Development of UNS N08031 Alloy Seamless Tubes for Chemical Heat Exchangers

Literature Overview

This paper by Wang Lianhua, Guo Ling, Liu Fuqiang, and Shao Haili, published in Steel Pipe (2020, Vol. 49, No. 3), documents the research and development process for UNS N08031 (Alloy 625) alloy seamless tubes intended for chemical heat exchanger applications. The tubes were produced at Xinxing Casting Pipes Co., Ltd. using a hot extrusion process for blank tube production followed by multi-pass cold rolling to achieve the final dimensions of Φ50.8 mm × 4 mm. The research validates that the produced tubes meet the requirements of ASME SB 622—2019 specification.

Core Technical Findings

UNS N08031 (Alloy 625) is a nickel-chromium-molybdenum superalloy known for its excellent resistance to corrosion, high-temperature oxidation, and mechanical strength at elevated temperatures. Its application in chemical heat exchangers is driven by the need for materials that can withstand aggressive chemical environments while maintaining structural integrity at operating temperatures.

Product Specifications and Requirements

Manufacturing Process Route

The production process followed a two-stage approach:

1. Hot extrusion stage: Production of blank tubes (荒管) from solid billets through hot extrusion. This stage establishes the initial tube geometry and initiates the recrystallization of the as-cast microstructure.
2. Multi-pass cold rolling stage: Progressive reduction of the blank tube dimensions through multiple cold rolling passes. This stage achieves the final dimensions, improves surface finish, and enhances mechanical properties through work hardening.

Metallurgical Analysis

The metallographic examination revealed a uniform grain distribution with an average grain size of Grade 6 (ASTM E112 equivalent). This indicates:

• Effective hot working during extrusion, achieving complete recrystallization of the as-cast structure.
• Appropriate cold work accumulation during multi-pass rolling, which refines the grain structure without introducing excessive texture.
• No evidence of abnormal grain growth, banding, or segregation that would compromise mechanical performance or corrosion resistance.

Engineering Practice Implications

The successful development of UNS N08031 seamless tubes for chemical heat exchangers has significant implications for the chemical processing industry, where heat exchanger tubes must withstand a combination of corrosive media, high temperatures, and mechanical loading.

Process Quality Control Measures

FMEA Analysis of Manufacturing Risks

Applying a Failure Mode and Effects Analysis to the manufacturing process:

1. Cracking during hot extrusion — caused by excessive extrusion temperature leading to grain boundary weakening or insufficient temperature leading to inadequate ductility. Countermeasure: strict temperature control with continuous thermocouple monitoring and adjustment of extrusion speed.
2. Excessive grain growth during annealing — results in reduced mechanical properties and potential corrosion susceptibility. Countermeasure: precise control of annealing temperature and time, with post-annealing grain size verification.
3. Cold cracking during rolling — occurs when the accumulated cold work exceeds the material's ductility limit. Countermeasure: intermediate annealing between rolling passes and careful control of reduction per pass.
4. Surface defects from oxide scale — oxide scale formed during hot extrusion may not be completely removed, leading to surface imperfections in the final product. Countermeasure: thorough pickling and passivation after hot extrusion and between cold rolling passes.

Key Questions and Reflections

An important consideration not fully addressed in the study is the long-term corrosion performance of the UNS N08031 tubes in specific chemical heat exchanger service conditions. While the tubes meet the mechanical and metallurgical requirements of ASME SB 622—2019, the actual corrosion resistance in service depends on the specific chemical environment, flow velocity, and temperature conditions. The study would benefit from supplementary corrosion testing in representative chemical media to validate the material's suitability for specific applications.

Another reflection concerns the dimensional tolerances achievable through the multi-pass cold rolling process. Chemical heat exchanger tubes must meet tight dimensional tolerances to ensure proper assembly into tube sheets and maintain uniform flow distribution. The study does not provide detailed dimensional accuracy data, which is critical for engineering applications where tube-to-tube-sheet fit and tube bundle assembly are essential for heat exchanger performance and leak tightness.

Study Insights and Implications

This research demonstrates the successful industrial-scale production of UNS N08031 seamless tubes for demanding chemical heat exchanger applications. The key insight is that the combination of hot extrusion and multi-pass cold rolling provides an effective manufacturing route for producing tubes with the required microstructural quality and mechanical properties. For engineers and procurement specialists in the chemical processing industry, this study provides evidence that domestically produced UNS N08031 tubes can meet international specification requirements, potentially reducing material costs and supply chain risks. The recommendation is to complement specification-based acceptance with application-specific corrosion testing and long-term performance monitoring to ensure reliable service in chemical heat exchanger environments. The successful development of this product also highlights the importance of process optimization and quality control in superalloy tube manufacturing, where the narrow processing windows and high material costs demand meticulous attention to every production step.