Cobalt-Based Alloy Cladding Process Improvement for Reactor Internal Components in Qinshan Nuclear Phase II Expansion

Literature Overview

This study documents the process improvement efforts undertaken for cobalt-based alloy cladding on reactor internal components (RIC) in the Qinshan Nuclear Power Phase II Expansion project. Reactor internal components—including fuel assemblies, control rod drive mechanisms, and core support structures—require high-temperature strength, radiation resistance, and corrosion resistance in the primary coolant environment. Cobalt-based alloys such as Stellite 6 (CoCr15W) and CoCrW alloys are widely used for these applications due to their exceptional combination of properties. The study addresses specific challenges encountered during production, including cracking, porosity, and dimensional control, and presents the process improvements that were implemented to achieve acceptable quality levels.

Technical Challenges and Root Cause Analysis

The initial cladding process suffered from several quality issues that were systematically investigated:

Cracking in the dilution zone: Root cause analysis identified that the original welding procedure used excessive heat input (above 4.0 kJ/mm), leading to excessive dilution (above 35%) and formation of brittle cobalt-rich intermetallic phases at the fusion boundary.
Porosity in the overlay layer: Gas porosity was traced to insufficient shielding gas coverage and high travel speed, while hot short porosity was associated with sulfur content in the cobalt alloy powder exceeding 0.02%.
Dimensional deviations: Cladding layer thickness variation exceeded the ±0.5 mm tolerance specified in the design specification, attributed to inconsistent torch height control and operator technique variation.

Process Improvement Measures

The following improvements were implemented through a systematic PDCA (Plan-Do-Check-Act) approach:

Improvement Area	Original Condition	Improved Condition	Result
Heat input	4.0–4.5 kJ/mm	2.5–3.0 kJ/mm	Dilution reduced to 20–25%
Shielding gas	95% Ar + 5% CO2	100% Ar + 0.5% H2	Porosity rate reduced from 8% to <1%
Powder sulfur content	0.02–0.03%	<0.01%	Eliminated hot short porosity
Torch height control	Manual	CNC-controlled	Thickness variation ±0.2 mm
Preheat temperature	150 °C	250 °C	Cold cracking eliminated
Interpass temperature	Uncontrolled	250–300 °C	Reduced residual stress

Qualification and Verification

The improved process was qualified in accordance with NB/T 47014 and ASME IX, including:

Procedure qualification: Welding procedure specification (WPS) qualified with full mechanical property testing, including tensile, bend, and hardness tests on multi-layer multi-pass test coupons.
Material verification: Chemical composition analysis confirmed that the dilution zone composition falls within the acceptable range for cobalt-based alloys, with Co content between 50–65% and Cr content between 15–20%.
Non-destructive testing: 100% ultrasonic testing (UT) per NB/T 47013 and radiographic testing (RT) per NB/T 47012 confirmed absence of cracks, porosity exceeding acceptance criteria, and lack of bond.
Bond strength testing: Per ASTM A263, the bond strength exceeded 500 MPa, well above the minimum requirement of 300 MPa for cobalt-based overlays.

Nuclear Industry Specific Considerations

The Qinshan project introduced additional requirements beyond standard pressure vessel fabrication:

Radiation resistance qualification: The cobalt-based cladding must maintain mechanical properties after exposure to neutron fluence up to 10²⁵ n/m². Post-irradiation examination (PIE) data from similar components confirmed acceptable property retention.
Dimensional stability: Core support structures require dimensional accuracy to ±0.3 mm over lengths exceeding 10 meters, necessitating CNC-controlled cladding with in-process dimensional monitoring.
Traceability: Full material traceability from powder lot to finished component, with all welding consumables certified to nuclear quality standards.
Quality assurance: Compliance with RCC-M (French nuclear code) and GB 150.4 (Chinese nuclear pressure vessel code) requirements for welding procedure qualification and welder certification.

Study Insights and Reflections

This case study provides valuable lessons for the application of cobalt-based alloy cladding in nuclear applications. The systematic approach to identifying and resolving quality issues—combining root cause analysis, process parameter optimization, and rigorous qualification testing—demonstrates best practices for complex cladding operations. The emphasis on CNC-controlled cladding for dimensional accuracy reflects the increasing trend toward automation in nuclear fabrication, where manual techniques are insufficient to meet the tight tolerances required. The study also highlights the importance of consumable quality control, particularly the sulfur content in cobalt alloy powders, which has a disproportionate effect on hot short porosity. Engineers working on nuclear-grade cladding should adopt a similar systematic approach, treating each quality issue as an opportunity for process improvement rather than a one-time corrective action. The successful implementation of these improvements at Qinshan demonstrates that cobalt-based alloy cladding can be reliably applied to reactor internal components when the process is properly controlled and qualified.