Press hardening

Hot stamping (also known as press hardening or hot press forming) is a relatively new technology which allows ultra-high strength steels (typically 22MnB5 boron steel) to be formed into complex shapes, which is not possible with regular cold stamping operations.[1]

Methods

1) Indirect Process: the blank is formed, trimmed, and pierced in cold condition (i.e., state 1 in below figure). It is later heated and quenched in a die to get high strength properties.

Indirect hotforming sketch

2) Direct Process: the unformed blank is heated in a furnace, formed in hot condition (state 2 in below figure), and quenched in the die to achieve the required properties.

Direct hot forming sketch, Direct hot forming

Selection of the process depends on part complexity and blank coating (Zn based coatings typically require indirect process). In either method, the blank is formed in a much softer and formable state and is later hardened in the dies, which have drilled cooling channels. A typical hot stamped components has 1000 MPa (145 ksi) Yield Stress and 1500 MPa (215 ksi) Ultimate tensile strength.[2][3]

Advantages

Higher strength steels may help reducing the weight by downgaging (i.e., use of thinner sheets), while increasing the crashworthiness.[4] However, one problem with many high strength steels is that their formability is generally lower than milder grades. In addition, springback and die wear also cause problems as the forming stresses and contact pressures are higher.[5]

References
  1. So, H., D. Faßmann, H. Hoffmann, R. Golle & M. Schaper. "An investigation of the blanking process of the quenchable boron alloyed steel 22MnB5 before and after hot stamping process", Journal of Materials Processing Technology 212, 437-449 (2012).
  2. Kurz, T., "New Developments in Zinc Coated Steel for Press Hardening", in Insight Edition Conference, September 20–21, Gothenburg, Sweden (2011).
  3. Watkins, J., "Material Development", in AP&T Press Hardening, Next Step Seminar, Novi, MI, October, (2011).
  4. Smith, D. "2011 Grand Cherokee", Presented in Great Designs in Steel, May 18th, Livonia, MI, (2011)
  5. Billur, E. & T. Altan, "Challenges in Forming Advanced High Strength Steels", in Proceedings of New Developments in Sheet Metal Forming 2010, pp285-304, May 2–4, Stuttgart, Germany (2010).
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