Research On The Preparation And Properties Of Ni-based Cladding Layer On The Surface Of High Strength Bolt Steel

High strength bolt is a widely used mechanical part,whose connection quality ensures the quality and the normal operation of the entire equipment.At present,its surface protection methods cannot fully meet the service requirements in harsh environment such as marine climate and saline alkali soil,because this could lead to corrosion on the bolt surface in real practices and significantly reduce the safety,reliability and service life.This paper carried out a research of perparing C22 alloy cladding on 40Cr steel surface and studied microstructure,element distribution and microhardness of C22 alloy cladding layer prepared by different laser parameters.It also examined theelectrochemical characteristics of 40Cr steel and C22 alloy cladding layer.The phase.composition of C22 alloy·cladding layer was analyzed,and the nano hardness,elastic modulus and.energy dissipation rate of different phases were measured by nano indentation instrument.The paper also investigated the tensile mechanical properties of laser cladding samples with different diameters and the fracture morphology of cladding layer and heat affected zone.The main phase of C22 alloy is solid solution formed by y-Ni and Cr,Mo,Fe,W and other elements,within which the content of Mo is reletively higher in the eutectic networks.With the laser power increases,the speed of scanning decreases and the depth of heat affected zone increases.As the heat input increases,the content of Fe element increases.The hardness of cladding changes in accordance with the content of Fe element,which can be reached up to 343HV.The self corrosion potential of C22 alloy cladding layer is greater than 40Cr steel and occurs passivation in the process of polarization.Among the samples of cladding layer,the bluff current density of P4 is 4.725×10-4 A·cm-2,the self corrosion current density is 3.440×10-6 A·cm-2,and the bluff potential is 1.02V,which shows that C22 alloy cladding layer can fully protect 40Cr steel.The cladding layer of C22 alloy on 40Cr steel surface changes from ductilefracture to brittle fracture.With the decrease of the diameter of cladding sample,the brittleness characteristics become more and more obvious.The tensile strength of C22 alloy cladding layer is higher than that of annealed 40Cr steel,but its plasticity is poor.The fracture mode is intergranular fracture with dimple characteristics.The result of nano indentation test on different phases of the cladding layer shows that compared with dendrite structure,the intergranular eutectic phase has higher hardness and elastic modulus,smaller energy dissipation rate,lower energy for fracture,and higher dendrite content which improves its plasticity.The tensile strength and plasticity of the bonding zone between the cladding layer and the matrix are greatly reduced as well as that of the heat affected zone.The result also demonstrates that there are very shallow parabola shaped dimples on the fracture surface of the bonding zone;the upper part of the heat affected zone is brittle fracture with cleavage fracture characteristics,and the lower part displays both brittle fracture and ductile fracture characteristics.