Meeting the challenge
High-temperature steels are being developed to improve the efficiency of power plants and to reduce CO2 emissions. Professor Scott Lockyer, Dr Mark Jepson and Dr David Allen* discuss the potential of martensitic steels to meet these requirements.
Materials must continually evolve to meet the increasing demands they face in operation. Take, for example, the creep strength enhanced ferritic (CSEF) steels, Grade 91 and 92, which are the materials of choice for steam pipework and boiler headers in conventional power generation plants. These 9%Cr martensitic steels are strengthened by sub-micron scale carbide and nitride precipitates to provide the high creep strength required to survive operation at steam temperatures and pressures of 560-610°C and 165-250bar, respectively.
However, there is now a drive to adopt ultra-supercritical (USC) plant designs with higher operating temperatures and increased efficiency and reduced CO2 production. Furthermore, increasing generation by intermittently available wind and solar plants requires compensating variations in conventional power plant operation, which promotes thermal fatigue damage, especially in thick components.
Such a conundrum requires stronger, high-temperature steels to meet the challenge. One of the most promising is MarBN – martensitic steels strengthened by controlled additions of boron and nitrogen. This is based on 9%Cr steel, as proposed by Professor Fujio Abe at the National Institute for Materials Science, Japan, who has produced small laboratory-scale melts and showed that their creep strengths were superior to grade 92 steel.
Making an impact
To develop MarBN steel for commercial scale production, a UK-based industrial collaborative project, IMPACT, led by Uniper Technologies, was formed. The project, funded by the UK Technology Strategy Board (now called Innovate UK), also included manufacturers Doosan Babcock, GE, Goodwin Steel Castings, Loughborough University, UK, and the National Physical Laboratory.
The core aims were to determine the optimum composition and heat treatment for MarBN steel, produce a large melt and assess creep strength. Loughborough University undertook thermodynamic modelling to analyse a range of compositions and trial melts were then produced for creep testing. An 8 tonnes melt was made by Goodwin and poured to produce several test blocks, ingots for forging and a large 3t bonnet-shaped casting replicating a turbine valve cover.
As part of IMPACT, MarBN boiler tubes were also produced and installed on the reheater drums on Units U2 and U3 of Uniper’s Ratcliffe-on-Soar power station in Nottinghamshire, UK.
The trial tubes were constructed with small sections of grade 91 tubes on either side, with grade 91 weld consumables used to join the MarBN to the tubes – the world’s first use of MarBN steel in a full-scale operational power plant.
At the end of January 2018, the tubes on U2 and U3 had experienced in excess of 10,000 hours and 300 starts. The tubes will likely be removed in the near future for analysis of oxidation and microstructural evolution, comparing the MarBN tubes with the grade 91 sections installed for the same period of time. Initial IMPACT creep data from the forged MarBN material used to make the trial boiler tubing show that, in the short term, MarBN is some 30% stronger than grade 92. If this is confirmed in longer-term testing, it will imply that MarBN could enable a 25°C increase in steam plant operating temperatures, with substantial efficiency improvements and hence the potential saving of millions of tonnes of CO2 emissions.
Competition is at the heart of this work. The IMPACT MarBN steel, known as IBN1, will compete with MarBN-based products from Europe, Japan and China, and also with the more ambitious concept of upscaling nickel-based superalloy technologies for tonnage-scale applications in steam power plants. Importantly, the extensive IMPACT alloy development database on variant alloy chemistries will enable IBN1 manufacturers to achieve adequate composition control without undue cost, while a heat treatment specification developed from modelling at Loughborough will enhance the creep performance of IBN1.
Modelling a future
In addition to this, recent work at Loughborough University has focused on the microstructures of MarBN welds and their associated heat affected zones (HAZ). Weldability is a crucial aspect of power plant materials’ development, as components must be welded together during manufacture and may also need to be repaired or replaced during service. Therefore, MarBN weld metals must demonstrate their toughness in order to be useful in pressurised components. Furthermore, the weld HAZ may be weakened by harmful thermal cycling, and so high temperature weld performance is often the critical link in plant life management.
