February 2021 Volume 3

EQUIPMENT & TECHNOLOGY

Many of the other hot work steel variations are chosen for their properties to address very specific application conditions, often related to extreme wear. The addition of chromium, molybdenum, cobalt, and other elements to the various steel formulas can make a

Heat treat experts have developed various methods that are specific to each steel type. For example, some steels must be heat treated in batches of uniformmaterial. It is not a one-size-fits-all process. In general, the initial heating and quenching process will result in steel that is very hard, but also brittle. The rate of quench is a balancing act – rapid quenching will produce a harder and stronger workpiece, but also can result in greater distortion. After hardening, the tempering cycles will decrease brittleness and increase toughness and ductility, while sacrificing some of the hardness. Heat Treat Rules ofThumb Regardless of the tooling being created, there are several parameters that must be considered, including: • Tool steel requires a double draw (triple draw is recommended) • Press tooling should never be lower than 44 Rc • Hammer tooling should run in a range of 36-45 Rc for best die life results Forging Parameters - Effects on Die Life Aside from the properties of the selected diematerial, heat treatment, and surface treatments, there are several forging process parameters that will also affect die life, including: • Billet shape – the greater the transformation required from the original billet shape to the completed forging shape, the greater the die wear. • Billet temperature – in general, mechanical properties of the workpiecemay be enhancedwith relatively lower billet temperatures. However, lower billet temperatures require greater forces to deform, causing higher flow stresses, and a greater risk of plastic deformation. • Work part material – there is variability in flow stresses across the range of work part materials. The greater the flow stress, the greater the die wear and risk of tool deformation and cracking. For

difference in die life and performance. Material and Contact Time

Defined as the amount of time a tool is in contact with the material under pressure, contact time is an important consideration when choosing forge tooling material. The various forging equipment types – presses (hydraulic, mechanical, screw), upsetters, and hammers – will vary in the amount of contact time with the part being forged. For example, in hammer forging, the contact time is very short with a very rapid increase in force during impact, while press forging has much longer contact times with lower impact loading. Therefore, with hammer forging, ductility and impact toughness are critically important, whereas in press forging heat resistance and wear resistance at an elevated temperature may take precedence. Higher contact times will drive the need for additional alloying elements (such as molybdenum, cobalt, chromium, etc.) to enhance the specific desired properties. In all cases, toughness and wear resistance must be optimized relative to one another based on each individual application, as other factors including die size will also lead to the proper material choice. Heat Treatment The heat treatment of tool steel serves to provide the desired properties of the finished tool, including hardness, toughness, ductility, and wear resistance. A typical heat treat process includes preheating, heating to a specified temperature for a precise time period, quenching, and a series of tempering cycles (draws) to achieve the desired properties, as illustrated by the timeline on the next page.

FIA MAGAZINE | FEBRUARY 2021 9