What plant owners should know about today’s materials before specifying pressure parts

Perhaps the most compelling takeaway from the HRSG Forum with Bob Anderson was “How innovations in steelmaking (sidebar) are changing the way pressure-part materials behave.” As attendees listened intently to Jeff Henry, one of the industry’s top materials experts, now affiliated with Applus+ RTD, a global leader in nondestructive testing, the details unfolded into a wakeup call. Henry also is chairman of the ASME Boiler & Pressure Vessel Code’s Section II (materials) committee.

Henry turned back the clock to 2015 when a Fluor representative alerted ASME to problems with some carbon-steel fittings and forgings at a work site. Unexpected cracking appeared during normal handling, at or below room temperature (Fig 1). All materials met ASME/ASTM specifications, but subsequent analysis showed poor toughness with brittle trans-granular fracture and nil lateral expansion. Re-normalizing did not restore toughness.

The following year, XGEN engineering’s inspectors at a nuclear site had troubling nondestructive examination (NDE) issues with stainless-steel and nickel-based materials. UT was showing false reflectors (indicating cracks). Inspectors were unable to transmit sound to critical areas.

Testing showed highly anisotropic mechanical properties (varying by measurement direction). Analysis also indicated accelerated growth rates for stress cracking in cast and banded structures in some well-known alloys.

Stated Henry, “Creep-strength-enhanced ferritic (CSEF) steels, such as Grade 91, are examples of what we consider advanced alloys for boiler and pressure-vessel applications.” He then added, “Implications for advanced alloys could be the most significant, particularly for those at elevated temperatures and under frequent cycling. These materials have more complex metallurgy, so small changes in their composition, heat treatment, or processing may produce significant adverse changes in properties.”

So what are we seeing and what are the implications?

We know there are potential changes in material behavior, but all of this is not yet clearly understood. Again in the past, OEMs filled part of this knowledge area with dedicated laboratories and veteran personnel, but they can no longer fill this role.

Material suppliers (steelmakers) are concerned with delivery and production costs.“Also,” Henry added, “most suppliers will charge a high premium to produce materials with more restrictive requirements for individual purchasers.”

Recent work at EPRI has focused on the effect of residual elements on both long-term creep rupture strength and ductility (Fig 2).

One finding might be even more concerning: “Significant differences in both strength and ductility have been found in different heats of Grade 91, all of which had been manufactured in the same manner and operated under exactly the same conditions.”

Henry then turned to compositional issues specific to Grade 91, noting ASTM’s requirements adopted in the mid-1980s. Details for more restrictive requirements (residuals) are now incorporated in a new Section I code case that was approved in 2016. And in February 2017, the responsible ASME groups voted to reduce allowable stress values for the existing Grade 91 steel (Fig 3). Other studies and potential actions are ongoing.

One message was clear: Don’t panic, but do be aware!

Although there were many questions and focused discussions during Henry’s presentation, there were many “still looking” statements. The primary conclusion: “Effects of fabrication processes on advanced alloys are not yet clearly understood,” but we all need to stay informed. We need to observe.

Henry summarized the implications: “There is no question that the concerns raised for performance of Grade 91 apply to all CSEF steels, and to other advanced alloys.” All codes will need to consider these “unintended consequences.”

In a more pointed remark, he noted that “Action has been taken on Grade 91 not because it is the only advanced alloy adversely affected by the changes, but because it is the only alloy that has received the attention necessary to define some of the required actions.”

He concluded with a look ahead, and the need to do the following:

      1. 1. Better understand these changes.

      2. 2. Implement the necessary Code changes.

      3. 3. Better define what constitutes critical service.

Closely related topics of personnel safety and plant downtime also were discussed.

Lively dialogue followed on the status, outlook, and use of 91, 91 Type 2, and 92. Topics included ASTM material and product processes, general upgrading of specifications including OEM’s lists of supplementary requirements (based on experience), and the troublesome challenge of management’s fallback: “If it’s not in the Code, we don’t need it.”

Time will tell.

Steelmaking evolves, impacts product quality

Traditional steelmaking reduces iron ore in a blast furnace to get high-carbon molten iron. This is desulfurized and charged into a furnace; carbon content is reduced and alloying elements are added. Residuals like sulfur, phosphorus, copper, and tin (even some dirt) exist, but at tolerable levels that are not worth the high cost of removal. At the known and allowable levels, these residuals should not change material behavior. Simple enough.

