Vessel PWHT
Vessel PWHT
(OP)
Can anyone give me any info. on how Tensile stress is affected after multiple PWHT's. I have a vessel (SA-516-70), that due to poor QA/QC and carelessness will have to go through a 3rd heat treatment. Any help will be appreciated.





RE: Vessel PWHT
The tensile stregnth of the Grade 70 steel plate may decrease slightly as a result of multiple PWHT cycles. However, it is not the tensile strength you should be concerned about, you might want to consider impact toughness, if applicable in your application.
Typically, tensile strength for SA 516 Grade 70 plate comes in well above the minimum requirement in SA 516. So, multiple PWHT cycle will not drop you below the minimum ultimate tensile strength requirement of 70 Ksi. Impact toughness, if applicable, is more of a concern and should be evaluated.
RE: Vessel PWHT
To know the exact answer, I would ask the shop to take a sample plate material (read left over pieces) and have a lab do a simulated PWHT at total simulated time to find out EXACTLY what the mechanical properties will be.
I learned long ago, not from a paper but from in-house experten and from the fabricators, that the mechanical properties can not be guarantee for more than 3 PWHT cycles. In the past, we had asked in our spec to ensure the vessel mat properties can be assured for 4 PWHT cycles and the fabricators come back saying the mill cannot guarantee that, and they took exception to the specification. More recently, equipment that were bought from Italy were able to guarantee 4 PWHT cycles. However, I did not look more into detail if they were just blowing wind.
I now do recall seeing a paper long ago that showed tensile strength drop considerably with prolonged heating time below transitional temp; I have to remember to go find it tonight.
blynn,
If your vessel is going through its 3rd shop PWHT, then it probably won't have much time left for the Owner have the requsite ONE future field repair. I suggest you hire a metallurgist to review the case and formally advise your project. Also, fire your crappy shop inspector who been sleeping on the job to let the fabricator get all this shoddy work pass by.
RE: Vessel PWHT
The web site from Mittal steel contains a brochure that explains the effect of PWHT time and temperature on mechanical properties of SA 516 and SA 387. Mittal has done a fine job of using the Larson Miller parametric equation to show the effect of time at temperature on tensile and yield strength behavior.
Some things to keep in mind when reviewing this data;
1. You must review the original mill test properties of the steel to see how far above you are from the minimum requirements in SA 516.
2. Focus on the graph of Figure 6 on page 4 of the brochure. The % of original tensile is what you need to understand, this relates to the drop in the TS as noted on the mill test report. If for example your original TS is 78 Ksi, you can effectively drop to 70 Ksi - ASME B&PV Code does not give extra credit for strength that exceeds the minimum - 70 Ksi.
3. You must know the aggregate or total time at temperature, not specific the number of cycles!
All may not be lost regarding tensile properties. As I stated in my first response, notch toughness degradation is more of my concern.
ww
RE: Vessel PWHT
RE: Vessel PWHT
Steve Jones
Materials & Corrosion Engineer
http://www.pdo.co.om/pdo/
RE: Vessel PWHT
RE: Vessel PWHT
RE: Vessel PWHT
RE: Vessel PWHT
rhg