Ion-Nitriding Of Maraging Steel (MDN 250A Grade) For Light Combat Aircraft Applications

Abstract

Maraging steels have got high strength and good toughness. Maraging steel MDN 250A grade is used for slat tracks on the leading edge of the wing of Light Combat Aircraft (LCA). This application requires high strength and good wear resistance. Required wear resistance for this application can be achieved by ion-nitriding (also called as Plasma nitriding or Glow discharge nitriding) process of case hardening. In conventional gas nitriding process the nitriding temperature is 500 – 550 0C, which is above the aging temperature of maraging steel. Ion-nitriding at temperatures lower than 480 0C is thought to be suitable for surface hardening for application of this material in slat tracks of the leading edge of the wing of LCA. Besides this ion-nitriding is stated to improve fatigue & wear resistance properties without significantly affecting other properties. Also this is a process where the thickness requirement is minimum for the wear properties accomplished. Further, no distortion, warpage and such other dimensional inhomogeneity problems are encountered. In ion-nitriding method glow discharge technology is used to introduce nascent (elemental) nitrogen to the surface of a metal part for subsequent diffusion into the material. The plasma assisted surface modification techniques offer a great flexibility and are capable in tailoring desirable chemical and structural surface properties independent of the bulk properties. It has other advantages like nil or very thin white layer after nitriding and there is no machining or grinding involved for complex parts after the process. The hardened surface layers become an integral part of the basic material without any significant reduction in the properties of base material. Aim of the present work is to study ion-nitriding process on Maraging steels, to carry out ion nitriding of Maraging steel MDN 250A grade at different temperatures and evaluation of properties. Presently, the ion-nitrided slat tracks have been cleared for limited usage instead of full expected life. Studies are also needed to understand the fatigue behavior upon nitriding. The main aspects considered in this study are, optimization of thickness of the ion-nitrided layer and properties required for aircraft applications like slat tracks on the leading edge of the wing where wear resistance, corrosion resistance, mechanical properties like tensile strength, impact strength and fatigue strength are important. To establish the variation in the basic properties of maraging steel (MDN 250A Grade) because of ion nitriding. Aging of Maraging steel is done at 480 ± 5 0C with very close temperature control. In industrial ion-nitriding furnaces the temperature control is of the order of ± 10 0C. As a part of this research work ion-nitriding at three different temperatures which are well below 480 0C were chosen. These temperatures are 435 0C, 450 0C and 465 0C and soaking time i.e., ion-nitriding duration is 10 hours. Microstructure is characterized using optical microscope, scanning electron microscope and X-ray Diffraction. Properties evaluated include Hardness, Case depth, Tensile, Low Cycle Fatigue, Impact and Corrosion by Salt Spray. Optical microscopy and SEM examination don’t show change in core microstructure of the material after ion-nitriding. The iron compound formed in the hardened layer is Fe4N for all the three nitriding temperatures. Ion-nitrided specimens exhibit higher surface hardness and good case depth. Salt spray test results shows that there is animprovement in corrosion resistance after nitriding. The nitrided samples shown higher UTS and 0.2 % PS values than un nitrided ones, where as % Elongation and % Reduction in Area are lower for nitrided samples as compared to un nitrided samples. This can be attributed to the surface layer getting hardened because of ion nitriding. Impact strength of nitrided samples is lower compared to un nitrided samples. This is again due to the surface layer getting hardened because of ion nitriding. Low cycle fatigue properties obtained on ion-nitrided samples show improvement in the fatigue strength due to introduction of the residual compressive stresses. The mechanical properties satisfy the design requirements. Thus ion-nitriding of this Maraging steel (MDN 250A Grade) is suitable for parts which are subjected to constant wear and are complex in shape.