Effect Of Cu And Ni On Corrosion Resistance And Delayed Fracture Of High Strength Bolt Steel

In recent years,with the development of larger-scale building and bridge engineering and the increasing of the strength level of construction steels,the bolts used in those constructions have been facing enhanced working stress level as well as deteriorated service conditions.Therefore,there has been increasing demands of enhancing both the strength level and performance properties of bolt steels.The purpose of the present study is to investigate the influence of alloying elements Cu and Ni on corrosion resistance and hydrogen-induced delayed fracture resistance of bolt steel based on formerly developed ADF steel(42CrMoVNb)by using wet/dry cyclic test,electrochemical analysis,constant load tensile(CLT)tests and slow strain rate tensile(SSRT)tests.The corrosion rust layer was analyzed by using scanning electron microscope(SEM),optical microscope(OM)and x-ray diffraction.The main conclusions are summarized in the following:The results show that the an addition of 0.42%Cu notably improve the corrosion resistance of the tested steel mainly due to the fast formation of a protective compact rust layer which could prevent the substrate from further corrosion.The result of electrochemical tests showed that the addition of 0.42%Cu also slightly enhances the self corrosion potential of the bare tested steel.Further results of SSRT and CLT tests revealed that the susceptibility of hydrogen embrittlement(HE)increases slightly increases for the Cu-containing steel which is ascribed mainly to the increased strength level due to its higher carbon content.SEM analysis of the fracture surfaces of the tested SSRT and CLT specimens shows a primary fracture of quasi-cleavage accompanied by a small amount of intergranular fracture.The results of the tested steels with different additions of Ni(0-0.99%)revealed that the corrosion rate decreases significantly while the density of rust layer increases with an addition of 0.42%Ni.The self corrosion potential of the bare tested steel with 0.42%Ni also moved towards positive side indicating an increased resistance to the initiation and propagation of corrosion pit.However,the corrosion resistance of the tested steel shows no further improvement with an addition of 0.99%from the aspect of corrosion rate,rust layer morphology and the self corrosion potential of the bare steel.The susceptibility to HE shows an up-and-down pattern for the SSRT test while it remains almost unchanged for the CLT test with increasing Ni content,which needs further investigation.