Application Issues of S30432 Steel Tubes for Power Station Boilers

Literature Overview

This 2021 paper by Jing Shiyu et al., published in the journal Steel Pipe, addresses practical engineering issues encountered during the domestication and application of S30432 steel tubes in power station boiler systems. The research, conducted by Dongfang Boiler Co., Ltd. in collaboration with Jiangsu Yinhuang Precision Steel Tube Co., Ltd., covers four critical aspects: domestic production quality assessment, grain size considerations, intergranular corrosion susceptibility, and the effects of internal shot peening on oxidation resistance.

Material Background and Application Context

S30432 is a stabilized austenitic stainless steel (essentially 321-type with controlled carbon and titanium content) used extensively in boiler superheaters, reheaters, and other high-temperature components where oxidation resistance, thermal stability, and mechanical strength are required simultaneously. The material typically operates in temperature ranges up to approximately 650°C in ultra-supercritical boiler applications.

Key Technical Issues and Findings

Domestication Status

The study confirms that domestically produced S30432 steel tubes meet relevant standard requirements and have reached the quality level of imported products. This is a significant development for cost reduction and supply chain security in China's power generation sector.

Grain Size Considerations

A critical finding is that pursuing excessively fine grain sizes may introduce other quality risks. While fine grains generally improve strength and toughness, in stainless steel tubes the grain size must be balanced against:

• Susceptibility to intergranular corrosion
• Fabrication and forming behavior
• Welding performance and heat-affected zone properties
• Long-term thermal stability

The paper advocates for a balanced approach to grain size control rather than an unqualified pursuit of finer structures.

Intergranular Corrosion Susceptibility

S30432 steel tubes inherently possess a certain degree of intergranular corrosion susceptibility due to the possibility of chromium carbide precipitation at grain boundaries under specific thermal exposure conditions. The study recommends heightened attention during:

1. Hydrostatic pressure testing (chloride contamination risk from test water)
2. Storage and transportation (atmospheric corrosion exposure)
3. Installation and fabrication (mechanical damage and contamination)
4. Initial boiler unit operation (transient thermal conditions)

Internal Shot Peening Effects

The study finds that internal shot peening does not adversely affect the oxidation resistance of S30432 steel tubes. Specifically, heat treatment at temperatures below 800°C after shot peening has essentially no impact on the material's oxidation performance. This finding supports the use of internal shot peening as a residual stress relief and fatigue life enhancement technique without compromising the primary corrosion resistance property.

Standards and Quality Control Framework

Engineering Practice Recommendations

For power station boiler manufacturers and operators, the following recommendations emerge from this study:

• Establish comprehensive incoming inspection protocols for S30432 tubes, including grain size verification, intergranular corrosion testing (ASTM A262 Practice E), and mechanical property confirmation.
• Implement strict water quality management during hydrostatic testing to minimize chloride-induced intergranular corrosion risks.
• Utilize domestic S30432 tube suppliers with confidence, subject to standard qualification and periodic requalification procedures.
• Incorporate internal shot peening into the manufacturing process where fatigue life improvement is desired, with the assurance that subsequent heat treatment below 800°C will not compromise oxidation performance.

Study Insights and Reflections

The practical orientation of this paper is particularly valuable for engineers involved in power station boiler procurement and quality assurance. The finding that domestic production has reached import-equivalent quality levels represents a mature industrial capability that should be leveraged for cost optimization without quality compromise. The warning against indiscriminate pursuit of fine grain sizes reflects a nuanced understanding of the competing requirements in stainless steel tube selection, where strength-gain from grain refinement must be balanced against corrosion resistance and fabrication behavior. The confirmation of oxidation resistance stability after internal shot peening removes a potential barrier to adopting this beneficial surface treatment technique in boiler tube manufacturing.