33 VOLUME THREE APRIL 2019 PLATFORM PLATFORM - A Journal of Engineering MICROSTRUCTURE EVALUATION OF DISSIMILAR LASER BEAM WELDS –MDN 250 AND 15CDV6 STEELS M. V. L. Ramesh 1 , P. Srinivasa Rao 2 , V. Venkateswara Rao 1 1 Advanced Systems Laboratory, Hyderabad - 500 058, India 2 Mechanical Engineering, Universiti Teknologi PETRONAS Email: srinivasa.pedapati@utp.edu.my ABSTRACT A novel dissimilar Laser Beam Weld of MDN 250 and 15CDV6 steels with adequate joint strength was successfully achieved using CO2 laser source. Electron probe microanalysis of the weld, weld microstructure, micro-hardness measurement across the weldment and transverse tensile properties of the weldment were evaluated. The fusion zone of the dissimilar welds exhibits an epitaxial grain growth. In dissimilar metal welds, HAZ (Heat Afected Zone) width is more on 15CDV6 steel side compared to MDN 250 steel side. An asymmetrical hardness profle across the dissimilar weld joint was observed, and HAZ region on 15CDV6 steel exhibited a high hardness. The tensile strength values of the welded specimens were close to that obtained for sheet specimens. Keywords: Laser beam welding, MDN 250 steel, 15CDV6 steel, Mechanical properties, Microstructure. INTRODUCTION The industrial application of ultra-high-strength steels is increasing day by day. In some cases, a diferent combination of these steels is used for technical reasons. The unique combination of MDN 250 and 15CDV6 steels are preferred in certain application due to the unique combination – ultrahigh strength and excellent fracture toughness [1], [2]. Chemically MDN250 maraging steel is having high nickel cobalt, molybdenum contents and very low carbon content. The combination of martensite and age hardening resulted in the term maraging[3]. The low carbon content in these steels provides excellent dimensional stability during the austenite-martensite transformation. The 15CDV6 steel is having low concentrations of its alloying elements - chromium, molybdenum and vanadium. Post weld heat treatment is usually not required for these steels as the properties are not afected by welding. Preheating is not recommended for welding of plates up to 10 mm thick[4,5]. Both the 15CDV6 steel and MDN250 steel exhibit good weldability and are being widely used in applications such as motor casings, landing gears etc. However, not much literature is available about the dissimilar combination of these steels [6],[ 7], [8]. In many engineering applications, welding is often performed between dissimilar metals to meet the end functional requirements. A strong weld joint between diferent metals should possess sufcient tensile strength and ductility so that joint will never fail when it is under service conditions [9]. The energy concentration of the laser beam is high. During the laser beam welding, workpiece receives small heat input resulting in negligible heat afected zone [10]. Hence laser beam welding was adopted for the study. Post weld heat treatment of the dissimilar weld joints are difcult, due to the diferent heat treatment schedules of the steels joined. Further, as the strength