August 2022 Volume 4

MATERIALS

Innovative Tool Steels for Forging Dies By Philipp Görts, M.Sc

Hot Work Tool Steels for Forging Dies The classic heat-treatable tool steels for forging tools are defined in relevant national and international standards such as DIN EN ISO 4957. The low-alloyed NiCrMoV-steels, as well as the three-percent and five-percent CrMoV steels usually meet the basic requirements very well. With the steel USD, for example, we offer a well made international standard steel based on H13/1.2344. If a higher heat resistance is required, we recommend the standard steel 1.2367 under the brand name RPU. As demands on tool steels continue to rise, standard steels can no longer meet the requirements for higher hardness or higher wear resistance. Based on the typical five-percent CrMoV steels, our company has in recent years developed various premium steels. Some of these will be briefly presented in the following sections. The premium hot-work tool steel Cr7V-L has established itself on the market and offers significant performance increases relative to standard steels. In comparison to H11/1.2343 the chromium content is increased for higher wear resistance. For higher hardness the carbon content is increased as well. This chemical composition allows Cr7V-L to concurrently provide greater hardness and wear resistance. Q10 is a premium steel based on the five-percent CrMoV steels. Its analysis and production have been adapted so that the steel achieves higher toughness than standard grades, while maintaining high temperature strength. Q10 has proven itself for forging dies that are prone to edge cracking. The premium steel CS1 offers excellent toughness with outstanding wear resistance and high hardness. Thanks to its higher carbon content, it has improved resistance to tempering and a classic quenched and tempered steel structure with a high proportion of carbides. This results in higher hot hardness and tempering resistance. Due to its manufacturing process the steel achieves high homogeneity and good isotropy of its mechanical properties. The steel is suitable for many different high demanding forging applications. Especially suitable for forging aluminum or titanium is HP1. When cracks are one of the main reasons for failure, this steel with its high toughness can improve die service life. Due to its adapted alloy concept and manufacturing process, outstanding toughness properties are obtained without having to forgo good high temperature strength.

The metalworking industry is undergoing major change and is under increasing pressure from global competition and the mobility shift that is already taking place. This particularly affects the forging industry, which has to cope with competing manufacturing processes, the demands for more complex workpiece shapes with tight tolerances, and with the elimination of some automotive forgings because of the trend toward fully electric vehicles (EVs). As a result, the economic pressure to continuously reduce manufacturing costs per component is growing. Tooling and setup costs represent a high proportion of production costs, as the share of tool costs alone can amount up to one fifth of total costs. Consequently, increasing tool performance by increasing service life represents a significant starting point for cost optimization. The service life of a forging die is a complex variable and is subjected to many different factors. The quality of the selected tool steel and its properties are particularly relevant in this context. The choice of tool steels for forging dies depends strongly on the application. The decisive factor is the right compromise between toughness, wear In order to form a workpiece, forging dies are subjected to high forces at high temperatures. To withstand these forces, hot-work tool steels must have high-temperature strength. To protect the dies from cracking and die breakage the tool steels require high hot toughness. The thermal load due to the forging temperature and the die base temperature also require a high hardening temperature. Additionally, a high tempering resistance is required of the hot work tool steels to withstand elevated process temperatures and the thermally-induced transformation of the microstructure to counteract a loss of hardness. If the surface of the die becomes too soft, plastic deformation may occur, thereby throwing off the tolerance of the forgings. The constant cycle between contact with the hot billet and the subsequent cooling of the tool surface with lubricant causes strong temperature changes that lead to high thermal shock stress. This can induce the formation of network-like cracks and further surface damage. A hot-work tool steel with a high thermal shock resistance, i.e. a high resistance to thermal fatigue, can withstand this stress for a longer time(1). Thermal shock resistance is a complex parameter that is mainly influenced by the hardness, strength and toughness of the steel. At the same time, the homogeneity of the material also plays an important role. resistance and resistance against cyclical thermal stress. Loads and Requirements on Forging Dies

FIA MAGAZINE | AUGUST 2022 34