Heavy Forgings

Heavy forgings are steel products of extraordinary dimensions and weights of up to around 600 tons. Today ever more forged components of remarkable size are demanded by sectors such as power generation, offshore engineering, ship construction, petrochemical production, mining & minerals processing, mechanical engineering, aerospace construction, etc.

The process of open die (free) forging is carried out in large hydraulic presses or by mechanized hammers. In open die forging the workpiece is usually larger than the forming tool. Accordingly, the deformation is applied to a smaller portions of the workpiece requiring numerous individual actions to achieve the final shape in a sequential manner. The finished forging part is subsequently machined to final dimensions and usually heat treated for adjusting the desired microstructure and properties.

Various standardized guidelines exist for designing large-sized forged components. In addition, the highly specialized manufacturers often build on own know how and experience. The principal properties when defining the steel alloy concept at the engineering design stage are strength and toughness. A key aspect is therefore the through-hardenability of these massive components. The aim is to achieve a uniform martensitic/bainitic heat-treated microstructure with homogeneous grain size distribution throughout the part cross-section. Plain carbon steels are therefore only applicable to simple applications of smaller dimensions. With increasing component size defined by the relevant cross-sectional diameter, molybdenum alloying is typically used in combination with other alloys such as Cr, Ni, and V.

Several international standards define the alloy composition of steels for heavy forgings. However, forging steel producers often deviate from the standard by producing proprietary alloys that have better performance than the related standard grade. Typically, these proprietary alloy involve an increased molybdenum content promoting through hardenability, strength and toughness. For instance, increasing the molybdenum content in grade 18CrNiMo7-6 from 0.25 to 0.50 mass-% resulted in a substantial performance improvement under typical operating conditions of main gear in wind turbines (cross reference: https://www.mdpi.com/2075-4701/7/10/415).