Takai Kenichi

Hydrogen and deuterium trapping sites in high-strength steels have been observed by secondary ion mass spectrometry (SIMS). High-strength steels with 1400 MPa tensile strength are loaded and dipped in D2O and 20% NH4SCN solution at 323 K to occlude hydrogen and deuterium. As a result, the depth profiles by SIMS show the presence of deuterium means hydrogen trapping site occluded during delayed fracture test. Secondary ion-image analysis by SIMS has made possible to observe hydrogen and deuterium trapping sites in high-strength steels. Hydrogen tends to accumulate at grain boundaries, in segregation bands, and in inclusions. Line scans by SIMS show that the concentration of hydrogen is 7.8 times as high at grain boundaries as in the matrix, 5.0 times as high in the segregation bands, and 11 times as high in the inclusions. Hydrogen trapping sites at grain boundaries can be observed by measuring after the delayed fracture test as soon as possible.

Medium carbon steels added 2.0% Si or 1.5% Si-30ppm Ca with 1400N/mm2 tensile strength show high resistance to delayed fracture. It is important to clarify the hydrogen occlusion behavior to delayed fracture of steels with high resistance to delayed fracture. The hydrogen diffusion coefficient and hydrogen content were measured by electrochemical permeation technique, and hydrogen evolution content after accelerated delayed fracture test was directly measured by hydrogen thermal analysis. It is found that high resistance to delayed fracture of 2.0% Si steel is due to the high hydrogen content needed for fracture, in spite of its high hydrogen occlusion rate. Also the high resistance to delayed fracture of 1.5% Si-30ppm Ca steel is found to be due to the low hydrogen occlusion rate and the low hydrogen content needed for fracture.

Effects of Si and Ca addition on delayed fracture of medium carbon steels with 1400N/mm^2 strength were investigated. Silicon and calcium were added at concentrations of 0∿2.0% and 30∿70ppm, respectively. The delayed fracture characteristics were evaluated by FIP (Federation Internationale de la Precontrainte) test which is a constant tensile load test in 20% NH_4SCN solution at 323 K. In order to make clear the effect of adding Ca and Si, the fracture surfaces were examined, and the hydrogen evolution behavior, the diffusion coefficient of hydrogen, and the hydrogen content were measured. It was found that 0.5%Si steels have no effect on the time to fracture regardless of Ca content, while 1.5%Si-30ppm Ca steel has the longest time to fracture. Fractography showed that adding Ca to 0.5%Si steels did not change the intergranular fracture area fraction. However, adding Ca to 1.5%Si steels changed the fracture from intergranular fracture to microvoid coalescence fracture. As for hydrogen behavior after three months from FIP test, 0.5%Si steel released hydrogen at the peak of 500 K, while for the 1.5%Si-30ppm Ca steel the peak was at 700 K. It was suggested that hydrogen released at around 500 K was crucial for delayed fracture characteristics.