An experimental investigation with 4 kW diodes laser system was carried out to study the effects of laser hardening process parameters on properties of P/M steels, based on prealloyed and diffusion-bonded powders. Eight different process parameters have been applied to optimize the process afterwards they have been set to two different compositions. The independent variables investigated have been: surface temperature, T, and laser travel speed (mm/min). The microstructural features of the laser hardened P/M steels were analyzed using LOM and the surface morphology has been characterized by SEM. Hardened depth both via hardness test and via optical measures were evaluated as well and used as responses for the ANOVA of the experimental data gathered. This work reveals the possibility to apply a selective and precise hardening treatment, like Laser Transformation hardening (LHT), to high-property P/M steel parts. Thanks to ANOVA analysis the correlation between energy density (ED) and optical dimension of treated zone has been identified. By increasing temperature and speed the density energy ED decreases, indeed at high temperature and speed the heated zone is larger. At the same speed, a temperature increase leads to a surface hardness decrease. A similar value of hardness depth (about 0.8 mm) has been detected by means of hardness profiles. The result has been strengthened by scratch test. The parabolic zone presents a very fine martensitic structure on the top of laser trail, and bainite microstructure on transition zone, as literature related. The transition zone between the laser affected zone and the base metal exhibits a micro hardness gradient. It is also possible to notice some Cu diffusion from outer to inner zone. By comparison among LHT specimens and sintering-hardened and stress-relieved one, (characterized by similar chemical compositions) penetration depth and hardness values appear similar. Transformation Hardening (LTH) is a suitable process for producing hard surface, on defined spots of P/M components, through the action of a scanning laser beam. The short heating time enables to decrease part distortion and surface oxidation, in comparison with sinter hardened. The possibility of very selective and precise hardening treatment seems to open new possibilities to widen the use of high-property P/M parts, based on advanced design, for demanding applications.
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|Data di pubblicazione:||2016|
|Titolo:||Tempra laser di acciai sinterizzati basso-legati|
|Autori:||Bocchini, G.F.; Colombini, E.; Mannini, A.; Parigi, G.; Poli, G.; Sola, R.; Veronesi, P.|
|Appare nelle tipologie:||Articolo su rivista|
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