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.

Simplified Iron–Carbon phase diagram showing key phases, eutectoid (727°C) and eutectic (1148°C) temperatures. Hover key points for details.

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.

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.

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.

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

17-4PH: σy ~1,100 MPa, σu ~1,170 MPa. Precipitation-hardened with copper. Widely used for aerospace structural fasteners and components requiring both high strength and corrosion resistance.

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)

Gray iron dominates brake rotors because of excellent thermal conductivity and damping (absorbs vibration). CGI is replacing gray iron in diesel blocks where higher cylinder pressures demand greater strength — Ford, Audi, and DAF use CGI blocks.

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