Investigation Of ??and ? Phases On The SCC And HIC Behavior In SUS301L-MT Stainless Steel Lahw Joints

In order to further realize the lightweight and reduce the maintenance cost of subway vehicls,SUS301L-MT stainless steels are used to manufacture the under frame of next generation subway vehicls using laser-arc hybrid welding(LAHW)process.The certain service environment of subway vehicls and discontinuous microstructures of SUS301L-MT stainless steel LAHW joints may cause many corrosive failure problems.Therefore,the following studies were performed on SUS301L-MT stainless steel LAHW joints:(1)the microstructures on the cross section of the joints were systematically investigated.Furthermore,the relationships between the microstructures and mechanical properties,corrosion properties were also discussed.(2)In combination with microstructure analysis,electrochemical analysis,fracture surface morphologies analysis and cracks propagation path analysis,the slow strain rate tensile(SSRT)methods were performed to investigate the stress corrosion cracking(SCC)behavior of the joints and base metal(BM).(3)“Applied cathodic potential+SSRT”method was performed to study the hydrogen induced SCC(HISCC)behavior of the joints and BM,combining with microstructure analysis,electrochemical analysis,fracture surface morphologies analysis and cracks propagation path analysis.(4)“In situ hydrogen charging+SSRT”method,in combining with microstructure analysis,fracture surface morphologies analysis and cracks propagation path analysis were performed to investigated the localized zone hydrogen induced cracking(HIC)behavior.(5)First principle calculation method was used to investigate the effect of strain on the H atom diffusion behavior in??martensite,?-ferrite and austenite.The main conclusions are as follows:The SUS301L-MT stainless steel joints showed almost no SCC susceptibility in 25?3.5 wt.%Na Cl solution with a I_SSRT value of 0.09.The BM showed certain SCC susceptibility with a I_SSRT value of 0.23,which was 2.56 times higher than that of WM.Low corrosive potential of?-ferrites in weld metal(WM)could result in the corrosion of?/?interface and proved cathode protection to the austenitic matrix,thus the joints could keep well plasticity and relate its failure.The certain SCC susceptibility of BM is due to the high SCC susceptibility of martensites formed during cold rolling and SSRT tests.The hydrogen concentration in the environment improved with the applied potential turning to more negative.As a result,during SSRT test in 25?3.5 wt.%Na Cl solution,the fracture location moved from WM to BM.However,the HISCC susceptibility of the joints was constantly lower than that of BM at the same cathodic potential.During-1200 m V and-1400 m V potentials were applied on BM,the HISCC susceptibility was about 0.6,while the joint was 0.3,two times lower than BM.A certain content of?-ferrites and grain boundaries in WM made it exhibit high hydrogen solubility and HIC resistance,resulted in lower HISCC susceptibility.High HIC susceptibility of??martensite induced during plastic deformation caused the high HISCC susceptibility of BM.In situ hydrogen charging SSRT tests of the localized zone specimens indicated that the HIC cracks in BM and HAZ initiated by??/?interface cleavage,and?/?interface cleavage in WM.The cracks propagation paths were controlled by the hydrogen distribution in front of crack tips through“hydrogen enhanced local plasticity”and“Hydrogen induced decohesion”mechanisms.The selective initiation and propagation of cracks in each zone are closely related to the diffusion behavior of H atom in the each phase.Hydrogen atom diffusion behavior in??martensite,?-ferrite and austenite using first principle calculation indicated that tensile strain could enhance H atoms diffusion in these three phases.However,the diffusion coefficient in??martensite and?-ferrite constantly lower than that of in austenite at least two order of magnitudes under the same strain.The difference between these three phases is due to their crystal structures and the interaction between H and surrounding Fe atoms.The SUS301L-MT stainless steel LAHW joints exhibited different microstructure features in each zone,resulted in different diffusion behavior of hydrogen atoms in different areas.And the hydrogen induced cracking cracks always initiated and propagated along hydrogen atoms enrichment areas.During a tensile strain was applied on the joints,H atoms intend to concentrate at the stress concentration zone,like the hard phases of??martensite and?-ferrite,phases interface and grain boundaries.As a result,the cracks initiated and propagated along??martensite,?-ferrite and grain boundaries.Therefore,the diffusion behavior of hydrogen atoms is the main factors which controlled the SCC,HISCC and HIC behavior of SUS301L-MT stainless steel LAHW joints.