4.1 Aircraft Ferrous and Nonferrous Metals & Heat Treatment

4.1 Aircraft Ferrous and Nonferrous Metals & Heat Treatment

Modern aircraft structures and powerplants depend upon a sophisticated spectrum of metallic alloys engineered to deliver maximum strength-to-weight ratios, fatigue endurance, fracture toughness, and environmental corrosion resistance. An Aviation Maintenance Technician (AMT) must master the metallurgical classifications, internal crystalline transformations, and precision thermal treatment processes that govern aircraft structural integrity under FAA-H-8083-30B (Aviation Maintenance Technician Handbook — General) and FAA AC 43.13-1B (Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair).

1. Ferrous Metals & SAE/AISI 4-Digit Steel Classification

Ferrous metals are alloys containing iron ($Fe$) as their primary constituent element. In aviation manufacturing, pure iron is rarely utilized due to its low tensile strength and high chemical reactivity; instead, iron is alloyed with carbon and specific metallic elements to produce aircraft-grade steels.

The SAE/AISI 4-Digit Numbering System

The Society of Automotive Engineers (SAE) and the American Iron and Steel Institute (AISI) established a standardized 4-digit numerical classification system for structural steels:

SAE/AISI 4-Digit Steel Numbering Breakdown:
  [ 4 ]                  [ 1 ]                  [ 3 0 ]
    │                      │                       │
    ▼                      ▼                       ▼
Primary Alloy Type    Major Modification      Carbon Content in Points
(4 = Molybdenum)      or Percentage (1% Cr)   (30 = 0.30% Carbon by Weight)

• First Digit: Designates the basic alloy group or primary alloying element:
  • 1xxx: Carbon steels
  • 2xxx: Nickel steels
  • 3xxx: Nickel-chromium steels
  • 4xxx: Molybdenum steels (e.g., Chrome-Molybdenum)
  • 5xxx: Chromium steels
  • 6xxx: Chrome-vanadium steels
  • 7xxx: Tungsten steels
  • 8xxx: Triple-alloy steels (Nickel-Chromium-Molybdenum)
  • 9xxx: Silicon-manganese steels
• Second Digit: Indicates the approximate percentage of the primary alloying element or a specific alloy series modification.
• Last Two Digits: Express the carbon content of the alloy in hundredths of one percent (0.01% or 'points' of carbon). For example, a steel with 20 points of carbon contains $0.20%$ carbon ($0.0020$ weight fraction).

Primary Aviation Structural Steels

2. Stainless Steels (Corrosion-Resistant Steels — CRES)

Corrosion-resistant steels (CRES) contain a minimum of $10.5%$ to $12%$ chromium, which forms an invisible, self-healing, passivated chromium-oxide ($Cr_2O_3$) surface film that prevents atmospheric oxidation and chemical corrosion. Aircraft stainless steels are divided into three primary metallurgical classes:

1. Austenitic Stainless Steels (AISI 300 Series)

• Composition: Known as 18-8 stainless steel ($18%$ chromium, $8%$ nickel). Common aerospace grades include AISI 301, 302, 304, 316, 321, and 347.
• Magnetic Properties: Completely non-magnetic in the fully annealed state, though they may exhibit slight magnetism after severe cold working.
• Heat Treatment Response: Cannot be hardened by thermal heat treatment. They can be hardened and strengthened exclusively by cold work (strain hardening).
• Intergranular Carbide Precipitation & Stabilization: When heated in the critical temperature range of $800^\circ\text{F}$ to $1500^\circ\text{F}$ ($427^\circ\text{C}$ to $816^\circ\text{C}$) during welding, chromium combines with carbon to form chromium carbides that precipitate at grain boundaries. This depletes chromium from adjacent grain regions, leaving them vulnerable to catastrophic intergranular corrosion (weld decay).
  • Stabilized Grades: AISI 321 (stabilized with Titanium) and AISI 347 (stabilized with Columbium/Niobium). Titanium and columbium possess a higher chemical affinity for carbon than chromium, forming stable titanium or columbium carbides and leaving the chromium intact in solid solution. These grades are mandatory for aircraft exhaust collectors, turbine heat blankets, and firewall shrouds.

