After half-pressure of the pipe, I went to the site and found a lot of cracks. I would like to ask some experts for advice.
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This post was last posted by shinaijian on 2019-4-20 16:13 The editing unit recently renovated a mixing kettle, mainly to replace the half pipe. The half pipe of the original equipment was cut off, then polished, and a new half pipe was installed. The specification is φ108×4. The head half pipe is made of bent pipe, and the upper half pipe of the cylinder is made of steel strip. The pressure of equipment hydrostatic test is 6MPa. After water pressure, pickling and passivation treatment is carried out, and then sand blasting is carried out. After the goods arrived at the site, it was discovered that there were many cracks on the half pipe. The characteristic is that there are no cracks at the head of the equipment, the cracks on the cylinder are mainly distributed in about 3-4 circles in the middle, and there are no cracks in the half pipes at both ends of the cylinder. I would like to ask the experts whether the cause of the cracks is due to excessive water pressure. (Note: The steel strip of the half-pipe of the cylinder has been used before, and no similar problems have occurred. In addition, there should be no cracks when the water pressure was applied, because the inspectors checked very carefully at that time, shining a flashlight everywhere, and if the water pressure leaked, the pressure could not be maintained at all).
After polishing the crack, it was discovered that the crack was not penetrating and was about 2mm deep.
Do a metallographic experiment. It's a bit like chlorine stress cracking. What kind of water is used for pressure testing?
Grinding probably won't be possible, because you just cut off the old half-pipe, then polish the barrel clean, and then install the new half-pipe.
At first, I was a little doubtful whether it was caused by the half-pipe production process, but after communicating with the outsourcing unit, I found that it should not be. The half-pipe production mold and process have not changed, and there are other 89 half-pipes being produced at the same time, and this problem has not occurred.
It's probably not chloride ion corrosion. The crack is on the surface of the half-pipe and does not penetrate into the interior. If there is corrosion, it should start from the inside, and I have tried pressing down. If there is corrosion, it cannot be that fast. The water pressure should be normal water.
Is there no follow-up to this? Have you found the reason? How come such a great post doesn’t get any participation?
You need to hit 6.0MPa for a half pipe, 0.60MPa is about the same. Is there any welding process before welding? Is heat treatment required?
The crack is outside the coil and does not come into contact with the test liquid, so it should not be caused by chloride ions. Hydraulic pressure test 6MPa tank? If it is a tank, then check whether the yield strength of the cylinder is sufficient during the hydrostatic test. The two ends of the cylinder are reinforced by heads, and the middle is the weakest. If the middle of the cylinder bulges under pressure, if the peripheral coil fibers that were originally stretched during processing have not been heat treated, they may be stretched to the limit and may break. The outermost part should be more serious. Similar to the situation in the photo.