November 2021 Volume 3

MATERIALS

the die (i.e., a negative effect). It is this important consideration that needs to be highlighted if higher die base hardness is implemented as an approach to addressing higher die operating temperatures. By ordering a Temper 1 (43-46 HRC) or Temper H (47-50 HRC) die block, for example, instead of Temper 2 (38-42 HRC) die block, the DBTT may be increased by 100° F to 200° F depending on the particular die steel grade and block size being used. This effect is illustrated in Figure 2. If the die preheating temperature and minimum steady-state operating temperature are not adjusted upward by this same amount, the die may drop below the temperature needed to operate under ductile conditions. This poses a threat of brittle fracture and catastrophic failure. As with hot strength, recommended die preheating temperatures for different base hardness levels are typically provided by die steel manufacturers on datasheets.

dies. Potentially an intermediate change is better than the somewhat extreme example given above. For example, a switch could be made from T2 to T1 while at the same time attempting to get the operating temperature down from 800 °F to 700 °F through a moderate coolant increase. This would involve only needing to preheat the die to 250 °F (100 °F hotter for the T1 die versus the T2 die) and operating in the range 250-700 °F. If even that relatively minor change to preheating practices is not possible, it would be inadvisable to use this approach of higher hardness dies to address an elevated temperature situation. Using a higher die base hardness for the occasional wayward temperature excursion such as a stuck forging is also not warranted. However, vigorous cooling to quench even occasional overheating is still a mistake. The safer approach is to allow some time for the die to air cool, or at least limit water cooling to a lighter, intermittent spray to avoid unnecessary stresses. Another limitation exists for applying higher base hardness to very large dies (not precisely defined, but generally with thickness over 18 inches or so). This limitation arises because very large dies experience an inherently less effective quench-hardening process compared to smaller dies. This is due to a smaller surface-to-volume ratio that retards heat transfer and effective quenching rate. The slower cooling rate produces a microstructure containing non- martensitic transformation products that has the effect lowering the as-quenched hardness. The lower tempering temperatures then required to achieve the higher hardness ranges like Temper 1 or Temper H can introduce new risks such as embrittlement. The efficacy of this approach for individual cases involving various die steel grades and block sizes should be discussed with your die steel supplier. In most cases, however, raising the base hardness of a die to address an expected elevated service temperature should offer improved die life. ■

Figure 2. Illustration of the shift in DBTT for a generic die steel with three different base hardness levels Let us pick back up with our previous example where we have switched from a Temper 2 (T2) die block to a Temper H (TH) die block to ensure we have suitable strength at 800 °F. While we are operating at 800 °F, the block will be well above the DBTT and thus not at risk for catastrophically cracking if the strength is exceeded. However, before the die settles into that range, we need to be very careful to follow proper preheating guidelines. For Finkl’s FX die steel, the recommended pre-heat temperature for T2 for a 10” thick block is 150 °F. This means a T2 block operating at 150 °F is comfortably above its DBTT. For an FX TH block of the same thickness, the recommended preheat temperature is 300 °F. Making this change therefore means that the die needs to be preheated a full 150 °F hotter, and the die must not fall below 300 °F during operation. This certainly stands to be a significant adjustment for a forge crew. Some forging operations may not be able to support the higher preheat and operating temperatures required of higher hardness

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.

FIA MAGAZINE | NOVEMBER 2021 35