Alloy Specification
Austenitic alloys of the chromium-nickel family represent the standard baseline for evaluating corrosion resistant structural metals in industrial eddy current testing. Commonly designated as 304 stainless steel, this material contains approximately eighteen percent chromium and eight percent nickel to maintain its non-magnetic phase under normal conditions. The nominal electrical conductivity of this metal is about two point five percent of the international annealed copper standard.
Electromagnetic Characteristic
Mechanical deformation or cold working of the metal causes a localized phase transition from austenite to martensite. This transition introduces ferromagnetic properties that alter the incremental magnetic permeability.
Calibration Procedure
Eddy current sorting relies on standard reference blocks to establish the instrument baseline. When calibrating for 304 stainless steel, the operator sets the operating frequency of the sensor to minimize lift-off effects and maximize phase separation between conductivity and permeability variations. These reference blocks must be verified against primary standards to ensure measurement traceability.
Measurement Uncertainty
Temperature changes during the test cycle introduce thermal drift that modifies the electrical resistivity of the sample. This thermal drift can be compensated by using a sensor with an integrated temperature transducer. The allowed tolerance for conductivity measurements is typically set at plus or minus zero point three percent of the international standard.
Failure to control specimen temperature leads to inaccurate sorting.
