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High-speed steel is a type of alloy tool steel characterized by a high content of alloying elements. After smelting, it forms a significant amount of primary eutectic carbides and secondary carbides, which can account for 18% to 22% of the total composition. These carbides play a crucial 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 even after quenching, 25% to 35% of retained austenite remains in the microstructure. This residual austenite makes the tool susceptible to cracking and corrosion. The following analysis explores the causes of quenching cracks and corrosion in high-speed steel tools and provides effective preventive measures.
1. **Metallurgical Defects in Raw Materials**
High-speed steel contains a large amount of hard and brittle carbides, which act as brittle phases within the matrix. These primary eutectic carbides are often distributed along the grain boundaries or in a dendritic structure. Even after rolling, some segregation may remain, leading to uneven distribution of carbides—such as strip-like, mesh-like, or clustered patterns along the rolling direction. As the size of the raw material increases, the non-uniformity of carbides becomes more pronounced. Conventional heat treatment struggles to eliminate these defects, resulting in stress concentration and potential quenching cracks. Impurities like sulfur and phosphorus, if present in excess, also contribute to quenching issues. Additionally, poor thermal conductivity and plasticity in high-speed steel make it prone to micro-cracks during hot working, which can propagate during quenching and lead to material failure.
To mitigate these issues, several preventive measures are recommended: (1) Use small ingots for rolling to reduce defects; (2) Utilize electroslag remelting (ESR) ingots with high purity and fine microstructure; (3) Forage and refine materials to reduce eutectic carbide non-uniformity below level 3; (4) Implement high-temperature step quenching and tempering processes to minimize crack formation.
2. **Overheating and Overburning Structures**
Overheated or over-burned structures in high-speed steel are marked by coarse grains and abnormal carbide distribution, such as adhesion, horns, or continuous networks along grain boundaries. In extreme cases, melting occurs, forming a burnt structure that significantly reduces intergranular bonding and toughness. Common causes include excessive quenching temperatures, inaccurate temperature control, or improper placement of tools near electrodes. To prevent this, strict control of raw material quality, regular inspection of equipment, and precise temperature management are essential.
3. **Naphthalene Fracture**
A naphthalene fracture is a common defect in high-speed steel, appearing as a fish-scale-like surface with a rough texture and coarse grains. It typically results from improper forging or deformation at critical strain levels. Preventive steps include optimizing forging temperatures, ensuring proper annealing before quenching, and avoiding critical deformation ranges.
4. **Stress Concentration from Mechanical Design and Cold Working**
Uneven thickness, sharp corners, or grooves on the tool can cause stress concentration during quenching, leading to cracks. Internal stresses from cold working, especially grinding, must be eliminated through annealing. Proper design and finishing techniques help reduce these risks.
5. **Quenching Internal Stress and Cooling Medium**
During quenching, internal stresses arise from volume changes, temperature differences, and structural inconsistencies. Choosing an appropriate cooling medium and using controlled quenching methods like step quenching or austempering can reduce these stresses and prevent cracking.
6. **Hydrogen Embrittlement**
Hydrogen embrittlement occurs when hydrogen atoms penetrate the steel during pickling or electroplating, causing internal pressure and cracks. Controlling acid concentration, neutralizing after pickling, and baking the tool at elevated temperatures can mitigate this issue.
7. **Cold Treatment Cracking**
Cold treatment at low temperatures can induce additional stress due to the transformation of retained austenite into martensite. Pre-heating and proper tempering help relieve this stress and avoid cracking.
8. **Grinding Cracks**
Excessive grinding speed, poor cooling, or residual austenite can lead to shallow surface cracks. Reducing speed, using proper coolants, and controlling quenching parameters are effective solutions.
9. **EDM Microcracks**
Electric discharge machining (EDM) can introduce microcracks due to rapid cooling of molten metal. Ensuring pre-treatment, controlling electrical parameters, and removing the metamorphic layer post-processing are key preventive strategies.
10. **Improper Tempering and Secondary Quenching Cracks**
Incorrect tempering procedures, such as rapid cooling, can lead to secondary quenching cracks. Proper tempering techniques, including slow cooling and multiple high-temperature treatments, are essential to avoid this.
11. **Tool Corrosion**
Corrosion often occurs during salt bath quenching and pickling. Using protective atmospheres, maintaining clean salt baths, and thorough rinsing after pickling are crucial to preventing corrosion.
By implementing these measures, the quality, durability, and performance of high-speed steel tools can be significantly improved.