16.3 Stress Relaxation Cracking (Reheat Cracking)

Stress Relaxation Cracking Overview — API RP 571 Section 3.54

1. Phenomenological Nature and Terminology

Stress Relaxation Cracking, historically and commonly designated in welding metallurgy as Reheat Cracking, is defined in API RP 571 Section 3.54 as intergranular cracking that develops in the coarse-grained heat-affected zone (CGHAZ) or weld metal of susceptible alloys during exposure to elevated temperatures. This cracking occurs either during:

1. The thermal heating and soaking cycle of Post-Weld Heat Treatment (PWHT) (typically 1100 °F to 1350 °F / 593 °C to 732 °C); or
2. The initial high-temperature operating exposure of un-PWHT or incompletely stress-relieved components in service at temperatures between 900 °F and 1200 °F (482 °C and 649 °C).

While Post-Weld Heat Treatment is legally mandated to relieve locked-in fabrication stresses and restore fracture toughness, the very process of thermal relaxation can tear susceptible alloys apart along grain boundaries if the rate of stress relaxation outpaces the grain boundary creep ductility of the metal.

                    THE PARADOX OF REHEAT CRACKING

       FABRICATION GOAL                            METALLURGICAL REALITY
   ┌──────────────────────────────┐          ┌──────────────────────────────┐
   │ Apply PWHT to relieve        │          │ Rapid heating causes fine    │
   │ residual welding stresses    │  Thermal │ intragranular carbides to    │
   │ and temper hard martensitic  │  Cycle   │ precipitate, locking grains; │
   │ HAZ microstructures.         │ ────────>│ all creep relaxation strain  │
   │                              │          │ is forced into brittle prior │
   │ [Intended: Ductile Vessel]   │          │ austenite grain boundaries!  │
   └──────────────────────────────┘          └──────────────────────────────┘

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Fundamental Metallurgical Mechanism

Stress relaxation cracking is fundamentally driven by a lethal metallurgical competition between the mechanical strength of the grain interior and the cohesive shear ductility of the grain boundary during high-temperature creep deformation:

                METALLURGICAL COMPETITION MECHANISM IN CGHAZ

       GRAIN INTERIOR MATRIX                     PRIOR AUSTENITE GRAIN BOUNDARY
   ┌──────────────────────────────┐          ┌──────────────────────────────┐
   │ • High peak welding temp     │          │ • Coarse grain size (ASTM 1-3│
   │   dissolves alloy carbides   │          │ • Tramp elements segregate:  │
   │ • Rapid cooling traps V, Mo, │  PWHT /  │   P, Sn, Sb, As, B weaken    │
   │   Nb, Ti in supersaturation  │ Service  │   atomic cohesion            │
   │ • Reheating to 900-1100 °F   │ 900-1200 │ • Denuded precipitate-free   │
   │   precipitates ultrafine,    │   °F     │   zones along boundary       │
   │   coherent intragranular     │ ────────>│ • 100% of creep strain is    │
   │   carbides (VC, Mo2C, NbC)   │          │   forced into boundaries     │
   │ • Grains become rigid & ultra│          │ • Micro-cavities nucleate,   │
   │   hard; dislocations pinned  │          │   coalesce, and link up      │
   └──────────────────────────────┘          └──────────────────────────────┘
                                                           │
                                                           ▼
                                            [INTERGRANULAR REHEAT CRACKING]

1. The Welding Thermal Cycle and Carbide Dissolution

During deposition of fusion weld passes, the narrow band of base metal immediately adjacent to the fusion line—the Coarse-Grained Heat-Affected Zone (CGHAZ)—is heated to extreme temperatures exceeding 2200 °F to 2400 °F (1204 °C to 1316 °C). At this temperature:

• Prior austenite grains undergo massive, uncontrolled grain growth, producing very coarse grains (ASTM Grain Size No. 1 to 3);
• Stable alloying carbides and carbonitrides (vanadium carbides $V_4C_3$, molybdenum carbides $Mo_2C$, niobium carbides $NbC$, and titanium carbonitrides $Ti(C,N)$) completely dissolve into solid solution in the austenite matrix;
• Upon subsequent rapid cooling, these carbide-forming elements are held in a thermodynamically unstable, supersaturated solid solution within the resulting bainitic or martensitic microstructure.

2. Intragranular Precipitation Hardening During Reheating

When the welded component is subsequently reheated into the stress relaxation window (900 °F to 1200 °F / 482 °C to 649 °C) during PWHT or service:

• Supersaturated alloying elements rapidly precipitate within the grain interiors as a dense, coherent dispersion of nanometer-scale intragranular carbides (e.g., $VC$, $Mo_2C$, or $NbC$);
• This coherent precipitation creates massive secondary hardening within the grain interior, pinning dislocations and dramatically elevating the room-temperature and elevated-temperature yield strength of the grains.

