May 2021 Volume 3

MATERIALS

Selecting the Right Die Material By Nick Cerwin and Benjamin Ritchey

A natural question for any forging company to ask is whether advancing technical knowledge will create a new die steel capable of dramatically improving die performance – one that overcomes the failure modes of accelerated wear and catastrophic cracking. Die steel suppliers recognize that poor die life threatens their own as well as their customers’ livelihoods as forgings face increased competition from advances in additive manufacturing, casting technology, machining efficiencies, and alternative materials. Improved die life is an essential component of maintaining, or gaining, an economic advantage for forged products in a competitive manufacturing environment. While new die materials may appear, the reality is that economically produced iron-based alloys already exist that can be heat-treated to a wide range of properties capable of making excellent, long- lasting dies for most forging conditions. In cases of unsatisfactory performance, the first action is to determine whether there were any serious departures from normal operations that may have contributed to the failure. If not, questions may arise about options available for changing die steel. Actually, both of these courses should be pursued together. It is essential to strategically match the demands of a particular forging operation with a die steel heat-treated to meet those specific demands. Obtaining technical information from a die steel supplier, coupled with a knowledge of one’s own forging history and failure modes is an effective route to optimizing die life. There are undoubtedly some forging operations that require exotic die materials, but they are largely outside the mainstream. Nickel- based superalloys such as Inconel® and Hastelloy® and Waspaloy® offer high strength with outstanding heat resistance and successfully serve as dies or inserts in specialized forging operations with extreme operating conditions. However, the cost of such materials is an order-of-magnitude over conventional die steels. These materials do not usually represent practical broad-based solutions for the average forging operation. The greater industry forges familiar ferrous alloys along with aluminum and copper-based alloys. Despite a long history of forging these familiar metals, there are innumerable operational details that introduce tripping points on the way to achieving peak die performance. Equipment, operating practices and engineering demands on the dies differ greatly between forging operations. Thoughtful matching of a die material to the demands of a forging operation is sometimes lost to the notion that a more expensive, more highly alloyed die steel is an assured route to improving die performance. This is a misguided notion. A better approach is to develop a thorough understanding of one’s own forging process and learn about available die materials that feature particular characteristics compatiblewith one’s unique application and practices.

Forging dies experience harsh conditions: high-pressure abrasion, high service temperatures, and severe thermal and mechanical shock. Each of these factors requires different properties from the die material. For example, reducing wear can be addressed by increasing die hardness, and reducing fracture incidents can be addressed by lowering die hardness to improve ductility and fracture toughness. Fortunately, for most die steel grades changing hardness is a relatively easy means to alter the basic properties of a die. Simply raising or lowering the tempering temperature following the quenching operation lowers or increases the final hardness, respectively. Commercially available temper ranges are noted in sales or marketing literature with hardness increments of about 4 HRC. For example, the Finkl T2 temper range corresponds to 38- 42HRC.This approach can provide significant adjustments to basic die block properties at little to no cost – just specify a change on future orders and evaluate the effect on die performance. In some instances, it may eventually become evident that simply changing the hardness of the die block is not enough to resolve a failure mode. Reexamining forging parameters and reviewing die design are essential steps, but at some point, the data may suggest that a change in die steel is the logical path forward. Changing grades is a major step for any forging operation. Since the underlying properties and performance characteristics of a die can differ significantly between grades, process adjustments may be required. For example, a higher alloy die steel will usually exhibit a higher Ductile-to-Brittle Transition Temperature (DBTT). The DBTT is an important critical service temperature of each die steel grade and hardness level. With an operating temperature above the DBTT, the die is ductile and fracture-resistant, while operating below the DBTT the die is brittle and fracture-prone. This is the basis for preheating and maintaining a certain minimum die operating temperature to avoid crack sensitivity. For a more detailed explanation of DBTT, please see The Effect of Ductile-to-Brittle Transition Temperature on Die Life , FIAMagazine, August 2019, p.26. The main advantages of increasing alloy content in die steel are: • Increased hardenability for improved heat-treating response. • Greater carbide density for improved wear resistance and strength. • Improved resistance to softening at service temperature. • Higher maximum die operating temperature. The steel property known as hardenability is a measure of the capability of a given steel composition to harden at depth with a conventional quench and temper heat treatment. Hardenability is commonly expressed as DI, a calculated value based on chemical composition having units of inches. Increasing the DI through increased alloy content improves the depth of martensite

FIA MAGAZINE | MAY 2021 39