Microstructures And Mechanical Properties Of 945 Steel Fabricated By Laser Melting Deposition Process

High-Strength Low-Alloy steel(HSLA)are widely used in ships,high-speed rail,and bridges,etc.,due to the advantages of high strength,high toughness,and low cost.945 steel is a HSLA alloy independently developed by our country,and is mainly used in the manufacturing of some ships.The development of ships requires methods and techniques suitable for the fast near-net shape fabrication and fast repairing of damaged parts.As an advanced near net-shaping additive manufacturing technique,Laser Melting Deposition(LMD)technique supplies a new method for the fabrication and repairing of some 945 steel components.In this study,945 steel thin walled samples have been prepared by LMD technique using different laser powers ranging from 600 W to 1600 W.Further heat treatments at 550 ?,600 ?,and 650 ? for 1 hour have been carried out to samples prepared using 800 W laser power,which have the best combination of strength of ductility.Microstructures and mechanical properties of the LMD sample and heat treated samples have been investigated using OM,SEM,EBSD,TEM,XRD,micro-hardness tester,tensile tester,etc.The effects of laser powder and heat treatment temperature on the microstructures and mechanical properties have been discussed.The main results and conclusions are as follows.All the LMD samples prepared using different laser powers have high densities larger than 99.6%.The microstructures of the LMD samples mainly contain lath bainite,residual austenite,polygonal ferrite,and pearlite.Samples prepared at lower laser powers mainly consist of lath bainite and residual austenite.As the laser power increases,the contents of residual austenite decreases,however,the polygonal ferrite and pearlite start to present.Meanwhile,the grain sizes of primary austenite and bainite block increase with the increasing laser power,from 150 ?m and 2 ?m to 1110 ?m and 8 ?m,respectively.The microstructural evolution is mainly related to the thermal history during the LMD process.For a single walled sample,the circular heat treatment on the deposited part and the heat accumulation lead to the formations of lath bainite.As the laser power increases,the cooling rate decreases,and the duration at high temperatures increases,leading to the coarsening of lath bainite,and the formations of ferrite as well as pearlite.Samples prepared at the 800 W laser power have a good combination of strength and ductility,showing a yield strength of 730 MPa,a tensile strength of 920 MPa,and a total elongation of 17.4%.In general,samples prepared by LMD have higher yield and tensile strength,but lower ductility than samples prepared by traditional methods.After heat treatments at 550-650 ? for samples prepared using 800 W laser power,no residual austenite can be observed,indicating a decomposition of austenite at these temperatures.OM observations find a blur of lath block boundary,and an increase of the width of lath block.The OM images have higher contrast after heat treatments,suggesting locally precipitations of cementite.TEM observations show a slightly coarsening of bainite lath,which is 503 nm compared to 480 nm before heat treatments.The tensile tests show that after heat treated at 550? for 1 hour,the mechanical properties of the LMD samples have been significantly improved.The yield strength,the tensile strength,and the total elongation have been improve by 15%,10%,and 32%,respectively.The yield strength and tensile strength are much higher than those of traditionally prepared samples,and the elongation is close to that of traditionally prepared samples.The improvement of mechanical properties may related to the precipitation of second phases,and the release of residual stress,of which the verifications require detailed TEM investigations on the microstructures,and the characterization of stresses.