15.3 Temper Embrittlement & Strain Aging

Temper Embrittlement Overview — API RP 571 Section 3.63

1. Phenomenological Definition and Affected Metallurgy

Temper Embrittlement is a metallurgical degradation mechanism characterized by the progressive reduction in ductility and notch impact toughness in certain low-alloy steels. It occurs as a result of prolonged exposure to operating temperatures within the range of 650 °F to 1070 °F (343 °C to 577 °C), or during slow, unconstrained cooling through this critical thermal window.

Temper embrittlement predominantly affects heavy-wall pressure vessels and piping fabricated from:

• 2.25Cr-1Mo (ASTM A387 Grade 22, ASTM A336 Grade F22)
• 3Cr-1Mo (ASTM A387 Grade 21, ASTM A336 Grade F21)
• Cr-Mo-V Steels (such as modern vanadium-modified 2.25Cr-1Mo-0.25V steels)
• Quenched-and-Tempered Low-Alloy Steels containing nickel and chromium (e.g., ASTM A542, ASTM A508, ASTM A533)

                      MECHANISM OF TEMPER EMBRITTLEMENT

            PRIOR AUSTENITE GRAIN BOUNDARY - ATOMIC SEGREGATION

       Bulk Ferrite / Bainite Grain                 Bulk Ferrite / Bainite Grain
     ┌───────────────────────────────┐           ┌───────────────────────────────┐
     │                               │           │                               │
     │    Fe, Cr, Mo Crystal Lattice │           │    Fe, Cr, Mo Crystal Lattice │
     │                               │           │                               │
     └───────────────────────────────┴───────────┴───────────────────────────────┘
                                     ▲           ▲
                                     │  P, Sn,   │ Tramp Impurity Elements
                                     │  Sb, As   │ Segregate in Solid Solution
                                     ▼           ▼
     ═════════════════════════════════════════════════════════════════════════════
                     PRIOR AUSTENITE GRAIN BOUNDARY REGION
     ═════════════════════════════════════════════════════════════════════════════
     • Monolayer equilibrium segregation of P, Sn, Sb, and As tramp elements.
     • Interfacial cohesive bond energy across grain boundary is severely degraded.
     • Microstructure and hardness remain identical; fracture mode shifts to intergranular.

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The Atomic Mechanism: Tramp Element Grain Boundary Segregation

1. Equilibrium Solid-Solution Segregation

Unlike other degradation mechanisms that involve the precipitation of visible second-phase particles (e.g., sigma phase or carbides), temper embrittlement is driven by the equilibrium segregation of non-metallic tramp impurity elements to prior austenite grain boundaries:

• The Four Key Tramp Impurities: Phosphorus ($\text{P}$), Tin ($\text{Sn}$), Antimony ($\text{Sb}$), and Arsenic ($\text{As}$).
• Grain Boundary Decohesion: These elements possess large atomic size mismatches with the host iron lattice. Within the 650 °F to 1070 °F temperature range, thermal diffusion allows these elements to migrate and concentrate within the disordered atomic structure of prior austenite grain boundaries.
• As these tramp elements accumulate into sub-nanometer-thick monolayers, they alter local electronic charge density and severely reduce the cohesive tensile bond strength across the grain boundary boundary plane.

2. Synergistic Co-Segregation with Alloying Elements

Alloying elements added for hardenability and deoxidation—specifically Silicon ($\text{Si}$) and Manganese ($\text{Mn}$)—exhibit strong chemical affinity for phosphorus and tin. In the presence of silicon and manganese, the thermodynamic driving force for phosphorus and tin to segregate to grain boundaries increases dramatically. Therefore, controlling bulk manganese and silicon is as essential as controlling the tramp elements themselves.

3. Thermal Reversibility (De-Embrittlement Heat Treatment)

A hallmark characteristic of temper embrittlement tested on the API 571 exam is its metallurgical reversibility:

• De-Embrittlement Soak: If an embrittled steel component is heated above the embrittlement range to 1100 °F to 1150 °F (593 °C to 621 °C) for 1 to 2 hours, the segregated tramp elements diffuse back away from the grain boundaries and redistribute uniformly into the bulk crystal grains.
• Rapid Quenching Requirement: If the component is then rapidly cooled (water quenched) through the 1070 °F to 650 °F window, ambient impact toughness is completely restored to its original, non-embrittled baseline.
• Operational Limitation: However, de-embrittlement is not a permanent cure for in-service equipment. Once the vessel is returned to high-temperature operating service within the 650 °F to 1070 °F range, tramp elements re-segregate to grain boundaries over time, re-establishing the embrittlement.

