Cold work die steel steel type selection

Generally speaking, the conditions for using cold-working dies can be categorized into four main scenarios:

(1) Small-sized cold-working dies with simple shapes and light loads. Examples include small punches or steel plate shears. These dies can be manufactured using carbon tool steels like T7A, T8A, T10A, or T12A. The benefits of these steels are their excellent machinability, affordable prices, and wide availability. However, they have drawbacks such as low hardenability, poor wear resistance, and significant quenching distortion. Consequently, they are best suited for creating tools or dies that are small, simple, and lightly loaded, where a deep hardened layer and high toughness are required.

(2) Cold-working dies that are large in size, complex in shape, and under light load. Steels commonly used here include low-alloy cutting steels such as 9SiCr, CrWMn, GCr15, and 9Mn2V. The hardened diameter of these steels in oil typically reaches up to 40mm or more. Among these, 9Mn2V steel is a relatively new option that doesn't contain chromium. It can serve as a replacement or partial substitute for chromium-containing steels.

The carbide non-uniformity and quenching cracking tendency of 9Mn2V steel are lower compared to CrWMn steel, and its decarburization tendency is less severe than that of 9SiCr steel. Additionally, it has better hardenability than carbon tool steel. Its cost is only about 30% higher than the latter, making it a steel grade worth promoting.

However, 9Mn2V steel also has some limitations, such as low impact toughness, and instances of cracking have been observed during production and usage. Furthermore, its tempering stability is poor, and the tempering temperature usually does not exceed 180°C. At 200°C tempering, bending strength and toughness begin to drop to low values. 9Mn2V steel can be quenched in quenching media with moderate cooling capacity, such as nitrate salts or hot oils. For molds with strict deformation requirements and low hardness demands, austenitic austempering can be employed.

(3) Cold-working dies that are large in size, complex in shape, and under heavy load. Medium or high alloy steels must be used, including Cr12Mo, Crl2MoV, Cr6WV, Cr4W2MoV, among others, as well as high-speed steels.

In recent years, there has been a growing trend toward using high-speed steel for cold-working molds. However, it's important to note that at this point, the unique red-hardness characteristic of high-speed steel is not being utilized. Instead, its high hardenability and wear resistance are being leveraged. This means that the heat treatment process should differ from that used for cutting tools.

When high-speed steel is used for cold-working molds, low-temperature quenching should be applied to enhance toughness. For instance, the quenching temperature typically used for W18Cr4V steel as a cutting tool is 1280-1290°C. For cold-working molds, however, a lower quenching temperature of 1190°C should be used. Similarly, with W6Mo5Cr4V2 steel, low-temperature quenching significantly improves lifespan, particularly reducing wear rates.

[4] Cold-working dies that endure impact loads and have thin blades. As mentioned earlier, the performance requirements for the first three types of cold-working die steels are based on high wear resistance and high-carbon hyper-eutectoid steel, or even eutectic steel. For certain cold-working molds, such as trimming floors or punching dies, the blades are thin. During use, they experience impact loads and should prioritize high impact toughness. To address this contradiction, several measures can be taken. First, reduce the carbon content to avoid toughness issues caused by primary and secondary carbides. Second, add alloying elements like silicon (Si) and chromium (Cr) to improve tempering stability and allow tempering at higher temperatures (240-270°C), which helps relieve quenching stress while maintaining hardness. Third, add elements like tungsten (W) to form refractory carbides, refining grain structure and enhancing toughness. Common high-toughness cold-working die steels include 6SiCr, 4CrW2Si, and 5CrW2Si.