Physical metallurgy of laser surface melted plastic mould steels: a case study

The purpose of this paper is to illustrate the potential of laser surface melting to improve the surface characteristics of plastic mould steels, using a typical plastic mould steel (DIN X43Cr12) as a case study. After laser surface melting the microstructure of this steel is formed by fine dendrites of austenite partially transformed into martensite. Although the equilibrium solidification phase is 8- ferrite, the formation of primary austenite is kinetically favored and this phase tends to predominate at the high solidification speeds used in laser processing. It was observed that the volume fraction of retained austenite depends critically on the laser processing parameters, so that the microstructure can change from almost completely martensitic to almost completely austenitic by changing the laser processing parameters. Laser melted tool steels show remarkable secondary hardening after tempering at suitable temperatures. In DIN X42Cr13 the secondary hardening peak temperature after LSM (600°C) is 100°C higher than after conventional heat treatment (500°C), due to the presence of large amounts of retained austenite. It was observed that this phase only destabilizes above 600°C, due to the precipitation of M7C3 and stress relieving. After destabilization, retained austenite transforms into martensite during cooling. Secondary hardening is due to the transformation of retained austenite into martensite and to the precipitation of M7C3 and M23C6 carbides. El objetivo del presente trabajo es ilustrar el potencial de la fusión superficial mediante láser para la mejora de las características estructurales de los moldes de acero para plásticos, centrándolo en el caso concreto del acero DIN X42Cr13. Tras el tratamiento de fusión superficial mediante láser, la microestructura del material está formada por dendritas finas de austenita parcialmente transformadas en martensita. Aunque la fase en equilibrio sería ferrita 8, la formación de austenita primaria está favorecida cinéticamente, por lo que esta fase tiende a ser predominante a las altas velocidades de solidificación que se consiguen en los tratamientos láser. Se ha observado que la fracción de volumen de austenita retenida depende de manera crítica de los parámetros del procesado con láser , así como que la microestructura puede cambiar de martensita a austenita en función de dichos parámetros. En la fusión superficial del acero DIN X42Cr13 se obtiene un pico de endurecimiento secundario después del tratamiento con láser a 600°C, mientras que, para los tratamientos térmicos convencionales éste aparece