November 2021 Volume 3

MATERIALS

Strategies for Success with Higher Hardness Dies By Nick Cerwin and Benjamin Ritchey

Dies experiencing unusually high service temperatures pose a special challenge with respect to achieving good die life. This issue has been discussed in prior articles (See FIA Magazine August 2019, The Effect of Ductile-to-Brittle Transition Temperature on Die Life, and FIAMagazine May 2020, Contending with Die Overheating ), but the proposed means of overcoming the detrimental effect of high die temperatures presented in those articles raises important collateral effects that warrant special mention. Dies usually function within a temperature range of about 200 °F to 600 °F with momentary temperature spikes at forging contact. Within this range the die hardness (strength) remains close to the hardness present at room temperature, which is the hardness specified by the forging company at the time of purchase with the expectation that it would provide good die life. In some cases, however, factors arise that result in higher die temperatures. If this occurs and the die body temperatures rise above 600 °F or so, the die begins to experience an accelerating and substantial strength loss. Note that strength and hardness are often used interchangeably since they are directly correlated: Tensile Strength equates closely to 500 psi x Brinell Hardness. The die wear equation , which is used to illustrate the effect of different variables on wear, is usually written without the explicit notation of hardness being a function of temperature (T) . However, writing it with this minor edit highlights the important dependence of die steel hardness on operating temperature as opposed to simply thinking of it as the purchased hardness: Wear = Pressure x Sliding Motion / Die Hardness (T) The hardness of die steel at temperature follows the general relationship shown in the graph in Figure 1. This graph may be confused at first glance with a tempering curve that has the same axes of hardness versus temperature. The tempering curve, however, relates the resulting hardness after a quenched steel returns to room temperature following a holding period at the indicated temperature (i.e., it shows the room temperature hardness after a tempering heat treatment). The graph of interest here is indicating the actual strength when measured at the indicated temperature (i.e, the hot strength ). This is typically accomplished with elevated temperature tensile testing, and here we show those strength values converted to Brinell hardness for ease of understanding. Actual hot strength test results are usually offered on die steel datasheets. Clearly, the die will suffer increasing wear with declining hardness, and there is a strong incentive to adopt a well-designed and effectively implemented cooling practice.

The usual means of addressing overheated dies is to use more aggressive cooling. While this approach quickly restores the die to a normal operating temperature that preserves good wear resistance, the high thermal shock introduced with this practice raises the potential for damage through heat-checking. One proposed solution to this dilemma detailed in our prior articles was to reduce the die cooling effort and resulting thermal shock by simply allowing the die to operate at the higher temperature. To mitigate the loss in die hardness resulting from the higher temperatures, one can start with a higher base hardness.

Figure 1. Example of hot hardness curves for a generic die steel with three different base hardness levels Let us consider for example a Temper 2 (352-388 HBW) die steel that settles into an operating temperature range of 800 °F during sustained production. Looking at Figure 1, we can see that this die could lose 90-100 Brinell hardness points (approximately 50 ksi of strength) at this operating temperature. In other words, the nominally 363 HBW (182 ksi) die will have the strength of a 269 HBW (133 ksi) die during operation. If we want to ensure that our die is still able to operate at 800 °F with the strength of a 363 HBW die, Figure 1 shows us that we can switch to a Temper H (444-477 HBW) base hardness. At 800 °F, the TH die in Figure 1 has the same hardness as the T2 die at room temperature. The tripping point to this approach is that raising the base hardness not only increases the hot hardness (i.e., a positive effect), but it also raises the Ductile-to-Brittle Transition Temperature (DBTT) of

FIA MAGAZINE | NOVEMBER 2021 34