Steel — Engineering Reference

See also (Tier 3 family index): Steel Grades Family Index

1. At a glance

Steel is the dominant structural metal of the industrial world. The World Steel Association reported crude-steel production of approximately 1.89 billion tonnes in 2023 and a similar figure in 2024 — roughly 2,000 kg of steel produced per second, about 30× the global aluminum production. By tonnage it is, after concrete, the single most-used engineered material on Earth.

Definition. Steel is an alloy of iron with carbon content normally ≤ 2.0 wt% (in practice, plain-carbon structural steels sit between 0.05 and 1.0 wt% C). Above ~2.0 wt% C the alloy becomes cast iron — the eutectic point in the Fe–C system at 4.30 wt% C, 1147 °C marks the divide between steel and the cast-iron family.

Five families:

Family	Typical C	Defining additions	Strength range (σ_y)	Example grades
Plain-carbon	0.05–1.0 %	Mn, Si only	200–700 MPa	AISI 1018, 1045, 1095
Alloy (low/medium)	0.20–0.55 %	Cr, Ni, Mo, V	400–1500 MPa Q&T	AISI 4140, 4340, 8620
Stainless	0.03–1.2 %	≥ 10.5 % Cr	200–1300 MPa	AISI 304, 316, 410, 17-4 PH
Tool	0.5–2.0 %	Cr, W, Mo, V	1500–2500 MPa	A2, D2, M2, H13, S7
HSLA / structural	0.05–0.25 %	Nb, V, Ti microalloying	250–700 MPa	ASTM A36, A572, A992, A1066

Why engineers reach for steel first. It offers the highest strength-to-cost ratio of any structural material (≈ 0.10 USD/kg commodity, < 0.001 USD/MPa·kg yield), a 200+ year tradition of standardised heat-treatment metallurgy, near-universal weldability, full recyclability (electric-arc-furnace scrap-fed mills now produce ~30 % of global tonnage), and design codes (AISC 360, EN 1993, AISI S100) backed by century-scale empirical experience.

Where it sits in the design stack. Steel is the default first pick for: load-bearing structure (beams, columns, foundations), rotating mechanical parts (shafts, gears, fasteners), pressure containment (vessels, pipe), and wear-critical tooling (dies, cutters). Aluminum and composites displace it only when the weight-driven cost (aerospace, racing, top-of-vehicle automotive) outruns the strength-cost advantage.

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2. First principles

2.1 The iron-carbon phase diagram

The Fe–Fe₃C metastable diagram is the foundation of steel metallurgy. Key phases and reactions every steel engineer carries in their head:

• α-ferrite (BCC iron) — stable below 912 °C; very low C solubility (0.022 wt% max at 727 °C; ~0.008 wt% at 20 °C). Soft and ductile. Magnetic below 770 °C (Curie point).
• γ-austenite (FCC iron) — stable 912–1394 °C in pure Fe; carbon-bearing austenite stable down to 727 °C. C solubility up to 2.14 wt% at 1147 °C. Non-magnetic, denser than ferrite, the high-temperature working phase for forging and rolling.
• δ-ferrite (BCC) — high-temperature ferrite 1394–1538 °C. Engineering-relevant only in welding and casting.
• Cementite (Fe₃C) — 6.67 wt% C iron carbide, orthorhombic, hard (~800 HV), brittle. Acts as the strengthening intermetallic in pearlite and bainite.
• Eutectoid reaction at 727 °C / 0.76 wt% C: γ (0.76 %C) → α (0.022 %C) + Fe₃C (6.67 %C). Slow-cooling product is pearlite — alternating lamellae of ferrite and cementite, ~120–250 nm lamellar spacing.

2.2 The non-equilibrium phases (the ones we actually exploit)

Equilibrium ferrite + pearlite gives modest strength. Engineering steels owe their high strengths to non-equilibrium decomposition products of austenite, controlled by cooling rate:

• Pearlite — slow cool (furnace or air). σ_y 250–450 MPa; ductile. Spacing decreases with cooling rate; finer = stronger (Hall-Petch-like).
• Bainite — intermediate cool (~250–550 °C isothermal hold). Acicular ferrite + dispersed cementite. σ_y 700–1400 MPa. Upper bainite (forms 550–400 °C) coarser, less tough; lower bainite (400–250 °C) finer, tougher.
• Martensite — quench (cool fast enough to miss the bainite nose on the TTT diagram). Body-centred-tetragonal (BCT) Fe supersaturated with C; lath or plate morphology. Extremely hard (up to 65 HRC at 0.8 %C) but brittle. Forms by diffusionless shear at M_s (~250 °C for 0.4 %C steel, drops with C content).
• Retained austenite — austenite that survives quenching because M_f falls below room temperature at high C. Soft and metastable; transforms under load (TRIP effect, basis of modern automotive TRIP and Q&P steels).

