Reheat Cracking (SR Cracking) in Cr-Mo Steels — Causes, Mechanisms, and Prevention

Reheat cracking — also called stress relief cracking (SR cracking) or stress relaxation cracking (SRC) — is one of the most insidious weld discontinuities encountered in the fabrication and repair of Cr-Mo pressure-vessel and piping steels. Unlike hydrogen-induced cold cracking, which announces itself near ambient temperature, reheat cracking waits until the joint is reheated during post-weld heat treatment (PWHT) or early service before fracturing, often without visible external warning. The consequences for P91 and P22 fabrications in power generation and petrochemical service are severe: intergranular cracks in the coarse-grained heat-affected zone (CGHAZ) that are difficult to detect, expensive to repair, and potentially catastrophic if missed.

This guide covers every facet of reheat cracking that a fabricator, welding engineer, or inspector working with creep-resistant Cr-Mo steels needs to understand: the thermodynamic and kinetic mechanisms driving the phenomenon, how to evaluate material susceptibility, the critical role of heat treatment parameters, NDE strategy, and the preventive measures that have become industry best practice under ASME, EN, and API standards.

What Is Reheat Cracking?

Reheat cracking is a form of elevated-temperature intergranular fracture that occurs in the heat-affected zone of welds — specifically in the coarse-grained region immediately adjacent to the fusion line — when the joint is subjected to a thermal cycle in the range of approximately 400–700°C. This temperature range corresponds to conditions encountered during:

• Post-weld heat treatment (PWHT) or stress relief heat treatment (SRHT)
• Subsequent weld passes that re-heat a previously deposited run
• Start-up and shutdown thermal transients in high-temperature service
• In-service creep conditions over extended operating periods

The fracture is always intergranular — propagating along prior austenite grain boundaries in the CGHAZ — and involves no hydrogen. The driving force is the inability of the material to accommodate residual welding stresses by distributed plastic deformation. Instead, the grain interior is rapidly strengthened by fine carbide and nitride precipitation during reheating, while the grain boundaries remain relatively weak. All creep deformation is forced to concentrate at the boundaries, which eventually rupture.

Terminology

Several terms are used interchangeably in the literature and standards. Understanding their nuances avoids confusion:

Metallurgical Mechanism

To prevent reheat cracking, the engineer must understand why it happens at the microstructural level. The mechanism unfolds in a precise sequence during the welding thermal cycle and subsequent reheating.

Step 1 — Grain Coarsening in the CGHAZ During Welding

The region of the base material immediately adjacent to the fusion boundary is heated above the prior austenite grain coarsening temperature — typically above 1100°C for most Cr-Mo steels. At these temperatures, grain boundary migration is rapid and the austenite grains grow substantially, sometimes reaching ASTM grain size 1 to 3 (mean grain diameter 250–500 microns). Large grains mean fewer grain boundaries per unit volume, so any given boundary must carry a proportionally higher stress when deformation is forced to occur there.

Step 2 — Dissolution of Carbides and Nitrides

During the same high-temperature excursion, pre-existing carbides and carbonitrides (M₂₃C₆, MC, MN) dissolve into solution. This leaves the austenite matrix highly supersaturated with Cr, Mo, V, Nb, Ti and their associated carbon and nitrogen. At peak weld temperature, this supersaturated solid solution has high ductility.

Step 3 — Rapid Cooling and Martensite / Bainite Formation

On cooling from the weld, the CGHAZ transforms to martensite or upper bainite (depending on the steel grade and cooling rate). This transformation creates high residual stresses locked into the microstructure. For P91, the as-welded CGHAZ is predominantly martensitic with very high hardness (400–550 HV) and virtually zero toughness in the absence of subsequent heat treatment.

Step 4 — Precipitation Hardening During Reheating

This is the critical step. When the joint is reheated for PWHT, the supersaturated matrix releases its alloying elements as fine, coherent precipitates — mainly M₂₃C₆ chromium carbides, vanadium carbonitrides (MX), and in P91 at higher temperatures, the Laves phase (Fe₂Mo). These precipitates are highly effective dislocation barriers within the grain interior. The grain interior becomes progressively stronger during heating through the 500–700°C range.

Step 5 — Grain Boundary Embrittlement and Fracture

While the grain interior hardens, the grain boundaries remain relatively weak and are further embrittled by segregation of trace elements (Sn, As, Sb, P) that diffuse to boundaries during the thermal cycle. Residual stresses, which the material must relax during PWHT, cannot be accommodated by distributed creep within the grains. All creep deformation is forced to concentrate at the few available grain boundaries. Voids nucleate at grain boundary triple points and at precipitate-boundary intersections, link up, and propagate as a purely intergranular crack.

