Date created: | Last Updated:

Category: Project

Description: Resistance spot welds made from Advanced High Strength Steels (AHSS) exhibit Heat Affected Zone (HAZ) softening due to the tempering of pre-existing martensite phase and the consequent decomposition into a mixture of ferrite and cementite. Despite the high strength level for the base metal, the occurrence of HAZ softening may lead to inferior joint strength during Tension-Shear (TS) and Cross-Tension (CT) testing. In this work, we investigated the effects of the HAZ softening on the global loading response for AHSS steels with three different volume fractions of martensite. Microhardness mapping was used as a measure of martensite tempering and extent of softening. Based on the data, the softening was identified in the sub-critical heat affected zone. Hardness drop with the magnitude of 6%, 18%, and 42% was observed in steels with 16%, 52% and 100% of martensite volume fraction (MVF), respectively. In order to model the welded joint loading response using finite element methods (FEM), there is a need to represent the softening in terms of stress-strain relationships. In this work, local stress-strain curves for different weld zones were obtained by scaling the base metal constitutive properties with local hardness ratio. Finite element (FE) simulations of Tension-Shear tests showed that HAZ softening can affect the Tension-Shear load capacity of specimens more significantly when the base metal tensile strength is above 1000 MPa. The paper will discuss the validity of the above finite element approach for describing experimental results and future directions.