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

Abstract

This study investigates the influence of negative bias voltage on the plasma nitriding behavior 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 discharge 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 increasing bias voltage, a progressive transformation toward the γ′-Fe₄N phase was observed, 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), indicating 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.