Mr. Hong, I’m using 40Cr and SKH55 high-speed steel punch rods for cold extrusion. The rods, with a hardness of 63–65 HRC, tend to break. I’ve had some last for over 600 cycles, while others break after just 5,000 cycles—the results are inconsistent. When I switch to LD die steel with a hardness of 60 HRC, the dies bulge; they expand after just over 1,000 cycles. What die steel would you recommend?
Your punch is a bit long. Since this is deep-draw cold extrusion, it requires high toughness and strength, as well as non-stick properties. Otherwise, the punch is prone to misalignment, which can lead to breakage.
The SKH55 you’re currently using is a high-speed steel with 5.5% cobalt and 6.0% tungsten, achieving a hardness of 63–65 HRC. While this high-hardness, high-alloy steel offers excellent wear resistance, it’s a standard cast grade with poor metallurgical quality. The microstructure exhibits segregation and numerous microcracks, resulting in high hardness but insufficient strength—hence the susceptibility to breakage. The fact that your current production volume is inconsistent—sometimes good, sometimes bad—and that all the parts end up breaking and being scrapped further confirms that the SKH55 is too brittle.
However, using low-hardness LD die steel causes the punch to expand, indicating that your operating conditions require high strength. You need to use high-hardness, high-alloy high-speed steel. Yet the SKH55 high-speed steel you are currently using is prone to breaking, which indicates a material selection error.
There are two solutions:
1) Reduce the total alloy content and use SKH51, a cobalt-free general-purpose high-speed steel with a hardness of 62–64HRC. This will improve toughness and reduce costs.
2) Improve smelting quality to eliminate microstructural segregation and refine the grain structure, thereby enhancing toughness. PM23 powder high-speed steel, with a hardness of HRC 64–66, can be used.
PM23 utilizes advanced powder metallurgy processes to eliminate segregation and microcracks in the microstructure, refine the grain size, and improve density. This enhances both the strength and toughness of the steel—properties not found in conventional cast high-speed steels. Additionally, it offers non-stick properties, resulting in high-surface-finish products after extrusion.
Furthermore, PM23 is cobalt-free and offers better toughness than SKH51 and SKH55 while maintaining high strength and wear resistance. However, its production costs are high, making it more expensive than SKH51 and SKH55.
Mr. Hong, when cold extruding 40Cr, using SKH55 high-speed steel for the punch tends to result in breakage, and the LD (die) is prone to expansion. Considering the comprehensive requirements for strength and toughness in the die, if a lower-cost option is desired, I recommend SKH51 general-purpose high-speed steel; for superior performance, I recommend PM23 powder-metallurgy high-speed steel.
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Wu Dejian’s tool steel, the chief of staff of the user, bought everything he had used.