2. Martensitic Stainless Steels (AISI 400 Series)

• Composition: Contain $12%$ to $18%$ chromium with higher carbon content ($0.15%$ to $1.20%$) and little or no nickel (e.g., AISI 410, 416, 420, 440C).
• Magnetic Properties: Strongly magnetic in all physical states.
• Heat Treatment Response: Can be hardened by standard thermal quenching and tempering, producing high hardness, yield strength, and wear resistance.
• Aviation Applications: Knife-edge valves, anti-friction ball bearings, hydraulic actuator valve spools, turbine compressor stator blades, and surgical cutting instruments.

3. Precipitation-Hardening (PH) Stainless Steels

• Grades: 17-7PH ($17%\text{ Cr}, 7%\text{ Ni}, 1.1%\text{ Al}$) and 15-5PH / 17-4PH ($15\text{-}17%\text{ Cr}, 4\text{-}5%\text{ Ni}, 3\text{-}5%\text{ Cu}$).
• Characteristics: Combine the superior corrosion resistance of austenitic grades with the ultra-high strength of martensitic grades. They are hardened via relatively low-temperature aging heat treatments ($900^\circ\text{F}$ to $1150^\circ\text{F}$), which minimizes quench warping, scaling, and dimensional distortion. Used extensively in transport aircraft wing attach fittings, high-strength engine pylon bolts, and pressure vessels.

3. Nonferrous Metals & Wrought Aluminum Alloy Classification

Nonferrous metals contain no iron in significant quantities. Aluminum ($Al$), titanium ($Ti$), magnesium ($Mg$), and copper ($Cu$) represent the primary nonferrous airframe materials. Pure aluminum is lightweight (density $\approx 0.10\text{ lb/in}^3$, roughly one-third the weight of steel at $0.283\text{ lb/in}^3$) and highly corrosion-resistant, but possesses insufficient tensile strength ($13\text{ ksi}$) for primary flight structures. Alloying aluminum transforms it into high-strength aerospace structural materials.

The 4-Digit Wrought Aluminum Alloy System (ANSI H35.1)

Wrought Aluminum 4-Digit Code Structure:
  [ 2 ]                  [ 0 ]                  [ 2 4 ]
    │                      │                       │
    ▼                      ▼                       ▼
Major Alloy Group     Alloy Modification      Specific Alloy Identifier
(2xxx = Copper)       (0 = Original Alloy)    (Original 24th experimental alloy)

• First Digit: Identifies the primary alloying element group:
  • 1xxx: Pure Aluminum ($99.00%$ minimum purity). Non-structural; used for fuel tank baffling, electrical busbars, and decorative cowlings.
  • 2xxx: Copper ($Cu$) primary alloy ($2.0% - 6.0%\text{ Cu}$). Delivers high tensile strength ($60 - 75\text{ ksi}$) and excellent fatigue resistance, but exhibits reduced corrosion resistance and poor weldability. The primary structural alloy for fuselage skins, tension wing skins, and forged bulkheads (e.g., 2024, 2014, 2017).
  • 3xxx: Manganese ($Mn$) primary alloy ($1.2%\text{ Mn}$). Moderate strength, non-heat-treatable, excellent formability (e.g., 3003 used for hydraulic oil tanks and utility fairings).
  • 4xxx: Silicon ($Si$) primary alloy ($4.5% - 12%\text{ Si}$). Lowers melting point; used almost exclusively as welding filler rod (4043) and brazing alloys.
  • 5xxx: Magnesium ($Mg$) primary alloy ($1.0% - 5.0%\text{ Mg}$). High corrosion resistance in marine/saltwater environments, moderate strength, easily welded. (e.g., 5052 for fuel/oil lines and 5056 for rivets driven into magnesium structures).
  • 6xxx: Magnesium and Silicon ($Mg_2Si$) alloys ($1.0%\text{ Mg}, 0.6%\text{ Si}$). Versatile, heat-treatable, medium strength ($45\text{ ksi}$), good corrosion resistance, easily welded (e.g., 6061 used for landing gear hydraulic lines, floor beams, and seat tracks).
  • 7xxx: Zinc ($Zn$) primary alloy ($5.0% - 8.0%\text{ Zn}$, often with $Mg$ and $Cu$). Delivers ultra-high tensile strength ($80 - 90\text{ ksi}$), high compressive strength, but is susceptible to stress-corrosion cracking if improperly aged (e.g., 7075, 7178 used for upper wing compression skins, main spar caps, and heavily loaded forged frames).
  • 8xxx: Other elements (e.g., Aluminum-Lithium alloys like 8090, which reduce density by $10%$ and increase stiffness/elastic modulus by $10%$).
• Second Digit: Indicates specific alloy modifications. A digit of 0 denotes the original alloy composition; digits 1 through 9 indicate successive metallurgical modifications (e.g., control of impurity limits such as iron and silicon in 2124 or 7475).
• Last Two Digits: In the 1xxx series, the last two digits represent the hundredths of one percent of pure aluminum above $99.00%$ (e.g., 1060 indicates $99.60%$ pure aluminum). In the 2xxx through 8xxx series, the last two digits have no numerical significance and simply serve as individual alloy identifiers.

