Reheat Cracking in Welded Steel: Metallurgical Mechanism and Control

Reheat cracking is an intergranular fracture mode that appears not during welding itself but when a joint is subsequently reheated, most often during post-weld heat treatment (PWHT) of creep-resistant Cr-Mo-V steels such as P91 and P92. This guide develops the underlying precipitation and grain-boundary metallurgy that drives the phenomenon, the composition-based susceptibility indices used to screen alloys, and the metallurgical basis for the prevention strategies used in creep-steel fabrication.

What Is Reheat Cracking?

Reheat cracking, also termed stress relief cracking or stress relaxation cracking, is intergranular fracture that develops along prior austenite grain boundaries in the CGHAZ when a welded joint is reheated through approximately 400-700°C. Unlike hydrogen-assisted cold cracking, which appears near ambient temperature shortly after welding and follows a transgranular or mixed fracture path, reheat cracking involves no hydrogen, occurs only on reheating, whether during PWHT or in long-term elevated-temperature service, and is entirely intergranular. Its metallurgical root cause connects directly to the HAZ microstructural evolution covered in our heat-affected zone microstructure guide and to the boundary-embrittlement concepts developed in our temper embrittlement article.

The Metallurgical Mechanism

Step 1: Grain Coarsening and Carbide Dissolution

Material immediately adjacent to the fusion boundary experiences peak temperatures above roughly 1100°C during welding. At these temperatures, grain boundary mobility is high and prior austenite grains coarsen substantially, while pre-existing carbides and nitrides dissolve into a highly supersaturated solid solution of Cr, Mo, V, Nb, and their associated carbon and nitrogen. Coarse grains mean lower total grain boundary area per unit volume, so any given boundary must later accommodate a disproportionately large share of relaxation strain.

Step 2: Transformation and Locked-In Residual Stress

On cooling, the CGHAZ transforms to martensite (in 9-12Cr steels) or bainite/martensite mixtures (in lower-alloy Cr-Mo grades), locking substantial residual stress into the joint. In P91, the as-welded CGHAZ is predominantly martensitic with hardness commonly in the 400-500 HV range and negligible toughness prior to tempering, a transformation behaviour rooted in the same principles covered in our martensite formation guide.

Step 3: Precipitation Hardening on Reheating

When the joint is reheated for PWHT, the supersaturated matrix rejects its dissolved alloying content as fine precipitates. M23C6 chromium-rich carbides nucleate first, largely on prior austenite and lath boundaries, while MX-type vanadium and niobium carbonitrides precipitate intragranularly with exceptional fineness, typically only a few nanometres, providing the most potent dislocation pinning of the carbide-forming species. In some 9-12Cr grades, the intermetallic Laves phase (Fe2Mo or Fe2W) forms at longer exposure and contributes further grain-interior strengthening, though it simultaneously depletes solid-solution strengthening elements from the surrounding matrix.

Step 4: Grain Boundary Embrittlement and Fracture

While the grain interior hardens through this reprecipitation, the grain boundaries remain comparatively weak and are further embrittled by segregation of residual trace elements, principally phosphorus, tin, antimony, and arsenic, that diffuse to the boundaries during the slow heating and holding stages of PWHT. Because residual stress must be relaxed by creep deformation somewhere, and the hardened grain interior increasingly resists that deformation, strain concentrates almost entirely at the boundaries. Once local strain exceeds the boundary’s creep ductility, voids nucleate at triple points and precipitate-boundary interfaces, link up, and propagate as a purely intergranular crack.

Lundin-Henning susceptibility index (weight percent, ladle analysis):

G = Cr + 3.3*Mo + 8.1*V - 2

G < 0            Low susceptibility
0 ≤ G < 2         Moderate susceptibility
G ≥ 2            High susceptibility

Example - P91 (9Cr-1Mo-V-Nb): Cr=8.8, Mo=0.95, V=0.20
G = 8.8 + 3.3(0.95) + 8.1(0.20) - 2 = 11.6   [High]

Example - P22 (2.25Cr-1Mo): Cr=2.3, Mo=0.98, V~0.01
G = 2.3 + 3.3(0.98) + 8.1(0.01) - 2 = 3.6    [High]

Composition Effects: Why V and Nb Dominate

Not all carbide-forming elements contribute equally to reheat cracking risk. The dominant variable is the fineness and coherency of the precipitate that a given element forms during the PWHT temperature window, since finer, more coherent precipitates pin dislocations more effectively per unit volume fraction. Vanadium and niobium MX-type carbonitrides precipitate at exceptional fineness in the 550-650°C range and are responsible for the sharpest grain-interior strengthening of the common alloying additions, which is why V- and Nb-bearing 9-12Cr creep steels such as P91 and P92 sit at the high end of the susceptibility scale despite their excellent creep strength being derived from exactly the same precipitation behaviour.