2.3 Role of alloying elements

Element	Primary effect	Typical level
C	Strength, hardness; reduces ductility and weldability	0.05–1.0 %
Mn	Deoxidiser; strength; austenite stabiliser; increases hardenability	0.3–2.0 %
Si	Deoxidiser; strength; key in electrical steels (resistivity)	0.15–2.5 %
Cr	Corrosion resistance (≥ 10.5 % for stainless); hardenability; high-temperature strength	0.5–25 %
Ni	Toughness (especially low-T); austenite stabiliser; corrosion in conjunction with Cr	0.5–20 %
Mo	Hardenability; creep resistance; chloride-pitting resistance in stainless; retards temper embrittlement	0.15–4 %
V	Grain refinement via VC precipitates; secondary hardening on tempering	0.05–0.3 %
Nb / Ti	Microalloying for HSLA — grain pinning, precipitation strengthening	0.01–0.1 %
W	Red-hardness (high-T strength) — HSS tooling	1–18 %
Co	Increases M_s; secondary hardening in HSS and maraging	5–10 %
Cu	Corrosion resistance (weathering steels); precipitation strengthening	0.2–1.5 %
B	Massive hardenability boost at 5–30 ppm; mild auto-fastener steels	0.0005–0.003 %
S, P	Generally tramp; deliberately added to free-machining 1215, 12L14	0.02–0.35 %

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3. Practical math / design equations

3.1 Hall-Petch grain-size strengthening

Yield stress rises as grain size falls:

σ_y = σ_0 + k_y · d^(−1/2)

For ferrite-pearlite steels, σ_0 ≈ 70 MPa, k_y ≈ 19 MPa·mm^(1/2). Refining grain size from ASTM 5 (~64 μm) to ASTM 10 (~11 μm) raises σ_y by ~110 MPa with no loss of ductility — the only strengthening mechanism that gives strength and toughness simultaneously, which is why HSLA microalloying lives by it.

3.2 Carbon equivalent (weldability)

International Institute of Welding (IIW) formula:

CE_IIW = C + Mn/6 + (Cr + Mo + V)/5 + (Ni + Cu)/15

CE	Weldability	Preheat
< 0.35	Excellent	None required
0.35–0.45	Good	Often optional, < 25 mm thick
0.45–0.55	Limited	100–200 °C preheat
> 0.55	Difficult	200+ °C preheat + post-weld heat treatment (PWHT)

For higher-strength low-alloy steels, the Pcm formula is more appropriate:

Pcm = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B

3.3 Jominy hardenability and Grossman ideal diameter

Hardenability — depth to which martensite forms on quenching — is independent of attainable peak hardness. Quantified by the Jominy end-quench test (ASTM A255): a 25 mm × 100 mm bar austenitised then water-jet quenched on one end. Hardness measured every 1.6 mm (1/16 in) along the axis gives a Jominy curve. J-distance correlates to local cooling rate; matching the J-distance for a given quenchant/section thickness predicts hardness at that radial position. A 4140 round bar of 50 mm diameter oil-quenched has ~J12 (3/4-radius) cooling rate, giving ~50 HRC if grain-size and C are nominal.

3.4 Time-temperature-transformation (TTT) and CCT diagrams

TTT diagrams plot constant-temperature transformation start/finish curves for austenite decomposition. Two “noses” — the upper (pearlite) and lower (bainite) — define the critical cooling rates:

• Critical cooling rate to bypass the pearlite nose for plain 0.4 %C steel: ~900 °C/s; for 4340: ~10 °C/s. Difference is why 4340 air-hardens 75 mm sections while 1040 needs brine.
• CCT (Continuous Cooling Transformation) diagrams are the practical version — they predict the microstructure from any continuous cooling path and are what you actually use in heat-treatment shop work.

3.5 Worked example — AISI 4140 round bar, quench and temper

Problem. A 25 mm (1 in) diameter shaft, AISI 4140 (0.40 %C, 1.0 %Cr, 0.20 %Mo, 0.85 %Mn). Heat treatment: oil quench from 845 °C, temper at 425 °C for 1 hour. Predict yield strength, ultimate, hardness. Compare to hot-rolled (HR) condition.

Step 1. As-quenched hardness. At 0.40 %C, fully-martensitic hardness ≈ 60 HRC per the Hodge–Orehoski correlation. With 25 mm dia oil-quench, centre cooling rate matches J6 on Jominy; 4140 retains > 95 % martensite to J9, so the centre is fully martensitic. As-quenched: ~58–60 HRC, σ_u > 2000 MPa, brittle — not a usable state.

Step 2. Tempering at 425 °C. Using the Hollomon–Jaffe tempering parameter:

P = T (K) · [log₁₀(t·hours) + C] / 1000, C ≈ 20 for low-alloy steels

P = 698 · [log₁₀(1) + 20] / 1000 = 698 · 20 / 1000 = 13.96

From Bain–Grossman tempering charts for 4140 at P ≈ 14:

• Hardness drops from 60 HRC as-quenched to ~45 HRC.
• σ_u ≈ 1500 MPa (218 ksi).
• σ_y ≈ 1380 MPa (200 ksi) — yield/ultimate ratio ~0.92 in tempered martensite.
• Elongation ≈ 12 % in 50 mm.
• Charpy V-notch (CVN) toughness ≈ 60 J at 20 °C; tempering above ~370 °C avoids the 350 °C temper embrittlement trough where CVN drops to ~25 J.

