Steels_ Metallurgy and Applications, Third Edition

364 Steels: Metallurgy and Applications

are particularly troublesome, it is sometimes necessary to use tubing which has

a surface cladding of stainless steel in order to achieve reasonable life. These

so-called co-extruded tubes will be discussed later.

Superheater/reheater tubing

Depending upon the position in the boiler and steam temperature, the tubing in

various stages of the superheater will be made from the following steel compo-

sitions:

Increasing

temperature

resistance

1% Cr, 
89 Mo

2~%, Cr, 1% Mo

18% Cr, 12% Ni, 2
89 Mo (Type 316)

16% Cr, 10% Ni, 8% Mn-Mo-V-Nb

(Esshete 1250)

Esshete 1250 was developed by the former United Steel Companies Ltd in the
1960s specifically as a high-temperature superheater material and the number
indicates the maximum operating temperature in degrees fahrenheit (675"C). As
indicated in Figure 4.35, the proof strength of the steel at 600*C is 40% higher
than that of Type 316, whereas its rupture strength is approximately 95% greater
than that of the standard grade. This provides a design cross-over temperature
from proof to rupture strength of 630"C in the case of Esshete 1250 compared
with 590~ for Type 316. This enables the use of substantially thinner tubing in
Esshete 1250 compared to other austenitic steels, which results in a considerable
cost saving. 34
Reheater tubing operates at the same temperature as superheater tubing and
this usually dictates the use of an austenitic stainless steel, such as Type 316,
in coal-fired stations. However, 2
88 % Cr 1% Mo steel can be used in oil-fired
boilers or in coal-fired stations where the design temperature has been reduced
deliberately to allow the use of ferritic steels. In this respect, steels such as 9% Cr
1% Mo can also be considered as candidate materials.

Steam headers

Although operating at high temperatures and pressure, steam headers are not
subjected to the corrosive conditions encountered by superheater or reheater
tubing and therefore headers are generally made from
89 % Cr-Mo-V, 1% Cr-Mo

or 21% Cr 1% Mo steel. For steam temperatures in excess of 565"C, highly

alloyed ferritic or austenitic steels would be required, as discussed in the next

section dealing with steam pipe.

Steam pipe
Steam pipes carry large volumes of steam from the headers to the turbine and,
like headers, they are not subjected to aggressive environments. In the UK,
89 %
Cr-Mo-V steel is used extensively for steam temperatures up to 565~ and
therefore the material is operating in its creep range.
Steam pipe is heavy-walled material and, according to Wyatt, 32 1% Cr-Mo-V
piping operating at a temperature of 565~ would have a wall thickness of