Deboronization Phenomena in Fe-Ni Alloys

Effects of atmosphere, holding time and temperature on deboronization phenomena at heating of Fe-36%Ni and Fe-78%Ni alloys have been investigated. The results obtained are as follows: (1) A considerable amount of boron removal was observed during the heat treatment at 1050°C in air. Neither argon nor hydrogen (dew point -40°C) atmosphere could eliminate the deboronization. (2) Boron concentration profiles after deboronization of Fe-36%Ni alloys containing 126ppm B or less in air at the temperature higher than 1050°C are explainable by diffusion controlled mechanism. (3) The diffusion coefficient of boron in Fe-Ni alloys may be represented by the following equations, DFe-36%Ni(cm2/s)=4.6×10-3e-95, 000/RT, DFe-78%Ni(cm2/s)=6.5×10-3e-98, 000/RT, (4) In Fe-Ni alloys, the subscale composed of metal/oxide mixed zone and intergranular oxide is observed. The subscale zone in 36%Ni alloy is far wider than that in 78%Ni alloy. Oxidized boron is solved in the oxide particles of the subscale. Boron concentration is the most depleted at the boundary of subscale and matrix, and increases gradually with increase of the distance from the boundary into the matrix.