extreme hardness and heat resistance up to $40\,m/min$ cutting speeds.

Aluminum Alloys

General Properties

• Specific weight: Approx $1/3$ of steel or copper ($2.66 - 2.85\,g/cm^{3}$").

• High corrosion resistance and electrical/thermal conductivity.

• Non-toxic and highly plastic (ductile/malleable).

• Low melting temperature: $510 - 650\,^{\circ}C$.

• Aluminum content in alloys: > 95\%.

Wrought Alloy Series (AA Designation)

• AA 1xxx: Pure Aluminum (> 99\%").

• AA 2xxx: Copper (+ Mg, Si) - Avional.

• AA 3xxx: Manganese.

• AA 4xxx: Silicon.

• AA 5xxx: Magnesium - Peraluman.

• AA 6xxx: Magnesium and Silicon - Anticorodal.

• AA 7xxx: Zinc, Magnesium, and Copper - Ergal.

Heat Treatment Designation (EN 515)

• F: As fabricated.

• O: Annealed or homogenized.

• W: Solution heat treated.

• T: Thermally treated (followed by digits).

• H: Strain hardened (followed by digits).

Specific Heat Treatment States (T and H)

• T1: Quenched from process and naturally aged.

• T4: Solution heat treated and naturally aged.

• T6: Solution heat treated and artificially aged.

• T7: Solution heat treated and overaged (stabilized).

• H1: Strain hardened only.

• H2: Strain hardened and partially annealed.

• H X2/X4/X8: Represents degree of hardening (1/4 hard, 1/2 hard, full hard).

Material Property Comparisons

Traducilo in italiano.