May 2021 Volume 3

MATERIALS

Cr-Mo-V steels are limited and higher alloyed tool steels will be required. This problem however operates against an economical tooling strategy. Figure 1 shows the Bohler product portfolio on hot work tool steels for drop- and press-forging applications accompanied with a description of the individual properties profile of the various Bohler hot work tool steels. Based on the universal properties of the 5 percent Cr-Mo-V alloy design a variety of branded, optimized tool steels had been developed and introduced by Bohler to the market in recent years.

Figure 2: Comparison of notched impact energy for different tool steels with different chemical composition and hardness levels Successful Applications in Hot Forging Figures 3 and 4 show the tool life figures for three different forging tool materials used for the second step in a 3-step manual drop forging equipment. Especially the upper dies made of standard hot work tool steel AISI ~ L6 (1.2714) at 40 to 44 HRC suffered from hot wear, whereas the lower dies showed early cracking due to stress concentrations related to the part design. First remarkable improvements could be achieved by changing to AISI H10 (1.2365) hot work tool steel. In order to address the different failure modes the hardness for the upper die was set at 48 to 50 HRC. The lower die was heat treated to 44 to 46 HRC in order to promote toughness and increase safety against fracture.

Figure 1: BOHLER product portfolio for hot work toolsteels in drop- and press-forging applications For example, with the twodifferent alloying strategies of BohlerW360 ISOBLOCandW350 ISOBLOCpremiumhot work tool steels a very distinguished, need-based property profile could be achieved. Depending on the required properties standard Bohler hot work tool steels are manufactured according to the traditional conventional melting route in the electric arc furnace with additional ladle metallurgy. Additionally for innovative, optimized tool steels protective gas electroslag remelting (PESR) or even vacuum arc remelting (VAR) is applied additionally resulting in homogeneous material quality with a high degree of purity. Mechanical properties e.g. isotropy and toughness are improved accordingly. As an example Bohler W360 ISOBLOC premium hot work tool steel is an innovative PESR-quality 5 percent Cr-Mo-V steel with higher carbon and molybdenum content and thus improved tempering resistance and a higher content of carbide formers in the classical heat- treated steel matrix. This modification results in a clear advantage regarding the hot hardness and softening behavior in the usual range of operating temperatures of hot working tools but keeping the toughness at extraordinary high level. Figure 2 confirms the high toughness of Bohler W360 ISOBLOC at 500°C and at an initial hardness of 57 HRC. Toughness is comparable to the classical hot work tool steels but all at lower initial hardness of 51 HRC. Based on these proved specific properties profile of the high performance hot work tool steel Bohler W360 ISOBLOC a competitive edge regarding tool life and productivity in the entire process chain of hot forging can be expected.

Figure 3: Tooling configuration, tools and forged parts of a 3-step drop forging equipment

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FIA MAGAZINE | MAY 2021