Effect of Tempering Temperature on Microstructure and Mechanical Properties of Hot-Rolled High-Strength Bainitic Steel Pipe

Literature Overview

This paper by Cheng Yefeng, Cheng Juqiang, and You Renjie (2020), published in the journal Steel Pipe, investigates the influence of tempering temperature on the microstructure and mechanical properties of a hot-rolled high-strength novel bainitic steel pipe. The research was conducted jointly by the School of Materials and Chemical Engineering at Xi'an Technological University and Jiangsu Xichuang Zhonghui Hydraulic Machinery Manufacturing Co., Ltd. The study addresses a practical metallurgical challenge: hot-rolled high-strength bainitic steel pipes exhibit high tensile strength but relatively low impact toughness, and the tempering process is identified as a means to improve toughness while maintaining adequate strength.

Metallurgical Background

Bainitic Microstructure in Steel Pipes

Bainitic microstructures are characterized by their intermediate position between pearlitic and martensitic transformations on the iron-carbon phase diagram. In the context of high-strength steel pipes, bainitic microstructures are attractive because they combine relatively high strength with reasonable ductility, offering a favorable balance of mechanical properties. The novel bainitic steel pipe studied in this paper is produced through a controlled hot-rolling process that promotes the formation of a bainitic microstructure without requiring subsequent quenching and tempering, which would add significant cost and complexity to the manufacturing process.

The microstructure of hot-rolled bainitic steel typically consists of lath bainite, granular bainite, ferrite, and retained austenite. Each of these phases contributes differently to the overall mechanical properties: lath bainite provides high strength through its fine lamellar structure; granular bainite offers a combination of strength and ductility; ferrite contributes to ductility and toughness; and retained austenite can influence both strength and toughness through its transformation behavior under stress.

The Tempering Challenge

The fundamental challenge identified in this study is that hot-rolled bainitic steel pipes, while achieving high strength through the bainitic transformation, exhibit relatively low impact toughness. This is attributed to the coarse grain structure resulting from the hot-rolling process and the specific morphology and distribution of the bainitic phases. Tempering, which involves reheating the steel to a temperature below the critical transformation temperature, can improve toughness by relieving internal stresses, promoting the coarsening of carbide precipitates, and modifying the morphology and distribution of retained austenite. However, tempering also reduces strength, and the optimal tempering temperature must therefore balance these competing effects.

Experimental Results and Analysis

Strength-Toughness Relationship with Tempering Temperature

The study systematically investigated the effect of tempering temperature over a wide range, from below 250°C to above 650°C. The key findings regarding the strength-toughness relationship are summarized in the following table:

Tempering Brittleness at 400°C

One of the most significant findings of this study is the observation of tempering brittleness at 400°C, where the impact toughness decreases rather than increases as the tempering temperature rises from 350°C. This phenomenon, known as temper embrittlement, is a well-documented issue in alloy steels and is attributed to the segregation of impurity elements such as phosphorus, antimony, and arsenic to grain boundaries during tempering. The presence of temper embrittlement at 400°C has direct practical implications: this temperature range must be avoided in the tempering process for this steel grade, or the steel composition must be modified to reduce the susceptibility to temper embrittlement.

Microstructural Evolution

The microstructural evolution with tempering temperature follows a clear progression. Below 500°C, the microstructure retains the basic features of the hot-rolled bainitic state, consisting of lath bainite, granular bainite, ferrite, and retained austenite. The tempering process at these temperatures primarily affects the carbide distribution and morphology within the bainitic phases and the stability of retained austenite. Above 500°C, the retained austenite undergoes complete decomposition, transforming into ferrite and cementite or other carbide phases. This decomposition eliminates the retained austenite phase entirely, resulting in a microstructure consisting of ferrite and granular bainite. The loss of retained austenite is associated with the significant improvement in impact toughness observed at higher tempering temperatures.

Practical Recommendations for Tempering Process Design

Based on the experimental results, the following tempering temperature ranges can be recommended for different performance requirements:

The avoidance of the 400°C temper embrittlement region is a critical practical consideration. In production tempering operations, the tempering temperature should be set either below 380°C or above 420°C to avoid the embrittlement zone. If the tempering temperature must pass through the 400°C range during heating or cooling, the cooling rate should be controlled to minimize the time spent in the embrittlement temperature range.

Engineering Practice Integration

Heat Treatment Process Design

For the practical application of this steel grade in pipe manufacturing, the tempering process must be carefully designed to achieve the desired balance of strength and toughness. The heating rate should be controlled to minimize thermal gradients and avoid thermal stresses that could cause cracking. The soaking time at tempering temperature should be sufficient to achieve uniform microstructural transformation throughout the pipe wall thickness, particularly for thick-walled pipes where thermal gradients can be significant. The cooling method—air cooling, furnace cooling, or oil quenching—should be selected based on the desired final microstructure and the susceptibility of the steel to temper embrittlement.

Quality Control Considerations

The quality control of tempered bainitic steel pipes should include verification of both mechanical properties and microstructure. Tensile testing should confirm that the strength meets the specified minimum requirements, while Charpy impact testing should verify that the toughness meets the specified impact energy at the relevant service temperature. Metallographic examination should confirm the expected microstructural state and identify any anomalous features such as excessive retained austenite, anomalous carbide distributions, or evidence of overheating. For applications where temper embrittlement is a concern, a temper embrittlement susceptibility test (such as the ASTM A370 Practice M) should be conducted to confirm that the steel is not susceptible to embrittlement at the operating temperature.

Welding Considerations

The tempering treatment of the base metal also has implications for the welding process. The heat-affected zone (HAZ) of a welded joint in tempered bainitic steel will experience a thermal cycle that can alter the microstructure and properties of the base metal near the weld. The tempering temperature of the base metal influences the HAZ microstructure evolution during welding, and therefore the selection of welding parameters should account for the tempering state of the base metal. Preheating and interpass temperature control during welding should be designed to prevent the formation of brittle microstructures in the HAZ.

Study Insights and Metallurgical Implications

This study provides valuable quantitative data on the tempering response of a hot-rolled high-strength bainitic steel pipe, which is directly applicable to production heat treatment process design. The identification of the 400°C temper embrittlement region is particularly important for process design, as it defines a temperature range that must be avoided in the tempering operation. The observation that retained austenite decomposition above 500°C leads to significantly improved toughness provides a metallurgical explanation for the observed property changes and offers a lever for process optimization. For engineers working with bainitic steel pipes, the key practical takeaway is that the tempering temperature must be carefully selected based on the specific application requirements, with particular attention to avoiding the temper embrittlement zone and ensuring adequate soaking time for uniform microstructural transformation.