Hydrogen embrittlement in the welding of high-strenght materials used in the automotive industry

Abstract

The increase in safety and energy efficiency requirements is one of the major concerns of the automotive industry. Advanced high-strength steels (AHSS) are preferred for mass reduction in structural areas of the vehicle designed for impact absorption in accidents. The 22MnB5 steel, which can be hardened by press hardening (PHS), is an ultra-high strength steel (UHSS) that achieves tensile strength of around 1500 MPa or higher after heat treatment during stamping in a cooled tool. The automotive industry also adopts technology for welding two flat sheets, enabling the production of reinforcements and structural parts during hot forming (patch weld), all the while increasing productivity. However, high-strength materials may exhibit hydrogen embrittlement, which reduces mechanical properties when exposed to a combination of tensile stresses and unfavorable microstructure. In this study, various resistance spot welding parameters used in patch welds were evaluated to ensure a representative sampling of the approved nugget diameter and weld parameters, to verify their influence on hydrogen embrittlement according to the production process. In the beginning of the present study, cracks were found to appear after welding. These cracks were located at the interface of the sheets at the edge of the spot weld and towards the fusion zone. The study was divided into two phases, a pilot and a test phase. The pilot phase focused on evaluating fused area diameters, residual stress measurements, and microhardness in all weld zones before and after hot stamping, which allowed for the selection of welding parameters for further assessment through inferential statistics. Results from the pilot phase indicated that the 22MnB5 steel, as received and after resistance spot welding, exhibited a martensitic structure in the fusion zone (FZ) with high hardness and at a normal distribution with low variance. All welding parameters showed an average microhardness of HV0.3 500 in the fusion zone, along with an increase in residual stress before the hot stamping process. After hot stamping, the high residual stress state of the surface was influenced by the welding parameters used prior to hot stamping, still showing residual tensile stress values. In the test phase, welding parameters with single pulse, double pulse (or double pulse) with post-weld heat treatment, selected during the pilot phase, were evaluated based on fused area diameters, residual stress measurements, microhardness, tensile shear tests, Charpy tests, metallography, hydrogen measurement, and a drop test designed to assess resistance spot welding energy absorption. Hydrogen measurement tests revealed significant hydrogen content between 5 ppm to 12 ppm throughout the welding samples. The welding parameters with single pulse, double pulse, or double pulse with post-weld heat treatment showed similar reductions in residual stress as observed in the pilot phase. Energy absorption measurements from tensile and Charpy tests yielded different results depending on whether the test samples were painted or just heat-treated. The tear test, using the drop test apparatus, revealed low plastic deformation in heat treated and painted samples, and fracture analysis confirmed these findings.