The quality of the smelting process is the most critical factor for drawing dies. If the die steel exhibits segregation or contains a large number of microscopic cracks invisible to the naked eye, the die will start to produce stringing after producing only a small number of products.
Mr. Zhou asked me, “What material is best for drawing dies?” He had asked me a similar question two years ago, and at that time, I gave him three principles for material selection. When he came to ask me this time, I asked him: “What problems are you experiencing with your drawing dies? How thick is the material you’re drawing, and what type of material is it?”
He told me that the drawn surface isn’t smooth, and that the DC53 gets scratched after drawing just a few hundred products. The material used for drawing is 0.8 mm thick 304 stainless steel, with a drawing height of 10 mm.
Stainless steel drawing is most prone to stringing, which places extremely high demands on the mold’s wear resistance and non-stick properties, as well as on the smelting quality of the mold steel. When DC53 is used to draw several hundred products, vertical striations appear on the side walls—this is due to the poor smelting quality of DC53, which contains microscopic cracks invisible to the naked eye.
There are currently two solutions:
If you do not want to switch mold steels, have your current DC53 mold surface-coated. The coating can cover some of the microscopic cracks, thereby alleviating the stringing issue. However, the coating is a one-time application; it cannot be polished and reused. Once it wears out, you’ll need to reapply the coating, and the cost—tens of yuan per kilogram—is quite high.
The second option is to switch the die steel. For drawing 0.8-thick 304 stainless steel, a material with suitable properties that doesn’t require a coating and has proven successful in real-world applications, I recommend using Yuhui SKH51 high-speed steel.
One of our customers told me that when drawing 1 mm thick 304 stainless steel to a height of 10 mm using Yuhui SKH-51 high-speed steel with a hardness of 62–64 HRC, they were able to produce 20,000 parts without any wire drawing defects, whereas other die steels would have developed such defects long ago. He praised the product, saying, “Your SKH-51 works very well for drawing dies.”
Of course, if you want an even better option, you can use cemented carbide (tungsten steel), but its price is beyond the reach of most people and most molds. You should consider this and first assess your mold costs. Alternatively, you could use PM23 powdered high-speed steel. It doesn’t stick to the mold, has very high hardness, and offers reliable wear resistance. It’s much cheaper than tungsten carbide, but it’s still more than twice the price of SKH51 high-speed steel—costing several hundred per kilogram—so you’ll need to evaluate this option carefully.
The most cost-effective solution is to coat your current DC53 steel or use high-quality Cr8Mo2VSi series cold-work die steel with minimal microcracks, achieving a hardness of 60–63 HRC, and then applying a coating. This is a low-cost solution.
Regardless of which die steel you choose, smelting quality is particularly important and should be your primary consideration. Stretching dies place the greatest demands on the smelting quality of the die steel; mold steel with poor smelting quality contains many microcracks, and no matter what you do, it will stick to the die and leave pull marks.
Of course, Mr. Zhou is drawing 0.8 mm thick 304 stainless steel. Since DC53 starts to show signs of wear after just a few hundred draws, it can no longer be used for this application. You could also use SKH51 high-speed steel; if the SKH51 has good metallurgical quality, no coating is needed, allowing the die to be repeatedly polished for reuse and making maintenance more convenient. Yuhui’s SKH51 high-speed steel can stretch 1mm-thick 304 stainless steel to a height of 10mm. Customers have praised it for being able to handle 20,000 stretches without developing stretch marks—other mold steels would have developed them long ago. This is a method you can put into practice right away.
Finally, a word of caution: when stretching stainless steel, you must absolutely avoid using Cr12MoV—it will inevitably develop stretch marks, and even a coating won’t help.
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Wu Dejian’s tool steel, the chief of staff of the user, bought everything he had used.