August 2019 Volume 1

MATERIALS

The Effect of Ductile-to-Brittle Transition Temperature on Die Life By Nick Cerwin and Benjamin Ritchey

Achieving good die life is crucial to the success of a forging company. The basic cost of a die block, coupled with extensive machining to the finished dimensions, entails considerable cost that must be amortized over a large number of forgings to yield a profitable operation. For example, a $30,000 die set that fractures after producing only 1,000 pieces requires that each forging sell for at least $30, just to cover the cost of the tooling. In contrast, a run of 30,000 pieces yields …well, you do the math. Achieving Proper Die Block Temperature Achieving successful die life involves managing many variables. Some of those get established before the actual forging operation while others fall within process control. Established variables, for example, might include the type of forging machine used, the complexity of the forging or the alloy being forged. Variables managed during the forging process may include the stock heating temperature, lubrication practice and die cooling procedures. Carefully managing all these variables plays a huge role in one key determinant of successful die life--achieving an operating temperature for that particular die block that is above the Ductile to-Brittle Transition Temperature (DBTT). The DBTT is the temperature at which the steel’s ability to absorb energy before fracture shifts significantly and the failure mode changes from a fibrous (ductile) fracture to a cleavage (brittle) fracture. Imposing forging loads on a die operating below theDBTT allows minor cracks to propagate readily, ending in catastrophic failure. Die steel operating above the DBTT, on the other hand, has a far greater resistance to crack propagation and thus a far greater chance of successfully finishing a forging run, even with minor surface cracking present. Simply put, die steelsmust operate above theirDBTT! The obvious question then becomes “What is the DBTT for my die?” Based on extensive experience using Charpy V-notch impact test data, a die steel supplier should be able to suggest a minimum operating temperature for most die blocks. However, the actual DBTT value can vary somewhat, depending on service conditions, which makes effective process control essential to establishing the DBTT with a good degree of confidence. The main variables affecting the DBTT for a particular die are:

3. Size of the die. 4. Grain flow direction (with respect to the orientation of the impression within the die) 5. Strain-rate characteristics of the forging process. Die Steel Alloy Die steels with higher concentrations of carbon and the carbide forming elements chromium, molybdenum and vanadium have higher DBTT values (all other variables being the same). This generally means that die steels with higher concentrations of these elements (Finkl die steel grades DC®, Shelldie®, ShelleX®, WF® and Durodi®) require higher pre-heat and service temperatures to remain fracture-tough during the forging process. Nickel is unique among the standard die steel alloys: It lowers the DBTT to allow dies with lower operating temperatures to still function above the DBTT. Finkl grade CX® (3 percent Ni) lowers the DBTT to below room temperature, often below 0 degrees F, to maintain fracture toughness at virtually any startup condition. This is especially important for peripheral components such as rams, piston rods and guides. However, nickel should be used judiciously for dies expected to run at higher temperatures because it also lowers the operating temperature ceiling. Exceeding this temperature ceiling on the surface of a die can create a brittle, crack-prone microstructure often called heat checking.

1. The die steel alloy composition. 2. Hardness, or temper condition

Figure 1

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