The impact of plasma nitriding bias voltages on mechanical characteristics of 25 Cr2Ni4W steel

This study investigates the influence of negative bias voltage on the plasma nitriding be-havior of 25Cr2Ni4W low-alloy steel to enhance its surface mechanical and tribological performance. The bias voltage was systematically varied from 2.0 to 3.5 kV, while dis-charge power, pressure, and treatment time were kept constant. Substrate heating was achieved through a self-induced mechanism. X-ray diffraction (XRD) analysis confirmed the formation of a compound layer consisting of ε-Fe₂₋₃N and γ′-Fe₄N phases. With in-creasing bias voltage, a progressive transformation toward the γ′-Fe₄N phase was ob-served, accompanied by a reduction in ε-Fe₂₋₃N content. This microstructural evolution significantly affected surface mechanical properties. Nanoindentation results revealed a marked increase in hardness compared with the untreated substrate. The sample treated at 2.0 kV (CW1) exhibited the highest and most stable nanohardness (≈ 6000 MPa), indi-cating a dense and coherent nitride layer. Conversely, the 3.5 kV (CW4) condition yielded a lower nanohardness (≈ 4000 MPa) due to excessive sputtering and reduced nitrogen diffusion. These results demonstrate that accurate control of negative bias voltage is a key factor in tailoring the nitride phase composition and optimizing surface hardening in plasma-nitrided 25Cr2Ni4W steel.