Steels_ Metallurgy and Applications, Third Edition

Stainless steels 355

2. Austenitic stainless steels

BS 1113 BS 5500

> 250 ~ C f ~. = Re(T) or Rm -- > 150 ~ f ~ = Re(T) or Rm -

1.35 2.7 - 1.35 2.5

whichever gives the lower value.

3. For time-dependent design stresses (creep range)

BS 1113 and BS 5500

SRt

fF 1.3

These rules give rise to the type of design curve shown in Figure 4.34 and
which is a graphical representation of the sort of information that is presented in
Table 4.13. As illustrated schematically in this figure, there is a marked change
in the design stress values at the cross-over temperature where the values based
on short-term tensile properties give way to those based on the time-dependent
creep rupture strength.
The actual design stress values for a series of ferritic grades and austenitic stain-
less steels are shown in Figure 4.35(a) and (b) respectively. The steels concerned
are in tube form (to BS 3059) and the design values in the creep range relate to
rupture in 100000 hours. As illustrated in Figure 4.35(a), the allowable stresses
increase progressively as the alloy content of the ferritic grades is increased.
Whereas the C-Mn steel exhibits a cross-over temperature of 430~ the alloy
steels undergo this change in the range 450-500~ In the austenitic series,
Figure 4.35(b), the design stress values increase progressively in the order:

9 Type 304- 18% Cr, 9% Ni
9 Type 316- 18% Cr, 12% Ni, 2.75% Mo
9 Esshete 1250- 16% Cr, 10% Ni, 8% Mn-Mo-V-Nb

E

C

.0.2% Proof stress

Rupture stress

3

Temperature

Figure 4.34 Design strength values for low-alloy steels according to BS 1113