Steel Grade Susceptibility and the Role of Alloying Elements

Not all steels are equally susceptible. The degree of grain-interior precipitation strengthening during reheating — which is the key variable — depends directly on the chemistry. The elements that most strongly promote susceptibility are those that form fine, coherent precipitates stable in the 500–700°C range.

The Lundin-Henning Susceptibility Index

The most widely cited empirical index for reheat cracking susceptibility in low-alloy steels is the G index, sometimes called the Lundin-Henning parameter:

Susceptibility of Common Cr-Mo Grades

For P91 and P92, the high vanadium and niobium content is the dominant factor. Vanadium forms V(C,N) carbonitrides that precipitate with exceptional fineness (2–5 nm) in the 550–650°C range, producing the most potent dislocation hardening of all the carbide-forming elements. Understanding the role of P91 material requirements and welding controls is essential before undertaking any Grade 91 fabrication.

Effect of Trace Elements

Residual trace elements — sometimes called “tramp” elements — have a disproportionate effect on reheat cracking susceptibility relative to their concentration. The key embrittling elements and their mechanisms are:

• Phosphorus (P): Segregates to prior austenite grain boundaries during slow cooling or isothermal holding; reduces grain boundary cohesive energy
• Tin (Sn), Antimony (Sb), Arsenic (As): Temper embrittlement agents; co-segregate with P at boundaries; even 50 ppm can shift the NDT temperature significantly
• Sulphur (S): Forms MnS inclusions that act as void nucleation sites at boundaries under stress
• Boron (B): In controlled amounts (50–100 ppm) can actually retard boundary segregation of P; excess B promotes grain boundary precipitation of Fe₂₃(C,B)₆ which reduces ductility

HAZ Zones and Where Cracks Form

The heat-affected zone is not homogeneous. Temperature gradients during welding create a series of distinct microstructural sub-zones, and reheat cracking occurs in a specific one. Recognising this helps direct NDE and understand repair scope.

Why Not the Fine-Grained HAZ?

The fine-grained HAZ (FGHAZ, peak temperature 900–1100°C) and the intercritical HAZ (ICHAZ, 720–900°C) are generally not susceptible to reheat cracking because their smaller grain size provides a much higher grain boundary surface area per unit volume. This means any given boundary carries a lower stress concentration and has better high-temperature ductility to accommodate relaxation. The CGHAZ is uniquely vulnerable due to the combination of large grain size and the highest concentration of carbide-forming elements in solid solution after the peak weld thermal cycle.

PWHT Parameters and the Cracking Temperature Window

Post-weld heat treatment is both the trigger and — if correctly specified — the primary mitigation for reheat cracking. The key is understanding that the cracking risk varies with temperature and time during the PWHT heating cycle, not just at the final hold temperature.

The Critical Temperature Window

For most Cr-Mo steels, reheat cracking is most likely to occur as the joint heats through the range 500–700°C. Within this window:

• M₂₃C₆ carbides precipitate rapidly from the supersaturated martensite, hardening the grain interior within minutes
• Vanadium carbonitrides begin precipitating in the 550–650°C sub-range, adding further dislocation strengthening
• Residual stresses are high (material has not yet stress-relieved)
• Creep ductility at these temperatures is low in the CGHAZ

PWHT Requirements by Steel Grade (ASME B31.3 and ASME BPVC VIII-1)

Heating Rate Considerations

ASME B31.1 specifies that the heating rate above 425°C for P5, P5A, and P5B materials shall not exceed 340°C per hour divided by the maximum wall thickness in inches (max 340°C/h). However, for very susceptible materials in heavy sections, many engineering specifications impose a lower limit — commonly 50–100°C/h above 400°C — to allow more uniform temperature distribution and more gradual stress redistribution. Always check the purchaser’s supplementary requirements and the WPS/PWHT procedure for specific limits.

The PSR Test and Material Susceptibility Assessment

For critical applications — particularly thick-section P91 and P22 pressure vessels and piping — pre-qualification testing to assess reheat cracking susceptibility is required by some standards and recommended by others. The most rigorous test available is the Post-Weld Simulated Stress-Relief (PSR) test.

PSR Test Procedure

The PSR test (described in BS 7363, later adopted by various engineering specifications) involves the following steps:

1. Weld a test joint using production-representative parameters (same WPS, preheat, heat input)
2. Machine specimens from the CGHAZ with a defined stress concentrator notch
3. Apply the thermal cycle representing PWHT (specified heating rate, target temperature, hold time)
4. Stress the specimen during heating at levels ranging from 50% to 100% of the yield strength at temperature
5. Examine fractured surfaces: a PSR factor is calculated from the ratio of notch ductility with and without heating
6. Plot a PSR temperature-stress curve; the critical stress below which no cracking occurs at each temperature defines the safe operating envelope

A material and procedure combination passes the PSR test if cracking does not occur at a stress level below the material’s 0.2% proof strength at the PWHT temperature. Failing the PSR test with a particular procedure requires modification of the PWHT parameters, the consumable selection, or the weld geometry before fabrication proceeds. The PSR test is mandatory in the UK power generation sector (per BWIF guidance) for all P91 pressure part welds above a certain section thickness.

