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Hot Forming Die Quenching (HFDQ)

In hot forming die quenching (HFDQ) steel sheets called “blanks” are first austenitized in a furnace and then transferred to a cooled die, where they are simultaneously formed and quenched in a single stroke. The parts are cooled fast enough to transform the austenitie into martensite, a steel phase having enormous hardness and tensile strength. The end result is a strong, lightweight part having superior crash performance. Since its inception in 1987, annual production of HFDQ parts has grown dramatically, reaching 350 million parts per year in 2013.

Our research group has worked on several aspects of HFDQ:

1. Heat Transfer Coefficients during Forming/Quenching

The objective of traditional HFDQ is to obtain parts having uniform martensitic microstructure, it is possible to achieve as-formed parts having distributed material properties by controlling the cooling rate. Achieving these cooling rates requires detailed knowledge of the heat transfer coefficient (HTC) between the blank and the die.

As part of a larger project led by Prof. Mike Worswick and sponsored by Honda, Cosma, and Arcelor Mittal, Dr. Etienne Caron inferred this parameter through inverse heat conduction analysis. Dr. Caron also showed that it was necessary to account for the contact resistance between the thermocouple within the die and the die cavity, which leads to a temporal lag. The HTCs obtained through experiment matched theoretical values. A subsequent work showed how the HTC varies with die temperatures. These values are now being used to design dies that have localized hot and cold zones, in order to produce stamped parts having distributed properties.

2. Furnace Modeling and Characterization

In almost all hot forming lines the blanks are austenitized using long roller hearth furnaces, in which the blanks are conveyed on ceramic rollers through a sequence of heating zones. In order to obtain the desired material properties in the formed blanks, the zone temperatures and roller speeds must be set so that the blanks are fully and uniformly austentized. In order to avoid oxidation and decarborization, most hot forming is done using Usibor® 1500 P, a 22MnB5 steel with a protective aluminum silicon dip coating. Furnace heating transforms the Al-Si coating into a permanent Al-Si-Fe layer that provides long term corrosion protection.

Our research group is working with Cosma International to develop numerical models of the roller hearth furnace. This is complicated by the fact that the radiative properties of this coating change throughout the furnace heating process. Blank heating is also influenced by the latent heat of austenitization, which is not fully characterized.

Noel Chester, a PhD candidate, is working to develop a coupled metallurgy/heat transfer model of blank heating, including a detailed (JMAK) submodel of austenitization, which will be developed based on dilatometry measurements made on blanks heated within a Gleeble® 3500 thermomechanical simulator. Noel is also characterizing the blank emissivity through ex situ means using an FTIR reflectometer, and in situ with an NIR spectrometer within the Gleeble chamber; this represents the first time that the Gleeble has been used to characterize the temperature-dependent emissivity of a material.

For his MASc work, Kamal Jhajj has developed detailed heat transfer models of roller hearth and muffle furnaces. These models are used to infer the specific heat of Usibor® 1500 P from temperature data obtained from instrumented blanks heated within these furnaces.

3. Direct Contact Heating

While almost all hot forming lines use roller hearth furnaces to austenitize the blanks, there are a number of disadvantages: they consume a large amount of energy and floor space; and the Al-Si coating melts and contaminates the ceramic rollers, shortening the roller lifetime and scoring the blanks.

Our research group is working with Mr. Mike d'Souza at F&P Mfg. Inc. to develop an alternative technique in which the blanks are conductively-heated using an electrically-fired monolith mounted within a hydraulic press. A fourth year capstone design group (Peter Plaisier, Cam Rush, Brad Froese, and Josh Rasera) designed and constructed a prototype lab-scale apparatus. Preliminary measurements showed that the blanks can be austenitized, and the Al-Si-Fe layer appears fully-formed, after 30 seconds of heating. Josh Rasera continued his studies as a MASc candidate to perfect the lab scale apparatus, and to investigate the possibility of tailoring during the heating stage through nonuniform heating/incomplete austenitization. Josh and Natalie Field are presently resizing the design to full size for pilot scale implementation.