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This paper systematises the destructive characteristics of ion-nitrided and carbonitrided layers on BH11 (4Cr5MoVSi) and D2 (Cr12MoV) tool steels produced in a glow-discharge plasma at 550 °C and ~400 Pa, and investigates their suitability for calibrating and validating non-destructive testing (NDT) methods. Two principal NDT techniques are examined—eddy current testing (ECT) and the Barkhausen noise method (BNM)—whose complementary depth sensitivity enables simultaneous estimation of effective case depth (δOB = 55–145 μm) and surface hardness (1150–1300 HV). Concrete calibration procedures, an integrated in-line station concept, and a validation strategy including statistical process control are proposed. Multi-frequency ECT (50–200 kHz) proves most sensitive to total diffusion-zone depth, while BNM (50–200 Hz magnetisation) captures near-surface hardness changes. A combined fusion model reduces mean absolute error in δOB estimation to ±10–15 μm and provides reliable compound-layer thickness classification.