In rock drilling operations, 23CrNi3Mo steel is widely used due to its excellent mechanical properties and wear resistance. However, a long-standing issue in the industry is the tendency of this steel to develop a banded structure after rolling, leading to uneven mechanical properties. In some cases, this results in cracking or breakage during use, significantly affecting the tool’s service life and operational efficiency. Thus, effectively controlling the formation of banded structures in 23CrNi3Mo steel has become key to improving the quality of rock drilling tools.
The Detrimental Effects of Banded Structure
Banded structure refers to the strip-like structure that forms parallel to the rolling direction during the rolling process of steel due to chemical composition inhomogeneity. In 23CrNi3Mo steel, this typically appears as variations in the distribution of M-A (martensite-austenite) islands within the bainitic matrix and carbon segregation. This structural inhomogeneity results in anisotropic mechanical properties—meaning that strength and toughness vary depending on the direction—compromising the tool’s overall performance and durability.
Specifically, banded structure leads to stress concentration under load, increasing the risk of cracking. During rock drilling, tools must withstand intense impact and friction. The presence of a banded structure significantly reduces fatigue resistance and wear performance, often leading to premature tool failure.
Causes of Banded Structure Formation
The formation of banded structures primarily stems from selective solidification during steel casting and elemental segregation during subsequent thermal processing. In 23CrNi3Mo steel, the main culprit is carbon segregation. As the molten steel solidifies, carbon tends to accumulate between dendrites, forming zones of carbon enrichment and depletion. These zones elongate during hot rolling, forming bands aligned with the rolling direction.
Although alloying elements such as chromium, nickel, and manganese can also contribute to banding due to uneven distribution, in 23CrNi3Mo steel, carbon segregation remains the dominant factor.
Measures to Control Banded Structure Formation
To address the root causes of banded structure in 23CrNi3Mo rock tool steel, the following measures can be implemented:
Optimize Smelting and Casting Processes
- Reduce Segregation: Employ technologies such as electromagnetic stirring and soft reduction during smelting and casting to minimize chemical segregation in billets. These methods help homogenize the molten steel and reduce the concentration of elements between dendrites.
- Control Pouring Temperature: Use low-superheat casting to minimize temperature gradients during solidification, promoting uniform chemical distribution.
Apply Appropriate Heat Treatments
- Diffusion Annealing: Heat the steel to a high temperature (such as above 980°C) and hold it at that temperature for several hours to allow the carbon and alloying elements to diffuse fully, thereby reducing or eliminating segregation. Slow cooling after diffusion annealing results in a more uniform microstructure.
- Normalizing: This process can partially reduce banded structure. By controlling the normalizing temperature and holding time, the microstructure of steel can be made more uniform. However, when the normalizing temperature is low and the holding time is short, banded microstructure may not be eliminated.
- Combined Diffusion Annealing + Normalizing: A combined approach offers greater effectiveness. Diffusion annealing enhances element homogenization while normalizing refines the microstructure and improves uniformity.
Optimize Rolling Processes
- Water Quenching After Rolling: Rapid water cooling immediately after rolling can suppress further diffusion of carbon and other elements, thus limiting band formation. The faster the cooling, the better the control over banding.
- Controlling rolling temperature: Proper control of rolling temperature is also an essential measure for reducing banding. Excessively high rolling temperatures may exacerbate element segregation, while excessively low rolling temperatures may impair the plasticity and deformability of the steel.
Conclusion
Controlling the formation of banded structures in 23CrNi3Mo tool steel is critical for enhancing the overall performance and longevity of rock drilling tools. By optimizing smelting and casting processes, adopting heat treatment processes, and optimizing rolling processes, we can effectively control the formation of banded structures, ensuring that rock drilling tools steel maintains excellent mechanical properties and stability even under complex and variable rock drilling conditions. It ensures that tool steel maintains excellent mechanical properties and stability even under the demanding conditions of rock drilling.
This challenge not only pushes the boundaries of materials science but also plays a significant role in improving manufacturing quality and efficiency. As technology continues to advance, we can expect more effective solutions to the banded structure issue in 23CrNi3Mo steel—delivering more reliable and efficient rock drilling tools for the industry.