Trial welding consumables have been developed to assess the effects of alloying elements on the creep performance and toughness of the weld metal, and to test the performance of the HAZ. The weld metals have been examined using advanced characterisation tools at Loughborough’s Materials Characterisation Centre (LMCC), including scanning electron microscopy (SEM) imaging, electron backscatter diffraction (EBSD), energy dispersive X-ray spectroscopy (EDS), focused ion beam (FIB) imaging, and transmission electron microscopy (TEM).
FIB-induced secondary electron imaging and SEM backscattered electron imaging can be used to identify different precipitates in the steel. Due to the high contrast observed within these images, it is possible to automatically count and size the particles observed and understand how the material evolves over time.
Impelled to act
Overall, MarBN has also evolved through the IMPEL project, led by GE, and a second Innovate UK-funded initiative, INMAP.
IMPEL is looking at the development of matching composition MarBN welding consumables, in collaboration with producer Metrode, for the manufacture of boiler components. INMAP, meanwhile, has pursued MarBN casting technology to develop the capability to produce thick-section steam turbine components.
However, there is a third strand. Known as IMPULSE, and led by Doosan Babcock and funded by Innovate UK, this project seeks to continue industrialisation of MarBN steel and produce a commercial scale steam pipe product by ingot casting, forging and pipe extrusion.
Metal component manufacturer Wyman Gordon has joined the programme to provide the necessary extrusion capability through their Livingston facility in Scotland, while the University of Birmingham, UK, has contributed specialist studies in extrusion flow testing and modelling to help adapt the process parameters.
Metrode has adapted the UK MarBN steel composition to produce a stick electrode – a manual-metal-arc (MMA) welding consumable – that deposits a near-matching MarBN weld metal with the improved impact toughness required to qualify the consumable for application in pressurised plants.
Plans for 2018 include the production of a large-scale ingot for forging, extrusion, welding and testing. This follows work by Goodwin, which produced one ingot in 2017 that has undergone forging in preparation for a first extrusion trial at Wyman Gordon.
IMPULSE partners have also pursued the data generation and metallurgical validation programmes required to underpin the design of high-temperature components. GE and Doosan Babcock have started long-term creep testing programmes, while the University of Nottingham, UK, has carried out short-term testing and modelling, finding that MarBN has sufficient creep ductility to achieve a reasonable performance in plant that is subjected to rapid cyclic loading at high temperatures.
Microstructural studies are also part of the equation, with Loughborough University looking at the causes and potential consequences of the reductions in long-term creep ductility observed in both MarBN steels and current high temperature alloys.
Ahead of the game
It is widely known that older coal-fired power plants are being closed in many countries due to concerns about climate change. However, economic factors are expected to drive substantial coal-fired generation in many emerging economies. China, where smog and water availability are also major concerns, wants future new coal-fired plants to be USC designed.
To support intermittent renewable energy generation with minimal CO2 production, there is also interest in increasing the operational flexibility of gas turbine power plants. MarBN could be used to design associated steam generating plants for higher temperature operation, eliminating austenitic stainless steel components, and thus removing the need to make mis-matching welds, which may not withstand the stresses of rapid on-off cyclic plant operation. While nickel superalloys are winning the race in terms of research and publicity, MarBN alloys stand a good chance of coming out on top for real applications.
*Professor Scott Lockyer is Technical Head of Materials and Corrosion at Uniper Technologies Limited, and Visiting Professor in Advanced Materials at Loughborough University.
Dr Mark Jepson is a Senior Lecturer in Metallurgy and Microscopy at Loughborough University, UK.
Dr David Allen is a Materials Consultant at IMPACT PowerTech, and a researcher at Loughborough working on the IMPULSE project.