IFMASS10

297

STEELS FOR ELEVATED TEMPERATURE APPLICATION:

HEAT-AFFECTED-ZONE SPECIFICS

Ljubica Milović

Faculty of Technology and Metallurgy, University of Belgrade, Belgrade, Serbia

1. INTRODUCTION

Ferritic steel 9Cr-1Mo and its modified versions are currently favoured structural

materials for steam generator applications as they offer useful combination of high

temperature mechanical properties and corrosion/oxidation resistance. The arrivals of

new generation power plants have increased the operating steam temperatures and

pressures to achieve higher efficiency and better environmental protection. This has led to

the development of modified versions of 9% Cr steels, with excellent combination of

creep strength and ductility. These steels include plain 9Cr-1Mo steel initially modified

by the addition of strong carbide forming elements such as Nb and V (9Cr-1Mo-V-Nb,

designated as T91 or P91) and further modified by the addition of W (9Cr-0.5Mo-1.8W-

V-Nb designated as T92 or P92), /1-3/.

Modified 9Cr-1Mo steel (T91/P91), with high creep strength, high thermal conduc-

tivity, low thermal expansion coefficient and good resistance to corrosion and stress-

corrosion cracking, is widely used as a structural material in power generation and

petrochemical industries. The alloy is recommended for use in the normalized and

tempered condition. Because of the transformable nature of the ferritic alloy, it is

susceptible to the formation of undesirable microstructures during fabrication and/or heat

treatment processes.

The non-destructive characterization of this steel microstructure is important for qua-

lity control during fabrication and heat treatment, to ensure the desired microstructure and

mechanical properties. The ultrasonic technique is promising tool (for characterization of

microstructures, assessment of defects and evaluation of material properties), /4, 5/.

Tungsten-bearing 9Cr and 12Cr steels have been favoured as advanced ferritic heat-

resistant steels with sufficient creep rupture strength for application to thick section boiler

and turbine components of coal-fired ultra-supercritical (USC) power plants at tempera-

tures higher than 600°C and to first wall and blanket structures of the fusion reactor.

Recent trends to increase steam temperature over 600°C and pressure of boiler and

turbine components enhance thermal efficiency, resulting in lower fuel consumption and

lower CO

emission. Under these circumstances, the construction of USC plants opera-

ting at 650°C is planned in Europe, The United States and Japan. This requires the deve-

lopment of advanced ferritic heat-resistant steels with sufficient creep rupture strength at

temperatures higher than 600°C. Tungsten-bearing 9Cr and 12Cr steels are also being

considered as alternate candidate structural materials to austenitic type 316 stainless steel

and conventional Cr-Mo steels for application to first wall and blanket structures of the

fusion reactor, because of their higher resistance to swelling than type 316 stainless steel

and because of their lower radio activation than Cr-Mo steels under high energy neutron

irradiation, /6/.