August 2020 Volume 2

EQUIPMENT & TECHNOLOGY MATERIALS

Alloying Die Steels for Strength, Toughness and Temper Resistance By Nick Cerwin and Benjamin Ritchey, Finkl Steel

Alloying elements in steel serve multiple purposes, but most often their primary role is to develop higher strength levels than what is possible in plain carbon steel. Ultimately carbon content determines the maximum achievable strength/hardness, but to reach this full hardening potential, a heat treating process is required that involves a very fast cooling rate from an elevated temperature (a process known as quenching). Unfortunately, during the quenching of large cross-section steel products such as die blocks, the cooling rate at internal locations is considerably slower than that occurring on the surface. This area falls below the critical cooling rate required to fully harden the steel. An average-sized die block made of an unalloyed steel might harden to 45 to 50 Rockwell C (HRC) on the surface, but it would fall to about 20 HRC just a half-inch into the block even if the steel has high carbon content and receives the best quench possible. The overall tensile strength of such a piece would be about 100,000 psi, which is well below the usual 180,000 to 250,000 psi strength required of most dies for service. Adding alloys to steel lowers the critical cooling rate required to reach full hardening potential. The effect on the overall reduction in critical quenching rate depends upon the quantity and specific alloy added. An overview of this effect for the key die steel alloying elements chromium, molybdenum, vanadium and nickel is shown in Figure 1. The graph shows the relative contribution of each alloy as a function of the amount added in terms of a variable called DI. The DI is a calculated number interpreted as the diameter of a bar, in inches, of that chemistry that will through-harden with an ideal quench. The values of the DI contribution for each alloy shown in the graph were generated using a method originally developed by researcher M. A. Grossmann in the 1930s. The procedure to calculate a DI value for a particular chemistry is available in specification A255 from the American Society for Testing and Materials (ASTM). From a die steel user viewpoint, the calculated DI of a die steel grade may be considered approximately as the thickness of a die block (in inches) that will harden through with a good quench. This depth of hardening capability of the steel is a property called hardenability. From the chart it is clear that increasing the content of the key alloys increases the hardenability of the steel, which means that larger pieces can achieve maximum strength through the full cross section. For a practical comparison of the role of different alloying elements, the calculated DI for SAE 1055 (a plain carbon steel), SAE 4140 (a low-alloy steel) and several die steels are shown in Table 1. This table highlights how small increases in multiple alloys can profoundly increase a die steel’s ability to fully harden through thick cross sections.

Quenching a die block is only half of the heat-treating process. The quench is always followed by a tempering process involving at least one thermal cycle, and sometimes two or more. Alloys in the steel that are present as dispersed atoms in themselves offer some hardening effect due to a mechanism known as solid solution strengthening. However, even greater strengthening occurs when certain alloys react with carbon to form carbides. Once formed, the dispersed carbides begin to coalesce to reach a certain critical size that maximizes their impact on hardness. Coalescence of the carbides beyond this size with continued heating starts a process of diminishing the overall hardness of the die block to a more tempered condition. For plain carbon steel, iron carbide formation during tempering (in technical language, carbide precipitation) occurs at about 400 °F. From that temperature upward, little is offered from iron carbides to improve the strength of the steel. However, the key hardenability elements of chromium, molybdenum and vanadium are also strong carbide formers. Carbide precipitation for these alloying elements does not start until the steel reaches much higher temperatures, around 1050 °F. The precipitation of alloy carbides can create a secondary hardening effect, and typically the best combination of strength and ductility is achieved when tempering just above the start of this range. For this reason, pre-hardened die blocks are commonly tempered at 1050 °F and above prior to delivery to the customer. Since die blocks in service often see surface temperatures in this same range, activation of the full hardening potential of an alloyed die steel may occur during service. This provides temper resistance, or resistance to softening in service, and in some cases can even result in further strengthening from use. Such resistance to softening of dies with exposure to elevated temperatures has limits, of course. If die temperatures linger above 1050 °F during service, tempering will continue beyond the optimum condition for temper resistance, and the die (usually limited to the working surface) will suffer a hardness loss leading to increased wear. Die temperatures will naturally spike with each forging cycle, but a quick application of water or water-based graphite lubricant immediately after removing the forging from the die will limit the exposure time at elevated temperatures. The tempering process is time dependent, and minimizing the time spent at elevated temperatures will minimize the cumulative time and prolong the life of the die. Nickel does not participate in carbide formation but contributes by replacing iron atoms in the crystal with the effect of increasing the fracture toughness of the steel. Some nickel is clearly beneficial for most dies, especially hammer dies, but there is a side effect

FIA MAGAZINE | AUGUST 2020 37