Fatigue Crack Growth Eqns in API 579

[xphiliac007]

Hello all,

I had a particular question concerning the fatigue crack growth data listed in Appendix F of API 579-1/ASME FFS-1 (2007 ed).

I am analyzing a pressure vessel constructed out of SA-302 Gr. B (mod.) material with a yield strength (Sy) of 100 ksi. Typically we use the Paris Law to determine our number number of cycles to failure using API 579 as a source for fatigue crack growth data. The constants C, n, and Eyr is determined based on Sy of the material, composition (grain structure), temperature, and outside environment that the material is exposed to. Since Sy is 100 ksi, I have to refer to Section F.5.3.3 to get my constants, but how do I determine if SA-302 Gr. B is a martensitic steel or ferritic-pearlite steel? Can that be determined simply by chemical composition, or if the material has been heat treated, etc.? Thanks in advance.

 

[chicopee]

According to my Kent ME hdbk "Design and Production", the 400 series SS are either Martensitic or Ferritic depending on the type number; the 300 SS series are all Autenitic

 

[DVWE]

SA-302 Gr B is nothing more than glorified Carbon Steel, and if you look at the spec, thickness under 2" are usually supplied as rolled (not heat treated at all). So it would not be considered martensitic. Even if your thicknesses are over 2" and you order it Normalized, that will not give you a martensitic structure.

 

[xphiliac007]

chicopee and DVWE, thanks for the input. I obtained the spec for SA-302 in ASME Sec. II, and it is a type of alloy steel. The plate is just over 3 inches in thickness so it is normalized as DVWE said.

So just to understand, in order to get a martensitic microstructure, it would have to depend on whether the steel has been heat-treated or not...? Do you get a martensitic microstructure for stainless steel only? Isn't normalizing a type of heat treatment, just removed from the furnace for air-cooling? What if the pressure vessel has experienced post-weld heat treatment?

 

[DVWE]

To get a martensitic structure the steel has to be rapidly cooled. So it doesn't depend on whether or not the steel has been heat treated, it depends on the cooling rate from what is called the austenitizing temperature - probably around 1800 F for this alloy. Perhaps a metallurgist can give you more detail, but I would think that it would be difficult to form martensite with a 3" thick plate of this alloy even if it was quenched.

Normalizing is exactly as you described it. This steady cooling heat treatment will allow the steel to form mainly pearlite.

PWHT for this alloy is probably going to be around 1150 F and is used to stress relieve the welds (which have cooled quickly). If carried out properly, it will not significantly alter the base metal.

 

[xphiliac007]

DVWE, that makes a lot of sense. Thanks for the clarification!