| A new composite treatment processes(Quenching and tempering treatment and low temperature gas multicomponent infiltration and cold galvanizing)was carried out.The mechanical properties of 45 steel and 20MnTiB alloy steel was improved,and the corrosion resistance and fatigue resistance were significantly improved.This technology is mainly used for surface treatment of high strength bolts,the surface of the sample is beautiful,with higher hardness,tensile strength and good iMPact toughness,as well as good corrosion resistance and high fatigue strength,the new composite treatment has good industrial value.Three different kinds of surface treatments(multicomponent infiltration,zinc plating,complex treatment)were used by 8.8 grade high strength bolt steel(45 steel in quenched and tempered)and 10.9 grade high strength bolt steel(20MnTiB alloy steel in quenched and tempered),the microstructure and morphology of the samples after different surface treatments were observed.The phase composition of the samples were analyzed by confocal laser,depth of field microscopy,X ray diffraction(XRD)and so on.The bonding strength of the zinc coating and the multi layer and the conventional mechanical properties were measured,Its anticorrosion performance and fatigue performance were analyzed and evaluated through salt spray corrosion test and high frequency fatigue test.The tensile and fatigue fracture mechanisms were analyzed by scanning electron microscopy(SEM).The experimental results show that 45 steel in quenched and tempered by surface composite treatment,the grain is finer,the microstructure is more homogeneous and compact,the bonding strength between the zinc coating and the multi layer is well.the thickness of the coating is only 50μm;The surface of the carbonitriding layer is mainly composed of Zn,iron nitride(Fe3N),carbide(Fe3C)and oxide(Fe3O4).through XRD analysis,which have the effect of dispersion strengthening.The maximum microhardness of the coating is about 636HV0.025,it is over two times the microhardness of the substrate,while the tensile strength is 830MPa,which is increased by one times of the original material,the impact absorption energy is 33J,which is 78%of the raw material,Its corrosion resistance after 845h protection level reached 9(a small amount of red rust appears the surface),is about 170 times of the raw material.The conditional fatigue limit value of notched specimens is 369MPa,which is higher than that of the raw materials.After the surface treatment of the quenched and tempered 20MnTiB alloy steel,the coating thickness is only 28μm since the effects of alloying eiements;the surface of the carbonitriding layer is mainly composed of Zinc,Fe3N,FezO3 and Fe3C.The microhardness of the coating is up to 756HV0.025,while the tensile strength is 904MPa,which is increased to 1.1 times of the original material,the impact absorption energy is 166.5J,which is lower than that of raw material,its corrosion protection after 846h to grade 9,it has been greatly improved.The conditional fatigue limit of the notched specimen is 382MPa,which is higher than that of the raw material. |
