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Cold-working dies are essential components in various industrial applications, and their optimal performance depends heavily on the material selection. These materials must meet specific conditions based on the size, complexity, and load of the dies. Let’s explore the different categories in detail.
For small-sized dies with simple shapes and minimal loads, materials like carbon tool steel, such as T7A, T8A, T10A, and T12A, are commonly used. These steels offer excellent machinability, affordability, and accessibility. However, they have limitations such as poor hardenability, wear resistance, and susceptibility to quenching distortion. Consequently, they are best suited for creating small, simple tools that require deep hardening layers while maintaining high toughness.
Moving to larger dies with intricate designs and lighter loads, low-alloy cutting steels like 9SiCr, CrWMn, GCr15, and 9Mn2V come into play. These materials typically achieve a hardened diameter of up to 40mm or more when quenched in oil. Notably, 9Mn2V steel has gained popularity as it doesn’t contain chromium, allowing it to replace or partly substitute chromium-containing alloys. This steel boasts better carbide uniformity, reduced quenching cracks, and less decarburization compared to CrWMn and 9SiCr. Additionally, its hardenability exceeds that of carbon tool steel, making it a cost-effective choice. Despite its benefits, 9Mn2V steel suffers from low impact toughness and cracking issues during production. Furthermore, its tempering stability is weak, with a maximum tempering temperature of 180°C, resulting in decreased bending strength and toughness beyond 200°C. Quenching in media like nitrate salts or hot oil is recommended, and austenitic austempering can be employed for dies with strict deformation constraints and low hardness needs.
When dealing with large dies featuring complex geometries and heavy loads, medium to high alloy steels such as Cr12Mo, Cr12MoV, Cr6WV, Cr4W2MoV, and even high-speed steels become necessary. High-speed steel usage is on the rise for cold-working dies, though its unique red-hardness property isn’t leveraged here; instead, its superior hardenability and wear resistance are utilized. Consequently, heat treatment processes need adjustment. For instance, using low-temperature quenching enhances toughness. Cutting tools often undergo quenching at 1280-1290°C, whereas cold-working dies benefit from a lower temperature of 1190°C. Similarly, W6Mo5Cr4V2 steel demonstrates significant lifespan improvements and reduced wear rates when subjected to low-temperature quenching.
Lastly, certain cold-working dies exposed to impact loads and possessing thin blades necessitate high-impact toughness over extreme wear resistance. To address this, measures include reducing carbon content, incorporating sub-eutectoid steel to prevent carbide-related brittleness, and adding elements like silicon and chromium to enhance tempering stability and temperature up to 240-270°C. Refractory carbides formed by adding tungsten refine grain structure and improve toughness. Popular high-toughness options include 6SiCr, 4CrW2Si, and 5CrW2Si.
In summary, selecting the right material for cold-working dies involves balancing multiple factors—size, shape, load, and application requirements—all of which influence durability, performance, and longevity.