Steels & Cast Irons
Steel is the backbone of both automotive and aerospace structures. It's cheap, strong, weldable, recyclable, and available in hundreds of grades tailored to specific applications. Understanding the iron-carbon system and steel classification is essential for any engineer working in these industries.
Iron-Carbon Phase Diagram — The Practical Version
You don't need to memorize phase boundaries, but you need to know the five key microstructures:
Ferrite (α) — Pure or near-pure iron, BCC crystal structure. Soft and ductile (σy ~100 MPa). The matrix phase in mild steels. Austenite (γ) — FCC iron, stable above ~727°C. Non-magnetic, highly formable. The starting point for most heat treatments. Retained in austenitic stainless steels (304, 316) at room temperature via nickel additions. Cementite (Fe₃C) — Iron carbide, very hard and brittle. Provides strength when finely distributed in the matrix. Pearlite — Alternating layers (lamellae) of ferrite and cementite. Forms during slow cooling from austenite. Moderate strength (~600 MPa UTS), good wear resistance. Found in rail steels and medium-carbon steels. Martensite — Formed by rapid quenching of austenite. Body-centered tetragonal (BCT) structure. Extremely hard (up to HRC 65) but brittle. Must be tempered to restore some ductility. The basis of all quenched-and-tempered high-strength steels.Carbon Steel Grades (AISI/SAE System)
The first two digits indicate the alloy system (10xx = plain carbon), and the last two indicate carbon content in hundredths of a percent.
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Low Carbon (< 0.25% C)
| Grade | C% | σy (MPa) | σu (MPa) | Applications |
|---|---|---|---|---|
| 1008 | 0.08 | ~180 | ~330 | Deep drawing — car body panels, fenders |
| 1010 | 0.10 | ~200 | ~365 | Sheet metal, tubing, wire |
| 1018 | 0.18 | ~235 | ~400 | General purpose — shafts, pins, case-hardened parts |
| 1020 | 0.20 | ~245 | ~420 | Structural, carburizing applications |
Medium Carbon (0.25–0.60% C)
| Grade | C% | σy (MPa) | σu (MPa) | Applications |
|---|---|---|---|---|
| 1045 | 0.45 | ~310 | ~565 | Shafts, gears, axles, bolts |
| 1050 | 0.50 | ~345 | ~620 | Springs, cutting tools |
High Carbon (> 0.60% C)
| Grade | C% | σy (MPa) | σu (MPa) | Applications |
|---|---|---|---|---|
| 1095 | 0.95 | ~460 | ~830 | Springs, blades, wear plates |
Alloy Steels
Alloying elements modify properties beyond what carbon alone provides:
| Element | Effect |
|---|---|
| Chromium (Cr) | Hardenability, corrosion resistance, wear resistance |
| Molybdenum (Mo) | High-temperature strength, temper resistance |
| Nickel (Ni) | Toughness, especially at low temperatures |
| Vanadium (V) | Grain refinement, precipitation strengthening |
| Manganese (Mn) | Hardenability, solid solution strengthening |
| Silicon (Si) | Deoxidizer, spring steels |
| Boron (B) | Hardenability at tiny additions (~0.002%) |
Key Alloy Steel Grades
4130 (Cr-Mo) — σy ~460 MPa, σu ~560 MPa. Excellent weldability and toughness. Used for aircraft tubing (fuselage frames in light aircraft), roll cages, bicycle frames. 4340 (Ni-Cr-Mo) — σy ~860 MPa, σu ~1,000 MPa (quenched & tempered). One of the most widely used high-strength steels. Landing gear components, high-strength shafts, connecting rods. 8620 (Ni-Cr-Mo, low carbon) — σy ~360 MPa core, case hardness HRC 60+. The go-to case-carburizing steel for gears and bearing races. Hard, wear-resistant surface with a tough core. 300M — Modified 4340 with Si and V additions. σy ~1,550 MPa, σu ~1,860 MPa, KIC ~60 MPa√m. The standard aircraft landing gear steel. Combines ultra-high strength with sufficient fracture toughness.Advanced High-Strength Steels (AHSS) for Automotive
Modern car bodies use a patchwork of steel grades, each optimized for its role:
Dual Phase (DP) Steels
Two-phase microstructure: soft ferrite matrix with hard martensite islands. Excellent combination of strength and formability.