Step 3. Compare to hot-rolled. AISI 4140 HR (ferrite + pearlite, ASTM grain size 5–7): σ_y ≈ 415 MPa (60 ksi), σ_u ≈ 655 MPa (95 ksi), 25 % elongation, ~200 HB.

Strength gain from heat treatment: 3.3× σ_y, 2.3× σ_u, at the cost of ductility (25 % → 12 %) and a roughly 4× increase in fabrication cost. This is the central trade in alloy-steel design.

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4. Reference data — common steel grades

Grade	Class	C	Other	Cond.	σ_y (MPa)	σ_u (MPa)	Elong.	Hardness
AISI 1018	Plain LC	0.18	0.75 Mn	HR	220	400	25 %	120 HB
AISI 1018				CD	370	440	15 %	130 HB
AISI 1045	Plain MC	0.45	0.80 Mn	HR	310	565	16 %	165 HB
AISI 1045				Q&T 540 °C	530	720	12 %	220 HB
AISI 1095	Plain HC	0.95	0.40 Mn	Spheroidised	380	685	13 %	192 HB
AISI 4140	Alloy	0.40	1.0 Cr, 0.2 Mo	Annealed	415	655	25 %	197 HB
AISI 4140				Q&T 425 °C	1380	1500	12 %	45 HRC
AISI 4340	Alloy	0.40	1.8 Ni, 0.8 Cr, 0.25 Mo	Annealed	745	745	22 %	217 HB
AISI 4340				Q&T 425 °C	1280	1470	12 %	45 HRC
AISI 8620	Carburising	0.20	0.5 Ni, 0.5 Cr, 0.2 Mo	Carburised + Q&T	690 core / surface 60 HRC	1100	14 %	60 HRC case
AISI 304	Aust. SS	0.08 max	18 Cr, 8 Ni	Annealed	215	505	70 %	85 HRB
AISI 316L	Aust. SS	0.03	16 Cr, 10 Ni, 2 Mo	Annealed	170	485	60 %	80 HRB
AISI 410	Mart. SS	0.15 max	12 Cr	Annealed	275	515	25 %	95 HRB
AISI 410				Q&T 200 °C	1380	1730	10 %	45 HRC
17-4 PH	PH SS	0.07 max	16 Cr, 4 Ni, 4 Cu	H900	1170	1310	10 %	44 HRC
2205	Duplex SS	0.03	22 Cr, 5 Ni, 3 Mo, 0.15 N	Annealed	450	620	25 %	30 HRC
ASTM A36	Struct.	0.26 max	0.80 Mn	HR	250	400–550	20 %	119–162 HB
ASTM A572 Gr 50	HSLA	0.23 max	Nb/V microalloy	HR	345	450	18 %	150 HB
ASTM A992	Struct.	0.23 max	V + Nb, capped CE	HR	345–450	450–620	18 %	160 HB
ASTM A1066 Gr 65	HSLA	0.18 max	Nb/V/Ti	TMCP	450	550	17 %	200 HB
ASTM A325	Fastener	0.30 max	medium-C alloy	Q&T	635 (8 dia ≤ 25 mm)	825	14 %	25–34 HRC
ASTM A490	Fastener	0.30 max	alloy	Q&T	895	1035–1210	14 %	33–39 HRC
D2	Tool	1.50	12 Cr, 1 Mo, 1 V	Hardened + temp	—	—	—	60 HRC
H13	Hot-work tool	0.40	5 Cr, 1 Mo, 1 V	Q&T	1380	1650	12 %	48 HRC
M2	HSS	0.85	6 W, 5 Mo, 4 Cr, 2 V	Q&T	—	—	—	64 HRC

Properties per ASTM E8 / E8M tensile (room-temperature), ASTM E18 Rockwell hardness, ASTM E10 Brinell. Charpy values per ASTM E23.

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5m. Composition & microstructure

5m.1 Plain-carbon steels (AISI 10xx)

The xx is the carbon content in hundredths-of-a-percent. Microstructure in normalised/HR state is ferrite + pearlite; the pearlite fraction rises linearly from 0 % at 0.022 %C to 100 % at 0.76 %C (the eutectoid). Above 0.76 %C the structure is pearlite + grain-boundary cementite (hypereutectoid).

• AISI 1018 (0.18 %C) — the canonical “cold-rolled mild steel.” Highly weldable, machinable, used for shafts that see modest stress, bolts, automotive body panels, machinery bases. CD bars are the workhorse stock for hobby and prototype machining.
• AISI 1045 (0.45 %C) — medium-carbon, harden-and-temperable. Used in axles, shafts, gears, crankshafts. Sweet spot of strength-and-formability.
• AISI 1095 (0.95 %C) — high-carbon. Hardens to 65 HRC. Used in springs, knives, files. Brittle if not tempered. Spheroidised condition (rounded cementite particles) used to enable cold-forming and machining before final heat treatment.
• AISI 12L14 — free-machining variant with 0.30 %S + 0.20 %Pb. Machinability rating 170 % (1212 = 100 %). Lead now being phased out (REACH, RoHS); replaced by Bi-bearing 12L14+ and tellurium variants.

