SA 387 Gr 11 Cl.1 vs Cl. 2

[rkg]

Is SA 387 Gr.11 Cl.1 superior to Cl.2 with respect to Creep? Can I substitute Cl.2 for Cl.1 when Design temperature is 500 deg.C? I am looking for some literature on this matter.

 

[bvi]

The approach you should take it to take a look at the allowable stress tables for the two materials, at temperatures in the creep regime. You will find, in this case, that at elevated temepratures, there is no difference in allowable stress.

 

[rkg]

Thank you for your answer. As a matter of fact, I had checked the stress values before posing the question. However, if you refer the note against each stress value row, you find that for Cl.1, creep is considered above 900F and for Cl.2, creep is considered above 850F. Does this imply that creep comes into picture at a higher temperature for Cl.1 ? I presume yes and that is why I posed this question. Would like to have your views.

 

[bvi]

Class 1 is higher strength than class 2, so below the creep regime, it has a higher allowable stress. When the note says creep is considered, it means that the criteria based on creep properties governs over the criteria based on yield and tensile strength. Even if, for example 2/3 average stress rupture in 100,000 hours (or one of the other two creep criteria in the allowable stress basis) gives the same stress value for class 1 and class 2, if it is less than the 2/3 yield/1/3.5 tensile stress values for class 1 at 850, it will goven, but if it is more than those properties for class 2 at 850, it will not. So creep governs at 850 for class 1 because the allowable stress based on yield and tensile strength are higher than that based on creep strength, because the yield and tensile strength is higher for class 1 than class 2. This explanation must seem convoluted, but it should be understandable after a few readings.

 

[rkg]

Dear cb4

Thank you for your answer. However, there is one basic error in your explanation, ie., UTS and YS of Cl.1 &2.

Actually, Cl. 2 has a higher strength and yield.

 

[bvi]

I accidently reversed class 1 and 2 in the answer. Switch them and the answer reads correctly.