Analysis of Transverse Cracks Beneath Stainless Steel Overlay Layer in AP1000 Nuclear Island Core Makeup Water Tank Shell

Literature Overview

This paper by Wang Peihe (2013), published in the journal Welding (焊接), investigates a critical quality issue encountered during the manufacture of the AP1000 nuclear island core makeup water tank. The component in question is a large forged steel shell on whose inner wall a stainless steel overlay layer has been deposited to provide corrosion resistance against the high-purity water environment inside the reactor vessel. During post-weld inspection, a significant number of transverse cracks were discovered beneath the overlay layer, threatening the structural integrity of this safety-critical nuclear component. The author conducted a systematic root-cause analysis and proposed preventive measures.

Root Cause Analysis of Transverse Cracks

The investigation identified the transverse cracks as cold cracks (延迟裂纹), which is a particularly insidious defect type in welding metallurgy because it can occur hours or even days after welding completion. The mechanism was traced through a chain of causative factors:

Insufficient preheating temperature: The preheat applied before overlay welding was too low, failing to adequately slow the cooling rate in the heat-affected zone (HAZ).
Excessive HAZ hardening: The rapid cooling resulted in a martensitic or bainitic microstructure in the HAZ of the forged parent material, significantly increasing the local hardness and reducing ductility.
High diffusible hydrogen concentration: The low preheat temperature also prevented adequate hydrogen diffusion from the weld metal into the atmosphere. Hydrogen atoms migrated into the hardened HAZ microstructure.
Crack initiation at weak regions: Under the combined action of welding residual stress, hydrogen embrittlement, and high hardness, cracks initiated at microstructural weak points in the parent material — specifically at loose grain boundary regions, non-metallic inclusions, or other internal discontinuities near the surface of the forging.
Crack propagation through thickness: Once initiated, the cracks propagated in the thickness direction of the forged shell, forming transverse (circumferential) cracks beneath the overlay weld metal.

Key Metallurgical Factors

Factor	Condition in This Case	Critical Threshold
Preheat temperature	Too low (below recommended minimum)	Typically 150–250°C for low-alloy steels
HAZ hardness	Excessive due to martensitic transformation	Should remain below 350 HV for crack sensitivity control
Diffusible hydrogen	High concentration in HAZ	Should be below 5–10 mL/100g for low-alloy steels
Welding residual stress	High tensile stress in HAZ	Controlled by proper sequence and post-weld treatment
Parent material microstructure	Loose regions and inclusions present	FGM (Forged Grain Map) inspection required

Engineering Practice and Preventive Measures

The author proposed a multi-layered prevention strategy consistent with the principles of FMEA (Failure Mode and Effects Analysis):

Optimized preheating: Increase the preheat temperature to a level that ensures the cooling rate in the HAZ does not exceed the threshold for martensite formation. For the specific low-alloy steel used in the AP1000 tank, a preheat of 200–250°C with interpass temperature control is recommended.
Hydrogen control: Use low-hydrogen consumables, ensure thorough drying of welding materials, and apply post-weld hydrogen bake-out treatment (300–350°C for 2–4 hours) immediately after welding.
Welding sequence optimization: Adopt a symmetric welding sequence to minimize residual stress concentration, and consider multi-pass strategies to reduce peak heat input per pass.
Parent material quality assurance: Implement strict ultrasonic and magnetic particle inspection of the forged blank before overlay welding to identify and reject material with internal looseness or inclusions.
Post-weld stress relief: Apply appropriate post-weld heat treatment (PWHT) to reduce residual stresses below the crack initiation threshold.

Study Insights and Reflections

This case study is highly instructive for nuclear-grade component manufacturing. The core makeup water tank is a Class MC (Main Coolant) component under ASME Section III, meaning any crack defect would require extensive repair qualification and potentially component replacement — with enormous cost and schedule implications. Several lessons emerge:

The interaction between preheat temperature, cooling rate, hydrogen content, and parent material microstructure is a classic cold crack mechanism, but its manifestation beneath an overlay layer (rather than in the weld metal itself) is particularly deceptive and difficult to detect by conventional surface NDT.
The fact that cracks initiated from internal looseness or inclusions in the forging highlights the importance of forging quality control — even when the welding process itself is technically sound, a flawed substrate can cause catastrophic failure.
The transverse orientation of the cracks (perpendicular to the welding direction) is consistent with the maximum tensile stress component in the HAZ, which is a useful diagnostic indicator for root cause analysis.
For nuclear applications, the demonstration of a qualified welding procedure must include not only weld metal qualification but also thorough HAZ evaluation, including hydrogen embrittlement susceptibility testing.

This paper serves as a valuable reminder that in nuclear component fabrication, the boundary between the weld metal and the parent material — the HAZ — often represents the weakest link in the system, and process parameters must be designed with the HAZ metallurgy as a primary consideration rather than an afterthought.