Manufacturing Technology for G115 Martensitic Heat-Resistant Steel Large-Diameter Thick-Wall Seamless Tubes

Literature Overview

The paper by Lei Bingwang, Li Yongqing, Pang Haiping, Hu Yongping, Liu Kehong, and Qin Ruiting, published in Metallic Functional Materials (Vol. 27, No. 5, 2020, pp. 14-19), details the manufacturing technology for G115 martensitic heat-resistant steel large-diameter thick-wall seamless steel tubes. This work was conducted at Inner Mongolia Northern Heavy Industries Group Co., Ltd. and supported by the National Key R&D Program of China (Project 2017YFB0305202). G115 steel, developed by the Institute of Iron and Steel Research over more than a decade, is the only material option available for the national 630°C advanced ultra-supercritical coal-fired power station demonstration project, making this manufacturing technology of critical national strategic importance.

Core Technical Approach

The manufacturing process employs a 36,000-ton vertical rolling extrusion mill to produce the G115 seamless tubes. The vertical rolling extrusion process is well-suited for producing large-diameter thick-wall tubes because it allows for high reduction ratios in a single pass and produces tubes with favorable grain flow patterns that follow the tube contour. The process route includes four main stages: raw material preparation, billet manufacturing, hot extrusion forming, and heat treatment.

Raw Material Preparation

The raw material for G115 steel is a high-alloy martensitic heat-resistant steel with a complex chemical composition designed to achieve exceptional creep resistance and oxidation resistance at 630°C. The composition typically includes elevated levels of Cr, Mo, V, and other alloying elements that promote the formation of stable precipitates and maintain a tempered martensitic microstructure at elevated temperatures. The raw material must meet stringent requirements for cleanliness, including low levels of sulfur, phosphorus, and non-metallic inclusions, as these impurities can severely degrade the creep performance and fatigue life of the final product.

Billet Manufacturing

The billet manufacturing stage involves vacuum induction melting or vacuum arc remelting followed by electroslag remelting to achieve the required purity and homogeneity. The billet is then homogenized through controlled heating and soaking to eliminate segregation and ensure uniform chemical composition throughout the cross-section. For large-diameter thick-wall tubes, the billet dimensions must be carefully calculated to ensure that the extrusion ratio falls within the optimal range for achieving the desired tube dimensions and microstructure.

Hot Extrusion Forming

The hot extrusion stage is the critical forming operation. The billet is heated to a temperature in the range of 1150-1250°C, depending on the specific alloy chemistry, and then extruded through a die system that shapes the solid billet into a hollow tube. The extrusion temperature, extrusion ratio, and extrusion speed must be carefully controlled to achieve the target tube dimensions while avoiding defects such as internal cracking, surface tearing, or excessive grain growth. The 36,000-ton capacity of the extrusion mill provides sufficient force to achieve the high reduction ratios required for thick-wall tubes.

Heat Treatment

The post-extrusion heat treatment is essential for achieving the required mechanical properties and microstructure. The typical heat treatment cycle for G115 steel includes normalization followed by tempering. Normalization is performed at a temperature in the range of 1050-1100°C to dissolve carbides and austenitize the microstructure, followed by air cooling to produce a fine-grained martensitic structure. The tempering treatment is conducted at 700-750°C to relieve residual stresses, improve toughness, and stabilize the microstructure for long-term service at 630°C. The tempering temperature and duration must be precisely controlled, as under-tempering results in excessive hardness and brittleness, while over-tempering degrades the creep strength by promoting excessive precipitate coarsening.

Quality Control and Performance Verification

The finished G115 tubes undergo comprehensive inspection to verify that all quality requirements are met. Surface quality is examined using visual inspection and magnetic particle testing to detect surface and near-surface defects. Internal quality is verified through ultrasonic testing to detect internal cracks, inclusions, and laminations. Mechanical property testing includes tensile tests, impact tests, and hardness measurements to confirm that the material meets the specified strength, toughness, and hardness requirements. Metallographic examination of the cross-section verifies the microstructure, grain size, and absence of internal defects.

Study Insights and Reflections

This paper documents a manufacturing technology of significant national importance, as G115 steel is the sole material option for the 630°C demonstration power station project. The vertical rolling extrusion process is particularly well-suited for this application because it produces tubes with excellent grain flow continuity, which is critical for creep resistance in thick-wall sections. The process has been solidified and can now produce G115 tubes in stable batch quantities, which is a major achievement in terms of manufacturing reliability and consistency.

From a metallurgical perspective, the key challenge in manufacturing G115 tubes lies in balancing the competing requirements of high creep strength, adequate toughness, and resistance to oxidation and thermal cycling degradation. The heat treatment parameters must be optimized to achieve a tempered martensitic microstructure with a fine, stable precipitate distribution that provides long-term creep resistance without compromising fracture toughness. The thick-wall geometry adds additional complexity because the thermal gradients during heating and cooling can lead to differential microstructures between the inner and outer surfaces of the tube wall, which can affect the overall mechanical performance.

For engineers involved in the design and procurement of G115 tubes for power station applications, this paper provides essential information about the manufacturing capabilities and quality assurance framework. However, long-term performance data from service exposure at 630°C is still limited, and ongoing monitoring of tubes in service is essential to validate the predicted creep life and ensure safe operation over the design lifetime. The successful commercialization of G115 tube manufacturing represents a significant milestone in China's power generation technology advancement, and continued research into microstructure optimization and process refinement will be important for extending the operational envelope of 630°C power plants.