May 2021 Volume 3

MATERIALS

for both hammers and press inserts. Larger hypereutectoid dies, however, are more likely to have carbide segregation at grain boundaries. Dies with this condition will still provide good wear resistance with good fracture toughness for hydraulic presses, or in forging operations with well-designed and carefully controlled process variables. Good process control, therefore, will create the basis for successful transition to higher alloy dies and the potential for significantly improved die life. Considering the properties of die steel discussed herein and the known characteristics of various types of forging equipment, it is possible to propose categories of die steel to be used with certain types of hammers or presses. Reasonable matches are shown in Figure 3. This arrangement takes into account the strain rate of the forging machine, which determines the level of mechanical shock to be absorbed by the die, and the likely operating temperature expected for the dies based on characteristic dwell times of the type of forging machine. This scheme may be used as a guide to a first application without a known performance history, or as a guide to suggest possible options to improve die life with a better match.

feeding technology permits tightly controlled post-vacuum calcium injection for sulfide and oxide morphology control. This process alters the remaining sulfides and oxides to reduce elongation of sulfides during the hot-working process and encapsulates oxides into complex oxy-sulfides to further improve ductility and toughness. The net effect of these background metallurgical improvements has undoubtedly contributed to better die life. ESR or VAR premium quality steel is not widely used for dies in the forging industry, but they are common in the aluminum die casting industry to endure the extreme thermal shock of that manufacturing process. The improved properties from double- melted steel are well established and may offer improvement paths for the forging industry.Their cost can be nearly double that of ingot cast steel but this may be justifiable as a solution for chronic cracking problems. The advantage of double melting is especially notable for high alloy hypereutectoid steel. The remelting technique greatly reduces detrimental segregation of alloys to produce a more uniform microstructure with a high density of fine, well-distributed carbides that promote good ductility and toughness. Future advances for die steel may be reasonably expected to be from advancements in surface engineering technology. Current methods of nitriding already provide solutions in many high wear applications, and these processes continue to be refined and improved. Additive manufacturing with precisely deposited specialty alloys, such as superalloys, over portions or all of a die surface could conceivably be engineered to endure localized high temperature demands while keeping the overall die cost low. This technology bears close watching for viable forging solutions. While continued technological improvements and major breakthroughs can be expected in the future, matching particular forging operations with appropriate die steel characteristics can realize gains right now. One cannot understate the importance of this approach. As an industry partner, Finkl Steel works closely with forgers to educate, develop, and troubleshoot die materials to consistently produce quality forgings. ■

Figure 3. Finkl die steel selection matrix. Temper hardness is presented in HRC. Returning to the question of technological advances, in one sense, nearly all forging dies currently offered are “new” even though the names of the die steel grades may have remained the same. Advances in melting technology over the past few decades have included desulfurization and deoxidation techniques used with ladle metallurgy and vacuum degassing. These processes currently produce sulfur levels under 0.005 %S on average and are capable of producing 0.001 %S. Compare this to an average of about 0.015%S just a few decades ago. Oxide levels in single-melt quality steels continue to approach premium double-melted steel (ESR: Electro-Slag-Remelted and VAR: Vacuum Arc Remelted) through advancements in shrouding and degassing technology. Together, the currently lower oxide and sulfide levels in die steel result in micro- cleanliness ratings under 1.0 per ASTM E45. Additionally, wire

Nick Cerwin Benjamin Ritchey Nick Cerwin is the retired Director of Technical Services at Finkl Steel. Benjamin Ritchey is the Technical Director for Finkl Steel and can be reached at BRitchey@Finkl.com.

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FIA MAGAZINE | MAY 2021