The issue of substituting materials for the manhole flange covers of pressure vessels
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According to the HG/21521-2005 standard, manhole covers are made entirely of sheet metal, but our company often uses forgings as a substitute. This time I encountered a heat exchanger whose design does not allow the use of forgings. I would like to ask everyone: what potential problems might arise if forgings are used as a substitute?
It’s definitely better to use forgings for manhole covers rather than sheets; forgings have finer grain structures, and their mechanical properties are more consistent compared to sheets. If the sheet is relatively thick, its mechanical properties in the middle section are significantly lower than those in other areas. I think it’s probably because the designer finds it troublesome and doesn’t want to make changes; talking to him properly should work fine.
It’s definitely better to use forgings for manhole covers rather than sheets, right? Forgings have better performance; presumably their allowable stress is also higher than that of sheets. Designers surely don’t want to make the changes because it’s too much trouble
For this heat exchanger, there is a requirement for the raw materials to undergo impact testing at -40 degrees Celsius; will this have an impact in this regard?
Yes, that’s what he meant – the allowable stress is not sufficient; but the allowable stress determined through actual testing of the forgings should be sufficient as well
Since a low-impact test at -40 degrees is being conducted, where should the sampling points for the manhole cover plates be located? If the plates are thick enough, will the material in the middle part of the plate meet the requirements regarding impact energy?
Yes, if you buy ordinary sheet metal, there’s a risk that the impact tests won’t pass, and since the steel plates are purchased in whole sheets, it’s very wasteful
If the designer insists on using sheet material without specifying the location for impact testing, it is recommended that sampling be carried out at the 1/4 mark for the impact test, which will increase the chances of success.
The allowable stress is probably not sufficient. Let’s look at an example. At 150 degrees, the allowable stress for 55mm thick Q345R plates, as specified in GB150 (16MnR), is 157MPa. Meanwhile, the 55mm 16MnⅡ forgings have only 147MPa.
It’s better to use forgings. First, if the plate thickness exceeds 30 mm (according to GB150), normalized plates must be used; second, forgings are usually supplied in a normalized state ； Moreover, purchasing sheets is not as convenient as purchasing forgings; you can buy just the number of pieces you need, unlike sheets, where you have to buy an entire sheet at once. It ultimately depends on the manufacturer.