Domestic Development and Application of S30432 Steel Pipe for Ultra-Supercritical Boilers

Literature Overview

This 2010 paper published in the Journal of Power Engineering by Yang Huachun, Peng Fangfang, and Yang Jinbing from Dongfang Boiler (Group) Co., Ltd. documents the domestic development and application of S30432 steel pipe for ultra-supercritical (USC) boilers in China. The paper covers the investigation of foreign S30432 steel grades, the optimization of domestic manufacturing processes, performance testing, welding performance evaluation, and the application of the domestically produced pipe in USC boiler projects.

Material Background and Requirements

S30432 is a high-temperature austenitic stainless steel developed for use in ultra-supercritical power plant boilers operating at steam temperatures exceeding 600°C and pressures above 25 MPa. The material designation follows the European EN 10216-2 standard for austenitic stainless steel pipes. The material is a variant of the well-known 310H/310S austenitic stainless steel family, with optimized chemical composition to enhance creep strength at ultra-high temperatures while maintaining adequate weldability and corrosion resistance.

The development of domestic S30432 steel pipe was driven by the need to reduce dependence on imported materials for China's rapidly expanding USC power plant fleet. Importing these specialized pipes was not only costly but also subject to supply chain risks and long lead times.

Manufacturing Process Optimization

The domestic manufacturing process for S30432 seamless steel pipes involves several critical stages:

1. Billet preparation: The steel is melted using vacuum arc remelting (VAR) or electro-slag remelting (ESR) to ensure low impurity levels and uniform composition. The billet is then homogenized at high temperature to eliminate segregation and ensure a uniform microstructure.
2. Piercing and rolling: The billet is heated to the appropriate forging temperature and pierced on a Mannesmann-type piercing mill or a plug piercing mill. The subsequent rolling stages (elongation, reduction, and finishing) are carefully controlled to achieve the target dimensions and microstructure.
3. Heat treatment: The rolled pipe undergoes solution annealing at approximately 1050–1100°C followed by controlled cooling (air cooling or furnace cooling depending on the required properties). The heat treatment is critical for dissolving carbides and achieving a uniform austenitic microstructure.
4. Cold working (if applicable): Some applications require cold-drawn pipe for improved dimensional accuracy and surface finish. The cold drawing process must be carefully controlled to avoid excessive work hardening, which can reduce the creep strength.

The paper reports that the domestic manufacturing process was optimized through systematic trials, with particular attention to the rolling temperature, reduction ratio, and cooling rate to achieve the target mechanical properties and microstructure.

Performance Testing Results

The domestic S30432 pipe was subjected to comprehensive performance testing, including:

• Mechanical properties: Tensile strength, yield strength, elongation, and reduction of area at room temperature and elevated temperatures (up to 650°C). The results demonstrated that the domestic pipe met or exceeded the specified requirements.
• Creep properties: Long-term creep tests at 600–650°C under various stress levels were conducted to verify the creep strength and rupture life. The creep rupture life was found to be comparable to that of imported material.
• Corrosion resistance: Oxidation tests at high temperatures and corrosion tests in simulated boiler water/steam environments were performed to verify the long-term durability.
• Welding performance: Weldability tests using GTAW (gas tungsten arc welding) and GMAW (gas metal arc welding) were conducted. The weld metal, heat-affected zone (HAZ), and base metal were examined for microstructure, mechanical properties, and intergranular corrosion resistance.

Welding Considerations

The welding of S30432 pipe presents several challenges:

• Hot cracking susceptibility: Austenitic stainless steels with high Cr and Ni content are susceptible to solidification cracking in the weld metal. Preheating is generally not required, but the welding parameters must be controlled to maintain a low solidification rate and avoid the formation of brittle δ-ferrite in excessive amounts.
• Intergranular sensitization: The HAZ of the base metal may be sensitized if the welding heat input is too high, leading to chromium carbide precipitation at grain boundaries and subsequent intergranular corrosion. Low-heat-input welding processes and rapid cooling are recommended.
• Weld metal composition: The filler metal must be carefully selected to match the base metal composition and provide adequate creep strength. Typically, a high-nickel filler metal such as ER310 or a specialized filler for S30432 is used.

The paper reports that the domestic pipe demonstrated satisfactory welding performance, with weld metal and HAZ properties meeting the requirements for USC boiler applications.

Engineering Application

The domestically produced S30432 pipe was successfully applied in ultra-supercritical boiler projects in China, replacing imported material. The application demonstrated that the domestic pipe performs reliably in actual service conditions, with no significant issues reported during commissioning and initial operation. This successful application marked a milestone in China's domestication of high-temperature materials for advanced power generation technology.

Key Reflections

The domestication of S30432 steel pipe represents a significant achievement in China's materials science and manufacturing capabilities. The systematic approach taken—investigating foreign materials, optimizing the manufacturing process, conducting comprehensive testing, and validating through actual application—is a model for the domestication of other specialized materials. The welding performance data is particularly valuable for engineers who must design and fabricate USC boiler components, as it provides confidence that the domestic material can be reliably joined using standard welding practices. The paper's documentation of the manufacturing process optimization trials is also instructive, highlighting the importance of process control in achieving consistent material properties. Future work should focus on long-term in-service monitoring and post-service examination to validate the predicted creep life and corrosion resistance under actual operating conditions.