Heat resisting steels and iron-containing superalloys
The typical operating life of a modern automobile is 2000 to 3000 hours. That of a civilian aircraft engine is ten times longer, while certain thermal power station components are expected to last more than 300000 hours (1 year = 8760 hours), often with a
- PDF / 2,316,246 Bytes
- 16 Pages / 439.37 x 666.142 pts Page_size
- 93 Downloads / 203 Views
20-1 Ferritic heat resisting steels The "ferritic" heat resisting steels in fact have martensitic or bai ni tic structures, strengthened by precipitation, mainly of secondary carbides. They are extensively employed in thermal power stations, for many different components, including boiler vessels, tub ing, disks, rotors and bolting. A wide range of different materials has been developed as plant efficiency has increased. Extensive efforts have been made in the USA, Japan and Europe to enhance the thermal yield of the steam turbine cycle, reducing fuel consumption and the associated CO 2 emissions. Since it is a well-established thermodynamic principle that efficiency increases with operating temperature, this has led to increasingly severe demands on materials performance. Thus, supercritical plants operate with steam temperatures of 580-600°C and pressures of around 250 bars, while ultrasupercritical processes involve temperatures up to 650°C and pressures of 300-350 bars. Such conditions are extremely demanding in terms of structural stability, creep strength and corrosion resistance, particularly
M. Durand-Charre, Microstructure of Steels and Cast Irons © Springer-Verlag Berlin Heidelberg 2004
THE MICROSTRUCTURE OF STEELS AND
I ~I I ~
CAST IRONS
+Mo
4MO,+V,+Nb
_V,_""
-
-M.
I II
+Mo. +V
35
_w
II
60
+W+CO
~,+W.N b
100
1O'h our creep rupture strength (MPa)
140
180
Figure 20-1-1: Schematic evolution of the three major families of ferriric creep resisrant sreels, containing respecrively 2.25, 9 and 12 % Cr. Standard grades are represented by ovals, while the rectangles indicate development sreps. Adapred from [MasOO].
since these properties must generally be associated with good weldability and roughness. A temperature of 650°C almost certainly represents an upper Iim it for ferritic materials. There are three main classes of ferritic heat resisting steels, which have each undergone successive improvements (Figure 20-1-1). The first group, corresponding ro 2.25Cr-lMo steels, includes the T22 grade developed in the 1950s, which is stiU used for certain components. A modified version developed by Sumiromo in Japan was included in the ASME standard as T23 grade in 1995. lts properties have been improved by controlled additions of tungsten, vanadium, niobium, nitrogen and boron (cf Table 20-1-2). Apart from enhanced creep strength, the new alloy can be readily welded, without the need for pre- or post-weld heat treatment. The second family represents the 9Cr-lMo steels, initially developed in the USA in the 1970s, with various designations (G91, P91, T91). Significant progress was made in Japan in the 1990s, particularly by the use of tungsten and nitrogen additions ro precipitate highly stable carbonitrides. These steels are now known under the standard designations P92 and T92. The third category corresponds to the 12 % Cr steels, whose chromium content is sufficient to be genuinely stainless. Similar improvements ro these grades, also made initially in Japan, have led to the recent P122 and Tl2
Data Loading...