5m.2 Alloy steels (AISI 4xxx, 8xxx)

• AISI 4140 (0.40 %C, 1.0 %Cr, 0.20 %Mo) — the “do-everything” medium-strength shaft, gear, and structural steel. Through-hardens to ~50 mm section, weldable with preheat, machinable annealed, available worldwide. Variants 4142, 4145 push C slightly higher for hardness at the expense of weldability.
• AISI 4340 (0.40 %C, 1.8 %Ni, 0.80 %Cr, 0.25 %Mo) — the ultra-high-strength workhorse. Through-hardens to ~150 mm section. Best toughness-to-strength of any common alloy steel; used in aircraft landing-gear, helicopter rotor shafts, gun barrels. Most-published mechanical-property database in metallurgy.
• AISI 8620 (0.20 %C, 0.55 %Ni, 0.50 %Cr, 0.20 %Mo) — low-C carburising steel. Surface carburised to 0.8–1.0 %C, then Q&T → hard wear-resistant case over tough core. Standard automotive gear steel.
• AISI 9310 — premium aerospace carburising steel; higher Ni than 8620.

5m.3 Stainless steels (corrosion-resistant, ≥ 10.5 %Cr)

The Cr forms a self-healing passive Cr₂O₃ film 1–5 nm thick — the chemical basis of stainless resistance. Five microstructural families:

• Austenitic (300-series) — FCC, non-magnetic, exceptional ductility, formability, weldability, cryogenic toughness. Cannot be hardened by heat treatment; only by cold work. Dominant in food, chemical, architectural.
  • 304 / 304L (18 Cr, 8 Ni) — general-purpose. L = low carbon (< 0.03 %) to avoid weld-zone sensitisation (Cr-carbide precipitation at grain boundaries → intergranular corrosion).
  • 316 / 316L (16 Cr, 10 Ni, 2 Mo) — Mo addition gives chloride-pitting resistance. Marine, biomedical implants (orthopaedic plates, screws), pharmaceutical reactors.
  • 321 / 347 — Ti or Nb stabilised; resistant to sensitisation at elevated service temperatures.
• Ferritic (400-series 405, 430, 444) — BCC, magnetic, cheaper (no Ni). Used in automotive exhaust, kitchen appliances. Limited weldability (grain growth in HAZ).
• Martensitic (410, 420, 440C) — heat-treatable. Used in cutlery (420), surgical scalpels (440C, 60 HRC), valve seats (410).
• Duplex (2205, 2507) — ~50/50 austenite-ferrite. Double the strength of austenitics at similar corrosion resistance. Oil/gas, desalination. Service temperature 50–280 °C only (475 °C embrittlement above, austenite-stabiliser segregation below).
• Precipitation-hardening (17-4 PH, 15-5 PH, 13-8 Mo) — austenitic or martensitic matrix, aged to precipitate Cu-rich or Ni-Al intermetallics. 17-4 PH H900 condition: σ_y 1170 MPa with full stainless corrosion resistance — used in aerospace fasteners, golf-club heads, pump shafts.

5m.4 Tool steels

Classified by hardening medium (AISI letter prefix):

• W (water-hardening) — high-C plain-carbon. Cheap; small parts only.
• O (oil-hardening) — O1, O6. General-purpose tools.
• A (air-hardening) — A2 is the most common machine-shop tool steel; minimal distortion.
• D (high-Cr, high-C) — D2 (1.5 %C, 12 %Cr) — cold-work dies, punches, blanking tools. Wear-resistant but limited toughness.
• S (shock-resistant) — S7. Chisels, jackhammer bits.
• H (hot-work) — H13 (5 %Cr, 1 %Mo, 1 %V). Aluminum-die-casting dies, forging dies, extrusion mandrels. Service temp up to ~600 °C.
• M / T (high-speed) — M2, M42, T1. Cutting tools; secondary-hardening to ~64 HRC; service to 600 °C red-hot without losing edge.

5m.5 Construction / structural steels

• ASTM A36 (σ_y = 250 MPa / 36 ksi) — the long-time US generic mild structural. Still common in plate, angle, channel. Largely superseded for W-shapes by A992.
• ASTM A572 Gr 50 (σ_y = 345 MPa / 50 ksi) — HSLA microalloyed, mostly Nb/V. Used in bridges, transmission towers, plate girders.
• ASTM A992 — current US standard for hot-rolled W-shapes since 1998. Like A572 but with maximum yield = 65 ksi and CE ≤ 0.45 capped to guarantee weldability and predictable plastic-design behaviour (Yield/Tensile ≤ 0.85 required).
• ASTM A1066 — TMCP (thermomechanically-controlled-process) plate, grades 50–80; ultra-high-strength HSLA for heavy plate construction, mining-equipment booms, wind-tower bases.
• ASTM A325 / A490 — high-strength structural bolts. A325 (Type 1 medium-C, Type 3 weathering): proof 585 MPa, σ_u 825 MPa. A490: proof 825 MPa, σ_u 1035 MPa minimum. Now consolidated with metric bolts under ASTM F3125 since 2015.