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Compositional Screening Formulas: J-Factor and X-Factor

To prevent temper embrittlement in modern heavy-wall pressure vessels, refining engineering specifications enforce strict chemical composition formulas during steel manufacturing and welding:

                    COMPOSITIONAL SCREENING METRICS FOR TEMPER EMBRITTLEMENT

           WATANABE J-FACTOR (Base Metal)               BRUSCATO X-FACTOR (Weld Metal)
     ┌────────────────────────────────────────┐   ┌────────────────────────────────────────┐
     │  Formula:                              │   │  Formula:                              │
     │  J = (%Si + %Mn) · (%P + %Sn) · 10^4   │   │  X = (10P + 5Sb + 4Sn + As) / 100      │
     │                                        │   │                                        │
     │  Inputs: Elemental weight percentages  │   │  Inputs: Elemental concentrations in   │
     │  Target: J <= 100 (or <= 80 premium)   │   │          parts per million (ppmw)      │
     │  Legacy: J > 200 - 300 (severe risk)   │   │  Target: X <= 15 ppm (or <= 12 ppm)    │
     └────────────────────────────────────────┘   └────────────────────────────────────────┘

1. The Watanabe J-Factor (Base Metal Specification)

Developed by J. Watanabe, the J-factor quantifies base metal susceptibility as a function of silicon, manganese, phosphorus, and tin contents (measured in weight percent, wt%): $J = (\%\text{Si} + \%\text{Mn}) \times (\%\text{P} + \%\text{Sn}) \times 10^4$

• Legacy Steels (Pre-1980s): Commonly had J-factors between 200 and 350, making vintage hydroprocessing reactors exceptionally vulnerable to severe in-service embrittlement.
• Modern Engineering Requirements: Standard specifications enforce $J \le 100$ (and frequently $J \le 80$ for ultra-clean heavy-wall reactor plates and forgings).

2. The Bruscato X-Factor (Weld Metal Specification)

Developed by R. M. Bruscato, the X-factor is the universal screening parameter for welding consumables (filler wire, fluxes, and shielded metal arc electrodes): $X = \frac{10\text{P} + 5\text{Sb} + 4\text{Sn} + \text{As}}{100}$ where phosphorus ($\text{P}$), antimony ($\text{Sb}$), tin ($\text{Sn}$), and arsenic ($\text{As}$) are expressed in parts per million (ppm) (where $1\text{ ppm} = 0.0001\text{ wt\%}$).

• Specification Threshold: Modern welding filler specifications mandate $X \le 15\text{ ppm}$ (or $X \le 12\text{ ppm}$ for critical reactor seams).

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Mechanical Property Changes and Catastrophic Operational Risks

           EFFECT OF TEMPER EMBRITTLEMENT ON CHARPY IMPACT TRANSITION (DBTT)

       Impact Energy (ft-lbs)
         ▲
         │         Un-Embrittled Baseline (New Steel)       Embrittled Steel (In-Service)
     100 │                 ┌───────────────                  ┌───────────────
         │                /                                 /
      80 │               /                                 /
         │              /                                 /
      60 │             /                                 /    DBTT Shift:
         │            /  40 ft-lb                       /   +150 °F to +300 °F
      40 │───────────┼─────────────────────────────────┼─────────────────────────
         │          /                                 /     (54 J threshold)
      20 │         /                                 /
         │  ──────┘                                 / ◄── Catastrophic brittle fracture
       0 └─────────────────────────────────────────┴───────────────► Temperature (°F)
                   -20 °F                         +200 °F to +250 °F

1. Shift in Ductile-to-Brittle Transition Temperature (DBTT)

• The single defining mechanical consequence of temper embrittlement is a massive upward shift in the Ductile-to-Brittle Transition Temperature (DBTT).
• In non-embrittled 2.25Cr-1Mo steel, the 40 ft-lb (54 J) Charpy V-notch (CVN) transition temperature typically sits comfortably below $-20\text{ °F }(-29\text{ °C})$.
• Following extended operating exposure, the transition temperature shifts upward by 150 °F to 300 °F (83 °C to 167 °C), placing the 40 ft-lb threshold between $+150\text{ °F and }+250\text{ °F }(+66\text{ °C and }+121\text{ °C})$.
• Under ambient conditions (e.g., 60 °F to 70 °F / 15 °C to 21 °C), the steel operates deep within its brittle lower-shelf regime.

