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This paper systematises the destructive reference characteristics of ion-nitrided and carbonitrided layers on BH11 (4CrSMoVSi, VS4659) and D2 (Cr12MoV) tool steels produced in 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 (δ, 55–145 µm) and surface hardness (1150–1300 HV₀.₁). Concrete calibration procedures, an integrated inline station concept, and a validation strategy including statistical process control (SPC) 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 data-fusion model reduces MAE in δ estimation to ±10–15 µm and provides reliable OSZ classification (accuracy > 92 %).