Direct Contact Heating for Hot Forming Die Quenching

Abstract

Most hot forming lines use slow, energy-intensive roller hearth furnaces to austenitize boron steel “blanks”. This paper describes an alternative heating technology in which blanks are austenitized by bringing them into contact with a hot monolith. The austenitizing temperature was reached in less than 30 seconds, and subsequent material characterization tests on oil-quenched blanks confirm that a fully martensitic structure is formed, and that the hardness and yield strength are comparable to furnace-treated samples. An Al-Si coating is typically used to prevent the oxidation and decarburization of the blanks within the furnace; preliminary tests found that the coating adheres to the monolith, impeding blank transfer and damaging the Al-Si-Fe ternary coating. Five interchangeable striking surfaces were assessed to see if they were less prone to adhering to the molten Al-Si coating.

In situ surface imaging: High temperature environmental SEM study of the surface changes during heat treatment of an Al Si coated boron steel

“…Most of the studies characterizing the Al-Si coating evolution during austenitization are carried out ex situ on cross-sections of the coated steel, i.e. only after the heat treatment [3,13,17]. However, to our knowledge, few studies have been focused on the surface evolution during austenitization.…”

“…In this context, ultra-high strength steels have been developed to minimize their amount, the most common being composed of boron and achieving tensile strengths of around 1500 MPa in quenched condition [1]. These materials, produced using a hot stamping process [2,3], have been significantly employed into vehicles since then, particularly in their body structure, such as door beams, bumpers or roof pillars. In 1995, they accounted for 1 wt.% of the body structure components, while they can now reach up to 60 wt.% [4].…”

Evolution of Microstructure and Properties of Steel 22MnB5 due to Short Austenitization with Subsequent Quenching

“…Contact-or press-heating: for a contact heating the material is placed between two plates, which previously can be preheated by resistance heating, by induction, or by conduction. [6,10,11] This type of heating is relatively new in comparison with other methods and seems like the best procedure for substituting conventional furnaces. The main advantages of this heating method are the homogeneity of temperature distribution in the whole blank and the relatively simple realization of partial heating for production of load-bearing components with tailored properties.…”

“…[8] The main downside of this heating concept is the high complexity of the induction heating equipment and the difficulty in achieving a uniform heating through the blank with complex shape. [6] 3. Contact-or press-heating: for a contact heating the material is placed between two plates, which previously can be preheated by resistance heating, by induction, or by conduction.…”

“…Among them are high investment and operating costs, large spaces, and long heating times, which cause significant oxidation of steel blanks when special protection coatings are not employed. [6] In addition, the demand of reducing energy [ costs and of an environment-friendly production process stimulates the development of new heating methods. During the last decade, the following rapid heating concepts have been developed with which the abovementioned drawbacks can be avoided:…”