Part 3: Allowable Stresses
While inspectability has been at the center of recent concerns, other issues such as chemistry control, ductility and excessive oxidation have also come under scrutiny. Perhaps most interesting is that while all of the preceding concerns have been debated, it has also been found that existing ASME allowable stresses for Grade 91 are not conservative relative to newer data techniques. This is beautifully illustrated in Figure 5 from , where the continual drop in allowable stress is plotted as more and more short term test data is removed from the analysis. The good news is that “new” material is not worse than older material (at least from a statistical and data analysis perspective). The bad news, as discussed in  and can be seen in Figure 6, is that even more modern techniques like “region-splitting” do not capture the apparent steep drop in strength for the longest tests now available. As Figure 5 shows, even the proposed reduced allowables that are expected to be published in the 2019 Edition of the ASME B&PV Code (see Figure 7, “Current Cl. 1 Proposed” or Figure 7 of ) are not likely to be conservative at 100,000 hours, and that likelihood decreases as target lifetime increases. While the 2/3 margin applied to the 100,000 hour stress in ASME will likely envelope this drop, life-based design methods, which are sorely needed for complex materials and operating conditions, could struggle to overcome this un-conservatism. This makes data analysis techniques like those described in  even more important.
While it would appear simply due to timing that the impending drop in allowable stresses is related to the Type IV damage detectability concerns, they are actually completely independent. In fact, the allowable stresses are based solely on data from smooth, polished specimens. Cross-weld specimens are used by the Code, but would apply to setting reduction factors for allowable stresses, which would be an added penalty over the allowable stress decreases shown in Figure 7. And as already discussed, an allowable stress penalty does not change the failure mode away from potentially sudden Type IV failure.
With all of this, it probably seems surprising that Grade 91 is used at all, much less in the massive quantities that it is. The reality is that there is currently no reliable alternative material available and the attempt to move beyond relatively simple Grade 22 and the ligament cracking problems that plagued industry have been far more difficult than once expected. Grade 91 is not alone; modern materials apply all of the science available to push limits as far as they possibly can to meet ever increasing process demands. The problem is not really Grade 91 at all, but rather incredibly demanding service conditions coupled with decreasing budgets;
“All materials have their limitations and the solution to high-temperature problems is often a compromise between careful materials selection . . . , process control . . ., and better design specifications to recognize mechanical constraints at elevated temperature or resulting from thermal cycling. The ultimate choice will be a compromise based on what is available and how much it costs.” 
There really are no magic materials, and for all high temperature applications, there is a finite lifetime; often much less than desired when cycling is involved if care is not taken.
For Grade 91, the key is not only implementing all the materials and fabrication learnings from the last 30 years, but also leveraging modern analysis tools to improve and create inherently safe designs (while understanding the limitations that Type IV failure imposes). At the same time, details that can’t be made safe for high temperature operation should obviously be obsoleted. This has really always been the plan (materials, design, fabrication and inspection working together to create something useful at a price someone is willing to pay) – at least until a magic material comes along . . .
- EPRI Report 3002012262, “Integrated Life Management of Grade 91 Steel Components: A Summary of Research Supporting the Electric Power Research Institute’s Well-Engineered Approach,” Electric Power Research Institute. May 2018.
- Maruyama, K. J. Nakamura and K. Yoshimi, “Assessment of Long-Term Creep Rupture Strength of T91 Steel by Multiregion Rupture Data Analysis,” Journal of Pressure Vessel Technology, Vol. 138. ASME, 2016.
- Corrosion Engineering, P. R. Roberge, McGraw-Hill, NY. 2008