Simpler Screening — Hardness Testing

While the PSR test provides the most rigorous quantitative assessment, hardness testing of the CGHAZ provides a rapid qualitative screen. The CGHAZ hardness in the as-welded condition correlates with susceptibility:

• P91 as-welded CGHAZ: typically 400–500 HV; acceptance criteria post-PWHT is ≤275 HBW per ASME B31.1
• P22 as-welded CGHAZ: typically 250–350 HV; acceptance post-PWHT is ≤225 HBW
• Failure to achieve the post-PWHT hardness limit indicates incomplete tempering and elevated residual stress — a red flag for reheat cracking

For more on mechanical test requirements and hardness acceptance criteria, see the guide to mechanical testing methods in welding qualification on WeldFabWorld.

Prevention Strategies

Prevention operates at multiple levels: material selection, joint design, welding procedure, and heat treatment. An effective programme addresses all four.

1. Material and Consumable Selection

Where grade selection is flexible, choosing steels with lower G index values reduces inherent risk. When P91 or P22 is mandated by design, material procurement specifications should include:

• Restriction on P + Sn + Sb + As (some specifications limit P + Sn + Sb + As ≤ 0.035 wt%)
• Chemistry certification per heat, not just per lot
• CMTR review against all specified chemistry limits before use

For consumables, use only those classified to the correct AWS or EN classification for the base metal grade. For welding consumable nomenclature and selection guidance, refer to the dedicated WeldFabWorld article. Verify that the deposited weld metal chemistry (from the diffusible hydrogen test or from PQR records) has V+Nb within specification.

2. Preheat and Interpass Temperature

Adequate preheat serves double duty for Cr-Mo steels: it prevents hydrogen-induced cold cracking and it controls the as-welded CGHAZ hardness. For P91, minimum preheat is 200°C (some specifications require 230°C for section thicknesses above 50 mm). The interpass temperature is limited to a maximum of 300°C to prevent excessive grain growth from extended elevated-temperature exposure. The joint must not be allowed to cool below preheat temperature between passes. Immediate PWHT on completion (without allowing the joint to cool to ambient) is sometimes specified for very heavy sections.

3. Heat Input Control

High heat input widens the CGHAZ and extends the time above the grain coarsening temperature, producing the largest prior austenite grains and hence the highest susceptibility. Targeted heat input limits for P91 are typically 15–45 kJ/cm (welding procedure-specific). Low heat input also reduces the width of the coarse-grained zone and the magnitude of residual stresses. The GTAW (TIG) welding process is preferred for the root passes of P91 piping for precisely this reason — its characteristically low heat input minimises the CGHAZ width.

4. Temper-Bead Technique

The temper-bead technique involves deliberately depositing subsequent weld passes so that each new bead reheats the underlying CGHAZ of the previous bead to the FGHAZ temperature range (900–1100°C), refining the coarse grains. A properly executed temper-bead sequence can substantially reduce the area of untreated CGHAZ at the cap pass. This technique is most relevant in repair welding where PWHT may not be practical (in-service repairs under ASME Code Case 2452 for P91, or API 582 for petrochemical service).

5. PWHT Optimisation

As discussed in Section 4, the PWHT procedure must achieve:

• Controlled, uniform heating through the critical 500–700°C range
• Final soak temperature within the specified window (e.g., 730–775°C for P91)
• Minimum hold time sufficient for full stress relaxation (verified by post-PWHT hardness)
• Controlled cooling rate after PWHT to avoid thermal shock and re-introduction of stresses
• Temperature measurement with calibrated, directly attached thermocouples

6. Joint Design Optimisation

Geometric stress concentration at the weld toe is a major driver of reheat cracking. Measures to reduce stress concentration include:

• Full penetration butt welds in preference to partial penetration or fillet welds at high-stress locations
• Smooth weld toe profile; avoid undercut or sharp weld-toe angles (<150° included angle)
• TIG dressing of the weld toe to improve geometry
• Avoidance of stiff attachments (lugs, clips) adjacent to susceptible welds in high-restraint configurations
• Access for post-PWHT NDE coverage at all identified susceptible locations

NDE Strategy for Reheat Cracking Detection

NDE is the last line of defence against reheat cracking reaching service. The fundamental constraint is timing: reheat cracks form during or after PWHT, so pre-PWHT NDE cannot detect them. A two-stage NDE approach is standard for high-risk joints.

