Ni effects on large diameter welded pipe delay fracture

As we all know, under large diameter welded pipe tin oxide chemical element enrichment, but no relevant research on the effects of the phenomenon of delay fracture characteristics, Therefore, with respect to iron oxide enriched below Ni, heat treatment is the use of the actual product sticky oxide scale PC steel bar delayed fracture characteristic evaluation, rather than using conventional machining of finished samples were evaluated.

Japan after the heat-treated steel tensile strength 1500MPa grade PC stick stress is applied to 80%, delayed fracture test (FIP test method) at 50℃ ×20% ammonium thiocyanate solution when Ni of delayed fracture time impact study. With the increase of Ni, the rupture time will be greatly extended, when Ni increased to more than 0.77mass%, large-diameter pipe did not rupture occurs when the 100h.

On the other hand, the surface of the dresser large diameter pipe surface after polishing performed in the same test, the results show that although the fracture time is reduced than the material under heat treatment, but the same effect can be seen adding Ni. However, never add Ni steel, the fracture time and material under the same heat treatment. You can think of this delayed fracture characteristic differences related to hydrogen penetration difference.

Therefore, the material under heat treatment and surface polishing large diameter pipe material placed under the same environmental delayed fracture test after dipping 5h, heating type analysis method using diffusion of hydrogen (in this case diffusion in the vicinity of room temperature in parts of Hydrogen Here, analysis using heating type analysis showed that the diffusion of hydrogen in a low temperature about 300℃lower than the precipitation of hydrogen) were determined. From the material point of view under heat treatment, increases the amount of Ni, the diffusion of hydrogen would be drastically reduced, while the grinding material point of view, than the hydrogen diffusion material under heat treatment increases, but after grinding can be seen with the Ni increases, the diffusion of hydrogen decreasing trend was.

Using EPMA surface of the material in the vicinity of large diameter pipe heat treatment state conducted a survey in the border iron oxide and ferrite can see Ni enrichment layer. If you add Ni steel surface was polished, it can clear the oxide scale and Ni-enriched layer, which indicates the grinding help curb infiltration hydrogen, so no matter where terms, there is a need for grinding. Here, although no added Ni steel oxide skin only, no Ni-rich layer, but since the amount of diffusible hydrogen is not changed before and after the polishing, and therefore has no effect on iron oxide skin penetration of hydrogen. It can be said that in the heat-treated steel surface Ni added, the effect of hydrogen infiltration of Ni-rich layer.

Hot rolling to expand through large diameter pipe of Ni-enriched layer can be suppressed hydrogen penetration, improving delayed fracture properties.