Influence of Heating Conditions on the Oxidation Behavior of Fe-36Ni Invar Alloy

The oxidation of Fe-36Ni invar alloy at 1273-1473 K in 15% CO2-(0, 3, 5, 6)% O-2-N-2 atmosphere and the effect of the grain structure on the compressed cracks were investigated using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and a microcomputer control universal material testing machine. The results showed that the oxygen content and the oxidation temperature had an obvious influence on the morphology of the external scales, which changed into only two-layer structures from three stratified structures when the oxygen content was 0% or the oxidation temperature was below or equal 1423 K. However, the subscales still consisted of a typical structure of the intergranular subscale and the intragranular subscale with different oxygen contents or oxidation temperatures. The growth rate of the scales did not depend on the partial pressure of oxygen when oxygen existed. The oxidation temperature had an obvious influence on the growth rate of the scales, the growth rate of the scales was slower below or equal 1423 K and faster over 1423 K. A linear relationship existed between the thickness of the scales and the square root of oxidation time, moreover, the estimated activation energy for the growth of the external scale, the intragranular scale and the intergranular scale were 165, 159 and 131 kJ/mol, respectively. The higher apparent activation energy corresponded to the thinner scale at each oxidation temperature. In addition, the compressed cracks were hard to propagate and grow in the specimen with equiaxed grain structures, which would result in the narrower and shallower compressed cracks and the better surface quality during the compressed process after oxidation.