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High-speed steel is a type of alloyed tool steel characterized by a high concentration of alloying elements. After smelting, it forms a significant amount of primary eutectic carbides and secondary carbides, which can account for approximately 18% to 22% of the total composition. These carbides play a critical role in determining the quenching quality and service life of high-speed steel cutting tools. The quenching temperature of high-speed steel is close to its melting point, and after quenching, about 25% to 35% of retained austenite remains in the microstructure. This retained austenite makes the tool more susceptible to cracking and corrosion. This text explores the causes of quenching cracks and corrosion in high-speed steel tools and provides preventive measures.
One major factor contributing to quenching cracks is the presence of metallurgical defects in the raw materials. High-speed steel contains a large amount of hard and brittle carbides, which can lead to stress concentration and crack initiation during heat treatment. If the carbides are unevenly distributed—such as in a strip, mesh, or pile-like structure—they can significantly increase the risk of cracking. Additionally, impurities like sulfur and phosphorus can also contribute to this issue. Poor thermal conductivity and low thermoplasticity make high-speed steel prone to micro-cracks during hot working, which can propagate during quenching, leading to failure.
To prevent these issues, several measures can be taken. Using smaller ingots, refining through electroslag remelting, forging to break up carbide structures, and employing controlled quenching processes can all help reduce the risk of cracking. Ensuring uniform carbide distribution and controlling the quenching temperature are also essential steps in improving the performance of high-speed steel tools.
Another common problem is overheating and over-burning, which results in coarse grain structures and unstable carbide distributions. This can severely weaken the material and increase the likelihood of cracking. Proper control of heating temperatures, regular inspection of equipment, and accurate temperature monitoring are key to preventing such issues.
Naphthalene fracture, a structural defect caused by improper forging or cooling, can also lead to quenching cracks. This defect is characterized by a rough, fish-scale-like fracture surface. To avoid this, careful control of forging temperatures and proper annealing before quenching are recommended.
Mechanical design flaws, such as sharp edges or uneven thickness, can cause stress concentration during quenching. Similarly, cold working processes that introduce internal stresses can compound with quenching stresses, increasing the risk of cracks. Design improvements, rounding sharp corners, and eliminating residual stresses through annealing can mitigate these risks.
Quenching cooling media and their rates play a crucial role in the formation of internal stresses. Choosing the right medium and using step quenching or austempering techniques can help reduce stress and improve tool integrity.
Hydrogen embrittlement is another concern, especially after pickling or plating. Hydrogen atoms can penetrate the metal and cause cracking. Controlling acid concentration, ensuring thorough rinsing, and baking the tool at elevated temperatures can help eliminate hydrogen-related damage.
Cold treatment and grinding can also introduce secondary quenching stresses, leading to cracks. Proper pre-treatment, controlled grinding parameters, and adequate cooling can help prevent this.
Electric discharge machining (EDM) can create microcracks due to rapid cooling and phase transformations. Reducing electrical parameters, removing the affected layer, and stress-relieving treatments can minimize this risk.
Improper tempering, especially rapid cooling after quenching, can result in secondary quenching cracks. Following proper tempering procedures, including slow cooling and multiple tempering cycles, can help stabilize the microstructure and prevent cracking.
Finally, corrosion from salt baths and pickling solutions can weaken the tool. Using high-purity salts, maintaining clean bath conditions, and properly rinsing and sealing the tool after pickling are effective ways to prevent corrosion.
By addressing these factors through careful material selection, precise heat treatment, and proper post-processing, the performance and longevity of high-speed steel tools can be significantly improved.