When Should High-Hardness Die Steel Be Selected for Hot Forging Dies?
The primary failure modes of hot forging dies are die collapse and cracking. This is because die temperatures during the hot forging process approach 600°C. At such high temperatures, many die steels with poor heat resistance undergo annealing, leading to die collapse and cracking.
To address the issue of die collapse in hot forging dies, the first priority is to select die steel with good heat resistance, followed by good toughness, and only then should high hardness be considered. Good heat resistance ensures that the die’s hardness is not lost through annealing and does not decrease—this is the foundation for solving die collapse in hot forging. Other properties like toughness and hardness come second; heat resistance comes first. So, under what operating conditions are high-hardness die steels suitable for hot forging dies?
Today’s video features a motor component mounted on a robotic arm. The part is only 13 mm tall, with a shallow cavity of just 6 mm deep between the upper and lower dies. The product has extremely stringent requirements for both appearance and dimensional accuracy. H13 dies failed after producing just over 2,000 parts. For small hot-forging dies with such shallow cavities, high-hardness die steel is essential.
H13 has a tempering temperature of only 530°C. This temperature is too low, resulting in poor tempering resistance. When operating at high temperatures of 600°C, H13 will quickly anneal, causing its hardness to drop rapidly and leading to die collapse and cracking. For small, shallow-cavity hot forging dies like this, if you want a long die life and have the budget, you should go straight for 8566 die steel.
The heat resistance and stability of 8566 are on par with those of high-speed steel SKH-51, with a hardness of 58–60 HRC and four times the toughness of SKH-51. 8566 strikes the perfect balance between high heat resistance, high hardness, and high toughness. Even when operating at temperatures approaching 600°C, its hardness remains virtually unchanged. This stability ensures red hardness, preventing die collapse; combined with its high hardness and excellent resistance to thermal wear, it produces parts with a superior surface finish, thereby extending the die’s service life.
For a small mold the size of a palm, the cost of the mold material is negligible. We can afford to use a slightly more expensive mold steel. The key is to avoid frequent mold repairs. The more products you produce, the higher your efficiency, and the greater your profits will naturally be.
If you use H13 to produce 2,000–3,000 products, you’ll either experience mold collapse or excessive wear, forcing you to lower the surface height and repair the mold. This back-and-forth process is unprofitable. A mold that lacks production efficiency is costly, no matter how cheap it is.
Take this client of ours, for example. They’ve learned the hard way before. Initially, they used H13, HD, and HM5—all available on the market—but the hardness could only reach 50–54 HRC. The thermal wear resistance was insufficient, so they couldn’t ramp up production. The dimensional tolerances and surface finish of the finished products didn’t meet customer requirements, and constant mold repairs were a major hassle. However, now that they’re using 8566, this problem has been solved. In their own words, the molds hardly need any maintenance. With fewer repairs, my efficiency has improved. With higher output, the cost per mold has gone down.
For shallow-cavity hot-forging dies, where toughness isn’t a major concern but thermal wear resistance is critical, you should consider using high-hardness die steel. For such applications, you should opt for 8566 die steel, which offers both high heat resistance and high hardness.
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Wu Dejian’s tool steel, the chief of staff of the user, bought everything he had used.