Trace Element Segregation and the Link to Temper Embrittlement

Phosphorus, tin, antimony, and arsenic, present as low-concentration residuals rather than deliberate alloying additions, diffuse to prior austenite grain boundaries under the slow thermal cycling of PWHT, reducing boundary cohesive energy in a mechanism directly analogous to classical temper embrittlement. Because carbide-boundary interfaces present higher interfacial energy sites than clean boundaries, segregation is often most concentrated at these interfaces, compounding the loss of local ductility exactly where precipitation hardening has already stiffened the adjacent grain interior. This shared segregation chemistry is why fabrication specifications for P91 and P22 typically cap the aggregate P + Sn + Sb + As content on certified mill chemistry, independent of the primary alloy specification limits.

The Critical PWHT Temperature Window

For Cr-Mo-V steels, the period of greatest cracking risk is not the final PWHT soak but the heating ramp through approximately 500-700°C, where M23C6 carbides precipitate within minutes and MX carbonitrides begin forming in the 550-650°C sub-range, hardening the grain interior while residual stress is still close to its as-welded peak. Because of this, slow, poorly controlled heating through this window, which extends dwell time at the most dangerous combination of high stress and active precipitation, is generally more hazardous than a properly ramped and monitored heating schedule, even though slower heating is intuitively associated with lower thermal stress.

Assessing Susceptibility: The PSR Test

The Post-weld Simulated stress-Relief (PSR) test provides the most rigorous experimental evaluation of reheat cracking risk for a specific steel, consumable, and welding procedure combination. A notched specimen machined from a representative CGHAZ is subjected to the PWHT thermal cycle while held under a controlled tensile stress, typically expressed as a percentage of the material’s proof strength at temperature. Testing across a range of stress and temperature combinations produces a PSR curve identifying the critical stress below which no cracking occurs at each temperature, which is then used to validate or adjust the PWHT schedule, consumable selection, or joint restraint before production welding proceeds. Where PSR testing is impractical, post-PWHT hardness provides a faster, if less rigorous, qualitative screen: incomplete tempering and residual hardness above the specified limit signal elevated risk and warrant further investigation, connecting to the hardness-based acceptance criteria discussed in our hardness testing methods guide.

Metallurgical Basis of Prevention

• Heat input control: lower heat input narrows the CGHAZ and limits time above the grain-coarsening temperature, restricting maximum prior austenite grain size and the resulting boundary-area deficit.
• Preheat and interpass temperature control: maintains a controlled as-welded hardness and cooling rate through the martensite transformation, limiting locked-in residual stress magnitude.
• Consumable chemistry control: selecting weld metal with V and Nb content toward the lower end of the specification range avoids adding further precipitation-hardening potential beyond what the base metal CGHAZ already contributes.
• Temper-bead technique: sequencing subsequent passes to reheat the underlying CGHAZ into the fine-grained temperature range refines the coarse prior austenite grains, directly increasing boundary area and relaxation capacity.
• Controlled PWHT heating rate: minimizing dwell time in the 500-700°C precipitation window reduces the opportunity for grain-interior hardening to outpace stress relaxation.
• Joint design: reducing geometric stress concentration at weld toes lowers the local stress that must be relaxed by the already-compromised boundary microstructure.

Industrial Significance

Reheat cracking is a primary metallurgical hazard in the fabrication of P91 and P92 power generation and petrochemical piping and pressure vessels, where the same vanadium- and niobium-driven precipitation strengthening that gives these steels their excellent creep strength also makes their welds intrinsically susceptible to intergranular fracture during PWHT. Correct alloy screening, procedure qualification through PSR or equivalent testing, and disciplined PWHT execution are essential to realizing the creep performance these steels are selected for without introducing a fabrication-stage integrity risk, a balance closely related to the long-term degradation mechanisms covered in our creep-fatigue interaction guide and sensitization in stainless steel article.

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