February 2020 Volume 2

MATERIALS

Methods and Practices for Controlling Die Temperatures

By Nick Cerwin and Elizabeth Bilitz Two previous articles emphasized the importance of maintaining tight temperature control on dies throughout the forging process. On the low end, die preheat and service temperatures must be above the DBTT (The Effect of Ductile-to-Brittle Transition Temperature on Die Life – FIAMagazine August 2019) to keep dies in a fracture tough condition to lower the risk of catastrophic cracking. On the high end, controlling heat input to the dies through effective use of a coolant/lubricant during the forging process will avoid excessively high die temperatures that weaken a die and cause accelerated wear (Controlling Maximum Die Temperature for Better Die Life – FIA Magazine November 2019). The concept is simple, but holding dies to a tight middle-range of temperature throughout a service campaign is challenging and requires more care and technology than is generally appreciated. Die preheating can be performed with a simple arrangement involving a straight or bent pipe with holes drilled along the length for distributing lazy gas flames over the faces or sides of the upper and lower dies. This method can be effective and is an improvement over direct flame, but the parameters involved need careful consideration. Details such as pipe size, hole size and spacing, gas pressure, air-gas ratio and proximity to the surfaces being heated as well as duration of heating are all relevant factors to be incorporated into the design. Without careful design and control, this approach may overheat localized areas of the die surface. Moderately elevated temperatures (1100° to 1200° F, or so) on the die surface will over temper the dies, producing a permanent softening and a reduction in wear resistance. Such overheated areas are usually shallow, about 0.010-inch or so, and mostly located on the tops of ribs, bosses or flashland edges that are already subjected to high wear rates. Dies just moderately overheated during the preheating process can dramatically lower die life. Severe overheating, to about 1400 °F or more, may occur during a poorly managed preheating process or under some extreme service conditions. Either way, overheating to this extent alters the microstructure of the die steel in the affected area, and likely leads to a severely heat-checked condition with significant loss of die life. Such overheating during the preheating process can possibly go unrecognized because the die temperature is not typically checked until after the heating source is removed. The large mass of the die block, compared to the relatively small layer of the overheated surface, is an effective heat-sink that quickly conducts heat from the affected area to yield anunrepresentatively lower surface temperature just seconds later. A recommended procedure to evaluate preheating performance is to mark the die surface with temperature sensing

crayon (or paint) before activating the heating source. If the die surface temperatures exceed the critical over-tempering or austenitic transformation temperatures during the heating process, the telltale discoloration of the selected heat sensing marks will be evidence of the need to adjust the heating procedure. Electrically based die preheating holds strong appeal, largely due to the degree of control available with this method. Pyrometric control involving immediate feedback to the heating source from strategically positioned thermocouples offers a means of precise control. Without such feedback, however, it is still necessary, as with gas heating, to provide a passive means of temperature control such as temperature sensing paint or crayon. Controlling high-side temperatures is largely through the use of water-based graphite that functions as both a lubricant and coolant. The graphite solution is sold in concentrated form and diluted to varying degrees according to forging requirements. Over-diluting may leave excess water puddled in the die with a reduced lubricity contribution from the lower level of graphite. This combinationmay cause increased difficulty with die filling and accelerated die wear. Inadequate dilution may also lower the effectiveness of the graphite as water between the graphite platelets is a necessary component for graphite lubrication. The adsorption of water reduces the bonding energy between the graphite platelets to a lower level than would occur between graphite platelets alone, as is the case with no water present. The difference is rather significant, with lubricity being about 5X better with water in the mix. Providers of die lubricants have guidelines available for assisting with optimal ratios of water/ graphite solutions that should be present during the actual forging process for the most effective lubrication performance. Differences in water evaporation in the time between application and the actual forging process will determine water/graphite ratio at the time of application. This will certainly differ between forging operations. Applying science to the water/graphite application will help to arrive at a proper ratio. Using the information on the specific heat of water and steel, and the heat of water vaporization: Specific Heat = Calories (or BTUs) required to raise one gram (or one pound) of material 1°C (or 1°F). Heat of Vaporization = Calories (or BTUs) to convert 1 gram (or 1 pound) of liquid at the boiling point to a gas phase at the same temperature. Specific Heat of Water = 1.0 calories/gram/°C (1.0 BTU/lb./°F) Specific heat of Steel = 0.1 calories/gram/°C (0.1 BTU/lb./°F) Heat of water Vaporization = 550 calories/gram (550 BTU/lb.)

FIA MAGAZINE | FEBRUARY 2020 40