Post-Forming Heat-Treatable (PFHT) Steel
Post-forming heat treatment is a general method to develop an alternative higher strength steel. The major issue holding back widespread implementation of HSS typically has been maintaining part geometry during and after the heat treatment process. Fixturing the part and then heating (furnace or induction) and immediate quenching appear to be a solution with production applications. In addition, the stamping is formed at a lower strength (ellipse 1) and then raised to a much higher strength by heat treatment (ellipse 2).
One process is water quenching of inexpensive steels with chemistries that allow in-part strengths between 900 and 1400 MPa tensile strength. In addition, some zinc coatings can survive the heat treating cycle because the time at temperature is very short. The wide assortment of chemistries to meet specific part specifications requires extra special coordination with the steel supplier. Another process is air-hardening of alloyed tempering steels that feature very good forming properties in the soft-state (deep-drawing properties) and high strength after heat treatment (air-hardening. Apart from direct application as sheet material, air-hardening steels are suitable for tube welding. These tubes are excellent for hydroforming applications.
The components can be heat treated in the furnace in a protective gas atmosphere (austenitized) and then hardened and tempered during natural cooling in air or a protective gas. The very good hardenability and resistance to tempering is achieved by adding, in addition to carbon and manganese, other alloying elements such as chrome, molybdenum, vanadium, boron, and titanium. The steel is very easy to weld in both its soft and air-hardened states, as well as in the combination of soft/air-hardened. This steel responds well to coating using standard coating methods (conventional batch galvanizing and high-temperature batch galvanizing).
A third option is in-die quenching. A version of Indirect Hot-Forming (described in AHSS Application Guidelines 1.B.7.) completes all forming of the part at room temperature, heats the part to about 850 - 900º C, and then uses a water cooled die to quench the part to martensite. This process is called Form Hardening.
