Hydrogen-assisted cold cracking in welded joints of press-hardened 22MnB5

Abstract

Over the last decade, press hardening has become all pervasive for automotive body-in-white design. Press-hardened boron micro-alloyed steels are widely used in modern body structure for high safety standard and lightweight achievement. Considering the welding of press-hardened components, some special considerations have to be taken into account compared to conventional deep-drawing steel grades. The mechanical properties of the welded joints are influenced by the hydrogen content in the base metal as well as the weld metal. Hydrogen absorption may result from the press-hardening process, as well as the welding process. Due to the martensitic microstructure and the high strength level, a critical hydrogen content may cause hydrogen embrittlement or hydrogen-assisted cold cracking (HACC). This paper focuses on the determination of the diffusible hydrogen content in the base metal after the press-hardening process, as well as in gas metal arc (GMA) welded joints of 22MnB5. Hydrogen source during press hardening is the humidity at the austenitizing temperature in the furnace atmosphere. During welding, for this study hydrogen was deliberately introduced by hydrogenous fluids on the sheet surface. The diffusible hydrogen content in the base metal and the weld metal was quantified by thermal desorption mass spectroscopy (TDMS) technique. The influence of the diffusible hydrogen on the mechanical properties of the welds was determined by four-point-bend testing. The time to fracture was identified using acoustic emission technique.