Stainless steels 357The last composition was developed specifically for use in power plant boilers
and will be discussed later on pp. 363-365. However, the main feature to note
with these steels is that the design stress values are relatively low at temperatures
up to about 500~ compared with the ferritic grades, whereas the cross-over
between short-term tensile properties and time-dependent properties is extended
to considerably higher temperatures. As discussed later, Esshete 1250 offers a
significant advantage over standard grades because of the facility to design on
the basis of proof strength at operating temperatures up to 630~
From the data presented in Figure 4.35, it can be appreciated that the economic
selection of steels for service at elevated temperature would dictate the use of
C-Mn or low-alloy grades at temperatures up to 500-525~ However, at higher
temperatures, use must be made of austenitic stainless grades which exhibit better
properties, particularly in the creep rupture range.
Steels for low-temperature applications
As illustrated on pp. 350-351 austenitic stainless steels possess excellent
impact properties at extremely low temperatures, which is a characteristic of
face-centred-cubic metals. Therefore stainless steels, together with alloys of
aluminium, copper and nickel, are used extensively for the construction of
equipment operating at temperatures below -200~ However, there are many
structures which operate at only moderately low temperatures which do not
require the high toughness levels of these materials and which are satisfied by
the use of less expensive steels. This situation is demonstrated very clearly by
Wigley 31 in Figure 4.36, which identifies the boiling temperatures for a wide
range of gases and the materials that are used for the associated structures and
containment vessels. Thus for temperatures down to about -50~ C-Mn steels
(to BS 4360) may be adequate and a range of nickel steels (288 3 89 and 9% Ni)
can be used progressively down to temperatures of the order of -200~ Wigley
states that 9% Ni steel is the only ferritic steel which is permitted for use at
liquid nitrogen temperatures (-196~ It is also economical for the construction
of storage tanks for liquid argon (-186~ oxygen (-183~ and methane
(-161"C).
The specifications, composition ranges and properties of these C-Mn and
nickel steels are summarized in Table 4.14. In this table, Wigley makes the point
that although specific impact test temperatures are laid down in the relevant
standards, this does not guarantee that the steel will be satisfactory for use at
those particular temperatures. This aspect is particularly pertinent if the mate-
rims are used in thick sections. Annex D of BS 5500:1997 therefore specifies
particular requirements for ferritic steels for use in vessels operating below 0*C.
These take the form of diagrams providing relationships between the lowest oper-
ating temperature (design reference temperature) and impact test temperature for
different thicknesses of material. These relationships are shown in Figures 4.37(a)
and (b) for as-welded and post-weld heat-treated conditions respectively. Given
the beneficial effect of post-weld stress-relieving treatments, the impact test
requirements are less demanding than those required in the as-welded condi-
tion. For example, 20 mm material operating at a minimum design temperature