May 2021 Volume 3

MATERIALS

die is to cool it aggressively with a vigorous water application. Applied to a glowing surface, this practice results in a brittle surface layer with greatly diminished resistance to thermal shock. A better approach to managing a die overheating is to use moderate cooling to avoid thermal shock and boost the strength of the now hotter-running die by employing a die with a higher base hardness (temper range). Additional recommended practices can be found in Contending with Die Overheating , FIAMagazine, May 2020, p. 35. From the above discussion, it has hopefully become clear that achieving optimal die performance is a matter of finding the right balance between wear resistance (strength) and fracture toughness for given die design and set of operating conditions. An overview arrangement of the relative merits of commercially available die steel alloys relative to these two key materials properties is presented for Finkl die steel in Figure 1. While changes to the heat-treated condition (temper range) of a die steel can shift the relative balance of wear characteristics and fracture toughness to some degree, larger swings can be expected when changing from one grade to another. Specific melting, forging and heat-treating procedures employed by different die steel manufacturers have some effect on final properties, but this ranking generally applies to all die steel grades having similar chemical compositions regardless of the alloy commercial name. For an understanding of the key chemical differences between the grades in Figure 1, please refer again to Table 1.

may result in a continuous carbide network along grain boundaries. This development, if present, offers a continuous pathway for cracking promoted by the brittle carbide phase. Die alloys with this microstructure exhibit excellent wear resistance due to the high concentration of carbides, but are prone to crack propagation and fast fracture. Despite greater crack sensitivity, properly processed dies from these grades are well suited for presses because of their excellent abrasion resistance, the recovery of some ductility and toughness at the typically higher operating temperatures, and the lower strain-rates of presses that largely avoid extreme stress spikes. Die steel grades to the right in Figure 1 starting with WF-Xtra® are hypoeutectoid and tend to develop ferrite at grain boundaries. This phase, whether intermittent or continuous along grain boundaries is ductile and serves to blunt crack tips thereby impeding crack propagation and making an inherently tougher die steel. These die steel grades perform well in hammers or screw presses where high impact forging loads coupled with lower operating temperatures demand more ductility. Another helpful arrangement of die steel grades relates to their tolerance of fluctuating forging conditions. The hypoeutectoid die steel grades perform well over a wide range of temperatures and stress profiles, and even survive some mismanaged use of coolant/lubricant. The higher alloy, hypereutectoid grades are less forgiving. Irregular and unregulated use of coolant/lubricant and unusual events such as cold stock or misalignment that depart from the focused operating requirements significantly raise the risk of catastrophic cracking. Tight process control, therefore, is more essential for high alloy dies than for the lower alloy dies with their wide tolerance for changes in temperature and stress conditions. These “Personality Types” of forging dies can be arranged in a pyramid as shown in Figure 2.

Figure 1. Finkl die steels arranged according to wear resistance and fracture toughness properties. Aside from simply having a higher carbide density, the higher alloy grades on the left side of Figure 1 tend to have a distribution of carbides in their microstructure that lend them to somewhat increased fracture sensitivity. Describing this carbide distribution characteristic requires the introduction of the metallurgical terms of hypereutectoid and hypoeutectoid . Higher alloy die steel (Shelldie® and left in Figure 1) are hypereutectoid and tend to precipitate carbides along grain boundaries. Ideally the grain-boundary carbide phase will only be intermittent, but certain thermal processing conditions

Figure 2. Finkl Die Steel Service Pyramid. The extent to which either hyper or hypo conditions develop in die steel is influenced by the thermal history of the die block. Smaller die blocks of the higher alloy steels capable of fast quenching rates should exhibit minimal carbide segregation patterns and be appropriate

FIA MAGAZINE | MAY 2021 41