November 2019 Volume 1

MATERIALS

Controlling Maximum Die Temperature for Better Die Life By Nick Cerwin and Benjamin Ritchey

In the August issue of this magazine, we discussed the importance of operating with a minimum die temperature to avoid a crack-prone, brittle condition in the die (FIA Magazine, Issue 2, August, 2019). This minimum temperature is designated as the Ductile-to-Brittle Transition Temperature (DBTT). The exact value of the DBTT for a particular die is determined by a number of factors such as the alloy content, hardness and microstructure of the die, plus some key variables in effect for that forging operation. Once established, it is essential to maintain die operating temperatures above the DBTT to minimize die breakage. For a forging process that recognizes and adheres to a DBTT requirement, the next logical question is “Is there also a maximum recommended die operating temperature to be observed?” The answer is “Yes”, although there is more than one critical temperature, and just approaching those temperatures has a deteriorating effect on die performance. Die wear resistance can be determined by the relationship: The value of the numerator in this proportionality is largely determined by the alloy being forged and the complexity of the forging. The denominator, however, is our focus for this article since die hardness is not a fixed value (as received from the die steel supplier), but varies with die temperature. The effect of elevated temperatures on a range of mechanical properties for the popular Finkl die steel grade FX® is provided in a technical, commercial data sheet for this grade. Most die steel suppliers offer similar tables of properties for other die steel options. Examining the hardness values from the Finkl FX® data shows the effect of rising temperatures on this particular die steel. Two popular, heat-treated hardness conditions, Temper 2 (352-388 HBW) and Temper 1 (401-429 HBW) are selected and shown here:

Although most forging operations control die temperatures at the lower values of the recommended range, it is possible to reach higher values when forging iron-based alloys without an effective means of die cooling in place. Dies often must be removed from service when high temperatures that develop in localized areas such as ribs and bosses cause those areas to weaken and suffer accelerated wear. For example, FX® Temper 2 dies that experience surface temperatures around 900 °F function essentially at annealed strength on those surfaces. This is not good for die life. The good news is that such weakened die steel will fully recover the starting hardness upon return to room temperature. The exception to this is if the elevated temperature exceeds the tempering temperature used to establish the heat-treated hardness of the die block. In this case, further tempering results, permanently softening the die by approximately one Brinell Hardness number for each one degree Fahrenheit over the original tempering temperature or by a full temper range (e.g., T1 to T2) for each 50°F over. This critical temperature, i.e., the actual tempering temperature used for your die block, is usually available from the die steel supplier or heat treater. The second critical temperature is the ferrite-to-austenite transformation temperature. This temperature varies a little depending upon the exact chemistry of the die steel, but in general is approximately 1380°F. Exceeding this temperature erases the provided microstructure of the die block, and the overheated area changes upon cooling to a microstructure that is usually less than-optimal for good die performance. Exceeding 1380 °F die temperature seems beyond the norm, but most forging operators have seen, at times, an orange glow in some areas of a die. This is evidence that the ferrite-to-austenite temperature has been exceeded for those areas. If die wear was the only consideration, then maintaining dies at room temperature would preserve the full hardness of a die and provide the best wear performance. However, from a fracture viewpoint, the dies must be raised to a temperature above the DBTT to provide full fracture toughness. The optimal service temperature for a die, therefore, must recognize these competing temperature requirements. As a minimum temperature, dies should be above the DBTT, but then as a maximum the temperature should be limited to as little as possible above the DBTT.

FIA MAGAZINE | NOVEMBER 2019 28