BRAZING FILLER METAL WITHOUT BORON AND SILICON FOR BRAZING OF HEAT-RESISTANT NICKEL ALLOY

摘要

The use of nickel brazing filler metals, containing boron and silicon as depressants, provides a good wetting of material brazed, allows a significant reduction in brazing temperature, but leads to formation of brittle phases and low-melting eutectics in the brazed welds. This work shows the possibility of forming brazed joints of cast heat-resistant nickel alloy ZhS6U applying brazing in vacuum with use of multicomponent nickel brazing filler metals which do not contain boron and silicon as depressants. Applying the method of high-temperature differential thermal analysis in atmosphere of high-purity helium, the temperatures of liquidus and solidus of brazing filler metals were determined. The results of metallographic and micro-X-ray spectral examinations on studying the features of structure formation of brazed welds are presented. The long-term strength of the brazed joints was evaluated. It is shown that nickel brazing filler metals, containing a large concentration of zirconium, are characterized by a lower melting point, however, in the brazed welds the precipitations of the phase Ni(Me)_xZr are formed. It was determined that decrease in the concentration of zirconium in the nickel brazing filler metal allows avoiding the formation of intermetallic phase enriched in zirconium and obtaining a solid nickel-based solution as the predominant phase in the weld. The results of tests of flat brazed (butt) specimens on long-term strength, carried out at the elevated temperature of 975°C and the stress of 140 MPa, showed that the joints preserve integrity and do not fracture after 41-60 h of testing. It is shown that zirconium can act as an alternative depressant (instead of mutual adding of boron and zirconium).