S235 P235 difference between structural versus pressure steel

[kapot]

I'm looking for the difference between S235 en P235? Concerning mechanical, or chemical composition? I'm asking this question as St37.2 (S235, 1.0037) was used instead of St35.8 (P235, 1.0305) for a pressure pipe (14bar) at elevated temperature (up to 400°C). As I look in standards both steels have comparable mechanical properties. Chemical composition differs only in specification of silicon content and manganese content. Have these two elements influence on properties at elevated temperatures or use in pressure applications? Recently one pipe failed. Now we need to decide if all St37.2 pipes need to be replaced.

S235 (C<0.17, P<0.040, S<0.035)
P235 (C<0.17, Si 0.1-0.35, Mn 0.4-0.8,P<0.04, S<0.04)

 

[metengr]

kapot;
Make sure you determine the cause of failure in the carbon steel pipe because this will provide the necessary information regarding the above comparison, and course of action for existing pipe material.

Let me provide you with an example - in elevated temperature service carbon steel pipe or tube material can fail from a damage mechanism known as graphitization. If this is the case in your pipe failure - silicon content plays a significant role as well as aluminum content regarding susceptibility to graphitization versus spheroidization. Normally, lower Si content results in increased susceptibility to graphitization failure in elevated temperature service.

 

[mrfailure]

And what metengr presents is but one example. You first need to conduct a failure analysis on the pipe to determine failure mode and mechanism. Then you can look at whether the difference in expected composition could account for the determined mechanism.

Using the metengr example: Your reported maximum operating temperature is a little below the graphitization range (generally above 425 deg. C) but nothing says actual temperature might not really be higher than expected to account for this. If so, your difference in steels would be meaningless as both alloys would be expected to graphitize; you would instead need some chrome in the steel to tie up the carbon in the form of carbides.

Aaron Tanzer

http://www.lehightesting.com