Study On Microstructure,Profromance And Manufactural Technology Of High-strength Fastener With High Corrosion And Fatigue Resistance

High strength is a tendency for the development of bolts.In 1980?1990s,researches on high strength bolts were focused on delayed fracture,but in recent years,with the growing competition for resources in petrochemical,marine and other complex environment,some special properties of high strength bolts,such as corrosion,fatigue,etc,are increasingly attracted researchers9 attention.Through selecting some applicable materials and optimizing the heat treatment and surface treatment processes,this paper developed a new kind of high strength bolts,which exhibits excellent corrosion resistance,fatigue resistance,delayed fracture resistance,etc.In this paper,45CrNiMoVA steel was selected as the new high strength bolts,material,and both the "Quenching+Gas multicomponent thermochemical treatment,process(Q+GMT)and the "Gas diffusion+multicomponent thermochemical treatment,process(G+GMT)were earried out as the bolts' heat treatment processes.Additionally,in order to further improve the corrosion resistance of bolts,electroless Ni-P plating technology was applied.Microstructure shows that a relatively hard and thick nitrocarburized layer is produced on the surface of bolts during both of the two heat treatment processes mentioned above.A relatively dense Ni-P coating is also obtained on the bolts' surface by the electroless Ni-P plating technology.Through a series of performance tests such as tensile,impact,anti-fatigue,anti-corrosion,wear,delayed fracture and so on,the bolts treated by the Q+GMT process were turnmed out to have the best comprehensive performance.Its strength level can reach 12.9.The delayed fracture lifetime increases one time compared with that obtained by the " Quenching+Tempering"(Q+T)process.The fatigue resistance property is generally higher than the high strength bolts sold in the market and 75%higher than the same material bolts treated by the Q+T process.The neutral salt spray corrosion resistance is dozens or hundreds of times better than the high strength bolts sold in the market,and even better than the 201/304 stainless steel bolts.Furthermore,it was still turned out to have excellelt wear resistance,SO2 corrosion resistance and impact strength.At last,trough building the model of the stress distribution,SEM analysis of the fatigue fracture and microstructure analysis of the nitrocarburized layer,two different kind of fatigue fracture models for the bolts treated by the GMT process were found.Meanwhile,it proved that the bolts treated by the GMT process have a relatively excellent delayed fracture resistance by first-principles calculations with the microstructure analysis of the compound layer.