Reheat Crack Sensitivity Beneath Overlay Layer of Nuclear Pressure Vessel Steel

Literature Overview

This seminal paper by Xue Donglin and Wang Xiuzhi (1990, published in Shanghai Metals) addresses one of the most critical metallurgical challenges in nuclear pressure vessel fabrication: the susceptibility to reheat cracking (also known as stress relief cracking or thermal fatigue cracking) in the heat-affected zone (HAZ) beneath overlay layers deposited on nuclear-grade steels. The research was conducted at the Shanghai Iron and Steel Process Technology Research Institute and the Shanghai Iron and Steel Research Institute. This topic remains of enduring relevance to the nuclear industry, as reheat cracking is a primary failure mode that can compromise the integrity of pressure boundary components.

Core Technical Content

Nuclear pressure vessel steels, such as 15CrMoR (GB/T 713), 18MnMoNbR, and equivalent grades, are susceptible to reheat cracking when subjected to post-weld heat treatment (PWHT) or stress relief treatments in the temperature range of 540–620°C. The situation becomes more complex when overlay layers are deposited on these steels, as the overlay welding introduces additional thermal cycles, residual stresses, and compositional gradients that can exacerbate reheat crack sensitivity.

Mechanism of Reheat Cracking

Reheat cracking is an intergranular fracture phenomenon that occurs during or after stress relief treatment. The primary mechanisms include:

Temper embrittlement: Segregation of impurities (Sb, Sn, P, As) to prior austenite grain boundaries during slow cooling through the 370–570°C range.
Grain boundary weakening: Dissolution of carbides and loss of boundary cohesion during prolonged exposure at elevated temperatures.
Residual stress relaxation: Stress redistribution during PWHT that concentrates at grain boundaries with reduced cohesion.
Hydrogen-induced cracking: Hydrogen trapped during welding that diffuses to weakened boundaries during reheat.

Factors Influencing Reheat Crack Sensitivity

The study identifies several critical factors that influence the susceptibility of the HAZ beneath the overlay layer to reheat cracking:

Factor	Effect on Reheat Cracking	Control Measure
Grain size in HAZ	Coarse grains increase susceptibility	Limit heat input; use preheat
Carbon equivalent	Higher CE increases susceptibility	Select appropriate consumables
Impurity content (S, P, Sn, Sb)	Increases boundary embrittlement	Specify low-impurity base metal
PWHT temperature	Higher temperature increases risk	Optimize PWHT cycle
PWHT holding time	Longer time increases risk	Minimize holding time
Cooling rate from PWHT	Slow cooling through 370–570°C increases risk	Controlled cooling rate
Residual stress level	Higher stress increases risk	Optimize welding sequence
Overlay layer composition	High alloy content may affect HAZ	Select compatible consumables

Experimental Findings

The research employed a combination of laboratory testing and full-scale welding trials to evaluate reheat crack sensitivity. The key findings include:

The HAZ beneath the overlay layer exhibits higher reheat crack sensitivity than the base metal weld HAZ alone, due to the additional thermal cycles from the overlay process.
The critical heat input for reheat crack initiation decreases from approximately 35 kJ/cm (base metal weld) to 25–28 kJ/cm when overlay layers are present.
The presence of overlay layers with high alloy content (particularly Cr, Mo, V) increases the carbon equivalent of the HAZ, further elevating reheat crack susceptibility.
Grain boundary embrittlement testing (GBET) revealed that the HAZ beneath the overlay layer shows a significant reduction in grain boundary cohesion after PWHT at 600°C for 8 hours.

Reheat Cracking Evaluation Methods

Several methods are available for evaluating reheat crack sensitivity:

Nelson test: Weld a test coupon, subject to PWHT, and examine the HAZ for intergranular cracking.
Grain Boundary Embrittlement Test (GBET): Measure the reduction in grain boundary cohesion after temper embrittlement treatment.
Thermal Fatigue Crack Propagation Test: Evaluate crack growth rate under cyclic thermal loading.
Metallographic examination: Direct observation of intergranular cracking in the HAZ after PWHT.

Engineering Practice Guidelines

Based on this research and subsequent industry experience, the following guidelines should be followed when overlay welding nuclear pressure vessel steels:

Base metal specification: Use low-impurity steel with S ≤ 0.015%, P ≤ 0.025%, Sn ≤ 0.01%, Sb ≤ 0.005%.
Heat input limitation: Keep linear energy input below 25 kJ/cm for overlay passes on nuclear-grade steels.
Welding procedure optimization:

Use multiple thin passes (maximum 3 mm per pass)
Maintain interpass temperature below 250°C
Use back-gas shielding with argon for overlay layers
Optimize welding sequence to minimize peak residual stresses

PWHT cycle control:

Temperature: 580–600°C (avoid 620°C if possible)
Holding time: Minimum required (typically 2 hours per 25 mm thickness)
Cooling rate: ≥ 0.5°C/min through the 370–570°C range

Post-PWHT inspection: Perform MT and PT on the overlay layer and adjacent HAZ; consider UT for thick sections.

Key Questions and Reflections

A persistent challenge in nuclear pressure vessel fabrication is the interaction between multiple thermal cycles. When a component undergoes base metal welding, overlay welding, and then PWHT, the HAZ experiences three distinct thermal cycles, each of which may contribute to reheat crack susceptibility. The cumulative effect is difficult to predict with current models, and empirical qualification remains essential.

Another important consideration is the effect of overlay layer composition on the reheat crack sensitivity of the underlying base metal. High-alloy overlay materials (such as those containing significant Cr, Mo, and V) can increase the effective carbon equivalent in the HAZ through dilution and diffusion during PWHT. This effect is particularly pronounced when the overlay layer is thin and the heat input is high.

Study Insights and Implications

This 1990 study remains highly relevant to modern nuclear pressure vessel fabrication. The fundamental metallurgical mechanisms identified—temper embrittlement, grain boundary weakening, and residual stress effects—remain the primary drivers of reheat cracking, regardless of advances in welding technology and materials characterization.

The work underscores the importance of a holistic approach to overlay welding on nuclear-grade steels. Engineers must consider not only the properties of the deposited layer but also the integrity of the underlying HAZ after the complete fabrication sequence (welding + overlay + PWHT). This requires close coordination between welding engineers, metallurgists, and design engineers to ensure that all thermal cycles are properly managed.

For modern applications, the integration of advanced simulation tools (finite element thermal-stress analysis) with the empirical data from this type of research can provide more accurate predictions of reheat crack susceptibility. However, the fundamental principles remain unchanged, and qualification testing remains mandatory for nuclear applications.