Laser Additive Manufacturing Of 12CrNi2 Alloy Steel And Heat Treatment Regulating Of Its Microstructures And Mechanical Properties

Laser additive manufacturing(LAM)that can realize high-precising and highperformance rapid manufacturing of the complex parts which is divided into two important technical methods:laser melting deposition(LMD)technology and selective laser melting(SLM)technology.The LAM can deposit powders or wires on the substrates through high-energy laser beams to achieve rapid prototyping of complex components.However,there are comparatively few studies on the LMD manufacturing of the low-alloy steels,which mainly because of its poor mechanical properties resulting from the difficulty control in the microstructure.First of all,the performance of the manufactured alloy steel tends to be unstable in case of the applied powders containing more or less impurities.Furthermore,due to the rapid non-equilibriummelting and solidification process in the LMD manufacturing process,it is difficult to effectively control the phase transformation process and microstructure of the formed alloy steel.In summary,it is necessary to optimize the characteristics of alloy steel powder and regulate the microstructure and comprehensive mechanical properties of LMDed alloy steel through post heat treatment.In this paper,12CrNi2 alloy steel is manufactured by LMD technology.The effect of powder O content on the microstructure and mechanical properties of the formed alloy steel is studied.At the same time,the LMDed alloy steel was subjected to austempering treatment with different processes.The excellent comprehensive mechanical properties of the heat-treated alloy were obtained by optimizing the corresponding microstructure.The main findings of this paper are as follows:1.Two kinds of 12CrNi2 alloy steel powders with different O contents(5300 ppm and 350 ppm)were used for LMD manufacturing.The porosity with smooth inner walls appear in the alloy steel manufacrued by applying the 12CrNi2 powder containing high O content.It was found that the formation of the porosity was thought resulting from the capture of the melting pool on the COx bubble which derived from the reaction of the residual O in powder and carbon in the alloy steel.A LMDed alloy steel without pores containing ferrite and austenite islands was finally obtained by reducing the powder O content.The elongation and tensile strength of the LMDed alloy steel was improved.2.The applied commercial powder adding 2wt.%,3 wt.%and 4 wt.%Cr particles were manufactured by LMD process respectively.The porosity in the manufactured alloy steel was prevented by mixing with 4 wt.%Cr particles.The element Cr preferentially reacts with element O to form more thermodynamically stable Cr2O3 which inhibiting the formation of the CO and CO2 gases.This also refines the microstructure of the manufactured 12CrNi2 alloy steel.3.The LMDed alloy steel composing of major ferrite and minor austenite,isothermal quenching treatment at 200-400℃ for 5-20 s was applied foe the enhancing of its mechanical properties.The formation of "lath bainitic ferrite/austenite/lath bainitic ferrite" multilayer structure with high volume fraction leads to the obvious improvement of strength of the LMDed alloy steel.Meanwhile,more coordinated deformation between proeutectoid ferrite and bainite benefits the ductility of the LMDed alloy steel.4.In order to further elevate the tensile strength of the LMDed alloy steel,different intercritical austempering processes,which were deliberately-designed for obtaining FB dual phase(DP)alloy steels with similar volume fraction but a different morphological feature of bainite,were used.The strength of the LMD manufactured 12CrNi2 alloy steel were greatly improved by forming the F-B DP microstructure.Furthermore,the larger-size ferrite-bainite DP microstructure exhibited higher strength and elongation than the fine DP microstructure which is mainly due to its higher work hardening ability benefiting from the larger difference in carbon content between bainite and ferrite and its pronounced heterogeneous carbon distribution in the bainite.5.The microstructural development of the isothermal-quenching bainite with a coarse prior-austinite and a fine prior-austinite under a thermal cycle treatment was studied.The morphology of bainite with finer bainite ferrite lath and austenite film with a decrease in prior austenite grain size was obtained.Both the bainitic ferrite lath width and the retained austenite film thickness decrease which resulting to the increased strength of the manufactured alloy steel.In the prior-austenite refined bainite,the relative lower dislocation density and a small amount of twins are conducive to release stress,which promote the excellent comprehensive mechanical properties in the formed alloy steel.