A netizen asked: For hot die forging of stainless steel, what type of die steel offers the best fatigue resistance and heat resistance? During last night’s 296th live stream,
Hot forging of stainless steel involves high operating temperatures; parts must be heated to nearly 1200°C. The high temperature of the parts causes the die to heat up significantly, making it prone to annealing, which can lead to die collapse and cracking.
Stainless steel is also a high-alloy material, so forming it requires extremely high resistance to thermal wear. The die steel must therefore be selected for its high heat resistance, high hardness, and high toughness. Many people assume that die collapse is caused by insufficient hardness in the die steel, so some try to increase the die’s hardness.
A couple of days ago, one of my clients—who specializes in hot forging stainless steel—was using SKH-9 high-speed steel for his punches. However, after producing only a few parts, the punch tips became rounded due to wear. He switched to M42 high-speed steel, but after producing just a few more parts, the punch tips snapped off completely. He asked me what tool steel should be used for punches in hot forging stainless steel.
Both SKH-9 and M42 are tungsten-containing high-speed steels; M42 also contains 8.5% cobalt. They both have excellent red hardness, very high hardness, and excellent wear resistance. However, hot forging of stainless steel requires not only high heat resistance and high red hardness, but also high toughness and high thermal conductivity. This is because the parts are water-cooled immediately after demolding. SKH-9 and M42 high-speed steels have poor thermal conductivity and low toughness, causing them to break upon contact with water.
He is currently using M42; after producing only a few parts, the punch head snapped off—which is sufficient proof that this material is unsuitable.
A die steel with excellent performance in a single aspect alone cannot extend the service life of a hot-forging die; hot-forging dies require a good combination of properties. For example, while tungsten carbide is hard, it is prone to brittle fracture. If your die structure does not place high demands on toughness and your primary concern is thermal wear, then using tungsten carbide as a hot-forging die material for stainless steel can be very durable.
Many people use H13 for hot forging stainless steel, but after just a few dozen or a hundred products, the punch tips become rounded and the workpiece gets swaged. The reason is that H13 has extremely poor heat resistance and poor resistance to thermal wear. For hot forging stainless steel, you should consider using 8566 die steel, which offers high heat resistance, high hardness, and high toughness.
The heat resistance of 8566 is on par with that of SKH51 high-speed steel, with a hardness of 58–60 HRC, while its toughness is four times that of SKH51. 8566 strikes the perfect balance between high hardness, high heat resistance, and high toughness—a combination that other die steels cannot match.
When used in hot forging dies for stainless steel, 8566 is less prone to problems such as die collapse and cracking. In the words of Mr. Zhang, 8566 earns a five-star rating for hot forging 304 stainless steel. The finished products have excellent surface finish, resembling those made from tubing.
In response to this user’s question regarding hot die forging of stainless steel—specifically, which die steel offers the best fatigue resistance and heat resistance—I’d like to share the following: A few days ago, a customer used SKH-9 and M42 high-speed steel for punches in the hot forging of stainless steel, but the punches broke after producing only a few parts. I recommended 8566 die steel, which offers high heat resistance, high hardness, and high toughness. We already have successful case studies with this material, so you can use it with confidence.
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Wu Dejian’s tool steel, the chief of staff of the user, bought everything he had used.