M2 coating prepared by ultra-high speed laser cladding: Microstructure and interfacial residual stress

The coating of M2 high speed steel was prepared on 42CrMo steel substrate using ultra-high speed laser cladding technique (EHLA in German). Our results show that the M2 cladded coating formed a dentate metallurgical bonding with 42CrMo steel substrate. The morphology of the M2 coating involved from the “corn” shape structure at the interface, to “column” shape in the middle zone, and final a “basket” shape near the surface. The near-surface zone of the M2 coating is composed of residual austenite + lamellar twinning martensite + network carbides, and the latter being composed of unstable V4C3 and unstable Cr3(W10C3)2, in accordance with the parallel phase relationship of [5̅15̅3]V4C3[110]Cr3W10C32and (015)V4C3(1̅15)Cr3W10C32. The residual stress was measured using nanoindentation with the assistance of atomic force microscope (AFM) for direction measurement of residual indents to account for pile-up. An analysis method was proposed by combining the AFM measurement and contact mechanics calculation. Our results show high tensile residual stress existing across the whole M2 coating and even extended over 100 µm deep into the heat affected zone in the substrate; the peak value of about 311 MPa was found near the interface area and then dropped sharply. Good agreement was found in results obtained using our method and Oliver & Giannakopoulos (G&S) method, hence validating the G&S method for future application in this material system without the need to use AFM for evaluation of pile-up.