2. Critical Diagnostic Trait: Unaltered Hardness and Strength

API RP 571 heavily emphasizes that temper embrittlement does not affect room-temperature tensile strength, yield strength, or hardness:

• Base metal hardness and tensile strength remain completely unchanged.
• Creep rupture properties and high-temperature ductility remain unaffected.
• Standard field non-destructive testing (NDE)—including hardness testing, ultrasonic thickness gauging, and field replica metallography—CANNOT detect temper embrittlement. The atomic monolayer segregation does not alter optical microstructure.

3. The Catastrophic Turnaround Scenario: Hydrotest and Pressurization

Because the steel displays nominal tensile strength and hardness, plant operators can be lulled into a false sense of security. The true operational hazard occurs during turnaround shutdowns and startups:

• Ambient Turnaround Testing: If a heavy-wall hydroprocessing reactor (operating at 1500 to 3000 psig design pressure) is subjected to an ambient-temperature hydrostatic pressure test (e.g., at 60 °F to 70 °F / 15 °C to 21 °C), or if high pressure is applied during startup before warming the vessel, the shell steel is entirely brittle.
• Catastrophic Brittle Rupture: A pre-existing crack-like flaw, weld toe undercut, or localized stress concentration can instantly initiate an unheralded, fast-running brittle fracture across the multi-inch-thick shell wall, destroying the vessel.

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Operational Mitigation: Minimum Pressurization Temperature (MPT)

To prevent catastrophic brittle fracture of embrittled heavy-wall equipment, refining operating protocols enforce strict Minimum Pressurization Temperature (MPT) procedures (also known as Minimum Metal Temperature, MMT):

                      MINIMUM PRESSURIZATION TEMPERATURE (MPT) ENVELOPE

       Internal Pressure (% of Design)
        100% ▲                                                ┌───────────────────────
             │                                                │ Full Operating
         75% │                                                │ Pressure Allowed
             │                                                │
         50% │                                                │
             │                                                │
         25% │──────────────────────────────┐                 │
             │ Max 20-25% Pressure Permitted│                 │
          0% └──────────────────────────────┴─────────────────┴───────────────────────►
             0 °F (Ambient)                250 °F            300 °F to 350 °F (MPT)
                                           Reactor Metal Temperature

1. The MPT Operational Rule:
   • Pressurization above 20% to 25% of design pressure is strictly prohibited until the reactor vessel wall temperature exceeds the established MPT.
   • For vintage 2.25Cr-1Mo reactors suffering temper embrittlement, the MPT typically ranges from 300 °F to 350 °F (149 °C to 177 °C).
2. Startup Heating Sequence:
   • During unit startup, circulating recycle gas or warm flush oil is used to heat the heavy-wall reactor above the MPT threshold under low pressure (typically $<250\text{ to }300\text{ psig}$). Only after thermocouples verify through-thickness metal temperatures exceed 300 °F to 350 °F is full system operating pressure applied.
3. Shutdown Depressurization Sequence:
   • During normal or emergency unit shutdowns, the unit must be depressurized before the metal wall temperature drops below the MPT.
4. Surveillance Coupon Testing:
   • Because NDE cannot detect grain boundary tramp segregation, surveillance Charpy V-notch coupon baskets are installed inside reactor beds during construction. Surveillance coupons are removed during major turnarounds and tested in accredited laboratories to track the real-time upward migration of the DBTT curve.

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Strain Aging — API RP 571 Section 3.60

1. Phenomenological Definition

Strain Aging is a metallurgical embrittlement mechanism affecting carbon steels and certain low-alloy steels that have undergone cold plastic deformation (cold working). When cold-worked steel is subsequently exposed to temperatures between 200 °F and 600 °F (93 °C and 316 °C)—or exposed to ambient room temperature over an extended period of time—the steel experiences an increase in hardness and tensile strength accompanied by a severe loss of ductility and notch impact toughness.

