| With the development of further enhancing the strength level and of extending the application of microalloyed(MA)medium carbon forging steels for the purpose of higher productivity and thus cost saving,therefore have been increasing attentions concerning the service safety especially the susceptibility to hydrogen embrittlement(HE)of this kind steel.Therefore,the purpose of the present study is to investigate the HE behavior of a newly developed MA bainitic forging steel 25MnCrVTi and a ferritic-pearlitic medium carbon forging steel microalloyed with different contents of V.One commercial low alloy structural steel 40Cr with quenched&tempered(Q&T)microstructure was also used for comparison.The tests conducted in the present study mainly include optical microscope(OM),scanning electron microscope(SEM),transmission electron microscope(TEM),electron backscattered diffraction(EBSD),thermal desorption spectrometry(TDS)and slow strain rate tests(SSRT).The results of the study are expected to provide theoretical basis and technical support for the safety application of the newly developed microalloyed forging steels.The main conclusions are summarized in the following:Mechanical properties of the newly developed MA bainitic steel were investigated after the hot forging and tempering process.The results show that the microstructure of the as-forged bainitic steel mainly consists of granular bainite and～20 vol.%martensite.The increase of tempering temperature leads to the decrease of the ultimate tensile strength but the increase of the yield strength,which is mainly attributed to the decomposition of M/A constituents,the tempering effects in martensite and the precipitation of carbide with increasing tempering temperature.In addition,tempering treatment could notably increase the impact toughness of the tested bainitic steel except the temper brittleness zone at around 400 ℃.Therefore,it is concluded that the tempering treatment of the forged bainitic steel at around 200 ℃ not only could relive the post-forging residual stress,but also results in an excellent combination of strength and toughness,which is comparable to that of the conventional Q&T 40Cr steel.The susceptibility to HE of the novel MA bainitic forging steel was evaluated using SSRT method.The results show that the susceptibility to HE of the as-forged bainitic forging steel is notably higher than that of the Q&T steel with high-temperature tempered martensitic microstructure and similar tensile strength level,which is mainly due to the presence of considerably high amount of untempered martensite and large size blocky M/A constituents of the former.It was further found that tempering at 200 ℃could reduce the susceptibility to HE by～35%of the as-forged bainitic forging steel at no expanse of strength and ductility,though its resistance to HE is still lower than that of the steel in the as-Q&T condition.It is thus suggested that the reduction or elimination of the large size blocky M/A as well as the martensite phase through optimizing chemical composition and processing route could help to further reduce the susceptibility to HE of the tested bainitic forging steel.The HE behavior of ferritic-pearlitic MA forging steels with different vanadium(V)additions(0 and 0.3 wt.%)was investigated by means of SSRT and TDS methods.The results show that most of the charged hydrogen in the tested V-containing steel was trapped by the finely distributed V(C,N)precipitates,and the hydrogen trapped by the V(C,N)precipitates has a high stability at room temperature.Whether the steel in the as-forging or as-Q&T conditions,the higher the strength level of the steel is,the greater its susceptibility to HE will be;both higher tensile strength and absorbed hydrogen content of the 0.3%V steel result in its higher susceptibility to HE than that of the 0%V steel. |
