Steel 91 Seamless Pipe Production and Application

Literature Overview

This paper by Lin Zhaojie, Cheng Shichang, Zhong Qianxia, and Liu Zhengdong from the Iron and Steel Research Institute of the Ministry of Metallurgy, published in "Special Steel" (Vol. 17, No. 6, 1996), provides a comprehensive overview of the production technology and application of Steel 91 seamless pipes. Steel 91, also known as 9Cr-1Mo-V-Nb-N, is a martensitic ferritic steel developed for advanced power plant applications, particularly for high-temperature creep-resistant components in supercritical and ultra-supercritical boilers. The paper covers chemical composition control, hot workability, smelting methods, pipe manufacturing processes, and field applications.

Chemical Composition and Metallurgical Design

Steel 91 is designed to achieve superior high-temperature strength and creep resistance through a carefully balanced alloying strategy. The key alloying elements and their roles are as follows:

The microstructure of Steel 91 in the tempered martensite condition consists of fine MX carbonitrides (VC, VN, NbC, NbCN) dispersed within a tempered martensite matrix. The MX phase is critical for maintaining creep strength at elevated temperatures (550-650°C) because it is thermodynamically stable and resistant to coarsening, unlike M23C6 and M6C3 carbides which can dissolve and coarsen during long-term exposure.

Hot Workability and Manufacturing Challenges

The paper emphasizes that Steel 91 exhibits limited hot workability due to its high alloy content and susceptibility to cracking during hot deformation. The hot rolling temperature window is narrow, typically between 1050°C and 1250°C, with a minimum finishing temperature of approximately 850°C to avoid strain-induced martensite transformation and subsequent cracking. The presence of chromium and molybdenum increases the hot cracking susceptibility, particularly at lower deformation temperatures where sulfide and oxide inclusions can act as crack initiation sites.

Smelting and Refining

Clean steel production is paramount for Steel 91. The paper describes the use of vacuum induction melting (VIM) or vacuum arc remelting (VAR) as secondary refining methods to reduce sulfur and oxygen content below 0.010% and 0.005%, respectively. Resulfurization to a controlled level of 0.020-0.030% is often performed to improve hot workability by forming elongated MnS inclusions that are more ductile than irregular CaS inclusions.

Seamless Pipe Manufacturing Process

The seamless pipe production route described in the paper involves the following key steps:

1. Billet preparation with controlled chemistry and cleanliness
2. Billet heating to the appropriate piercing temperature
3. Mannesmann piercing to produce a hollow shell
4. Multi-stand rolling to reduce wall thickness and achieve target dimensions
5. Straightening and cutting to length
6. Normalizing and tempering heat treatment (typically 1020-1060°C normalization followed by 720-760°C tempering)
7. Final mechanical property verification

The tempering step is critical: it transforms the as-normalized martensite into tempered martensite, reduces residual stresses, and precipitates fine MX carbonitrides that provide the desired high-temperature strength.

Application and Engineering Considerations

Steel 91 seamless pipes have found extensive application in supercritical and ultra-supercritical power boilers, particularly for main steam pipes, reheater outlet pipes, and superheater tubes operating at temperatures up to 620°C and pressures exceeding 25 MPa. The paper notes that the improved creep rupture strength of Steel 91 compared to the conventional P91 steel (9Cr-1Mo-V-Nb) allows for higher operating temperatures and pressures, thereby improving thermal efficiency of power generation systems.

A critical concern in the application of Steel 91 pipes is the susceptibility to temper embrittlement during long-term service at 350-550°C. The paper implicitly acknowledges this challenge, as the presence of impurity elements such as Sn, P, and As can segregate to grain boundaries and promote embrittlement. Quality control measures must therefore include strict control of these elements during steelmaking and periodic surveillance testing during service.

Study Insights and Implications

Reading this paper in the context of current industry practice reveals how foundational this work was for the subsequent adoption of 9Cr-1Mo-V-Nb-N steels in advanced power plant technology. The paper's emphasis on the interplay between chemical composition, microstructure, and manufacturing process is a classic example of metallurgical systems thinking. For engineers today working on high-temperature pipe specifications, the lessons from Steel 91 development remain relevant: achieving the target microstructure requires not only correct chemistry but also precise control of every manufacturing step from smelting through final heat treatment. The paper also serves as a reminder that material development is inherently interdisciplinary, requiring integration of metallurgical science with manufacturing engineering and application-specific performance requirements.