Coal Power Plant Materials and Life Assessment

Developments and Applications

A. Shibli

• Publisher: Woodhead Publishing
• Year: 2014
• Print ISBN: 9780857094315
• eBook ISBN: 9780857097323
• Topic: Technology & Engineering
• Subtopic: Fossil Fuels
• Index: Technology & Engineering

Due to their continuing role in electricity generation, it is important that coal power plants operate as efficiently and cleanly as possible. Coal Power Plant Materials and Life Assessment reviews the materials used in coal plants, and how they can be assessed and managed to optimize plant operation. Part I considers the structural alloys used in coal plants. Part II then reviews performance modelling and life assessment techniques, explains the inspection and life-management approaches that can be adopted to optimize long term plant operation, and considers the technical and economic issues involved in meeting variable energy demands. - Summarizes key research on coal-fired power plant materials, their behavior under operational loads, and approaches to life assessment and defect management - Details the range of structural alloys used in coal power plants, and the life assessment techniques applicable to defect-free components under operational loads - Reviews the life assessment techniques applicable to components containing defects and the approaches that can be adopted to optimize plant operation and new plant and component design

Part I

Materials for coal power plant construction

Outline

1 Grade 91 heat-resistant martensitic steel

2 Grade 92 creep-strength-enhanced ferritic steel

3 T23 and T24 – new generation low alloyed steels

4 Traditional low alloy steels in power plant design

5 Creep strength of austenitic stainless steels for boiler applications

6 Nickel-base alloys for advanced power plant components

1

Grade 91 heat-resistant martensitic steel

F. Abe, National Institute for Materials Science, Japan

Abstract:

Key issues for application of Grade 91 (Gr.91) to power plants, such as long-term creep strength of base metal and welded joints, microstructure evolution during exposure at elevated temperature, degradation in welded joints due to type IV fracture, creep–fatigue properties and steam oxidation behaviour, are comprehensively described, together with chemical, heat treatment, tensile and process requirements for Gr.91. Greater attention will be paid to technical issues on estimation of 100 000 h creep rupture strength, creep life prediction by creep strain analysis, and microstructure degradation causing a loss of creep strength at long times, which have been extensively investigated for Gr.91.

Key words

creep strength; 100 000 h creep rupture strength; type IV fracture; martensitic microstructure; M₂₃C₆ carbide; creep–fatigue interaction; oxide scale

1.1 Introduction

9Cr-1Mo-0.2 V-0.05Nb steel specified as ASME SA213/SA213M T91 (seamless tube), SA335/SA335M P91 (seamless pipe), SA387/SA387M 91 (plate) and so on,¹ which are denoted Gr.91 as a general name in this text, was developed by Oak Ridge National Laboratory and Combustion Engineering in the late 1970s, originally for application to the steam generator of liquid metal-cooled fast-breeder reactors with a service temperature not exceeding 550°C.² At present, however, Gr.91 is widely used in pressure vessel and piping systems in the fossil power plants with steam temperatures of up to 600°C and the petrochemical industries, because it has a higher creep strength than low alloy steels such as 2.25Cr-1Mo steel and better thermophysical properties. These are shown by the fact that it has lower thermal expansion and larger thermal conductivity than austenitic steels such as the 300 series stainless steels.³ Gr.91 is also one of the candidate materials for future Generation IV Nuclear Energy Systems.

Critical issues for long-term safe operation of Gr.91 components such as main steam pipe, header and tube in coal-fired ultra-supercritical (USC) power plants are oxidation resistance in steam as well as long-term creep rupture strength of base metal and welded joints. The resistance to type IV cracking strength loss in welded joints is serious for thick-section boiler components. Furthermore, the thermal-cycling capabilities of thick-section components in USC power plants would be severely restricted by creep–fatigue damage. Not only chemical compositions, but also production routes and heat treatments, determine microstructure before operation in power plants, which affects subsequent microstructure evolution and mechanical performance during operation in power plants at elevated temperatures.

In the following sections, key issues for application to power plants are comprehensively described for Gr.91, in terms of long-term creep strength of base metal and welded joints, microstructure evolution during exposure at elevated temperatures, degradation in welded joints due to type IV fracture, creep–fatigue properties and steam oxidation behaviour, together with chemical compositions, heat treatments and processing. Greater attention will be paid to technical issues on the estimation of 100 000 h creep rupture strength and on microstructure degradation causing a loss of creep strength at long times.

1.2 Chemical compositions, heat treatments and microstructure

The Gr.91 steel is a modification of 9Cr-1Mo steel specified as ASME T9 by the combined addition of vanadium (V), niobium (Nb) and nitrogen (N), which causes precipitation strengthening due to fine MX carbonitrides, where M is V, and Nb and X are carbon and nitrogen. Tables 1.1 and 1.2 give the chemical, heat treatment, and tensile requirements for tube, pipe and plate of Gr.91 listed in ASME (American Society of Mechanical Engineers) Section II, Part A.¹ The requirements for forgings, fittings, forged pipe and casting are also listed in ASME Section II, Part A. Gr.91 products are intended to exhibit a 100% martensitic microstructure after normalizing heat treatment. Therefore, the chemical composition is balanced according to a chromium–nickel balanced (CNB) formulation in Eq. 1.1:

$\begin{array}{l}\mathrm{CNB}=\mathrm{Cr}+6\mathrm{Si}+\mathrm{4...}\hfill \end{array}$

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Woodhead Publishing Series in Energy
• Part I: Materials for coal power plant construction
• Part II: Performance assessment of coal power plants
• Index