5m.6 Microstructure quick map

Cooling path	Microstructure	Typical hardness	Comment
Slow furnace	Coarse ferrite + pearlite	< 200 HB	Annealed — softest, most ductile
Air (normalise)	Fine ferrite + pearlite	150–250 HB	Refined grain, baseline mechanical state
Salt-bath 300–550 °C hold	Bainite	30–50 HRC	Excellent toughness-strength balance
Oil quench	Mostly martensite + retained austenite	50–60 HRC	Hard, brittle until tempered
Water/brine quench	Fully martensite	up to 65 HRC	Risk of quench cracking
Temper 150–250 °C	Tempered martensite	55–62 HRC	Stress-relieved hardness retained
Temper 400–550 °C	Tempered martensite + secondary carbides	35–48 HRC	Engineering Q&T condition
Temper 600+ °C	Spheroidised cementite + ferrite	20–30 HRC	Softened for machinability

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6m. Mechanical properties

Quoted per ASTM E8/E8M-22 (tensile), ASTM E18-22 (Rockwell), ASTM E10-23 (Brinell), ASTM E23-23a (Charpy V-notch), ASTM E647-15e1 (fatigue crack growth). Service temperature 20 °C unless noted.

Stiffness (essentially constant across all steels):

• Young’s modulus E = 200 ± 10 GPa (29 × 10⁶ psi)
• Shear modulus G = 79 GPa
• Poisson’s ratio ν = 0.29
• Heat treatment does not change stiffness. This is critical: a deflection-limited design cannot be improved by heat-treating to higher strength.

Density ρ = 7.85 g/cm³ (austenitic stainless slightly higher at ~7.95 g/cm³ from Ni; martensite slightly lower at 7.75 g/cm³ from BCT distortion).

Fatigue endurance limit (R = −1, rotating-beam, polished):

• Plain-carbon & alloy steels: σ_e ≈ 0.5 · σ_u (for σ_u ≤ 1400 MPa; plateaus at ~700 MPa above)
• Stainless steels (austenitic): no true endurance limit — fatigue strength at 10⁸ cycles ≈ 0.35 · σ_u

Fracture toughness K_IC (per ASTM E399):

• A36 ~60 MPa·√m
• 4340 Q&T 425 °C: 50–80 MPa·√m
• Maraging 300: 110 MPa·√m
• 17-4 PH H900: 50 MPa·√m
• Tool steel D2: 15 MPa·√m

Charpy V-notch trends. All BCC steels (plain-carbon, ferritic, martensitic) show a ductile-to-brittle transition (DBTT) — at low temperature, CVN drops sharply (e.g. A36 from 100 J at 20 °C to < 20 J at −40 °C). Austenitic stainless 304/316: no DBTT, stays ductile to liquid-He temperatures — the reason cryogenic vessels are austenitic.

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7m. Thermal / electrical / chemical properties

Property	Carbon steel	304 stainless	4340
Thermal conductivity (W/m·K, 20 °C)	50	16	44
Specific heat (J/kg·K)	480	500	475
Coefficient of thermal expansion α (10⁻⁶ /°C, 20–200 °C)	12.0	17.3	12.3
Electrical resistivity (μΩ·cm)	17	72	22
Melting range (°C)	1425–1540	1400–1450	1415–1455
Service-temperature ceiling (continuous)	~400 °C (creep)	870 °C oxidation; 425 °C structural (sensitisation risk)	~480 °C

Stainless steels conduct heat 3× worse than carbon steel — a frequent design surprise in heat-exchanger and weld-heat-affected zone analysis. The high α of austenitic stainless (17 × 10⁻⁶ /°C, ~45 % higher than carbon steel) drives the well-known distortion problems in stainless welding.

Corrosion in general atmospheric exposure:

• Plain-carbon steel: ~50–100 μm/year urban, ~200–400 μm/year marine. Use protective coating or galvanising.
• Weathering steel (ASTM A588 / Corten) — Cu+Cr+Ni alloying forms protective rust patina, ~10× longer maintenance-free life in non-chloride atmospheres.
• Stainless 304: passive in clean atmospheres; pits in chlorides. Critical pitting temperature (CPT) ~15 °C in 6 % FeCl₃.
• Stainless 316: CPT ~25 °C; standard for marine atmospheric.
• Duplex 2205: CPT ~35 °C; super-duplex 2507 ~70 °C.

Galvanic series in flowing seawater (active → noble): Mg < Zn < Al < CS < CI < Pb < Sn < Brass < Cu < bronze < Monel < 304 (passive) < 316 (passive) < Ti < Pt. Dissimilar metals in electrical contact corrode at the more-active member: stainless bolts in aluminum will pit the aluminum, not the stainless.

