Low-Temperature Deformation Annealing of 20G and 12Cr1MoVG Seamless Steel Pipes

Literature Overview

This paper by Xu Xiaohua, Chen Junde, Xiao Xueshan, and Xue Gangyi, published in Hot Working Technology (2010, Vol. 39, No. 18, pp. 187-190), extends the investigation of low-temperature deformation annealing (LTDA) to two widely used carbon-manganese and low-alloy steel grades: 20G and 12Cr1MoVG. These grades are extensively employed in boiler and pressure vessel applications in China, where 20G serves as a general-purpose boiler steel and 12Cr1MoVG is a 1Cr-0.5Mo-V-Gb low-alloy steel used in high-temperature boiler tubes and structural components. The study evaluates whether the LTDA process, previously validated for P91, can be successfully applied to these lower-alloy grades for hot-expanded seamless pipes, with results benchmarked against GB 5310-2008 requirements.

Technical Content and Process Parameters

The hot expansion process for 20G and 12Cr1MoVG pipes involves heating the pipe to the austenitization temperature range, followed by mechanical expansion over a mandrel. For 20G, the expansion temperature is typically around 1050-1100°C, while for 12Cr1MoVG, it is slightly lower at approximately 1020-1080°C due to the higher alloy content that raises the austenitization temperature. After expansion, the conventional route involves normalizing followed by tempering to restore the mechanical properties degraded by the thermal cycling. The LTDA alternative involves a controlled deformation at a sub-austenitizing temperature followed by annealing, bypassing the need for a separate normalizing furnace.

Microstructural and Mechanical Evaluation

The microstructural analysis confirms that both grades retain their characteristic ferrite-pearlite structure after LTDA, with grain sizes falling well within the acceptable range specified by GB 5310-2008. The mechanical properties meet or exceed the minimum requirements of the standard, demonstrating that the LTDA process does not compromise the mechanical integrity of the hot-expanded pipes.

Comparison with Conventional Heat Treatment

The study compares LTDA-treated pipes with those subjected to the conventional normalizing plus tempering route. The results show that the mechanical properties are closely aligned between the two treatment methods, with differences generally within ±5% for yield and tensile strengths. The microstructural differences are minimal, with both routes producing fine, uniform ferrite-pearlite structures. The non-metallic inclusion levels are comparable, indicating that the LTDA process does not introduce additional inclusions or alter the existing inclusion morphology.

Engineering Practice Implications

The successful application of LTDA to 20G and 12Cr1MoVG pipes has significant implications for boiler tube manufacturing in China. These two grades account for a substantial portion of the domestic boiler tube market, and the ability to simplify the production process while maintaining quality compliance offers considerable economic benefits. The reduction in production steps translates directly to lower manufacturing costs, shorter delivery times, and reduced energy consumption.

Quality Control Considerations

1. The LTDA process requires precise temperature control during the deformation step to ensure uniform property distribution, particularly for thicker-walled pipes where through-thickness gradients may develop.
2. Visual inspection and ultrasonic testing should be performed after LTDA to detect any surface defects or internal discontinuities that may have been introduced during the mechanical deformation.
3. The hardness profile across the pipe wall should be mapped to verify uniformity, as localized hardness variations may indicate incomplete or non-uniform deformation.
4. For 12Cr1MoVG, which is more susceptible to temper embrittlement, the LTDA parameters should be carefully optimized to avoid the temper embrittlement temperature range (350-550°C) during the annealing step.

Study Insights and Reflections

The extension of LTDA technology to carbon-manganese and low-alloy boiler steels demonstrates the versatility and robustness of this process approach. Unlike P91, which relies on precipitation strengthening and martensitic transformation for its mechanical properties, 20G and 12Cr1MoVG derive their strength primarily from solid solution strengthening and pearlite fraction. The fact that LTDA can accommodate the hot expansion-induced microstructural changes in these grades without requiring a full normalizing cycle suggests that the deformation annealing process effectively recovers the ductility and toughness that are typically degraded by the thermal cycling of hot expansion. Engineers considering the adoption of LTDA for these grades should note that the process window is relatively wide compared to P91, providing greater manufacturing flexibility. However, the long-term high-temperature performance, particularly for 12Cr1MoVG in boiler applications where creep and oxidation resistance are critical, should be validated through accelerated aging tests before widespread implementation.