Mold Materials Pages 25 to 34 The Benefits of Alloying Tool Steel Image courtesy of Doerrenberg Edelstahl/Tremblay Tool Steels. By Dr.-Ing. Jens Jonas Wilzer and Bill Ross Plastic injection molding has various chal-lenges beginning with tool steel selection, which requires a look at corrosion and wear resistance, hardness, machinability, polishing ability and dimensional stability. Martensitic stainless steels are high alloyed mold materials to consider for meeting these requirements for large mold plates, molds and inserts and plastic pressure dies. Chromium Steels with a chromium content of at least 12% are generally considered corrosion-resistant. Martensitic tool steels have a high chromium content of 16% and a carbon content between 0.33%-0.38% that provides quenching and tempering during which only tempering car-bides appear in the microstructure, leaving behind enough chromium to ensure corrosion resistance. The resistance to corrosion is based on the formation of surface layers, protecting the metal from further reaction with oxygen. The most effective element is chromium. In combination with oxygen, it forms an oxide layer (chemical coniguration: Cr2O3) on the surface, which shows a dense and amorphous structure as the chromium content increases. The formation of this Cr2O3 surface layer of approximately one nanometer promotes passivation, a process that protects the material against surface corrosion and ensures the func-tionality of the mold. Molybdenum Another form of corrosion is pitting. Processing aggressive polymers or using aggressive detergents can create damage at the passivating layer. This chemical attack leads to a point-like corrosion phenomenon caused by removing the oxygen atoms, allowing chloride ions to move into the passivating layer, and preventing it from re-passivating. Active metal dissolution is the consequence, leading to hole formation underneath the Cr2O3 layer. The dissolved metal reacts with the chloride ions of the electrolyte, forming salts which then react with the H2O of the electrolyte via a hydrolysis process. The hydrolysis process increases the proportion of hydrogen ions in the hole ground, causing a decrease in the pH value that makes the environment acidic. So, oxygen diffusion is Tool steel selection is one of the many challenges of plastic injection molding because mold builders must consider corrosion and wear resistance, hardness, machinability, polishing ability, and dimensional stability. inhibited in the hole ground, which leads to an accelerated metal dissolution. Alloying with molybdenum improves resistance to pitting corrosion, as it contributes to stabilizing the passivating Cr2O3 layer. In addition, molybdenum builds into the struc-ture of the layer and strengthens it by preventing the remov-al of oxygen from the respective layer. Using stainless steels alloyed with molybdenum can also avoid corrosive attacks during mold cleaning, preventing hazardous products from entering the production process. Sulfur Modern steel production sets the sulfur content as low as possible. Sulfur is typically an undesirable by-element as it decreases steel toughness. However, in combination with manganese it forms sulides that improve chipping during machining. Therefore, several pre-hardened, corrosion-resis-tant mold steels are alloyed with sulfur and manganese. Due to the improved machinability, the overall tooling cost can be reduced. FOR MORE INFORMATION Dörrenberg Edelstahl GmbH / Germany / +49 2263 79-0 info@doerrenberg.de / doerrenberg.de/en/home-en Dr.-Ing. Jens Jonas Wilzer, Head of Technical Sales Special Steels Daniel Stibe, Technical Sales Tremblay Tool Steels, LLC / 330-467-4725 info@tremblaytoolsteel.com / tremblaytoolsteel.com Bill Ross, Director of Sales & Marketing 3 34 M MoldMaking g Technology y — — JU JULY 2022