Effect of the dwell time on the microstructure and tensile strength of vacuum-brazed tool steels using BNi-2 filler metal

Abstract

Nickel-based brazing alloys are widely used to vacuum braze numerous base materials such as tool steels, heat resistant steels, austenitic steels, and nickel-based alloys. The formation of intermetallic phases like Ni3Si, Ni3B, or CrxBy can cause a significant embrittlement of the joint. A sufficient diffusion of the melting point depressants boron and silicon will avoid such phase formations and can be primarily affected by the temperature-time-cycle. The process parameters required to achieve an entire nickel solid solution microstructure can be thermodynamically predicted, but usually exceeds the specifications of the heat treatment of the base material by far or necessitate hardly practicable small brazing gaps. This research is focused on the microstructure and the properties of vacuum-brazed joints, using a lower process temperature compared to thermodynamically optimized brazing parameters of AISI H11/BNi-2 joints. In order to investigate the influence of the dwell time, the tool steels AISI H11 and AISI 420 were brazed at 1050\xa0°C for 25 and for 90\xa0min with a BNi-2 amorphous foil (50.8\xa0μm). The extended dwell time has mainly led to a higher Fe/Ni ratio within the brazed metal. Therefore, the average tensile strength was improved from 668 to 1304\xa0MPa for AISI H11 joints as well as from 815 to 1351\xa0MPa for AISI 420 joints. Furthermore, the fracture path was located at the interface brazed metal/diffusion area and could be attributed to a high disparity of the microhardness as well as a weakening by Kirkendall porosity.