13 Stress Assessment in Precipitation Hardened IN718 Nickel-Base Superalloy Based on Hall Coefficient Measurements

The feasibility of residual stress assessment in precipitation hardened IN718 nickel-base superalloy based on Hall coefficient measurement is investigated through studying the influence of thermal hardening, cold work, and applied stress. Measurements in IN718 specimens of various hardness levels show that the Hall coefficient increases from 8 +/- 0.1 x 10(-11) m(3)/C in the fully annealed state of 14 HRC to 9.4 +/- 0.1 x 10(-11) m(3)/C in the fully hardened state of 45 HRC. Measurements in IN718 specimens of various cold work levels show that plastic deformation exerts negligible effect on the Hall coefficient of fully annealed IN718, while in fully hardened IN718 the Hall coefficient decreases more or less linearly with cold work from its peak value of 9.4 +/- 0.1 x 10(-11) m3/C in its intact state to 8.9 +/- 0.1 x 10(-11) m(3)/C in its most deformed state of 22% plastic strain. Measurements taken under applied stress show that elastic strain significantly increases the Hall coefficient of IN718 regardless of the state of hardening. The relative sensitivity of the Hall coefficient to elastic strain is called the galvanomagnetic gauge factor and defined as the ratio of the relative change of the Hall coefficient divided by the axial strain under applied uniaxial stress. The gauge factor of IN718 is in the range of 2.6-2.9 depending on the hardness level. Besides the fairly high value of the gauge factor, it is important that it is positive, which means that compressive stress in surface-treated components decreases the Hall coefficient in a similar way as plastic deformation does, therefore the unfortunate cancellation that occurs in fully hardened IN718 in the case of electric conductivity measurements does not happen in this case. In addition, the influence of thermal exposure up to 700 degrees C and the reversible temperature dependence of the Hall coefficient at room temperature are studied in IN718 at different hardness levels.