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8m. Processing & joining

8m.1 Heat treatment

• Annealing — heat into γ, slow furnace cool. Softens to lowest available hardness for cold-forming or machining. Spheroidising anneal (sub-critical, hold near A_1 for hours) for high-C steels to roundthe carbides.
• Normalising — heat into γ, air cool. Refines grain to ASTM 6–8; baseline for forged or as-cast parts before final HT.
• Stress-relieving — sub-critical hold 550–650 °C, slow cool. Relieves residual stress from welding, machining; does not change microstructure significantly.
• Quenching — austenitise then rapid cool in water, brine, oil, polymer, or air. Quench severity H factor: water = 1.0, oil = 0.3, air = 0.02.
• Tempering — reheat quenched part to 150–650 °C to convert brittle martensite to tougher tempered martensite. Always temper after quench; un-tempered martensite is a stress-cracking time bomb.
• Case hardening:
  • Carburising — pack, gas (endothermic atmosphere), or vacuum at 870–950 °C; C diffuses in to 0.8–1.0 % at surface. Case depth 0.5–2 mm. Then Q&T.
  • Carbonitriding — N added; thinner case, less distortion.
  • Nitriding — gas (NH₃) or plasma at 500–550 °C; N forms hard nitrides. Sub-critical, so no distortion. Used on H13, 4140, nitralloys.
  • Induction hardening — surface only, RF coil. Used on crankshaft journals, gear teeth, axle shafts.
  • Flame hardening — surface, oxyfuel torch. Less precise than induction.

8m.2 Welding

Process	Acronym	Use
Shielded metal arc	SMAW (stick)	Field, structural, repair
Gas metal arc	GMAW (MIG)	Production, semi-automatic
Flux-cored arc	FCAW	Outdoor structural, high deposition
Gas tungsten arc	GTAW (TIG)	Stainless, thin-section, root passes
Submerged arc	SAW	Heavy plate, pipe, vessel longitudinal seams
Resistance spot	RSW	Automotive body
Electron beam	EBW	Deep, narrow welds aerospace
Laser	LBW	Precision, automotive tailored blanks

Steel weldability rules of thumb:

• C ≤ 0.30 %: weldable without preheat in thin section.
• CE_IIW ≤ 0.40: weldable, often no preheat.
• CE_IIW > 0.55: high cracking risk; mandatory preheat + low-H₂ consumables (basic-coated E7018) + PWHT.
• Cold cracking (hydrogen-induced) — at HAZ, hours-to-days after weld. Mitigation: dry electrodes, preheat, low-H₂ practice.
• Hot cracking — austenitic stainless 304; mitigate with ferrite content 5–10 FN in weld (filler with deliberate Cr-Ni balance per Schaeffler-DeLong diagram).
• Sensitisation of austenitic stainless — chromium-carbide precipitates at grain boundaries 425–815 °C → intergranular corrosion. Use L-grades (304L, 316L) or stabilised grades (321, 347).
• Distortion control in stainless — high α + low k = severe distortion. Use back-step welding, intermittent passes, fixturing.

8m.3 Machining

• 1018 CD: ideal for general machining (machinability 78 % of 1212 reference).
• 12L14: machinability 170 % — best ferrous chip control.
• 4140 annealed: 65 % — machinable; Q&T 4140 drops to 30–40 %, needs carbide tooling and slow speeds.
• 304/316 austenitic stainless: 45 % — work-hardens severely. Use sharp, positive-rake tools, slow speeds (60–90 m/min), heavy feeds (force the cut below the work-hardened layer), flood coolant.
• D2/M2 tool steels (in annealed state): 25 % — only machined before hardening; ground to final shape after.

8m.4 Forming

• Cold-rolled vs hot-rolled: Hot-rolled (HR) processed above recrystallisation (~900 °C), rough scaled surface, oversize tolerance, lower σ_y. Cold-rolled / cold-drawn (CR/CD) processed below recrystallisation, smooth surface, tight tolerance, ~30 % higher σ_y from cold work.
• Springback in cold forming: bend radius needs overbend by 1–3° (low-strength) up to 15–20° for advanced high-strength (AHSS) automotive sheet.
• Deep drawing: austenitic stainless 304 (LDR ~2.1), AISI 1008/1010 IF steel (LDR ~2.3) preferred; martensitic stainless unsuitable.
• Roll forming, brake forming, stretch forming, hydroforming — same process families across all flat-rolled steels with grade-specific limits per ASTM E290 bend test and n-value (strain hardening exponent).

8m.5 Surface treatments

• Hot-dip galvanising (ASTM A123) — immerse in 450 °C molten Zn, 50–150 μm coating, sacrificial protection ~50 years atmospheric.
• Electrogalvanising — thinner (~10 μm), automotive body panels.
• Paint / powder coating — barrier protection; powder is solvent-free, baked-cure.
• Black oxide — magnetite Fe₃O₄ conversion coating; mild corrosion resistance only; aesthetic on tools and firearms.
• Passivation of stainless (ASTM A967) — nitric or citric acid bath to remove free iron and re-establish thicker Cr₂O₃ film after machining.
• Electroless nickel — Ni-P coating; uniform thickness regardless of geometry; mild corrosion + wear.
• PVD coatings (TiN, TiAlN) — on tools and dies; 60 % friction reduction.