                       MECHANISM OF STRAIN AGING (COTTRELL ATMOSPHERES)

       1. Cold Plastic Deformation              2. Thermal Exposure (200 °F to 600 °F)
     ┌───────────────────────────────┐        ┌───────────────────────────────┐
     │  High density of mobile       │        │  Interstitial Nitrogen (N)    │
     │  dislocations generated       │        │  and Carbon (C) atoms diffuse │
     │  by cold bending/rolling.     │ ─────> │  rapidly to dislocation cores │
     │  (Plastic strain > 5%)        │        │  forming "Cottrell atmospheres"│
     └───────────────────────────────┘        └───────────────────────────────┘
                                                              │
                                                              ▼
       4. Embrittlement Consequences            3. Dislocation Pinning
     ┌───────────────────────────────┐        ┌───────────────────────────────┐
     │  • Sharp yield drop & Lüders  │        │  Dislocations are locked in   │
     │    band formation             │ <───── │  potential energy wells;      │
     │  • Substantial loss of CVN    │        │  massive shear stress required│
     │  • Catastrophic brittle crack │        │  to unpin dislocations        │
     └───────────────────────────────┘        └───────────────────────────────┘

2. Dislocation Pinning by Interstitial Solute Atoms

• Generation of Dislocations: Cold plastic deformation (such as cold pipe bending, rolling plate into vessel cylinders, or cold flanging of vessel heads) generates a massive density of tangled, mobile line defects (dislocations) within the ferrite crystal lattice.
• Diffusion of Interstitial Solutes: Carbon and nitrogen atoms reside in interstitial spaces between iron atoms. Because nitrogen has higher solubility and faster diffusion kinetics than carbon in alpha-ferrite at moderate temperatures, free uncombined nitrogen is the primary driver of strain aging.
• Cottrell Atmospheres: At temperatures between 200 °F and 600 °F (93 °C and 316 °C), interstitial nitrogen and carbon atoms diffuse toward the strain fields surrounding dislocation cores, forming concentrated clusters known as Cottrell atmospheres.
• Mechanical Consequences: These solute atmospheres lock the dislocations in place. When external stress is applied, dislocations cannot glide smoothly. A high threshold stress is required to break dislocations free from their solute traps, creating an abrupt upper yield point, yield point elongation (Lüders bands), increased hardness, increased ultimate tensile strength, and a dramatic decrease in fracture toughness.

3. Critical Factors and Material Susceptibility

• Degree of Cold Work: Susceptibility escalates when localized plastic strain exceeds 5% (e.g., tight-radius cold-formed pipe elbows, dished head knuckle regions, cold-punched bolt holes).
• Steel Deoxidation Practice:
  • Rimmed and Semi-Killed Steels: Historical steels deoxidized without sufficient aluminum contain substantial concentrations of free interstitial nitrogen, rendering them exceptionally prone to severe strain aging.
  • Fully Aluminum-Killed Steels: Steels manufactured with fine-grained aluminum-killing practice tie up dissolved nitrogen as stable aluminum nitride ($\text{AlN}$) precipitates, substantially suppressing strain aging.
• Thermal Acceleration: While strain aging can occur over years at ambient room temperature, exposure to temperatures between 300 °F and 450 °F (149 °C and 232 °C) accelerates aging kinetics to complete embrittlement within hours.

4. Prevention and Mitigation of Strain Aging

• Post-Forming Thermal Stress Relief: Carbon steel components subjected to cold strain exceeding 5% should receive post-weld heat treatment (PWHT) or thermal stress relief at 1100 °F to 1250 °F (593 °C to 677 °C). Thermal stress relief relieves residual fabrication stresses and allows dislocations to rearrange and annihilate before interstitial locking can cause embrittlement.
• Material Specification: Require fully aluminum-killed, fine-grained carbon steels (e.g., ASTM A106 Grade B, ASTM A516 Grade 70) for components undergoing cold forming.
• Hot Forming: Performing pipe bending or head forming at elevated temperatures (above 1200 °F / 649 °C or within the normalizing range) eliminates cold deformation strain fields entirely.

RP 571 notes that strain aging is mainly a concern for older (roughly pre-1980s) carbon steels with a large grain size and for C-0.5Mo steels; modern fully killed, fine-grain steels are much less susceptible. When deformation happens at the intermediate temperature itself, the effect is called dynamic strain aging (blue brittleness is one form).