The Failure Characterizations Research On The High Strength Bolts Hydrogen Embrittlement

The high strength bolt hydrogen embrittlement fracture is a quite complex problem at present, and there are various testing methods related to hydrogen embrittlement available, but many of them only qualitatively judge whether the hydrogen embrittlement is in existence or not. An accelerated method has been introduced by American Society for Testing Materials (ASTM) to quantitatively measure the hydrogen embrittlement critical value of steels. This paper focused on the testing methods of the hydrogen embrittlement failure characterizations and the hydrogen embrittlement sensitivity, verified the feasibility of the incremental step loading test to measure the hydrogen embrittlement critical value, and conducted research on the tensile rate during the process of the tensile testing.The results of the present measurement of the hydrogen embrittlement failure blots with the preload testing, slow strain rate tensile testing and gap tensile testing indicated that there was certain difficulty to measure the hydrogen embrittlement sensibility of the failure bolts due to the limitation of testing time and sample processing conditions.The testing results also indicated that incremental step loading testing could determine the hydrogen embrittlement fracture value or the hydrogen embrittlement critical value of actual samples, and this is a sufficient supplement to the current hydrogen embrittlement sensitivity measurement methods. This testing method has been revised as the national standard "Standard Test Method for Measurement of Hydrogen Embrittlement Threshold in Steel" in the recent research process, and now this standard is being requested authorization. The simple incremental step loading device can be used to measure and record the testing results of hydrogen embrittlement fracture value, which could provide the mentality of designing the autoloading device in future.The results of SEM analysis on the hydrogen embrittlement fracture bolts indicated that intergranular secondary cracks were existed in the hydrogen embrittlement fracture, and the fracture appearance was intergranular and quasi-cleavage. The appearance of crack source was basically intergranular and quasi-cleavage, and there were some secondary cracks existed along the crack sources, which reduced gradually far away from the crack source.When the slow strain rate tensile testing and other hydrogen embrittlement sensitivity testing were carried on, grip separate rate will be usually adopted for controlling. And the characteristics of strain rate changing in the hydrogen embrittlement testing process will be understood by conducting the research on the tensile rate testing. The research discovered that theoretical strain rate was higher than actual strain rate before the force reached to the maximum value, the difference between the two strain rates were very large in the elastic deformation stage, nearly 8-10 times, but in the uniform deformation stage, the difference was smaller. The reason of the phenomenon mainly attribute to the testing machine rigidity.