Low-Temperature Deformation Annealing of Large-Diameter P91 Seamless Steel Pipe for Hot Expansion

Literature Overview

The paper by Xu Xiaohua, Chen Junde, Yao Jing, Xue Gangyi, Xiao Xueshan, and Chen Yi (Shanghai University Institute of Materials Research and Wuxi Dexin Steel Pipe Co., Ltd.) published in Metal Heat Treatment (2010, Vol. 35, No. 4, pp. 69-72) addresses a critical manufacturing challenge in the production of large-diameter P91 seamless steel pipes through the hot expansion process. P91 steel, a 9Cr-1Mo-V-Nb-N martensitic ferritic steel, is widely used in supercritical and ultra-supercritical power boiler main steam and hot reheat pipelines, where it must withstand temperatures up to 620°C and pressures exceeding 25 MPa. The hot expansion process, which involves heating a smaller-diameter seamless pipe and expanding it over a mandrel to achieve the required larger diameter and thinner wall thickness, introduces significant microstructural degradation in the heat-affected zone and the pipe body due to the high expansion temperatures typically ranging from 1050°C to 1150°C. This paper proposes and validates a low-temperature deformation annealing (LTDA) approach as a post-expansion heat treatment alternative to the conventional normalizing plus tempering route, demonstrating that acceptable mechanical properties and microstructural integrity can be maintained while simplifying the production workflow.

Core Technical Approach

The low-temperature deformation annealing process investigated in this study involves a controlled mechanical deformation step performed at a temperature significantly below the conventional normalizing range, followed by an annealing step. The push-type production method is employed, where the heated pipe is pushed over a conical mandrel to achieve radial expansion. After the hot expansion, the pipe undergoes LTDA at a temperature window that avoids the austenitization regime entirely, thereby preventing grain coarsening and excessive carbide dissolution. The key advantage of this approach is that it eliminates the need for a separate normalizing furnace, reducing energy consumption, shortening production cycle time, and minimizing the risk of intergranular carbide precipitation that can occur during prolonged high-temperature exposure.

Microstructural Analysis

The LTDA-treated P91 hot-expanded pipe exhibits a tempered martensite microstructure, which is the desired condition for this alloy grade. The grain size is measured at Grade 6 according to the ASTM E112 standard, indicating a fine and uniform grain structure that is comparable to the original seamless pipe before expansion. Non-metallic inclusions are observed to remain at levels consistent with the pre-expansion condition, suggesting that the LTDA process does not introduce additional inclusions or alter the distribution of existing sulfide and oxide inclusions. The carbide morphology, particularly the M23C6 and MX-type carbides (Nb(V)C), appears to be preserved without significant coarsening, which is critical for maintaining the high-temperature creep strength of P91 steel.

Mechanical Properties Comparison

The mechanical properties after LTDA show a slight decrease compared to the original pipe, which is expected due to the thermal and mechanical cycling during hot expansion. However, the values are remarkably close to those obtained from the conventional normalizing plus tempering route, confirming that LTDA achieves equivalent mechanical performance. The yield strength of 565 MPa and tensile strength of 740 MPa in the longitudinal direction satisfy the ASTM A335 specification for P91, which requires a minimum yield strength of 345 MPa and minimum tensile strength of 517 MPa. The elongation of 25% also meets the minimum requirement of 20% specified in the standard.

Standards Compliance and Engineering Significance

The results are evaluated against both ASTM A335 P91 and GB 5310-2008 for 10Cr9Mo1VNbN, demonstrating dual-standard compliance. This is particularly significant for Chinese manufacturers serving both domestic and international markets, as it confirms that the LTDA process produces pipes meeting the stringent requirements of both Western and Chinese specifications. The microstructural stability, grain size control, and non-metallic inclusion levels all fall within acceptable limits defined by these standards.

From an engineering practice perspective, the adoption of LTDA for P91 hot-expanded pipes offers several practical advantages. First, the elimination of the normalizing step reduces the overall production cycle by approximately 30-40%, which is economically significant for large-diameter pipe production where furnace capacity is a bottleneck. Second, the lower processing temperature reduces thermal distortion and dimensional variation, improving the geometric accuracy of the final product. Third, the reduced energy consumption contributes to lower carbon footprint, which is increasingly important in the context of environmental regulations.

Key Considerations for Implementation

1. The deformation temperature during LTDA must be carefully controlled to avoid entering the recrystallization range, which would alter the martensitic microstructure.
2. The strain rate and total strain during deformation should be optimized to achieve uniform property distribution across the pipe cross-section.
3. Post-LTDA dimensional inspection is critical, as the mechanical deformation may introduce residual stresses that need to be assessed for stress-corrosion cracking susceptibility.
4. Long-term creep testing at service temperatures (550-620°C) would be advisable to confirm that the LTDA-treated pipes maintain their high-temperature strength equivalent to conventionally heat-treated pipes.

Study Insights and Reflections

This research represents a pragmatic approach to process optimization in specialty steel pipe manufacturing. The LTDA method effectively bridges the gap between the aggressive microstructural changes introduced by hot expansion and the need for a simplified, economical post-expansion treatment. The fact that P91, a highly alloyed martensitic steel with complex precipitation strengthening mechanisms, can tolerate this simplified treatment without significant property degradation is noteworthy. However, engineers should remain cautious about extrapolating these results to other applications where the pipe may be subjected to cyclic thermal loading or high-temperature creep conditions, as the carbide distribution and grain boundary stability after LTDA may differ subtly from conventionally heat-treated material in terms of long-term durability. Further investigation into the creep rupture behavior and intergranular fracture susceptibility of LTDA-treated P91 pipes would strengthen the engineering confidence in this process.