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| Grade | σy (MPa) | σu (MPa) | Elongation (%) | Application |
|---|---|---|---|---|
| DP590 | ~340 | ~590 | ~20 | Floor panels, cross members |
| DP780 | ~450 | ~780 | ~14 | Roof rails, side impact beams |
| DP980 | ~600 | ~980 | ~10 | B-pillar reinforcements |
TRIP Steels (Transformation-Induced Plasticity)
Contain retained austenite that transforms to martensite during deformation, providing exceptional energy absorption. Used in front rails and crash structures.
Complex Phase (CP) Steels
Ultra-fine grain structure with bainite, martensite, and precipitation hardening. High hole-expansion ratio — good for stretch-flanged parts like suspension components.
Press-Hardened Steel (PHS) / Hot Stamping
22MnB5 — Heated to ~900°C (fully austenitic), formed in a cooled die, and quenched in a single operation. Final properties: σy ~1,100 MPa, σu ~1,500 MPa.Used for the A-pillar, B-pillar, rocker panels, roof rails, and door intrusion beams — the safety cage of the car. Some newer PHS grades reach 2,000 MPa.
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Stainless Steels
Minimum 10.5% chromium, which forms a self-healing chromium oxide passive layer.
| Type | Example | Structure | Key Properties | Application |
|---|---|---|---|---|
| Austenitic | 304, 316 | FCC | Excellent corrosion, non-magnetic, formable | Exhaust systems, food equipment, chemical plants |
| Ferritic | 430 | BCC | Moderate corrosion, magnetic, cheaper | Automotive exhaust trim, kitchen sinks |
| Martensitic | 410, 440C | BCT | Hardenable, moderate corrosion | Cutlery, valve components, turbine blades |
| PH | 17-4PH | Martensite + precipitates | High strength + corrosion resistance | Aerospace fasteners, landing gear pins |
Cast Irons
Cast irons contain 2–4% carbon (vs. < 2% for steels). The form of graphite determines properties:
| Type | Graphite Form | σu (MPa) | Elongation (%) | Application |
|---|---|---|---|---|
| Gray iron | Flakes | 150–400 | < 1 | Brake rotors, older engine blocks |
| Ductile (nodular) | Spheroids | 400–700 | 2–18 | Crankshafts, differential housings, suspension knuckles |
| CGI (compacted graphite) | Vermicular | 300–500 | 1–6 | Modern diesel engine blocks (75% stronger than gray, better thermal conductivity than ductile) |
Heat Treatments
| Treatment | Process | Result | Example |
|---|---|---|---|
| Annealing | Heat to ~850°C, slow furnace cool | Softens, relieves stress, improves machinability | Cold-worked sheet before further forming |
| Normalizing | Heat to ~870°C, air cool | Refines grain, moderate strength | Forgings, structural steels |
| Quench & Temper | Heat to austenite, water/oil quench, then temper at 200–650°C | High strength + controlled toughness | 4340 landing gear, 1045 shafts |
| Carburizing | Heat in carbon-rich atmosphere at ~930°C | Hard case (HRC 58–62) with tough core | 8620 gears, bearing races |
| Nitriding | Heat in nitrogen atmosphere at ~500°C | Very hard surface, no quench needed | Crankshafts, cylinder liners |
Where Steels Go in a Car
- Body-in-White (BIW): AHSS — DP590-980, PHS 22MnB5, TRIP steels
- Chassis/Suspension: HSLA steels, DP590, spring steels
- Powertrain: 4140/4340 crankshafts, 8620 gears, gray/CGI iron blocks
- Fasteners: Grade 8.8 (medium carbon, Q&T), Grade 10.9 (alloy, Q&T), Grade 12.9 (alloy, highest strength)
Where Steels Go in Aircraft
- Landing gear: 300M, 4340 (ultra-high strength + toughness)
- Engine mounts/tubing: 4130 (weldable Cr-Mo)
- Fasteners: A286 (iron-nickel superalloy, high-temp), 17-4PH
- Bearings: 52100 (high-carbon chromium)
Key Takeaways
- The AISI/SAE numbering system tells you alloy type (first two digits) and carbon content (last two digits)
- AHSS grades (DP, TRIP, CP, PHS) enable lighter, safer car bodies by using the right grade in the right location
- Hot-stamped boron steel (22MnB5) at 1,500 MPa is the strongest steel in a typical car body
- Cast irons are classified by graphite morphology: flakes (gray), spheroids (ductile), vermicular (CGI)
- Heat treatment transforms the same steel composition into vastly different properties