Now add some more-modern activities. Quantities of scrap metal are charged into an electric furnace, saving time and money. But the potentially pesky residuals are now determined more and more by the scrap content. According to Jeff Henry of Applus+ RTD, “The number of residual elements—such as copper, tin, chromium, nickel, niobium, and titanium—have increased with the greater use of scrap.”

But even residuals are a complex topic. In carbon steel, chromium is residual. In Grade 22, it is alloying.

Now more progress. Years ago, hefty ingots were sent out from the mills as the main starting point for steel parts. The industry knew that as these large ingots slowly cooled, their cast structure became a mix of grain structures with pronounced macro and micro segregation. Breaking down the ingots into sheets improved product homogeneity. Knowingly, the process chain added several mechanical steps with intermediate heat treatments. This produced a relatively consistent and uniform structure and composition.

But today there’s more continuous casting, again saving time and money. Molten metal is solidified into intermediate shapes more closely aligned with the final product. On the positive side, the continuously cast products cool more quickly, eliminating much of the macro segregation and structural variety. But the effects of micro segregation and finer-scale structural inconsistency remain.

Because the product is now close to final size, the amounts of mechanical working and heat treating are reduced, and some remnants of the original cast heterogeneous structure can move freely into the final part. And don’t forget the pesky residuals.

More on materials

There were two additional presentations focusing on materials—one by Jean-Francois Galopin of CMI Energy, who used WebEx technology to deliver his message and answer questions from Brussels, the other by Kent Coleman of EPRI.

Galopin discussed higher efficiencies and the resulting higher exhaust gas temperatures entering the HRSGs. “We are already operating,” he noted, “at the upper limits of Grade 91 materials.”

CMI’s recent involvement includes Bouchain in France and Hamitabat in Turkey, both high-temperature plants now in operation. He is also involved in advanced ultra-supercritical installations.

His discussion covered material selection and design, weldability, steam oxide resistance, allowable stress, and Code approvals (both ASME and EN). For the highest temperatures, he discussed Super 304H, good for resistance to stress corrosion cracking and stress relaxation cracking. Galopin also reviewed experience with headers mode of Grades 91 and 92. To show variety and characteristics he discussed Super 304H (fine grain structure), TP347H (course grain structure), and high-cost Incoloy 617.

Specific to maximum allowable stress/creep and impact on design, he presented the data for component thickness and cycle fatigue performance (Fig 4).

Other items included header mockups for research, welding procedures, and metallurgical aspects for dissimilar welds, and alternative weld locations. In one summary Galopin stated, “Welding procedures and metallurgical aspects of dissimilar welds exist, but long-term thermal cyclic behavior of dissimilar welds is not well established.”

He concluded with finite element analysis and a focus on cyclic behavior.

Questions included the pros and cons of horizontal and vertical HRSG design, cold metal working and shot peening, and stress-relief applications.

EPRI’s Coleman discussed issues with CSEF piping, including Grade 91. He first noted several design and manufacturing flaws that have led to premature damage in Grade 91 piping systems including:

      • Dissimilar metal welds.

      • Improper materials.

      • Fabricated fittings and wyes.

      • Fabrication/design errors.

      • Soft materials.

      • Longitudinal seam risks.

      • Thermal fatigue at sky vents and drip pots.

      • Lower alloy filler metal/base metal with and without heat treatment for attachments and lining lugs.

      • Use of high nickel and manganese fillers for improved toughness.

Many of these practices, he stated, “are not prohibited by Code.” Similar to the discussions with Henry, therein could lie part of the problem.

For specifics, Coleman stated that “dissimilar metal weld failures can be quite dangerous and can occur between Grade 91 and austenitic alloys as well as lower-alloy ferritic materials.” Main areas of concern include flow elements, thermowells, RT plugs, bypasses around stop valves (warmup lines), and material transitions (Fig 5).

Coleman also highlighted longitudinal welds, stating that current reinforcement rules are unacceptable for materials in the creep range where HAZ damage is likely.

He also questioned various cross and attachment welds and then offered some specific suggestions to minimize the problems:

      • Require lifting and alignment lugs to be nominally the same composition as the base metal to which they are attached.

      • Follow PWHT requirements.

      • Protect in a dry environment until PWHT is performed.

      • Ensure PWHT on lifting lugs before they are used or stressed.

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