One user mentioned that when hot-forging stainless steel square boxes using 8418 mold steel, the mold collapses after producing 1,200–1,300 parts. Even after re-profiling the mold surface and performing stress relief treatment on the mold cavity, as well as shot peening the cavity, the mold life remains short. A single mold can be re-profiled a maximum of four times, and it becomes scrap after producing 8,000–9,000 parts—which is already its maximum lifespan. Could you recommend any mold steels suitable for hot-forged stainless steel?
They added that production volumes have increased, and frequent mold repairs can no longer keep up with production demands. They are looking for a mold steel capable of producing 20,000 to 30,000 parts.
Stainless steel hot-forging dies operate at high temperatures, with parts heated to 1200°C. The die steel is prone to annealing; once annealed, the die loses its hardness, leading to die collapse and cracking. The primary issues with stainless steel hot-forging dies are rapid die collapse and cracking, resulting in a short service life.
8418 is a medium-carbon hot-work die steel with a carbon content of 0.38%. While it offers good toughness, its hardness is limited to 50 HRC. Its heat-resistant alloy content—specifically molybdenum (Mo)—is only 2.2%, which is too low to meet the heat resistance requirements of stainless steel hot-forging dies. The fact that it collapses after forging approximately 1,200 parts further confirms this.
Shot peening is intended to pre-apply compressive stress to the mold cavity to reduce the tensile stress that causes cracking. This process is generally used in die-casting molds to prevent premature cracking or erosion, but it is ineffective for hot forging molds. It serves merely as a palliative measure and does not help extend mold life. The fact that the mold has undergone multiple surface roughening and shot peening treatments without any increase in service life further confirms this.
The key to hot forging dies lies in improving heat resistance and thermal wear resistance. Only when high-temperature strength is enhanced can red hardness be guaranteed, thereby preventing die collapse and cracking. The focus should be on increasing the molybdenum content in heat-resistant alloys. Given the harsh operating conditions of stainless steel hot forging dies, I still recommend 8566 die steel for its high heat resistance, high hardness, and high toughness.
8566 die steel offers the same heat resistance as SKH51 high-speed steel, 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. When used for hot forging of stainless steel, it prevents die collapse and cracking, resists wear, and offers a long service life; Other mold steels simply cannot achieve such excellent results.
Mr. Zhang said that for hot forging 304 stainless steel, 8566 deserves a five-star rating. The surface finish of the product is excellent; after finishing, it looks just like a tube. The punch has been ground down six times, yet its service life remains unchanged.
Hot forging dies undergo surface grinding, stress relief, and shot peening, yet they become scrap after producing just 1,200 parts. This lifespan is indeed too short. Putting in so much effort only to have them last such a short time is simply not cost-effective. The time spent making the dies is longer than the time they are actually used—this kind of imbalance makes it impossible to achieve efficiency.
In this situation, there is an even greater need for mold steel with excellent heat resistance to extend the mold’s service life. As long as the mold lasts longer, requiring fewer repairs, and efficiency improves, the investment will pay off no matter how you look at it.
Therefore, regarding the user’s stainless steel hot forging mold—which requires surface reduction and repair after just 1,200 pieces using 8418 steel—I recommend you try 8566 die steel, which offers high heat resistance, high hardness, and high toughness.
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Wu Dejian’s tool steel, the chief of staff of the user, bought everything he had used.