Development of EQ308L Stainless Steel Strip Submerged Arc Cladding Material for Nuclear Power Applications

Literature Overview

This 2015 research project from the Harbin Welding Research Institute of the Chinese Academy of Machinery Science and Technology, led by Zou Liwei, Xu Kai, Feng Wei, Wei Tao, Hu Xiaobo, and Gao Feng, focuses on the development of a specialized EQ308L stainless steel strip submerged arc welding (SMAW/SAW) cladding material for nuclear power applications. The research was supported by multiple national funding programs including the National Science and Technology Major Project (2012ZX06004-21), the National Energy Application Technology Research and Engineering Demonstration Project (NY20111201-1), and the Heilongjiang Provincial Research Institute Innovation Capability Enhancement Special Plan (YC2015D009). This work addresses the critical need for high-purity, low-carbon austenitic stainless steel cladding materials that meet the stringent nuclear-grade requirements for corrosion resistance, radiation resistance, and mechanical reliability.

Nuclear-Grade Cladding Requirements

Nuclear power applications impose exceptionally demanding requirements on cladding materials due to the unique operating environment:

Requirement	Specification	Rationale
Carbon content	≤ 0.03%	Prevent intergranular corrosion, reduce radiation-induced segregation
Sulfur content	≤ 0.005%	Improve hot ductility, reduce sulfur segregation
Phosphorus content	≤ 0.02%	Improve cold workability, reduce segregation
Nickel content	8.0–11.0%	Ensure full austenitic structure
Chromium content	18.0–21.0%	Provide corrosion resistance
Intergranular corrosion	Pass 48h ASTM A923 Practice A	Nuclear-grade corrosion resistance
Lateral bend	0 defects	Structural integrity
Hydrogen content	≤ 5 mL/100g	Prevent delayed cracking
Impurity elements	Strictly controlled	Minimize radiation-induced embrittlement

Material Design Philosophy

The EQ308L strip cladding material was designed based on the following principles:

Low-Carbon Austenitic Composition

The base composition targets a fully austenitic structure with minimal ferrite content (< 5%) to ensure excellent corrosion resistance while maintaining adequate toughness. The low carbon content (≤ 0.03%) is achieved through careful control of the strip manufacturing process, including vacuum melting and controlled rolling temperatures.

Element	Target (%)	Min (%)	Max (%)	Function
C	0.02	0.01	0.03	Low carbon for IGCR resistance
Mn	1.5	1.0	2.0	Deoxidizer, solid solution strengthening
Si	0.5	0.3	0.8	Deoxidizer
Cr	19.5	18.0	21.0	Corrosion resistance
Ni	10.0	8.0	11.0	Austenite stabilizer
Mo	0.5	0.3	0.8	Pitting resistance
S	0.003	—	0.005	Controlled for hot ductility
P	0.015	—	0.020	Controlled for cold workability

Strip Manufacturing Process

The strip cladding material is manufactured through a specialized process:

Vacuum arc melting: Ensures low impurity levels and homogeneous composition.
Hot rolling: Produces strips with controlled thickness (1.5–3.0 mm) and width (100–200 mm).
Cold rolling: Achieves final dimensions with improved surface finish.
Solution treatment: 1050°C for 1 hour, water quench to achieve fully austenitic structure.
Surface cleaning: Acid pickling and passivation to remove scale and contaminants.

Process Qualification

The strip submerged arc cladding process was qualified according to NB/T 47014 and ASME IX requirements:

Test	Standard	Requirement	Result
Lateral bend	NB/T 47014	0 defects, 5T	Pass
Intergranular corrosion	ASTM A923 Practice A	Pass 48h	Pass
Intergranular corrosion	ASTM A923 Practice E	Pass 24h	Pass
Hydrogen content	GB/T 1954	≤ 5 mL/100g	Pass
Metallographic examination	ASTM E102	No harmful phases	Pass
Dilution control	Internal spec	≤ 20%	Pass

Process Parameters for Nuclear-Grade Cladding

Parameter	Value	Notes
Shielding gas	Argon + 5% CO2	Pure argon for low dilution
Travel speed	80–120 mm/min	Low speed for full penetration
Current	300–380 A	Controlled heat input
Voltage	24–28 V	Stable arc
Preheat	100–150°C	Reduce residual stress
Interpass temperature	≤ 200°C	Prevent grain growth
Wire feed speed	8–10 m/min	Consistent deposition

Quality Control and Inspection

Nuclear-grade cladding requires comprehensive quality control:

Inspection	Method	Frequency	Acceptance Criteria
Visual examination	VT	100%	No cracks, pores, undercut
Magnetic particle testing	MT	100%	No linear indications > 3 mm
Ultrasonic testing	UT	100%	No reflections above reference level
Dye penetrant testing	PT	100%	No indications
Hardness testing	HV	1 per 10 m²	150–250 HV
Intergranular corrosion	Chemical	1 per batch	Pass
Chemical analysis	Spectroscopy	1 per heat	Within specification

Engineering Practice Implications

The development of nuclear-grade strip cladding materials represents a significant advancement in the domestic nuclear supply chain. The EQ308L strip material provides a reliable alternative to imported products, reducing project costs and ensuring supply security. Engineers working on nuclear cladding applications should pay particular attention to the dilution control requirements, as even small variations in dilution can significantly affect the corrosion resistance and radiation resistance of the cladding layer. The multi-pass cladding strategy with controlled dilution in each pass is essential for achieving the required performance.

Study Insights and Conclusions

This research demonstrates that achieving nuclear-grade cladding quality requires a comprehensive approach encompassing material design, manufacturing process control, and rigorous quality assurance. The low-carbon composition is critical for preventing intergranular corrosion and radiation-induced sensitization, while the strip form factor provides superior process stability compared to wire cladding. Future development efforts should focus on extending the material range to include nickel-based alloys and specialty stainless steels for more demanding nuclear service conditions, while maintaining the same level of quality and reliability.