Effects Of Microstructures On Hydrogen Embrittlement Susceptibility And Mechanism For High Strength Steels

High-strength steel is prone to hydrogen embrittlement during service in a hydrogen-containing environment,which often leads to sudden failure fracture without warning.Hydrogen-induced cracking(HIC)is generally characterized by intergranular or quasi-cleavage cracking.By observing the fracture feature,it is possible to reveal the crack initiation sites and the microstructures on the propagation path during hydrogen induced cracking.High-strength steel has many microscopic characteristics depending on the different strengthening methods.The hydrogen-induced cracking process reflects the interaction between the microstructure and hydrogen.The essence is that hydrogen attacks the weakest link in the microstructure and induces crack formation.Therefore,investigations of the microstructures in high-strength steels help to reveal the mechanism of hydrogen embrittlement.In this thesis,the HIC mechanisms of the high-strength steels with different strength grades have been studied.Microstructure observations,phase analyses and three-dimensional atomic probe(3DAP)elemental analyses are carried out to reveal the microstructure of high-strength steel and the hydrogen distribution at the characteristic region.Combined with hydrogen permeation,hydrogen thermal desorption spectrum and hydrogen content measurement,the hydrogen trapping sites in high-strength steel,the hydrogen diffusion coefficient and the critical hydrogen concentration causing hydrogen-induced cracking have been analyzed.The hydrogen embrittlement susceptibility of high strength steel was evaluated by pre-charged hydrogen and dynamic hydrogen charging tensile method.The results show that AISI 4140 steel after quenching and tempering treatment contains mainly the tempered sorbite.The average diameter of precipitated carbides is 200 nm and its volume fraction is about 20.2%.The carbides were found to be in both the interior grain and the grain boundary.3DAP results show that the type of-carbide is M23Ch or M3C(M is Cr,Mn,Mo or Fe).When the carbide precipitated in the inner grain,there was no obvious hydrogen segregation at the carbide/matrix interface.The average hydrogen concentration was 2.3 at.%,which was slightly higher than the average hydrogen concentration of the matrix.However,hydrogen segregation mainly occurs at the carbide/ferrite interfaces in thegrain boundary,with peak concentrations of 5.9 at.%.This result indicates that the precipitation of carbides at the grain boundaries acts as the main hydrogen trapping sites.The dynamic hydrogen charging tensile test results show that the hydrogen embrittlement susceptibility of AISI 4140 steel is as high as 58.1%,and the f-racture exhibits mixed characteristics of intergranular and quasi-cleavage cracking.Therefore,hydrogen segregation at the carbide/ferrite interfaces in the grain boundary is responsible for the observed intergranular cracking.Direct observations of the hydrogen distribution contributes to a better understanding of the hydrogen embrittlement mechanism.The initial microstructure of AISI 4140 steel is constituted of ferrite and pearlite.Until now,the real reason of hydrogen-induced cracking of ferritic/pearlite steels has not been fully understood.Observations of the microstructure beneath the fracture feature have been performed.The results show that the pearlite island boundary,the ferrite/pearlite boundary and the adjacent ferrite matrix are hydrogen-induced crack initiation and propagation sites.As the stress intensity factor increases,the fracture mode changes from the mixed intergranular and quasi-cleavage feature to a complete quasi-cleavage cracking feature(the crack propagates only along the ferrite matrix).Since the ferrite/cementite interface belongs to a low-energy interface,there was no cracking observed at this interface.A PHS1800 hot stamping martensitic steel with strength up to 2 GPa has been investigated,The thin plate steel is mainly constituted of martensite.The dynamic hydrogen charging tensile tests show that the sample fractures in the elastic stage,and the highest tensile strength is 740 MPa.The fracture is characterized by intergranular cracking.The results of 3DAP indicate that carbon segregation occurs with a peak concentration of 0.82 at.%on the prior austenite grain boundary,which is responsible for the intergranular cracking.As for the 18Ni 300 steel(maraging steel),scanning electron microscopy(SEM)observations show that the microstructure is martensite with a large number of intermetal lic compounds dispersed in the grain.3DAP reveals that the precipitated intermetallic compound is composed of Ni3Mo,Ni3Al and Ni3Ti,which have coherent relationship with the matrix.It is also found that carbon segregation occurs with a peak concentration of 0.09 at.%and a hydrogen concentration of 4.3 at%at the prior austenite grain boundaries.The pre-charged hydrogen tensile evaluation results show that the sample fractures in the elastic stage.and the fracture feature is intergranular crackinig.These results indicate that the segregation of carbon on the prior austenite grain boundary is also the main reason for excessive hydrogen concentration at the grain boundaries and fracture with intergranular cracking.In order to improve the hydrogen embrittlement resistance of PHS 1800 and 18Ni 300,surface treated by intensity pulsed ion beam(IPIB)and surf-ace mechanical rolling treatment(SMRT)were carried out,respectively.After IPIB treatment of PHS 1800 steel,the high carbon martensite with twin structure is formed in the melting zone due to the rapid melting and solidification of the surface.The formation of high-carbon martensite absorbs the carbon on the prior austenite grain boundary and reduces its concentration to 0.45 at.%.Meanwhile,it acts as a strong hydrogen trapping site and thus it inhibits hydrogen concentration at the grain boundaries.The dynamic hydrogen charging tensile tests show that the tensile strength of the irradiated sample reaches 1300 MPa,which is much higher than that of the unirradiated sample.Fracture observations show that fracture feature of the irradiated region is quasi-cleavage cracking.This result indicates that the IPIB treatment can inhibit the intergranular cracking of PHS 1800 high strength steel.After SMRT treatment of 18Ni 300 steel,a gradient nanograin structure is formed with the grain size from the intermediate layer of 1 μm to the matrix of 10 pm.After annealing,the over-aging treatment promotes the growth of the precipitation.The hydrogen embrittlement resistance is found to be improved.The fracture has complete quasi-cleavage characteristics without intergranular cracking.After grain refinement,the carbon concentration at the grain boundaries is reduced,which is probably responsible for the enhanced HIC resistance.