Investigations on hydrogen-assisted cold cracking of laser welded AHSS

Abstract

This study aims to investigate the impact of various surface conditions prior to welding on the susceptibility of materials to cold cracking, including an analysis of fracture surfaces. Additionally, a novel method is introduced for quantifying the presence of diffusible hydrogen using thermal desorption mass spectroscopy (TDMS). This method allows for the determination of diffusible hydrogen concentration in thin sheet welded joints without use of welding consumables. Three different cold-rolled Advanced High-Strength Steel (AHSS) samples with different surface conditions, such as coatings, lubrication, or water, are examined to assess their susceptibility to cold cracking. In addition to measuring the diffusible hydrogen content in both the base material and the coating, the overall hydrogen content of the base material is also measured using the melt extraction (ME) method. The new method for quantifying diffusible hydrogen in weld metal is applied to investigate different welding variations, intentionally introducing hydrogen through coatings and hydrogenous fluids on the sheet surface. By combining the assessment of cracking susceptibility and hydrogen content, a better understanding of critical hydrogen levels leading to hydrogen-assisted cracking (HAC) is achieved. The results of this study demonstrate that the occurrence of cold cracking in specific AHSS samples increases when either lubrication or both coating and water or lubricant are added. Additionally, the presence of diffusible hydrogen in the welds of all materials is found to increase with the introduction of hydrogenous layers to the material. Although a critical hydrogen content is identified, no clear correlation between the amount of hydrogen and cracking susceptibility can be determined. These findings have significant implications for the welding of cold-formed AHSS, particularly in the automotive industry where safety and lightweight design are of paramount importance.