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Becht Engineering Blog

In this section of the site contributing authors submit interesting articles relating to the various services, industries and research & development efforts of Becht Engineering.

Grade 91 Steel - How Did We Get Here? Part 2

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Read Part 1 Part 2: Type IV Cracking and Inspectability The current concerns with Grade 91 are fundamentally and firmly rooted in inspectability of Type IV damage; while sensitivity of the material can be managed (see for example [1]), Type IV cracking is perhaps the Achilles heel of Grade 91.  At a high level, the thermal cycle(s) due to welding will create a thin band of material in the heat affected zone (HAZ) with properties much closer to Grade 9 than Grade 91.  While full re-normalization and tempering of the entire component after welding can greatly improve the situation, simple (subcritical) post-weld heat treatment (PWHT) does not.  Damage is overwhelmingly concentrated in this thin band of material during high temperature operation, such that when failure finally occurs, it has an almost brittle appearance since there has been little if any creep deformation or damage outside of the HAZ (see Figure 3...
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Grade 91 Steel - How Did We Get Here? Part 3

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Read Part 1 or Read Part 2 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 [2], 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 [2] and can be seen in Figure 6, is that even more modern techniques like “region-splitting” do not capture the apparent...
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