3. Creep Strain Localization and Grain Boundary Rupture

• The Relaxation Mandate: Residual welding stresses in heavy-wall weldments regularly equal the material's yield strength (40 to 80 ksi / 275 to 550 MPa). At elevated temperatures, these residual elastic stresses must relax through plastic creep deformation (converting elastic strain into permanent plastic strain).
• The Failure Point: Because the grain interiors have been intensely strengthened by intragranular carbide precipitation, they cannot deform plastically via dislocation glide. Consequently, virtually 100% of the required relaxation creep strain is forced into the narrow, unpinned prior austenite grain boundaries.
• Decohesion and Cavitation: Coarse-grained boundaries have significantly reduced total boundary surface area per unit volume, concentrating shear strain. Under this localized strain, grain boundary sliding initiates. Micro-cavities nucleate at grain boundary triple points and link up into extensive networks of macroscopic intergranular cracks.

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Critical Susceptibility Factors

                     KEY FACTORS DRIVING REHEAT CRACKING

       Material & Metallurgical                      Geometric & Mechanical
  ┌──────────────────────────────────┐       ┌──────────────────────────────────┐
  │ 1. Susceptible Metallurgy:       │       │ 4. Heavy Wall Thickness:         │
  │    Cr-Mo-V low alloys, 321H,     │       │    >1.5-2 inches (38-50 mm)      │
  │    347H, Alloy 800H              │       │    High triaxial constraint      │
  │                                  │       │                                  │
  │ 2. Tramp Impurity Elements:      │       │ 5. Geometric Stress Risers:      │
  │    P, Sn, Sb, As, B segregate    │       │    Sharp weld toes, undercut,    │
  │    and poison boundary cohesion  │       │    lack of penetration, notches  │
  │                                  │       │                                  │
  │ 3. Carbide-Forming Elements:     │       │ 6. Fast PWHT Heating Rates:      │
  │    V, Mo, Nb, Ti drive secondary │       │    Precipitation occurs before   │
  │    intragranular hardening       │       │    stresses can relax smoothly   │
  └──────────────────────────────────┘       └──────────────────────────────────┘

1. Material Metallurgy and Alloying Elements

Susceptibility is governed by the concentration of carbide-forming elements that drive secondary hardening during reheating:

• Vanadium (V): The most potent promoter of reheat cracking in ferritic steels. Steels with >0.10 wt% V form fine $V_4C_3$ precipitates that severely harden grain interiors.
• Molybdenum (Mo): Forms fine $Mo_2C$ needles, contributing strongly to secondary hardening.
• Niobium (Nb) and Titanium (Ti): In stabilized austenitic stainless steels (347H and 321H), Nb and Ti dissolve in the HAZ during welding and precipitate as fine NbC or TiC during reheating, making these grades highly prone to stress relaxation cracking.

2. Tramp Impurity Elements and Embrittlement Indices

Trace tramp elements that segregate to prior austenite grain boundaries severely reduce atomic cohesion:

• Elements include Phosphorus (P), Tin (Sn), Antimony (Sb), Arsenic (As), and Boron (B).
• In low-alloy Cr-Mo and Cr-Mo-V steels, susceptibility is evaluated using empirical composition parameters such as the Reheat Cracking Sensitivity Parameter (Delta G): $\Delta G = \%\text{Cr} + 3.3(\%\text{Mo}) + 8.1(\%\text{V}) - 2$ When $\Delta G > 0$, the alloy is considered highly susceptible to stress relaxation cracking.

3. Component Wall Thickness and Mechanical Constraint

• Heavy Section Risk: Stress relaxation cracking is predominantly a disease of heavy-wall components (>1.5 to 2.0 inches / 38 to 50 mm).
• Triaxial Constraint: In thick plates and forgings, the rigid surrounding mass of metal prevents through-thickness deformation, imposing severe plane-strain triaxial constraint. This multiplies peak residual stresses and prevents the plastic yielding that would otherwise blunten micro-cavities.

4. Stress Concentrations and Weld Joint Geometry

Reheat cracks almost always initiate at geometric stress concentrations along the weld perimeter:

• Sharp weld toes, incomplete joint penetration, root pass undercut, backing ring corners, and abrupt thickness transitions concentrate relaxation strain by factors of 3 to 5, initiating cracks that propagate into the CGHAZ.

