Research On Microstructure And Properties Of Laser Cladding Layer Reinforced By Boron Nitride On The Surface Of Martensitic Stainless Steel

Martensitic stainless steel is widely used as hydraulic turbine flow-parts.Its surface is subject to erosion and wear from sand particles,as well as the corrosion from the p H of the water flow medium.In this thesis,high-performance cladding layers were prepared on the surface of martensitic stainless steel（ZG06Cr13Ni4Mo）by laser cladding technology.It has great engineering application value,which can prolong the service life of flow-parts and reduce material loss.In this thesis,Ni60AACu Mo and 316L powders were used as cladding materials respectively,and the Ni coated c BN powders（Ni/c BN）were used as reinforcement phase.The RFL-A2000D semiconductor laser was used to prepare Ni-based,316L and Ni/c BN composite cladding layers on the surface of martensitic stainless steel substrate by equiaxial synchronous powder feeding.The influence of Ni/c BN on the microstructure and properties of Ni-based and 316L cladding layers was characterized and analyzed by XRD,OM,SEM,EDS and other methods,as well as microhardness test,wear and erosion test,electrochemical corrosion test and other tests.The main findings are as follows:Optimized process parameters of Ni60AACu Mo cladding layer are as follows:laser power 1100 W,scanning speed 9 mm/s and powder feeding rate 9 g/min.Optimized process parameters of 316L cladding layer:laser power 1100 W,scanning speed 6 mm/s and powder feeding rate 8 g/min.The Ni60AACu Mo and 316L cladding layers with good bonding with the substrate were successfully prepared with a lap rate of 45%.The Ni60AACu Mo cladding layer is composed ofγ-Ni solid solution,Fe Ni₃,carbides M₂₃C₆ and Cr₇C₃,borides Cr B,Cr₅B₃ and eutectic product Ni₄B₃.Theγ-phase solid solution matrix is island-like and dendrite-like,surrounded by a large number of eutectic substances,few borides,and few carbides.The phase and microstructure of the cladding layer change after the addition of Ni/c BN.A new phase,Ni₃B,appeares in the Ni-5 wt.%Ni/c BN composite cladding layer,and Cr₅B₃disappears when the addition of Ni/c BN increase to 15 wt.%.The content of eutectic structure increase with the increase of the addition amount,and the microstructure is obviously coarsened at 15 wt.%.The 316L cladding layer is mainly composed ofγ-phase solid solution(Fe Cr_0.29Ni_0.16C_0.06),and the microstructures are plane,columnar,dendrites and equiaxed crystals from the junction to the surface.The 316L composite cladding layers reinforced by Ni/c BN are composed ofγ-phase solid solution(Fe Cr_0.29Ni_0.16C_0.06)and a small amount of undecomposed boron nitride particles.The microstructure of the cladding layers tends to be refined with the increase of the addition amount.The microhardness and wear resistance of the Ni-based layer are better than the substrate.The microhardness and wear resistance increase and then decrease with the increase of Ni/c BN addition.The highest are obtained when the addition amount was5 wt.%,and the microhardness and wear amount are 878.8 HV and 11.4 mg,respectively.The microhardness and wear resistance of the 316L cladding layer are improved under the strengthening effect of Ni/c BN reinforced phase.With the increase of Ni/c BN addition,it shows a trendency of first increasing and then decreasing.When the addition amount is 5 wt.%,the microhardness and wear resistance are the highest,and the microhardness and wear amount are 264.4 HV and44.9 mg,respectively.The wear mechanism of the composite cladding layer is mainly abrasive wear,and the wear morphology appears as furrow.The erosion resistances of Ni-based and 316L composite cladding layers are better than the matrix,and the erosion rate shows a trend of first decreasing and then increasing with the increase of Ni/c BN addition.When the addition amount of Ni/c BN is 5 wt.%,the erosion rates of Ni-based and 316L composite cladding layers are the lowest,which are 26.3 g·m^-2·h^-1 and 33.3 g·m^-2·h^-1,respectively.Compared with the substrate,it decreased by 63.8%and 54.2%respectively.The erosion morphology of the Ni-based composite cladding layer is mainly short-range furrows.While the erosion morphology of the 316L composite cladding layer is dominated by impact pits.Both Ni-based and 316L composite cladding layers show good passivation states.With the increase of Ni/c BN addition,the corrosion resistance of Ni-based cladding layer increases first and then decreases,and Ni/c BN addition of 5 wt.%has the best corrosion resistance;The corrosion resistance of the 316L cladding layer also increased first and then decreased,and the corrosion resistance was the best when the addition amount reaches 10 wt.%;The passivation state is the most stable when the Ni/c BN addition amount is 5 wt.%.Considering the influence of Ni/c BN addition on the forming quality,microstructure,and performances of Ni-based and 316L cladding layers,it is recommended that Ni/c BN be added in both Ni-based and 316L cladding layers to be5 wt.%,and the comprehensive performance of Ni60AACu Mo-5 wt.%Ni/c BN cladding layer is better.