Strength Properties pf 25CrMoV Steel Modified by Complex Ion Plasma Treatment with Deposition of Interlayer Metal Coatings

Abstract

To improve erosion resistance, strength, and other protective properties, a comprehensive modification of the surface layers of 25CrMoV steel, which is widely used in turbine construction, was performed. For comparative studies, modifications with different interlayer materials (Mo and Ti) and modifications without interlayer were used. The Mo and Ti layers were deposited on a nitrided ion plasma surface. The outer protective layer for all modifications was unchanged and consisted of a Mo2N coating. To determine the role of the deposition of interlayer metal coatings on the strength properties of the complex modified coatings, the distributions of hardness (H, GPa), Young's modulus (E, GPa), and other strength parameters (H/E and H3/E2) measured by cross sections (h, µm) were investigated. The hardness of the Mo₂N coating was ~30 GPa, and the hardness of the nitrided layer was ~12 GPa. The modulus of elasticity for the Mo₂N coating was ⁓415 GPa, and for the nitrided steel - ⁓270 GPa. It was found that the main factor influencing the strength properties of a multilayer structure is related to the different materials of the metal layers. For the Mo and Ti layers, the values of E differ significantly (⁓ 340 GPa and ⁓ 180 GPa, respectively), with almost identical values of H (⁓ 6.5 GPa). The distributions of elastic modulus E = f(h) measured in the modified layers correlate well with the distributions of nitrogen concentration C_N = f(h). The distributions of H/E = f(h) and H³/E² = f(h) for the modifications with Mo and Ti layers show a decrease in mechanical properties in the areas of the intermediate layers (Mo and Ti). For the modification without interlayer, the distributions of these indicators do not show such a drawback. The cavitation resistance of the comprehensively modified 25CrMoV steel is up to 2 times higher than that of the steel in the original condition. NSC KIPT performed extensive ion plasma modification on a pilot batch of turbine parts. These products, which are part of the steam distribution mechanisms, were manufactured by Ukrainian Power Machines JSC (Kharkiv) for the thermal power industry.