Alclad Aluminum Sheet Protection

High-strength aluminum alloys (such as 2024 and 7075) are susceptible to surface pitting and intergranular corrosion. To protect structural sheets, manufacturers produce Alclad (or Clad) aluminum:

• Construction: A core of high-strength alloy is metallurgically roll-bonded on both sides with a protective surface layer of high-purity aluminum (typically 1100 or a specialized non-copper alloy) representing $1%$ to $5%$ of the total sheet thickness per side ($2%$ to $10%$ total cladding).
• Galvanic Sacrificial Protection: The pure aluminum cladding has a higher negative electrode potential than the copper-bearing alloy core. If a scratch penetrates through the cladding, the pure aluminum cladding acts as a sacrificial anode, galvanically protecting the core metal from corrosive attack.
• Maintenance Limitation: Severe surface scratching, harsh chemical stripping, or aggressive mechanical buffing that removes more than the thin cladding layer exposes the core alloy to direct corrosion, severely reducing airframe fatigue life.

4. Aluminum Temper & Heat Treatment Designations

Wrought aluminum alloys are classified into non-heat-treatable (hardened only by cold work) and heat-treatable (hardened by solution heat treatment and precipitation aging) alloys. The complete mechanical condition is denoted by a letter-number suffix separated by a hyphen from the 4-digit alloy designation (e.g., 2024-T3, 6061-T6, 5052-H34).

Aluminum Temper Designation Suffixes:
  -F : As Fabricated (No mechanical or thermal property limits guaranteed)
  -O : Annealed (Fully recrystallized to lowest strength, maximum ductility)
  -H : Strain Hardened (Cold worked; non-heat-treatable alloys only)
       ├─ H1x: Strain hardened only
       ├─ H2x: Strain hardened and partially annealed
       └─ H3x: Strain hardened and stabilized
          └─ (Second digit: 2 = 1/4 hard, 4 = 1/2 hard, 6 = 3/4 hard, 8 = full hard)
  -W : Solution Heat-Treated (Unstable condition, spontaneously naturally aging)
  -T : Thermally Treated to Stable Tempers (Heat-treatable alloys)

Stable Thermal Tempers (-T1 through -T10)

5. Heat Treatment Principles for Ferrous Metals

Heat treatment of steel alters its internal crystalline microstructure through controlled thermal cycles of heating, soaking, and cooling. Pure iron undergoes allotropic phase changes at specific critical temperatures:

Iron-Carbon Microstructural Phase Changes:
  Room Temperature: Ferrite (BCC α-iron) + Cementite (Fe3C Pearlite)
         │
         ▼  Heating through Lower Critical Temperature A1 (1333°F / 723°C)
  Austenite Formation Begins (FCC γ-iron solid solution)
         │
         ▼  Heating through Upper Critical Temperature A3 (1400°F - 1650°F)
  100% Homogeneous Austenite (Carbon fully dissolved)
         │
         ├──────────────────────────────┬──────────────────────────────┐
         ▼ (Slow Furnace Cool)          ▼ (Still Air Cool)             ▼ (Rapid Liquid Quench)
      Annealing                     Normalizing                     Hardening
  (Coarse Pearlite: Softest)   (Fine Pearlite: Relieved)    (Martensite: BCT, Hard/Brittle)
                                                                       │
                                                                       ▼ (Reheat 300°F - 1100°F)
                                                                    Tempering
                                                            (Toughened Tempered Martensite)