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9m. Applications & selection trade-offs

Quick-pick guide by service requirement:

Need	First pick	Reason
Cheap general fabrication	A36 / 1018 HR	Lowest cost, weldable, available
Structural building frame	A992 W-shape	Capped CE, predictable σ_y, AISC-blessed
Bridge plate, high stress	A572 Gr 50 / A709 Gr 50W	Higher strength than A36 at small cost premium
Pressure vessel	A516 Gr 70 (carbon) or A240 304L (stainless)	ASME BPVC qualified
Shaft, gear, axle (medium duty)	1045 Q&T or 4140 Q&T	Through-hardenable; balanced cost
High-strength shaft (impact)	4340 Q&T 425 °C	Best toughness-strength balance
Carburised gears	8620 (commodity) or 9310 (aero)	Hard case + tough core
Springs	5160 (auto leaf), Music Wire (small)	High σ_y, fatigue endurance
Food / pharma	316L	Mo-Cr stainless, cleanable, no carbide pickup
Marine atmospheric	316L or duplex 2205	Chloride pitting resistance
Cryogenic vessel	304L or 9 % Ni steel	Retains CVN to 4 K
Cold-work cutting die	D2	High wear, OK toughness, air-hardens
Hot forging die	H13	Tempering resistance to 600 °C
HSS cutting tools	M2 / M42	Secondary hardening, red-hot edge retention
High-strength fastener	A325 (≤ 1380 MPa zone) / A490	Code-compliant for structural bolted connections
Architectural / corrosion-cosmetic	304 (interior), 316 (exterior)	Cheap, recognised, cleanable
Lightweight automotive	DP, TRIP, Q&P AHSS	σ_y up to 1200 MPa with formability
Eccentric strength + corrosion	17-4 PH H900	σ_y 1170 MPa + stainless

Trade-off discussion

• 4140 vs 4340. 4340 has 1.5–2× the Charpy CVN at equal σ_y due to Ni content. Pick 4340 when (a) section thickness > 60 mm (better hardenability), (b) impact loading dominates, (c) operating temperature < −30 °C. 4140 is cheaper (~20 %), more widely stocked, and adequate for general industrial shafts at room temperature.
• 316L vs 17-4 PH. Both stainless. 316L: σ_y 170 MPa, exceptional ductility and weldability. 17-4 PH: σ_y up to 1170 MPa in H900 condition, machinable then aged, but harder to weld (intermetallic-zone embrittlement in HAZ). Use 17-4 PH only when you need stainless and high strength.
• A36 vs A992. A36 has lower σ_y (250 vs 345 MPa) but is uncapped on the upper end and on CE — old A36 plates routinely tested at σ_y up to 410 MPa, which breaks plastic-design assumptions about predictable beam buckling. A992 was created to fix this; mandatory in W-shapes since 1998 per AISC.
• Carbon steel + paint vs stainless 304. Crossover cost depends on environment. In dry indoor service, painted carbon steel wins for 50+ years. In marine atmospheric, stainless 316 (not 304 — chloride pitting kills 304) wins on lifecycle by year 8–10 vs repainted carbon.

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10m. Failure modes

1. Yielding under monotonic overload. Ductile failure; large permanent deformation; usually obvious. Design factor of safety FS = σ_y / σ_design typically 1.5–3.0 (static), 4+ (dynamic, unknown loading).

2. Fatigue. Cyclic loading at σ < σ_y. Endurance limit σ_e ≈ 0.5 σ_u in clean rotating-beam testing; in service derated by:

   • Surface finish factor k_a (0.5 for as-forged, 0.9 for ground, 1.0 for polished)
   • Size factor k_b
   • Reliability factor (0.897 at 90 % survival, 0.620 at 99.99 %)
   • Stress concentration K_f at notches, fillets, holes Result: in-service σ_e often 60–150 MPa for plain-carbon steel. Crankshafts, gear teeth, weld toes — classic fatigue failure sites.

3. Brittle fracture. Low temperature + sharp notch + high σ_u + plane-strain section thickness. Liberty-ship hull failures (1943) — A36 plate, low-T North Atlantic, riveted-stress concentration. Mitigation: stay above DBTT; use Charpy-qualified steels (ASTM A20 specifies CVN for vessel plate); avoid sharp corners; specify fine grain (ASTM 6+).

4. Stress-corrosion cracking (SCC). Combination of tensile stress + corrosive environment + susceptible alloy. Austenitic stainless in chloride (304/316 with σ > 70 % yield in > 60 °C chloride) — intergranular cracking, can fail in hours. Carbon steel in caustic (NaOH) or amine, brass in ammonia. Mitigation: stress relieve (PWHT), use ferritic or duplex stainless in chloride service, eliminate residual tension (shot peen).

5. Hydrogen embrittlement. High-strength steels with hardness > 35 HRC (σ_u > 1100 MPa) are vulnerable. H sources: electroplating (Cd, Zn), cathodic protection, acid pickling, cathodic side of galvanic couples. ASTM F1940 specifies baking (190 °C, 4–24 hr) within 4 hours of plating for high-strength fasteners. Failures: delayed brittle cracking 1 hour – 30 days after assembly; intergranular fracture surfaces with no plastic deformation.

6. Creep. Time-dependent deformation under stress at T > ~0.4 T_melt (in K). For plain-carbon steel, above ~400 °C; for Cr-Mo (e.g., 1.25Cr-0.5Mo, ASTM A335 P11): up to 540 °C; for austenitic stainless 304H: up to 650 °C; for refractory tool steels and superalloys: higher still. Design to ASME BPVC Section II allowable stresses, which embed Larson-Miller parameter extrapolation to 100,000 hr service life.