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Susceptible Materials and Equipment

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Crack Morphology and Metallographic Features

                 REHEAT CRACK MORPHOLOGY IN WELD CGHAZ

        Weld Toe Notch
              ▼
    ══════════╗ ┌─────────────────────────────────────────────────────────┐
    Base      ║ │                      WELD METAL                         │
    Metal     ║ └─────────────────────────────────────────────────────────┘
              ║  ░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░░
              ║  Coarse-Grained HAZ (CGHAZ) - ASTM 1 to 3 Grain Size
              ╚═══╗ ◄── Intergranular, branching, jagged crack network
                  ║     propagating along prior austenite grain boundaries
                  ║     strictly within 1-2 grain diameters of fusion line!
                  ║
                  ▼

1. Macroscopic Features:
   • Location: Cracking is confined strictly to the Coarse-Grained HAZ immediately adjacent to the fusion line, or less frequently within the coarse columnar grains of the weld metal itself.
   • Initiation: Cracks initiate at external weld toes, root notches, or internal geometric discontinuities where stress concentration is highest.
   • Trajectory: Cracks run parallel to the fusion line and exhibit a distinctively jagged, stepped, multi-branched macroscopic trajectory.
2. Microscopic Metallographic Features:
   • Intergranular Separation: At high optical magnification and SEM, the crack exhibits 100% intergranular separation along prior austenite grain boundaries.
   • Grain Boundary Cavitation: Ahead of the main crack tip, micro-cavities (creep voids) decorate transverse grain boundaries. Individual grains show virtually no internal deformation or slip lines, confirming that all strain occurred in the boundary zone.
   • Oxidation Signatures: For cracks occurring during PWHT in open air furnaces, fracture surfaces are covered with a thick, uniform high-temperature thermal oxide scale.

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Prevention, Mitigation, and Fabrication Controls

                    REHEAT CRACKING MITIGATION FRAMEWORK

      Welding Metallurgy & HAZ Refinement              PWHT & Thermal Management
  ┌────────────────────────────────────┐       ┌────────────────────────────────────┐
  │ • Utilize temper-bead welding or   │       │ • Control PWHT heating rate:       │
  │   buttering to refine the CGHAZ    │       │   Slow heating (<=100-200 °F/hr)   │
  │ • Grind weld toes to smooth,       │       │   from 600 °F to soak temp         │
  │   generous transition radii        │       │ • For 347H stainless steel:        │
  │ • Specify high-purity metallurgy   │       │   Execute thermal stabilization    │
  │   (low P, Sn, Sb, As tramp levels) │       │   heat treatment (1600-1650 °F)    │
  │ • Ensure Delta G parameter < 0     │       │   prior to PWHT or hot service     │
  └────────────────────────────────────┘       └────────────────────────────────────┘

1. Weld Joint Geometry and Stress Minimization

• Contour Grinding: Eliminate sharp notches by grinding weld toes smooth with generous blend radii ($R \ge 0.25\text{ to }0.50\text{ times plate thickness}$) to minimize localized stress concentration.
• Full Penetration Joints: Ban partial penetration welds, permanent backing rings, and fillet attachment welds in heavy-wall components intended for elevated-temperature service.

2. Grain Refinement via Temper-Bead Welding

• Temper-Bead Technique: Deposit multi-pass weld layers such that the heat input of subsequent weld passes re-heats and normalizes/recrystallizes the coarse-grained HAZ of earlier passes into a fine-grained, ductile microstructure. A refined, fine-grained HAZ possesses significantly higher grain boundary area per unit volume, which disperses creep strain and eliminates reheat cracking susceptibility.

3. Thermal Stabilization for Type 321H and 347H Stainless Steels

• Heavy-wall 347H and 321H fabrications are exceptionally vulnerable to stress relaxation cracking during initial service in the 950 °F to 1200 °F (510 °C to 649 °C) range.
• Thermal Stabilization Heat Treatment: Prior to placing components in service or performing PWHT, subject the entire weldment to a thermal stabilization treatment at 1600 °F to 1650 °F (871 °C to 899 °C) for 2 to 4 hours:
  • At this elevated temperature, coarse niobium carbides ($NbC$) and titanium carbides ($TiC$) precipitate harmlessly out of solid solution without hardening the grain matrix;
  • Thermal residual stresses relax while the material retains high creep ductility, preventing subsequent strain localization and cracking during service.

4. Controlled PWHT Thermal Cycles

• Control heating rates above 800 °F (427 °C): ASME Section VIII (UCS-56) limits the heating rate above 800 °F to 400 °F/hr divided by the maximum metal thickness in inches, and never more than 400 °F/hr. Slower rates and intermediate holds (for example, at 650 °F to 750 °F) are often specified for reheat-crack-sensitive alloys.

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Inspection and Non-Destructive Examination (NDE)

Detecting reheat cracking requires advanced techniques capable of identifying tight, jagged intergranular planar defects located beneath weld toes or embedded along the CGHAZ: