Higher hardness isn’t always better for hot forging dies; in some cases, excessive hardness can actually accelerate cracking. A netizen used “Fake 8566”—a low-grade, deep-pit product for hot forging 45 steel costing 160 yuan per kilogram—and after just a few hundred strikes, the die cracked at the opening. They asked me: What die steel should be used for hot forging dies to prevent cracking?（8566 die steel）
Using high-hardness 8566 die steel to hot-forge deep-dish products made of 45 steel is a waste of resources—it’s the wrong choice of die steel.
8566 die steel has the same heat resistance as high-speed steel SKH51, with a hardness of 58–60 HRC and four times the toughness of SKH51. 8566 die steel strikes the perfect balance between high heat resistance, high hardness, and high toughness. Although 8566 die steel performs very well, it is not necessarily the best choice for some deep-cavity hot-forging dies.
For example, when hot-forming deep-cavity products made of 45 steel, the heat-resistant steel 8433—which offers better toughness at a lower price—is actually a better choice. Since 45 steel is heated to relatively low temperatures and contains no alloying elements, the stress on the die during hot forming is not particularly high. Instead, the requirement for heat resistance is extremely high: only if the die’s hardness is not lost through annealing can its red hardness be guaranteed, thereby preventing die collapse and cracking.
Heat resistance is the foundation for preventing die collapse in hot-working dies; high toughness is the next priority. Only with sufficient toughness can the “mouth” area avoid thermal fatigue cracking during rapid cooling and heating cycles. For deep-drawn products made from hot-forged 45 steel, the heat-resistant steel 8433 is a better choice.
The heat resistance and stability of the heat-resistant steel 8433 are 2–3 times those of H13 mold steel, with a hardness of 50–54 HRC. It will not anneal out over its service life, so hardness will not decrease, and red hardness is guaranteed. It is especially worth emphasizing that even if the die is repaired to the point of scrapping, the hardness on the back surface will not decrease.
We have a customer who produces deep-drawn parts made of hot-forged 45 steel. Using heat-resistant steel 8433, the die has maintained its service life even after four surface-hardening treatments, with each treatment lasting over 10,000 cycles. Previously, when using H13 , the die would collapse and crack after just 3,000 products.
In contrast, this user is using “Fake 8566” at 160 yuan per kilogram—which is very expensive and has extremely high hardness—but when hot-forging the same 45 steel products, the mold cracked at the cutting edge after producing only a few hundred pieces. The reason is that the fourth-grade 8566 die steel he purchased had extremely poor toughness, which led to thermal fatigue cracking during rapid cooling and heating cycles. Eventually, the die was torn apart, resulting in deep cracks. This is a classic case of using the wrong material—a costly mistake.
For deep-cavity hot-forged 45 steel products, using heat-resistant steel 8433 —which offers superior heat resistance and greater toughness—actually results in a longer mold life. Our client has successfully verified this: the service life is three times that of H13 , and even after four mold surface regrinds, the service life remains unchanged, with each mold exceeding 10,000 cycles.
The service life per mold is three times that of H13 , and even after four surface regrinds, the service life remains unchanged—meaning one heat-resistant steel 8433 is equivalent to four H13 . When you do the math, using heat-resistant steel 8433 for hot-forged 45 steel parts actually lowers mold costs compared to H13 , increases efficiency by reducing the frequency of mold replacements, and reduces worker fatigue. When hot-work dies suffer from collapse or cracking, using heat-resistant steel 8433 is even more satisfying than eating braised pork.
If you’re also dealing with the challenges of hot-forging 45 steel, don’t waste money on 8566 die steel —and definitely avoid “Fake 8566”. Instead, use heat-resistant steel 8433, which offers longer service life at a lower cost, saving you money and boosting efficiency.
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Wu Dejian’s tool steel, the chief of staff of the user, bought everything he had used.