Domestic Manufacturing Process Analysis of SUPER304H Steel Pipe for Ultra-Supercritical Units

Literature Overview

The paper by Peng Fangfang, Zhu Guoliang, and Song Jianxin (2008, Power Engineering, Vol. 28, No. 5, pp. 803–806) addresses a critical challenge in the domestic production of SUPER304H austenitic stainless steel pipe for ultra-supercritical (USC) power generation units. The authors conducted a comparative analysis of chemical compositions between domestically manufactured and imported SUPER304H pipe, proposed the application of inner-wall shot peening technology as a key process innovation, and experimentally validated the effectiveness of this approach through metallographic examination of foreign-made SUPER304H pipe specimens. This work represents a significant milestone in China's effort to achieve self-sufficiency in high-temperature creep-resistant austenitic materials for advanced thermal power stations.

Core Technical Points and Interpretation

SUPER304H is a high-columbium austenitic stainless steel designed for steam temperatures exceeding 600°C in ultra-supercritical boiler superheaters and reheaters. The alloying strategy relies on multiple strengthening mechanisms: solid solution strengthening by columbium (Cb), precipitation hardening by Cb carbides, and fine grain strengthening. The paper's analysis of actual chemical compositions reveals that the key differentiators between domestic and imported pipe lie not only in compositional control but also in the manufacturing process parameters that govern microstructural evolution.

The central innovation proposed is the application of inner-wall shot peening (IWSP) during the cold-working stage of pipe production. This technique introduces compressive residual stresses and a strain-induced martensite layer on the inner surface, which directly enhances creep resistance and oxidation resistance at elevated temperatures. The authors examined imported SUPER304H pipe specimens and measured the strain layer depth to be approximately 40–70 μm, with a jet pressure exceeding 0.7 MPa.

Process Analysis and Engineering Practice Integration

The manufacturing route for SUPER304H pipe typically involves hot rolling or piercing of ingots, followed by cold drawing or cold rolling to achieve final dimensions, intermediate annealing, and finally solution treatment. The critical process windows identified in the paper include:

1. Chemical composition control: Precise control of Cb content (typically 0.5–1.0%) is essential for precipitation hardening. Excessive Cb can lead to intergranular carbide precipitation during high-temperature service, while insufficient Cb compromises creep strength.
2. Cold working deformation: The cumulative cold work percentage must be carefully controlled to achieve the desired balance between strength and ductility. Excessive cold work introduces excessive dislocation density that may accelerate creep deformation.
3. Inner-wall shot peening: This is the differentiating process step. The shot peening must be applied after cold working but before final heat treatment, ensuring that the strain layer is preserved while allowing carbide dissolution during solution annealing.
4. Solution annealing: Typically performed at 1050–1100°C followed by rapid cooling, this step dissolves carbides and eliminates processing-induced residual stresses while preserving the fine grain structure.

From an engineering practice perspective, the implementation of IWSP introduces several quality control challenges. The shot peening coverage must be verified by magnetic particle inspection (MPI) to ensure uniform surface coverage without introducing surface cracks. The strain layer depth should be periodically verified by metallographic sectioning using a dedicated etchant that differentiates the strain-hardened layer from the base material. Additionally, the roughness profile after shot peening must be characterized, as excessive roughness can serve as crack initiation sites during cyclic thermal loading.

Key Questions and Reflections

The paper raises several questions that deserve further investigation. First, the long-term creep performance of domestically manufactured SUPER304H pipe with IWSP treatment has not been validated through extended creep testing at 650°C and above, which is the target service condition for next-generation USC units. Second, the interaction between the strain-hardened layer and the heat-affected zone during field welding of pipe spools remains an open question—welding thermal cycles may partially or fully eliminate the beneficial strain layer, negating the IWSP benefit. Third, the economic feasibility of implementing IWSP at scale in Chinese pipe mills requires consideration of equipment investment, cycle time, and scrap rate implications.

Study Insights and Implications

This paper provides a clear roadmap for domestic SUPER304H pipe production that is grounded in both metallurgical understanding and practical process engineering. The IWSP technique, while conceptually straightforward, requires careful parameter optimization and rigorous quality control to ensure consistent results. The measured strain layer depth of 40–70 μm represents a narrow but achievable window that balances creep enhancement against potential surface degradation. For engineers involved in material specification and procurement, this work underscores the importance of evaluating not only chemical composition but also process documentation and microstructural evidence when qualifying domestic alternatives to imported pipe. The study exemplifies how targeted metallurgical analysis can bridge the gap between imported technology and domestic manufacturing capability, a paradigm that remains relevant for other high-performance alloy applications in power generation.