Primary Steel Heat Treatment Operations

1. Hardening (Austenitizing & Quenching): Steel is heated to approximately $50^\circ\text{F}$ to $100^\circ\text{F}$ above its upper critical temperature ($A_3$), soaked until completely transformed into homogeneous Austenite (face-centered cubic $\gamma$-iron), and rapidly quenched in liquid. The rapid cooling prevents carbon from diffusing out of solid solution, trapping carbon atoms in a distorted, highly stressed crystal lattice known as Martensite (body-centered tetragonal). Martensite is the hardest and most brittle microstructure in steel.
2. Quenching Media: The cooling severity depends upon the quench medium:
   • Brine ($10%\text{ NaCl}$ in water): Most severe quench; fastest heat extraction.
   • Water ($65^\circ\text{F} - 70^\circ\text{F}$): Rapid quench for plain carbon steels.
   • Oil (Mineral Quench Oil): Slower, uniform cooling; standard for aircraft alloy steels (4130, 4340) to prevent thermal shock, severe warping, and quench cracking.
   • Still Air / Inert Gas: Used strictly for specialized air-hardening tool steels.
3. Tempering (Drawing Back): As-quenched martensite is too brittle for structural service. Steel is immediately reheated to a predetermined temperature below the lower critical temperature ($300^\circ\text{F}$ to $1100^\circ\text{F}$ / $150^\circ\text{C}$ to $600^\circ\text{C}$), soaked, and cooled in air. Tempering relieves severe internal residual stresses, transforms brittle martensite into tempered martensite, restores ductility and impact toughness, and establishes the final required tensile strength.
4. Normalizing: The steel is heated to $100^\circ\text{F}$ above its upper critical temperature, soaked, and cooled in STILL AIR at room temperature. Normalizing refines grain size, relieves internal residual stresses from forging, casting, machining, or gas/TIG welding, and establishes a uniform, homogeneous microstructure.
5. Annealing (Full Anneal): The steel is heated to $50^\circ\text{F}$ above its upper critical temperature, soaked, and cooled at a controlled ultra-slow rate inside the CLOSED FURNACE ($10^\circ\text{F}$ to $30^\circ\text{F}$ per hour). Annealing produces maximum softness, coarse pearlite grains, lowest tensile strength, maximum ductility, and optimal machinability.

6. Case Hardening Processes

Case hardening produces an extremely hard, wear-resistant outer surface ('case') while retaining a tough, ductile, shock-absorbing inner core. It is mandatory for aircraft engine crankshaft journals, camshaft lobes, reduction gears, and wristpins.

1. Carburizing

• Low-carbon steel ($0.15% - 0.25%\text{ C}$) is heated in contact with a carbonaceous material (solid pack carburizing, liquid cyanide/barium bath, or methane/propane gas) to $1650^\circ\text{F} - 1700^\circ\text{F}$. Carbon diffuses into the outer surface, raising case carbon content to $0.80% - 1.0%$. The part is subsequently quenched and tempered to harden the high-carbon case.

2. Nitriding

• Specialized alloy steels containing aluminum ($0.85% - 1.2%\text{ Al}$, such as Nitralloy), chromium, and molybdenum are finish-machined, hardened, tempered, and ground before nitriding.
• Process: The parts are placed in an airtight retort chamber, heated to $950^\circ\text{F}$ to $1000^\circ\text{F}$ ($510^\circ\text{C} - 538^\circ\text{C}$), and exposed to a continuous stream of cracked anhydrous ammonia gas ($NH_3$) for $20$ to $100$ hours. Ammonia dissociates into nitrogen and hydrogen ($2NH_3 \rightarrow 2N + 3H_2$). Atomic nitrogen diffuses into the steel surface to form ultra-hard iron, aluminum, and chromium nitrides.
• CRITICAL EXAM DISTINCTION: NO QUENCHING IS REQUIRED AFTER NITRIDING. Because the process operates below the critical transformation temperature and requires no liquid quench, nitriding produces zero thermal distortion, warping, or dimensional change. Case hardness reaches $65 - 72\text{ HRC}$ (harder than carburized cases), with extreme wear and fatigue resistance. Nitriding is the standard surface treatment for aircraft reciprocating engine cylinder barrel walls.