Pre-PWHT NDE

Pre-PWHT inspection serves to document the as-welded condition and identify any hydrogen cracks or other welding discontinuities before PWHT can mask or alter their characteristics. Typical pre-PWHT NDE includes:

• 100% visual examination (VT) of all welds
• Magnetic particle testing (MT) of all accessible weld surfaces and adjacent HAZ
• Ultrasonic testing (UT) or PAUT for volumetric inspection in critical joints

Post-PWHT NDE

Post-PWHT NDE is mandatory for detecting reheat cracks. The CGHAZ, where cracks form, lies beneath the weld metal and base metal surface — inaccessible to surface methods alone. The recommended post-PWHT NDE methods are:

• PAUT (Phased Array UT): The preferred volumetric method. Multiple focal laws allow simultaneous coverage of the CGHAZ from the weld cap, with electronic scanning eliminating the need to manually raster a single probe. Detection capability for tight intergranular cracks ≥2 mm in height has been demonstrated in qualification studies.
• TOFD (Time-of-Flight Diffraction): Highly sensitive to planar discontinuities; the diffraction signal from the crack tip is independent of crack orientation, making it effective for the near-vertical intergranular cracks typical of reheat cracking. See the guide to PAUT and TOFD in pressure vessel inspection for methodology details.
• MT (Magnetic Particle Testing): For surface-breaking or near-surface cracks in ferritic Cr-Mo steels; less effective for sub-surface or tight cracks
• ET (Eddy Current Testing): Can supplement MT for surface-breaking cracks; limited penetration depth

Understanding the full capability and limitations of advanced NDE methods is essential for a P91 fabrication QA programme. The guide to mechanical testing and NDE in weld qualification provides a broader framework for test selection.

In-Service Stress Relaxation Cracking

The same mechanism that drives reheat cracking during PWHT can operate slowly over years of high-temperature service. In-service stress relaxation cracking (SRC) is documented in P91 and P22 power plant components and represents an important long-term integrity concern.

Where In-Service SRC Occurs

In-service SRC concentrates at geometric stress raisers where residual and operational stresses combine, and where the material temperature remains in the susceptible range (400–600°C for most Cr-Mo steels):

• Weld toes at attachment welds (support brackets, instrument bosses, thermowell nozzles)
• Header nozzle-to-body welds in superheaters and reheaters
• Weld caps adjacent to stiff support structures in boiler headers
• Transition welds between dissimilar materials (e.g., P91-to-P22)
• Repair welds on aged material where grain boundaries may already be segregated

Life Prediction and Fitness-for-Service

For components showing evidence of SRC, fitness-for-service assessment under API 579-1/ASME FFS-1 (Part 7 — Creep Damage) provides a structured framework to determine remaining life and inspection intervals. The assessment requires characterisation of crack dimensions by PAUT/TOFD, material properties at service temperature, and a stress analysis (often FEA) at the cracked location. The ASME Section VIII principles and fitness-for-service assessment resource on WeldFabWorld provides relevant background on pressure vessel integrity assessment.

Related Phenomena and Distinctions

Relation to Temper Embrittlement

Temper embrittlement (TE) shares trace-element embrittlement of grain boundaries with reheat cracking but is mechanically distinct: TE is a loss of impact toughness at ambient temperature (measured by a shift in DBTT) without necessarily producing cracks. Reheat cracking produces actual fracture at elevated temperature during the thermal cycle. Both can occur in the same steel and from the same trace element segregation mechanism, making chemistry control for both phenomena consistent and complementary.

Relation to Hydrogen-Induced Cracking

As noted earlier, hydrogen cracking in sour-service environments and during welding are distinct from reheat cracking in mechanism, temperature, and microstructural location. However, a joint that has suffered undetected hydrogen cracking pre-PWHT may exhibit crack propagation during PWHT that appears to be reheat cracking. Post-PWHT metallographic examination (fracture surface appearance, crack morphology) is essential to distinguish the two when the failure mode is ambiguous.

Relation to Stress Corrosion Cracking

Stress corrosion cracking (SCC) in service — for example, caustic SCC or amine SCC in petrochemical environments — also produces intergranular fracture in the HAZ. The distinguishing factors are: SCC requires a corrosive medium and occurs at or below operating temperature; reheat cracking occurs only during thermal exposure above 400°C in the absence of any corrosive medium. Post-PWHT NDE prior to service exposure is therefore necessary to confirm that what is found after service entry is not residual fabrication cracking.

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