| H13 hot tool steel is a kind of hot working die steel and widely used in die and mould industry.It has attracted great interesting because of its good hardenability,strength,toughness and thermal fatigue resistance.Conventional mould manufacturing methods have some drawbacks,such as tedious fabrication process,long process flow and process cycle,high costs,etc.It also can’t meet the needs of people for complex and personalized mold.In this study,laser additive manufacturing(LAM)technology was adopted to produce H13 tool steel.The LAM technology has the advantage of fabricating complex components and easy to automation as well as high effciency.The microstructure,mechanical properties and tribological properties of the specimens under different heat treatment conditions were studied in detail.Finally,the surface of LAMed H13 steel was coated with Ni/WC coating by surface modification technology to improve its surface properties.The purposes of this research are to provide theoretical support and practice guidance for laser additive manufacturing technology in the field of die manufacturing and remanufacturing.The microstructure,phases composition,morphology of precipitates,mechanical properties and tribological properties of H13 and surface coating were studied by using optical microscope(OM),scanning electron microscopy(SEM),energy spectrum analyzer(EDS),transmission electron microscopy(TEM),XRD diffraction,hardness tester,electronic universal tensile test machine,nano-indentation tester and high temperature wear test machine as well as other modern materials analysis and testing equipment.The results show that,on the condition of optimizing the process parameters,the H13 specimens produced by LAM technology have a compact structure,without visible cracks and pores.The microstructure of the as-deposited H13 steel consists of martensite and retained austensite as well as some fine carbides distributed at the inter-dendrite.The retained austensite decomposes and the martensite converts into tempered martensite during the process of tempering.There are lots of short rod like and acicular carbides(VC)precipitated in the matrix when tempered at 550℃.As tempered at 650℃,there are two kinds of carbides in the martix.One is the spheroidal carbides with the size of 5～15 nm;the other is ellipsoidal carbides with the size of 50～100 nm.The average microhardness of the as-deposited samples is 535 HV0.3.At the beginning,the hardness increases with the rising of the temperature,while further rising the tempering temperature,the hardness decreases.Secondary hardneding peak occurs with the value of 600 HV0.3 when tempered at 550 ℃.The tendency of nanoindentation hardness is similar with that of microhardness.The maximum and minimum nano-hardness are 6119.38 MPa and 5145.38 MPa when tempered at 550℃ and 650℃ respectively.The elastic modulus of the samples under different tempered conditions is about 320 GPa.Tensile tests show that when the tempering temperature below 550℃,the main characteristic of the broken pieces is cleavage fracture mode.The greatest ultimate tensile strength is 1928.2 MPa with the elongation of 6.4%at the temperature of 550℃.As the temperature ranging from 600℃ to 650℃,the ductile fracture mode can be observed on the broken pieces and the tensile strength decreases while the elongation increases.The impact energy of the H13 steel under different tempered conditions is low with the value ranging from 6 to 12 J.The wear test time is 75 min at room temperature.The results show that the value of minimum volume loss is 0.0124 mm3 when tempered at 550℃.Compared with the as-deposited samples,the wear resistance is three times as good as that as-deposited ones.The wear mechanism is mainly adhesive wear and mild oxidative wear.When the ambient temperature is 400℃ and the test time is 75 min.The results show that the specimens with the minlium volume loss is 0.2192 mm3 when treated at 650℃.Compared with as-deposited ones,the wear resistance of treated samples is three times better than the as-deposited ones.High temperature wear mechansim is mainly oxidative wear as well as abrasive wear.After surface modification of LAMed H13 steel,the phases of the clad layer are mainly composed of γ-Ni,M23C6 and Ni3B.There are good metallurgical combination between ceramic phase WC particles and Ni-based alloy as well as H13 and clad layer.The dilution rate of the clad layer is about 5.5%.The average hardness of the clad layer is 16%higher than that of the matrix.The wear resistance of the cald layer is twice as good as the martrix,and the wear mechanism is abrasive wear.Last,the reaserch results of this project have been applied in the H13 mode products of a well known aluminum production enterprise. |
PDF Full Text Request