7. Galling (adhesive wear). Cold-welding between sliding surfaces of similar metallurgy. Austenitic stainless 304/316 fasteners are notorious — nut seizes on bolt at 50–70 % of torque-spec because protective oxide breaks down under contact pressure and clean metal cold-welds. Mitigation: dissimilar pairs (Nitronic 60 nut on 316 bolt), anti-seize compound (MoS₂, Cu-graphite), Ni-plating one mating surface.

8. Pitting and crevice corrosion in stainless. Chloride breaks passive film locally. Pitting Resistance Equivalent Number: PREN = %Cr + 3.3 %Mo + 16 %N. 304: PREN ~19, 316: PREN ~25, 2205 duplex: PREN ~35, 2507 super-duplex: PREN ~42. PREN > 40 needed for hot seawater service.

9. Temper embrittlement. Slow cooling through 375–575 °C (or extended service in this range) embrittles Cr-Mo and Cr-Mn-Ni steels via P/Sb/Sn/As segregation to prior-austenite grain boundaries. Mitigation: rapid post-temper cool; clean low-residual chemistry (e.g., ASTM A387 with J-factor restriction).

10. 475 °C embrittlement (ferritic and duplex stainless). α’ (Cr-rich) precipitation during 400–500 °C exposure. Pre-service annealing destroys it; service above 280 °C in duplex creeps slowly into the danger zone.

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11. Cross-references

• [[Engineering/mechanics-of-materials]] — stress, strain, beam bending, torsion; consumes steel properties
• [[Engineering/materials-aluminum]] — sibling material; the other major structural metal
• [[Engineering/materials-selection]] — Ashby method comparing steels with composites, aluminum, polymers
• [[Engineering/structural-analysis]] — steel-frame analysis using A992, A572 properties
• [[Engineering/steel-design]] — application to building, bridge, vessel design per AISC, AWS, ASME
• [[Engineering/joining-welding]] — welding processes and procedures by steel grade
• [[Engineering/machining]] — machining parameters, tool life, surface finish
• [[Engineering/Tier3/surface-treatments]] — Q&T, carburising, nitriding processes in detail
• [[Engineering/fatigue-analysis]] — fatigue and fracture mechanics with steel data
• [[Robotics/manipulator-design]] — steel for robot links, shafts, harmonic-drive gears
• [[Robotics/end-effectors]] — tool-steel selection for grippers and fixturing
• [[Languages/Tier3/construction-bim]] — STEP material assignments and naming conventions
• [[Languages/Tier3/industrial-automation]] — AWS D1.1, ASME Section IX welding-procedure qualifications

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12. Citations

1. Callister, W. D. & Rethwisch, D. G. Materials Science and Engineering: An Introduction, 10th ed. (Wiley, 2018). Foundational textbook; Fe–C diagram and heat-treatment metallurgy chapters.
2. Ashby, M. F. & Jones, D. R. H. Engineering Materials 1: An Introduction to Properties, Applications and Design, 5th ed. (Butterworth-Heinemann, 2019).
3. Ashby, M. F. & Jones, D. R. H. Engineering Materials 2: An Introduction to Microstructures and Processing, 5th ed. (Butterworth-Heinemann, 2019).
4. Bringas, J. E. (ed.) Handbook of Comparative World Steel Standards, 5th ed. (ASTM International, 2016).
5. ASM International. ASM Handbook Volume 1: Properties and Selection: Irons, Steels, and High-Performance Alloys (10th ed, 1990, with online updates).
6. ASM International. ASM Handbook Volume 4D: Heat Treating of Irons and Steels (2014).
7. ASTM A36 / A36M-19 — Standard Specification for Carbon Structural Steel.
8. ASTM A572 / A572M-21e1 — Standard Specification for High-Strength Low-Alloy Columbium-Vanadium Structural Steel.
9. ASTM A992 / A992M-22 — Standard Specification for Structural Steel Shapes.
10. ASTM A1066 / A1066M-19 — Standard Specification for High-Strength Low-Alloy Structural Steel Plate Produced by Thermo-Mechanical Controlled Process (TMCP).
11. ASTM F3125 / F3125M-22 — Standard Specification for High Strength Structural Bolts (consolidates A325, A490, F1852, F2280).
12. ASTM E8 / E8M-22 — Standard Test Methods for Tension Testing of Metallic Materials.
13. ASTM E18-22 — Standard Test Methods for Rockwell Hardness of Metallic Materials.
14. ASTM A255-20a — Standard Test Methods for Determining Hardenability of Steel (Jominy end-quench).
15. SAE J403_202109 — Chemical Compositions of SAE Carbon Steels (current AISI/SAE designations).
16. SAE J404_202205 — Chemical Compositions of SAE Alloy Steels.
17. AISC 360-22 — Specification for Structural Steel Buildings (American Institute of Steel Construction).
18. World Steel Association. World Steel in Figures 2024. https://worldsteel.org
19. Krauss, G. Steels: Processing, Structure, and Performance, 2nd ed. (ASM International, 2015) — definitive academic reference on steel microstructure–property relationships.
20. Totten, G. E. (ed.) Steel Heat Treatment Handbook, 2nd ed. (CRC Press, 2006).