EFFECTS OF NITROGEN CONTENT ON MICROSTRUCTURE AND MECHANICAL PROPERTY OF ALLOY 690

The microstructure and mechanical property at room temperature of Ni-30Cr-10Fe- x N( x=0.001, 0.011, 0.018, 0.030, mass fraction, %) base alloy 690 were investigated. The OM obser- vation shows that the amounts of precipitated nitrides and the annealing twins in the matrix increase with more nitrogen added in the alloy. The carbide precipitation along boundary and the chromium de- pletion near the grain boundary are modified by nitrogen addition, carbide becomes less and chromium depletion is mitigated. The tensile test result shows that both of the yield and tensile strengths are increased by about 50 MPa when nitrogen content increased from 0.001% to 0.030%, while the ductility lowers a little. The fractographs exhibit typical ductile dimple pattern and the sizes of the dimples reduce with increasing nitrogen content in the alloy.