Furnace Brazing of AlSi-coated and Uncoated 22MnB5 Steel at Austenite Temperatures

Sammanfattning: By continued advances in mechanical strength of press hardened boron alloyed steel (22MnB5) weight savings can be achieved by the use of less material. However, with reduced material thickness stiffness problems such as buckling arises and the high strength of the material can’t fully be taken advantage of. A solution to the stiffness problem could be to create a sandwich structure using 22MnB5 steel. To create a sandwich structure the faces have to be joined to the core. Work has been done concerning various methods of joining both coated and uncoated 22Mnb5 after being press hardened. A more direct approach would be to implement a joining process in the hot stamping line prior to forming and quenching. A joining method suitable for this task is Controlled Atmosphere Brazing (CAB), which today is used for both aluminum and steel brazing and could be implemented in a hot stamping furnace. Using a tube furnace with continuous gas flow of N2 overlap braze joints were produced on both AlSi-coated 22MnB5 (USIBOR® 1500P AS150) and plain 22MnB5. Material combination evaluated included two different brazing foil and two different flux paste in combination with various heat cycles. Both one-step braze cycles with brazing directly at austenite temperature and two-step braze cycles with brazing at a lower temperature followed by heat treatment at austenite temperature were developed. Evaluation of braze joint strength was done using tensile testing and the same was done to evaluate the coating strength of USIBOR® 1500P AS150 after heat treatment using adhesive to create an overlap joint. An adhesion pull-off test was used to determine USIBOR® 1500P  AS150 coating strength after various heat cycles. Scanning Electron Microscope (SEM) with Energy Dispersive Spectrometry (EDS) was used to investigate amount of Fe-diffusion from substrate into both coating and joint due to heat cycles and to determine phases connected to fracture location of joints. With NiCr-based brazing foil joints between plain 22MnB5 were produced with a braze time of 10 minutes at 950° C that had an avg. shear strength of 20 MPa. Brazing above the liquidus temperature of the filler material for 5 minutes with a higher furnace temperature showed no decrease in shear strength of joints, but reduced time in furnace by 16 minutes giving a total furnace time of 9 minutes. Best results for joints between USIBOR® 1500P AS150 substrates were achieved using a two-step brazing cycle. Brazing was done at 593° C for 35 minutes with AlSi12 brazing foil and flux paste recommended for aluminum brazing. Afterwards specimens where heated for 4 minutes above austenite temperature and had an avg. a shear strength of 5,4 MPa. EDS-analysis showed that fracture in braze joints and in coating of USIBOR® 1500P AS150 was connected to the intermetallic phase Al5Fe2 as well as Fe-diffusion from substrate was higher than in as received conventionally press hardened USIBOR® 1500P AS150. Adhesion pull-off tests indicated that the heat cycles used in this study significantly reduced coating strength compared to as received conventionally press hardened USIBOR® 1500P AS150.