Predicting Transient Softening in the Sub-Critical Heat-Affected Zone of Dual-Phase and Martensitic Steel Welds

Abstract

To improve vehicle fuel economy and crash worthiness the automotive industry has been redesigning parts from advanced high strength steels such as dual-phase and martensitic steels. These steels have high strengths with the higher formability characteristics when compared to lower strength conventional steels of similar ductility. These steels derive their unique properties from their complex microstructures containing ferrite and martensite. During assembly welding, the martensite within the sub-critical region of the heat-affected zone tempers, which locally reduces mechanical properties. Although this phenomenon is well studied, it has yet to be quantified. The present work proposes a technique to measure the softening kinetics of dual-phase and martensite steels using rapid isothermal tempering. The resulting model was then validated by predicting the heat-affect zone softening that occurs in laser and resistance spot welds as well as by comparing the microstructures of the rapid tempered samples to the microstructures found in the sub-critical heat-affected zone of welded samples.

Advanced Characterization of HAZ Softening of AHSS for Crash Modeling

“…Tempering of martensite is controlled by diffusion of alloying elements in the hierarchical structure of martensite. The mechanisms and kinetics of martensite tempering has been extensively reported elsewhere, 3,4) and Part I of this study has studied the modeling of tempering kinetics. 5) Tempering of martensite results in the reduction of hardness.…”

“…The magnitude of the hardness drop depends on the extent of HAZ tempering, and welding parameters, such as the maximum local temperature during welding. [3][4][5][6] In high strength steels, such as DP590, 6) there is minimal HAZ softening and the HAZ is not considered as separate area of elements in FEA modeling. However, in higher strength AHSS containing high volume fraction of martensite there is significant martensite tempering in the HAZ with large associated hardness changes across the weld.…”

“…M1500) with 100% volume fraction of martensite show the highest HAZ softening, and the extent of softening depends on chemical composition. 3,4) Recently, FEA modeling of full car crash or components include a separate set of elements representing HAZ properties. Previously, the ratio of the strength of elements making up the weld HAZ compared to those representing the base material were calculated from hardness values to scale the material properties.…”

Identification of Plasticity Model Parameters of the Heat-Affected Zone in Resistance Spot Welded Martensitic Boron Steel

“…In particular the so-called heat-affected zones that are found around resistance spot welds in fully hardened boron steels are potential areas for fracture initiation, which have to be considered in the simulation of the crashworthiness of a vehicle. The martensitic microstructure of hot stamped components, that is obtained when cooling rates exceed the critical threshold of 27 °C/s, is metastable and tends to decompose into softer microstructures when it is reheated to temperatures just below Ac1 (approximately 720 °C for the 22MnB5 used in this work [1,2]). This softened zone is generally known as the sub-critical HAZ, in which martensite tempering is the controlling mechanism [3].…”

Effects of Welding Schedules on Resistance Spot Welding of DP600 Steel

“…Dancette et al 17) detected martensite tempering in SCHAZ of DP980, which was responsible for strength reduction and ductility increase of spot welds. Biro et al 18) predicted the transient softening phenomenon in the HAZ of spot welded DP steel using rapid isothermal tempering technique and validated by experiments.…”