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With increased pressure for reduced CO2 emissions, automakers are trying to reduce the weight of vehicles by adopting advanced high strength steels (AHSS). Use of AHSS allows parts to be down-gauged without a loss in part strength. These steels get their high strength from special processing and complex chemistries with higher C and alloying contents than conventional steels.

In North America, most AHSS are being produced using existing steelmaking lines that were built between the 1960鈥檚 and 1990鈥檚. These steel lines were designed for low carbon and HSLA steels, so the processing needs of AHSS with their high hardenabilities were not considered. One of the outcomes of this is that coil welds in between AHSS grades are much too hard for lines to process without significant risk of weld breaks, which can lead to costly downtime. There are three major ways these issues can currently been handled: checkboarding, weld recipe development, and retrofitting induction postweld heat treating (PWHT) equipment into the lines. All of these are difficult or costly to implement resulting in either scheduling difficulties or capital improvements that are hard to justify in this economy.

This report summarizes the work completed to determine if low cost flame tempering is a feasible solution to temper welds. Press hardened steel (PHS) weld samples were collected from both a pickle line and galvanizing line and flame tempered at various speeds. Afterwards they were tested mechanically and the microstructures of the samples were examined. This work showed that flame tempering improved the weld properties sufficiently to be processed through the lines, without increasing processing times beyond the current amount of time allowed to make the coil weld. Flame tempering showed itself to be a low cost alternative to deal with hard coil joints in AHSS.