In the vast field of metal processing and materials science, 23CrNi3Mo rock drilling tool steel is highly regarded for its exceptional mechanical properties and wear resistance, making it the preferred material for high-strength applications, particularly in the manufacturing of rock drilling tools. However, like many high-performance materials, 23CrNi3Mo rock drilling tool steel is prone to a structural defect known as banded structure during processing and use. This banded structure not only compromises the mechanical properties of the steel but also increases the risk of fractures during use, significantly reducing the service life of tools. Therefore, effectively controlling the formation of banded structures in 23CrNi3Mo rock drilling tool steel has become a critical challenge in the industry. This article explores several control methods to help manufacturers optimize production processes, improve product quality, and ensure the full performance potential of 23CrNi3Mo rock drilling tool steel.
Hazards and Causes of Banded Structure
The banded structure refers to parallel, layered patterns that form along the rolling direction in steel, caused primarily by the segregation of carbon and alloying elements. This phenomenon leads to anisotropy in mechanical properties, reducing impact toughness and increasing the likelihood of tool cracking under stress. During rock drilling, the internal and external threads and the rock drill bit are particularly prone to fractures caused by banded structures, severely affecting tool lifespan and safety.
Key Hazards of Banded Structure:
- Reduced Mechanical Properties: Banded structure decreases the steel’s impact toughness, tensile strength, and yield strength.
- Increased Deformation: During heat treatment, banded structures will increase the quenching deformation, affecting the dimensional accuracy and shape stability.
- Shortened Service Life: Under high-intensity conditions, banded structures become stress concentration points, leading to cracks and tool failure.
Causes of Banded Structure:
Banded structures primarily result from selective crystallization during steel solidification and rolling deformation. During solidification, dendritic segregation causes carbon, alloying elements, and impurities to accumulate in certain regions. As the steel billet undergoes forging and rolling, these dendrites and segregated regions elongate into a fibrous shape, forming primary banded structures. Secondary banded structures can occur in low- to medium-carbon steels after hot rolling and slow cooling, resulting in alternating eutectoid ferrite and pearlite microstructures.
For 23CrNi3Mo rock drilling tool steel, banded structures are mainly attributed to the distribution variations of M-A islands (martensite-austenite) in bainite and carbon segregation.
Control Measures for Banded Structure
To address the banded structure issue in 23CrNi3Mo rock drilling tool steel, comprehensive control measures can be implemented across the smelting, casting, rolling, and heat treatment processes.
Optimizing Smelting and Casting Processes
- Electromagnetic Stirring: During smelting, electromagnetic stirring can effectively reduce element segregation and impurities in molten steel, minimizing the formation of banded structures.
- Soft Reduction Technology: In continuous casting, soft reduction helps improve the solidification structure of billets and reduces internal defects and segregation.
- Low Superheat Casting: Controlling the casting temperature and reducing superheat minimizes dendritic segregation during solidification, reducing banded structure formation.
Controlling Rolling Processes
- Optimizing Rolling Temperature: Proper control of rolling temperatures ensures uniform deformation, reducing internal stress concentrations and the formation of banded structures.
- Increasing Cooling After Final Rolling: Accelerating the cooling rate after the final rolling suppresses ferrite nucleation and promotes uniform pearlite distribution, thereby mitigating banded structures.
- Post-Rolling Water Cooling: Water cooling after rolling inhibits the diffusion of carbon and other elements, further reducing banded structure formation.
Optimizing Heat Treatment Processes
- Diffusion Annealing: Heating the steel above 1250°C and holding it for several hours facilitates atomic diffusion, eliminating segregation and banded structures in billets.
- Normalizing Treatment: Normalizing reduces the effects of segregation-related banded structures. While not removed, multiple normalizing cycles with precise temperature control can mitigate their impact.
- Combined Heat Treatment: Combining diffusion annealing and normalizing treatments can more effectively eliminate or minimize banded structures. For example, diffusion annealing removes segregation, followed by normalizing to optimize the microstructure.
Conclusion
Controlling the formation of banded structures in 23CrNi3Mo rock drilling tool steel presents a systematic engineering challenge that requires a holistic approach, considering smelting, heat treatment, alloy composition adjustments, and subsequent processing stages. By optimizing smelting parameters, precisely controlling cooling rates, adjusting the chemical composition, and implementing effective heat treatment processes, the occurrence of banded structures can be significantly reduced or eliminated. It ensures improved mechanical properties and extended service life for 23CrNi3Mo rock drilling tool steel.
It is worth noting that production conditions can vary for each batch. Therefore, practical operations require flexible adjustments tailored to specific conditions to achieve the best control results. With continuous technological advancements and accumulated experience, we can confidently anticipate more mature and effective control over the formation of banded structures, driving high-quality development in related industries.
