May 2021 Volume 3

MATERIALS

BOHLER Steels for Hot Forging Tools – Productivity Meets Innovation By Johann Mayerhofer

Forging tools are counted among the category of tools for massive forming. The processes of forging are grouped into cold forging, hot forging and semi-hot forging. On the one hand forging allows a high degree of functional integration, large deformations, and large cross- sectional and dimensional changes. On the other hand, the tools are facing heavy forces and high wear depending on the deformed materials i.e. the amount of forged parts made of Nickel- and Titanium-Alloys is increasing due to lightweight technology. Hot forging of such materials is a big challenge due to the difficult deformability. In hot forging a very wide range of applications is covered e.g. close die forging of large parts for aerospace and power generation usually at lower nominal number of parts per die, e.g. in the range of a few thousand parts. Smaller parts mainly for automotive industry are produced on automated multiple stage forging machines expecting a higher tool life in the range of a few hundred-thousand parts per die. During the hot forging process a complex loading situation occurs on forging tools which is defined by the forging procedure, the process parameters, the forged alloy and the design of the forging tool. In particular, the requirements on hot forging tools are steadily increasing. One non-negligible influence lies in the increasing use of alternative sprays for ecological reasons. Another reason for this can be found in the increasing complexity of hot forged parts as a result of permanently realized savings in weight and also in the reduction of machining expenditures. We at voestalpine BOHLER Edelstahl as one of the world leading manufacturer of all kinds of tool steels know these requirements well and offer custom-made hot work tool steel of different quality Mechanical and thermal fatigue, wear and plastic deformation in the highly stressed tool zones are the dominant failure mechanisms observed in hot forging tools. All of these must be considered individually depending on the application. The properties of tool steels selected for hot forging are mainly determined by the failure mechanisms that occur in hot forging. In general, the required material properties are always temperature- and time-dependent. Hot wear resistance of tool steels is mainly affected by the hardness of the hardened and tempered steel matrix and the type and distribution of the existing precipitates and hard phases, i.e. carbides. Resistance against plastic de-formation at the prevalent temperature is directly proportional to the yield strength of the materials. Mechanical fatigue strength simplified is considered mainly as a question of yield strength but also the levels for all kinds of commercial and technical needs. Properties of Tool Steels for Hot Forging

surface condition of the tool and the homogeneity of the materials. Thermal fatigue in practice is a very complex failure mechanism and is multi- directionally connected with various material properties. High thermal stability of the tool materials, indicated by the hot hardness and the resistance against softening, is also of high importance. Any kind of defects and inhomogeneity in the tool steel can be the initial point for the frequently observed heat checking cracks in hot forging applications. Tool Steels for Forging Applications Depending on the requirements related to specific applications various steel types are used for hot forging tools. Low alloyed Ni-Cr-Mo-V steels and 3 and 5 percent Cr-Mo-V steels are used as standard tool steels for hot forging tools. Such kinds of tool steels are also anchored in relevant national and international standards. The most important standards are DIN EN ISO 4957 and ASTM A681. Low alloyed Ni-Cr- Mo-V steels e.g. AISI ~L6 (DIN 1.2714) and 3 and 5 percent Cr-Mo-V steels e.g. AISI H10 (DIN 1.2365) and AISI H11 (DIN 1.2343), H13 (DIN 1.2344) and H13 mod. (DIN 1.2367) usually meet the basic requirements on productivity of common hot forging processes. Within the various standard tool steel grades mentioned above productivity in hot forging application generally increases with increasing alloy content, especially the content of Mo and V, which results in higher thermal stability and higher hot wear resistance of such alloys. Today many standard and highly productive tooling solutions are based upon standard AISI H13 hot work tool steel - Bohler grade W302 ISODISC. However the forging industry related to automotive industry e.g. tooling for bevel gears and ring gears for T-shaped transmissions frequently reports increase of tool life in the range of 50 up to 60 percent when changing the tool steel from H13 simply to standard 1.2367 hot work tool steel - Bohler gradeW303 ISODISC. Tool steel producers did a lot of optimization work on production technology and alloy design over the last years in order to improve the performance of these typical hot work tool steels. For advanced applications special tooling materials offering higher hot wear resistance and higher thermal stability are required and frequently complex alloyed hot work tool steels and matrix high speed steels are used as well. Of course the cavities of the dies usually are nitrided in order to ensure adequate wear resistance. In the case of large forging tools with complex and deep cavities the through- hardenability and therefore the achieved strength and toughness over the whole cross-section of the classical 5 percent

FIA MAGAZINE | MAY 2021 33