| Martensitic stainless steels,with a combination of high strength,favorable toughness and exceptional resistance to corrosion,are widely used in energy,aerospace and manufacturing fields,such as steam turbine blades,aircraft landing gear and high-strength fasteners.However,hydrogen embrittlement(HE)problem of higher-strength steel limits its application and development.The microstructure of martensitic stainless steel is complex,including lath martensite and the precipitates related to the alloy composition,and some materials also contain the primary δ ferrite phase.The precipitates and δ-ferrite phase can act as hydrogen traps,affecting the diffusion and enrichment of hydrogen,and then affecting the behavior of hydrogen-induced cracking.Therefore,it is necessary to study the influence of microstructure on the hydrogen-induced fracture behavior of martensitic stainless steel and explore the hydrogen embrittlement mechanism of martensitic stainless steel,which would provide theoretical guidance for safe service.In this research,three martensitic stainless steels PH13-8Mo,15-6PH and 174PH with different microstructures are investigated.The characteristics of hydrogen trapping of the microstructure and its influence on HE behavior and HE mechanism are investigated through a combination of constant load test(CLT),slow strain rate tensile(SSRT)test,scanning electron microscopy(SEM),electron backscatter diffraction(EBSD),X-ray diffraction(XRD),transmission electron microscopy(TEM),thermal desorption mass spectrometry(TDS),scanning Kelvin probe force microscopy(SKPFM),hydrogen microprinting technique(HMT)and other means,and the main conclusions obtained are as follows:(1)The hydrogen-induced fracture strength of PH13-8Mo,15-6PH and 17-4PH with hydrogen concentration was investigated by CLT and SSRT,and the fracture morphology was analyzed.Then the microstructure and hydrogen trapping were characterized by SEM,EBSD,TEM,TDS and HMT.The results of the analysis indicated that the variation of hydrogen-induced delayed cracking(HIDC)threshold strength(σth)with hydrogen concentration(C)under CLT wereσth/σb=-0.18lnC+0.84 for PH13-8Mo and σth/σb=-0.641nC+2.01 for 15-6PH;while for 17-4PH,the HIDC critical hydrogen concentration(≥23.99 ppm)for CLT was much higher than that of PHI3-8Mo(<7.23 ppm)and 15-6PH(<10.22 ppm).Under the same hydrogen charging conditions,for PH13-8Mo and 17-4PH,the HIDC threshold strength for CLT was slightly lower(<0.1σb)than the fracture strength for SSRT(1×10-7 s-1),while it presented the different pattern for 15-6PH.For PH13-8Mo and 17-4PH,the crack source for CLT and SSRT was located inside the specimens,but the crack source of 15-6PH for SSRT was located near the surface of the specimens.According to the results of microstructure observation and statistics,there was large-size carbides(average 350 nm)in 15-6PH,and hydrogeninduced cracks appeared at the interface of large-size carbides during the SSRT.Hydrogen transported by moving dislocation during the dynamic loading of SSRT would promote the accumulation of hydrogen around large-sized carbides,while during the CLT,there was little hydrogen transported by dislocation motion,and little hydrogen accumulated around large-sized carbides,which resulted in the lower fracture strength of 15-6PH under SSRT(<1×10-6 s-1)than the threshold strength under CLT.(2)For 17-4PH containing primary δ-ferrite phase,the results of SKPFM combined with HMT indicated that the δ-ferrite/martensite interface was hydrogen trap sites.The initiation and propagation of hydrogen-induced cracks during the SSRT and CLT were analyzed by EBSD.The results of the analysis indicated that:the elongated δ-ferrite phase is distributed along the rolling direction.The cracks originated at the δ-ferrite/martensite interface for SSRT and CLT,but the propagation paths were different.During the SSRT,the cracks propagated preferentially along the δ-ferrite/martensite interface,which was parallel to the tensile direction.During the CLT,cracks propagated through martensite or δ ferrite.The former was attributed to the higher degree of strain-induced local hydrogen accumulation at the δ-ferrite/martensite interface during the SSRT at a lower strain rate.while during the CLT,hydrogen enrichment at the phase interface was not evident.(3)For 15-6PH,aging treatment at 482℃ for 60 min,120 min and 240 min was applied to obtain the three specimens containing Cu precipitates with different structure.The effect of Cu precipitates on HE behaviors was investigated by TEM,TDS and geometric phase analysis(GPA).The results of the analysis indicated that there were the similar the grain size,dislocation density and tensile strength in three aged specimens,but the crystal structure and size of the Cu precipitates was different.When aging for 60 min,Cu precipitates are B2 structure and bodycentered cubic(bcc)structure,which are coherent with matrix,and it translated into 9R structure when aging for 120 min,finally,the Cu precipitate was face-centered cubic(fcc)structure which was semi-coherent with the matrix when aging for 240 min.GPA analysis showed that the strained interface of Cu precipitate was the hydrogen trap sites,which was consistent with the high hydrogen concentration in the aged samples.The HE resistance is highest in the 240 min aged specimens,which is attributed to the higher hydrogen binding capacity of the semi-coherent fcc-Cu precipitate/matrix interface in the 240 min aged specimens.(4)The SSRT with hydrogen charging was carried out at a temperature of 80℃,which simulated the actual service temperature of the last stage blade in steam turbine,compared with the HE property at room temperature,the effect of temperature on the HE behavior of PH13-8Mo steel and 15-6PH steel was investigated.The results of the analysis indicated that the HE susceptibility of both materials at 80℃ was lower than that at room temperature.The brittle area in fracture surface of 15-6PH at 80℃ showed quasi-cleavage characteristics,while a mixed characteristics with the intergranular and quasi-cleavage was presented at room temperature,which was because temperature affects the degree and location of local hydrogen accumulation.Hydrogen was distributed at the grain boundary at room temperature,while at both the grain boundary and matrix at 80℃.The strength of PH13-8Mo steel is higher than that of 15-6PH steel,while at room temperature,the HE susceptibility of 15-6PH was higher than that of PHI 3-8Mo,which was attributed to strain-assisted hydrogen accumulation and cracking around the carbide/matrix interface in 15-6PH,then,at 80℃,the HE susceptibility of 156PH at lower strain rate(1×10-6 s-1)was lower than that of PH13-8Mo,which was related to the reduced influence of carbide/matrix interface cracking on hydrogeninduced fracture.At 80℃,the degree of local hydrogen accumulation decreased,and the hydrogen concentration increased at lower strain rate,then the crack also originated at the grain boundary. |
PDF Full Text Request