T24 revisited

Above Image: RDK8, left (photo: EnBW/Daniel Meier-Gerber)

Nearly three decades ago, the development of grade T24 steel — 7CrMoVTiB10-10 — was underway in Europe. Since then numerous projects including, field test operations (eg, AVIF A077, COMTES700), have been executed to qualify T24 steel for use in membrane walls of ultrasupercritical (USC) coal-fired boilers, demonstrating that T24 steel can deliver better material properties for reduced wall thickness, significantly better plant efficiency, and reduced specific CO2 emissions.

In addition, T24 promises improved creep strength at higher temperatures than its predecessor, 10CrMo9-10 or T22 steel. With its lower carbon content, T24 provides improved weldability as well as a moderate hardness increase in the heat affected zone (HAZ) and weld metal. This means that T24 boiler tubes can be welded in compliance with applicable codes without the need for post weld heat treatment (PWHT), which results in significant cost and time saving benefits in terms of site work. In addition, the risk of stress and temperature related damage

because of thermal expansion is restricted. By eliminating the need for PWHT, T24 promises a key advantage as a material for membrane walls.

As of 2019, nine coal fired power plants with GE (formerly Alstom) supplied boilers employing T24 membrane walls were operating successfully in Europe. They have amassed more than 300 000 operational hours and have experienced no issues related to stress corrosion cracking (SCC).

This group of plants includes the 912 MWe Rheinhafen-Dampfkraftwerk 8 (RDK8) plant in Karlsruhe (pictured above), which has achieved a record net electrical efficiency of 47.5%, operating at 600°C/275 bar for the main steam and at 620°C for reheated steam.

The T24 membrane walls are a key enabler in realising this significant performance benchmark. And T24 boiler walls are expected to play an important role in the plant’s future efforts to increase plant efficiency and reduce CO2 emissions using double-reheat with USC steam conditions, or advanced ultrasupercritical (AUSC) steam conditions up to 700°C or above.

A stressful time

However, while GE’s experience with T24 has been generally positive, in 2010, public reports surfaced from Germany about sudden and unexpected serious tube butt weld issues related to hydrogen induced stress corrosion cracking discovered in T24 membrane walls during the commissioning of newly built boilers that were planned, designed, and built by companies other than GE.

These reports created enormous uncertainty about the reliability of T24 and prompted intense discussion and led to extreme uncertainty among power plant operators, boiler manufacturers, steel and weld metal manufacturers, and research organisations. Besides field test experience gained via R&D projects investigating

T24 utilisation in membrane walls, there were no other commercial T24 references available at the time. The threat of reduced plant efficiency and associated financial risks loomed large for units under construction.

With SCC indicated as the damage mechanism, an extensive bundle of technical measures — based on material, environmental and stress factors — was defined by the boiler community and integrated into plant construction projects.

A full evaluation of the technical necessity of each measure was not feasible at the time, and no positive operational experiences were available.

Because SCC requires a specific combination of stress, material conditions and environmental factors (see Figure 1), the wet evaporator component was identified as the most critical area.

Figure 1. Factors contributing to stress corrosion cracking

GE launched a T24 task force to define measures needed to reduce or avoid SCC-related damage and help ensure successful future installations.

Although GE — as a boiler manufacturer — had not experienced similar SCC-related issues, a detailed T24 investigation was launched to analyse all available information and verify GE’s design and procedures.

Across the industry, operators and research institutes started time-sensitive risk mitigation investigations of their own.

The result was the design and implementation of a technically based package of measures for risk mitigation covering all damage-relevant parameters, for the commissioning phase and during construction. See Figure 2.

Figure 2. Risk mitigation measures applied to tower boiler T24 membrane walls, post 2010

In-depth investigations

Over the years, in-depth investigations of the condition of welds in operation have revealed no incipient cracks, strengthening confidence in future plant reliability. However, based on market feedback, manufacturers must continue to optimise boiler design and processes to deliver economic solutions with the highest performance.

Material-science-based research activities and projects implemented since 2010 have delivered additional insights regarding the behaviour of T24 weldments.

For example, a design change required the replacement of wall segments in the T24 evaporator of one of GE’s previously commissioned boilers. Nearly 320 butt welds were properly executed without any additional commissioning procedure using GE’s advanced welding technology.

Today, these welds have been operating problem-free for well over 20 000 hours. Over time, relaxation effects have progressed, and protective oxide layers have built up. Based on the significant number of operating hours of this unit, additional future problems with SCC are not expected. This case was used as the first industry reference to suggest that the additional cost- intensive T24-related commissioning measures should be reviewed and optimised.

Local PWHT: a seemingly simple solution

Local post weld heat treatment still remains today a reliable way to temper welded areas for mitigating the risks of SCC in T24 membrane walls. But local treatments for tube butt welds in membrane walls increase the risk of creating dimensional distortions outside design tolerances. Because local membrane wall heat treatments take place in a framework with restricted thermal expansion, a critical stress level — that is not directly measurable — can be reached that causes the formation of intergranular stress relief cracks. These cracks can occur as wall penetrating cracks or microcracks. Intergranular preliminary damage potentially decreases pressure part component lifetimes and is not always detectable using non- destructive testing methods.

These pros and cons of local PWHT need to be considered carefully in the decision-making processes relating to site erection of membrane walls. GE decided not to apply local PWHT at membrane walls for projects in Europe. However, it made the decision to perform controlled and uniform furnace heat treatment in the workshop on the highly stress loaded transition piece from spiral to vertical wall, which contains in addition a large number of manual welds.

The latest approach: back to the roots

The experiences gained during boiler operation across the industry are in line with the outcome of a public research and development project funded by the Research Fund for Coal and Steel (RFCS). The project — Crack mechanism understanding and failure avoiding treatment of T24 tube material in advanced super critical coal fired steam generators (CRAMUFAT24) — studied the behaviour of T24 steel during operation, with an emphasis on evaluating SCC and stress relief cracking. The laboratory test results, in combination with operational experience, provide strong evidence that stress-optimised design, manufacturing and assembling, along with proper weld execution is enough to provide reliable boiler operation.

Figure 3 summarises GE’s view on the appropriate manufacturing philosophy for T24, based on available experience as of 2019.

Figure 2. Risk mitigation measures applied to tower boiler T24 membrane walls, post 2010

SteamH

Coal still plays a substantial role in power generation worldwide, which is why efficiency- improvement — such as that achievable through the adoption of the improved steam conditions that use of T24 makes possible — is a critical issue.

GE has harnessed a broad array of technological advancements in its latest AUSC offering, SteamH, which drives towards 50% efficiency. Main steam conditions of 650°C/330 bar and up to 670°C in the reheat section are only achievable using Ni-based superalloys and advanced Martensitic stainless steel materials.

Additionally, greater operational flexibility becomes more relevant with the global expansion of renewables, and this increased flexibility can be achieved by reducing the wall thickness of specific components through the use of advanced materials.

All the advanced technology concepts leading to future AUSC plants consider T24 as a key material for boiler membrane walls. Lessons gained from manufacturing, site erection, commissioning and operational experience to date as well as detailed analysis over the last decade have resulted in reliability improvements. With the material qualification programmes completed or under execution for Ni-based superalloys, GE is in a position to offer the next generation of coal-fired steam plants, based on its SteamH technology.