7. Solution Heat Treatment & Precipitation Aging of Aluminum Alloys

Heat-treatable aluminum alloys (2xxx, 6xxx, 7xxx) are hardened via a two-stage thermal cycle:

Aluminum Solution Heat Treatment & Quench Sequence:
  [ Heat-Treatable Alloy (e.g., 2024 Sheet) ]
                     │
                     ▼  Heat in Salt Bath / Air Furnace to 910°F - 930°F (Soak Time: 10-60 min)
  [ Solid Solution: Copper/Alloys Dissolved in Al-Matrix ]
                     │
                     ▼  CRITICAL QUENCH DELAY: < 5 to 10 Seconds into Cold Water (<85°F)
  [ Super-Saturated Solid Solution (Soft & Ductile -W Temper) ]
                     │
         ┌───────────┴───────────┐
         ▼ (Room Temp Aging)     ▼ (Elevated Temp: 250°F - 375°F for 8-24 hrs)
  Natural Aging (e.g. -T4, -T3)  Artificial Aging / Precipitation Hardening (-T6)

Solution Heat Treatment Protocol

• The alloy is heated in an electrically heated molten sodium nitrate/potassium nitrate salt bath or recirculating air furnace to a precise temperature window ($820^\circ\text{F}$ to $980^\circ\text{F}$, typically $920^\circ\text{F} \pm 10^\circ\text{F}$ for 2024). This dissolves alloying constituents (such as copper aluminide, $CuAl_2$) into a single homogeneous solid solution.
• CRITICAL SAFETY PARAMETER — QUENCH DELAY: The time elapsed from opening the furnace door to total immersion in the cold water quench tank ($<85^\circ\text{F}$) must not exceed $5\text{ to }10\text{ seconds}$ (governed strictly by sheet thickness in Mil-Specs):
  • Consequence of Quench Delay: If cooling is delayed, dissolved copper aluminide precipitates out prematurely along internal crystal grain boundaries. This grain-boundary precipitation creates localized galvanic micro-cells between copper-rich precipitates and copper-depleted matrix zones, inducing catastrophic, invisible Intergranular Corrosion (IGC) under environmental moisture exposure.

Natural vs. Artificial Aging

• Natural Aging: Alloys such as 2017 and 2024 spontaneously age-harden at ambient room temperature ($70^\circ\text{F}$), reaching full strength within 4 to 5 days ($90%$ within the first 24 hours).
• Artificial Aging (Precipitation Hardening): Alloys such as 6061 and 7075 remain relatively stable after quenching and require reheating in a precipitation oven to $250^\circ\text{F} to $375^\circ\text{F}$ for $8$ to $24$ hours to precipitate fine submicroscopic particles uniformly throughout the lattice, achieving full -T6 temper hardness.

8. 2017 (D) and 2024 (DD) 'Icebox' Rivets

Solid shank aluminum alloy rivets must be driven (bucked) while soft and malleable to form a structurally sound shop head without cracking.

Icebox Rivet Life-Cycle Protocol:
  [ 2017-T4 (D) / 2024-T4 (DD) Rivets ]
                 │
                 ▼  Solution Heat Treat at 910°F - 940°F in Salt Bath
  [ Cold Water Quench ]
                 │
                 ▼  IMMEDIATELY Transfer within 10-15 min to Sub-Zero Freezer (< 32°F / -10°F)
  [ Cryogenic Storage: Natural Precipitation Aging Arrested for up to 2 Weeks ]
                 │
                 ▼  Remove from Freezer for Airframe Assembly
  [ Drive Rivets Immediately within 15-20 min before Room Temperature Age Hardening ]

Rivet Classification & Driving Requirements

• 2117-T4 (Type AD — Dimple on Head): Known as 'Field Rivets'. They possess moderate shear strength ($26\text{ ksi}$) and can be stored indefinitely at room temperature and driven as-received without any heat treatment.
• 2017-T4 (Type D — Raised Dot on Head): High shear strength ($30\text{ ksi}$). Once solution heat-treated and quenched, natural age hardening begins immediately. If left at room temperature, they become too hard to drive within 1 hour.
• 2024-T4 (Type DD — Raised Double Dash on Head): Ultra-high shear strength ($35\text{ ksi}$). Natural age hardening progresses so rapidly that they must be driven within 10 to 20 minutes of quenching if left at room temperature.

Refrigeration & Storage Rules

• Arresting Age Hardening: Storing heat-treated 2017 and 2024 rivets in sub-zero refrigeration freezers at temperatures below $32^\circ\text{F}$ ($0^\circ\text{C}$) (typically maintained at $-10^\circ\text{F}$ to $-20^\circ\text{F}$) retards natural precipitation age-hardening, keeping the rivets soft and ductile for up to 1 to 2 weeks.
• Driving Window: Once removed from the sub-zero freezer, icebox rivets must be driven within 15 to 20 minutes. Rivets not driven within this window or allowed to warm up must be re-solution heat-treated and quenched before use (re-heat treatment is generally permitted up to 3 times per specification).