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 57Fe Mössbauer 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 ɛ-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 ɛ-Cu precipitates as well as the transforming of retained austenite into lath martensite.

► Surface hardening of 17-4PH is highly achieved by laser transformation hardening.
► A hardened layer with a thickness of 1.75 mm is formed.
► The phase similar to ɛ-Cu precipitates re-segregation after dissolved.
► The strengthening phase is composed of classic lath martensite, coarse NbC and a lot of finer fcc copper-rich phases.