August 2019 Volume 1

MATERIALS

Hardness, or Temper Condition All other conditions being the same, higher hardness raises the DBTT, requiring a higher pre-heat and service temperature to remain fracture-tough. Size of the Die The DBTT is governed by changes in microstructure that develop during the heat treatment of the die block. Large die blocks cool more slowly than small die blocks during the quenching stage of the hardening process. Interior regions of a die block cool more slowly than regions near the surface. Since quenching rate determines the percentage of desirable martensitic microstructure, differences in martensitic content (and therefore DBTT) are expected with block size and testing location. Die steel alloys are available to provide deeper hardening responses during the heat-treating process (high hardenability, or DI, value). Grain FlowDirection DBTT values can be somewhat higher (less favorable) in transverse testing (or loading) directions. Since die steel is tri-axially stressed, die manufacturers usually try to minimize this disparity during the melting and forging of die blocks. Effective argon stirring during the vacuum treatment stage of the modern melting process minimizes impurities in steel that contribute to this effect. However, there is still some advantage to aligning any stress-rising features of the die Faster forging deformation rates characteristic of hammers and screw presses are said to have higher strain rates compared to mechanical or hydraulic presses. Higher strain rates effectively raise the DBTT, all other conditions being the same. Therefore, die alloys for hammers and screw presses should be biased toward lower DBTT values (Curve 1 in the DBTT graph). This ensures that the die operating temperature falls on the upper shelf of the Charpy curve to provide fracture-tough conditions. Mechanical and hydraulic presses can use higher alloy dies at higher hardness due to their lower strain rates and tendency to transfer more heat to the dies, which shifts the die operating temperatures higher and into a fracture-tough condition. design across the grain direction. Strain-rate Characteristics

Figure 2 Though exceptions always occur, Figure 2 shows a basic matrix that arranges die steel characteristics according to general types of forging equipment. In summary, many factors in die steel and operating conditions effectively determine the DBTT for a die. Sudden and early cracking of dies, which is more likely to occur when dies operate below the DBTT, makes a strong case for fully appreciating and managing these factors. Good technical dialogue between the die steel supplier and forging company is essential to match the metallurgical characteristics of a die to a particular forging operation. Many individual details enter the die selection process and maintaining an open dialogue with your die steel supplier will further the goal of good die longevity. Nick Cerwin is the retired director of technical service at Finkl Steel. Benjamin Ritchey is the group chief metallurgist for Finkl Steel and can be reached at BRitchey@Finkl.com.

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