Microstructure and hardness investigation of 17-4PH stainless steel by laser quenching

Surface hardening of 17-4PH was achieved by laser transformation hardening using 5 kW continuous wave CO2 laser system. The microstructure of the laser-quenched sample was investigated by optical microscopy, transmission electron microscope and Fe-57 Mossbauer spectrometer. The hardness profile was determined by a Vickers hardness tester. The hardened layer with a thickness of 1.75 mm was formed, and it was composed of classic lath martensite, coarse NbC and a lot of finer fcc copper-rich phases which were similar to epsilon-Cu precipitates. The maximal hardness value of hardened zone is 446 HV which is 50 HV higher than that of the substrate (386-397 HV). The higher hardness in laser transformation layer of the 17-4PH steel could be attributed to the following aspects: the matrix with a high dislocation density; the fine microstructure; the finer fcc copper-rich phases that were similar to the epsilon-Cu precipitates as well as the transforming of retained austenite into lath martensite. (C) 2011 Elsevier B.V. All rights reserved.