November 2019 Volume 1

MATERIALS

This is a challenging effort requiring a well-designed temperature control program for the dies. Pre-heating with gas or electrical heaters should be performed in a way that does not locally overheat any region of the die surface, and a cooling/lubrication process should be practiced that gives careful thought to the duration and spray pattern as well as droplet size to achieve the desired cooling effects.

In contrast, if a die surface is allowed to dwell above the transformation temperature, the transformation to austenite will occur, and the transformation back to the ferritic condition upon eventual cooling almost always results in a less-than-optimal microstructure and degraded die performance. Even short dwell times less than a minute at these high temperatures can cause irreversible damage. Avoiding such high die temperatures with an effective cooling procedure is essential for good die life. In cases where the best efforts to contain die temperatures are unsuccessful, there are die steel alloys available to address such severe conditions. Higher alloy and higher heat-treated hardness can provide additional temper resistance and hot-strength, and some die steel alloys provide higher transformation temperatures that effectively raise the operating-temperature ceiling for the dies. Such options, however, generally incur added cost that can possibly be avoided with a good program of temperature control for the dies. From an economic viewpoint, the most reasonable approach for improving die life is through implementing an effective program that includes consistent and effective preheating and cooling practices to ensure both a minimum and maximum operating temperature throughout the die campaign. Once dies are preheated and forging is underway, regular application of a die lubricant/ coolant to maintain die temperatures within a tight operating range is the basis for 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.

Another important concept is that there is a time component associated with both the tempering and microstructural transformation processes when the critical temperatures are exceeded. The tempering process proceeds according to the relationship known as the Larson-Miller parameter (also known as the Hollomon-Jaffe parameter) which is expressed by the equation: where T = temperature in K, t = time in hours and C = material related constant. The relevance to the forging process is that once a tempering process starts at temperature T, further tempering at that temperature proceeds according to the log of the time. This is accumulated time at temperature, so minimizing the temperature dwell to a brief spike will greatly mitigate the effects of high temperature excursions. One way to accomplish this is practice would be to apply an immediate and effective coolant upon removing the forging from the die. An effective and quick recovery from high temperatures will greatly retard permanent hardness loss due to overtempering and bring significant benefit to die life. The second of the critical high-temperatures, the ferrite-to-austenite transformation temperature is effectively displaced upward with a high heating rate. If the spiking temperature is immediately reduced with aggressive cooling, before the displaced transformation temperature can recover, the transformation and its adverse consequences can be avoided.

Nick Cerwin

Benjamin Ritchy

FIA MAGAZINE | NOVEMBER 2019 29