Abstract
Cold formed Advanced High Strength Steels (AHSS) are used for automotive body-in-white components due to their high strength, formability and impact resistance. Advanced high strength steels can offer a promising reduction of weight in automotive applications and as a consequence a positive environmental impact. However, the excellent mechanical properties of the AHSS cannot be fully exploited due to their susceptibility to Hydrogen Embrittlement (HE). This is especially true for steels with the highest strengths. Hydrogen can be absorbed into the steel during several steps of the manufacturing line of cold formed AHSS, such as during pickling, electro galvanising, blanking, forming, phosphatizing, cutting, welding and in-service due to corrosion. Literature has shown a positive effect of vanadium on the hydrogen embrittlement resistance of high strength steels, where Vanadium carbides act as traps for diffusible hydrogen. Spot welding is performed on cold formed AHSS to join different components of the body-in-white. Spot welding might not introduce hydrogen in the steel, but can lead to changes in the microstructure and introduction of thermal residual stresses. This can modify the efficiency of the hydrogen traps, such as vanadium carbides, and increase the risk to HE. Moreover, the thermal residual stresses created during welding can also increase the risk of HE. Currently, there is no available knowledge on the effect of spot welding on the HE resistance of V-containing HSS.
This work shows the effect of Vanadium on the microstructure and hydrogen embrittlement sensitivity of spot welded cold formed AHSS for automotive applications. AHSS with a strength of 1700MPa, with additions of 0.15 wt.% V and 0.30 wt.% V were casted, hot rolled, coiled, cold rolled and annealed. After annealing in Gleeble the materials were electrogalvanized and spot welded. The effect of spot welding on the sensitivity to hydrogen embrittlement and the trapping efficiency of V-carbides is evaluated by constant load testing (CLT) at 80% Rm in 0.5M Na2SO4 under a current density of -0.2 mA/cm2 and by TDA coupled with a mass spectrometer (TDMS).
By a correlation between microstructure analysis by Transmission Electron Microscopy (TEM), mechanical degradation by CLT and hydrogen quantification by TDMS, this study shows whether the HE sensitivity of V-containing AHSS is affected by (i) spot welding, (ii) residual stresses from welding or (iii) electrogalvanising process.