CLADDING TECHNOLOGY SHANXI CO., LTDInvestigation of Reheat Cracking Sensitivity Below the Cladding Layer of Nuclear Container Steel
Literature Overview
This 1990 paper by Xue Donglin and Wang Xiuzhi from the Shanghai Steel Metallurgical Process Research Institute and the Shanghai Steel Research Institute investigates the reheat cracking sensitivity of the heat-affected zone (HAZ) beneath the cladding layer of nuclear pressure vessel steels. Reheat cracking is a form of weld cracking that occurs during or after post-weld heat treatment (PWHT) at temperatures typically between 500°C and 700°C. It is a critical concern in nuclear pressure vessel fabrication because the PWHT required to relieve welding residual stresses is precisely the condition under which reheat cracking can occur. The paper addresses a particularly important scenario: the interaction between the cladding weld and the base metal HAZ, where the metallurgical conditions may be more susceptible to reheat cracking than in a conventional weld.
Core Technical Content
Nuclear pressure vessel steels are typically low-alloy steels with controlled chemistry, designed to achieve a balance of strength, toughness, and weldability. Common grades include SA-516 Gr.70, SA-387 Gr.III, and their Chinese equivalents (15MnVR, 18MnMoNbR). These steels contain alloying elements such as Cr, Mo, V, and Nb that contribute to strength but also increase susceptibility to reheat cracking.
Mechanism of Reheat Cracking
Reheat cracking is a form of intergranular cracking that occurs along prior austenite grain boundaries in the coarse-grained HAZ of low-alloy steel welds. The mechanism involves:
- Solute segregation: During welding, alloying elements (particularly Cr, Mo, V, and Nb) segregate to austenite grain boundaries, forming precipitates that embrittle the grain boundaries.
- Grain boundary sliding: During PWHT, the combination of residual stresses and thermal stresses causes grain boundary sliding in the coarse-grained HAZ, where the grain size can exceed 100 μm.
- Cavitation and cracking: As grain boundary sliding progresses, microvoids form at grain boundary triple junctions and at precipitate-matrix interfaces. These voids coalesce to form intergranular cracks.
The cladding weld introduces an additional thermal cycle that modifies the HAZ microstructure beneath the cladding layer. This modified HAZ may have a different grain size, precipitate distribution, and residual stress state compared to the HAZ of a conventional weld, potentially altering the reheat cracking susceptibility.
Factors Influencing Reheat Cracking Sensitivity
The reheat cracking sensitivity of the HAZ beneath the cladding layer is influenced by several factors:
| Factor | Effect on Reheat Cracking | Control Measure |
|---|---|---|
| Grain size in CGHAZ | Larger grains increase susceptibility | Lower heat input, multiple thin passes |
| Cr content | Higher Cr increases susceptibility | Limit Cr to < 0.60% in base material |
| Mo content | Higher Mo increases susceptibility | Limit Mo to < 0.50% in base material |
| V + Nb content | Higher V + Nb increases susceptibility | Limit V + Nb to < 0.05% |
| PWHT temperature | Higher temperature increases susceptibility | Use lowest effective PWHT temperature |
| PWHT duration | Longer duration increases susceptibility | Use shortest effective PWHT duration |
| Residual stress level | Higher stress increases susceptibility | Optimize welding sequence, use low-stress procedures |
| Cladding weld heat input | Higher heat input coarsens HAZ grains | Use multiple thin cladding passes |
Assessment Methods
The reheat cracking sensitivity of nuclear container steel HAZs is typically assessed using one of the following methods:
- Ferrite Creep Test (FCT): A ferrite block is placed on the weld surface during welding, and the creep distance of the ferrite block during PWHT is measured. Greater creep distance indicates higher reheat cracking susceptibility.
- Reheat Cracking Test (RHT): A standard test coupon is welded and subjected to PWHT, then examined for intergranular cracking in the HAZ. The crack length and morphology are evaluated.
- Metallographic Examination: The HAZ microstructure is examined for grain boundary precipitation, grain boundary embrittlement, and evidence of intergranular cracking.
- Grain Size Measurement: The prior austenite grain size in the coarse-grained HAZ is measured and correlated with reheat cracking susceptibility.
Engineering Practice and Standards
The fabrication of nuclear pressure vessels with cladding layers is governed by stringent standards and quality requirements. The following standards are particularly relevant:
- ASME BPV Section VIII, Division 1 and 2: Specify the design, fabrication, and inspection requirements for nuclear pressure vessels, including PWHT requirements and reheat cracking assessment.
- ASME Section IX: Qualify welding procedures and welders, including procedures for cladding welds.
- RCC-M (French Nuclear Code): Specify additional requirements for nuclear pressure vessel fabrication, including reheat cracking assessment criteria.
- GB/T 150 and NB/T 47002: Chinese standards for pressure vessel design and fabrication, including requirements for nuclear-grade vessels.
The key engineering challenge is to select a cladding procedure that minimizes the reheat cracking susceptibility of the underlying HAZ while achieving the required cladding performance. This requires a careful balance of heat input, travel speed, and pass configuration.
Key Findings and Reflections
The research by Xue and Wang highlights a critical but often overlooked aspect of cladding weld quality: the effect of the cladding weld on the reheat cracking susceptibility of the base metal HAZ. In conventional welding practice, the focus is often on the weld metal quality and the immediate HAZ properties, but the long-term behavior during and after PWHT is equally important, particularly for nuclear applications where the consequences of failure are catastrophic.
The key insight from this research is that the cladding weld can act as a second thermal cycle on the base metal, modifying the HAZ microstructure in ways that may either increase or decrease reheat cracking susceptibility. If the cladding weld heat input is too high, it can coarsen the HAZ grains and increase the volume fraction of embrittling precipitates, thereby increasing reheat cracking susceptibility. Conversely, if the cladding weld is performed with low heat input and multiple thin passes, it can refine the HAZ grains and reduce reheat cracking susceptibility.
This finding has important implications for welding procedure qualification (WPQ) of cladding welds on nuclear pressure vessels. The qualification procedure should include assessment of the reheat cracking susceptibility of the HAZ beneath the cladding layer, not merely the cladding layer properties themselves. This requires additional testing beyond the standard WPQ requirements, including FCT or RHT testing of the cladding weld HAZ.
Study Insights
The work of Xue and Wang represents an important contribution to the understanding of reheat cracking in nuclear pressure vessel fabrication. The key takeaway for engineers is that cladding welds must be designed and executed with awareness of their impact on the base metal HAZ, not just the overlay layer. This requires a holistic approach to welding procedure design that considers the entire thermal history of the component, including all welds and heat treatments.
The research also underscores the importance of materials selection in minimizing reheat cracking susceptibility. Nuclear pressure vessel steels should be selected with low reheat cracking susceptibility in mind, limiting the content of Cr, Mo, V, and Nb to levels that provide adequate strength without excessive cracking risk. The interaction between base material chemistry, welding procedure, and PWHT parameters must be carefully managed to ensure the long-term integrity of the cladded nuclear pressure vessel.