Steels_ Metallurgy and Applications, Third Edition

Stainless steels 331

1. The lack of facilities and expense involved in canting out the ageing
   treatments - typically 700-750~ for 24 hours.


2. The reduction in corrosion resistance produced by the precipitation of carbides,
   phosphides or intermetallic compounds.


3. The loss of strength in the heat-affected zones of welds.


4. The reluctance of plant designers to consider the very large reductions in plate
   thickness etc. that could be provided by these high-strength steels.

As illustrated in Table 4.8, a large number of standard and proprietary grades
have been developed with the objective of increasing the resistance to reducing
acids, pitting and crevice corrosion. Many of the steels can be regarded as simple
extensions of Type 316 (18% Cr, 12% Ni, 2.5% Mo) with higher molybdenum
contents and with increased levels of nickel to preserve a fully austenitic structure.
Some of these steels also contain up to 0.25% N which increases the strength
and augments the chromium and molybdenum contents in improving the pitting
resistance, according to the following pitting index formula:

Pitting index = Cr + 3.3% Mo + 16% N

Copper additions of up to 2% feature in some of the compositions which improve
the resistance to sulphuric acid.
Many of the compositions listed in Table 4.8 are used in flue gas desulphuriza-
tion plant where acid condensates, contaminated with chlorides, produce severe
corrosive conditions. Steels such as 94L are also used in the food industry and
also for heat exchangers, particularly where salt water is involved. This grade

Table 4.8 Compositions of high-alloy austenitic and duplex stainless steels

Alloy Chemical composition (wt %)

designation C cr ..... Mo' Ni " Cu N Others

316L 0.03 17 2.25 12 - -
316LM 0.03 17 2.75 12 - -
317L 0.03 18 .5 15 - -
317LN 0.03 18 4.5 13 - 0.15
317LM 0.03 18 4.5 15 - -
317LMN 0.04 18 6 14 - 0.15
94L 0.02 20 3 25 -
254SMo 0.02 20 6.5 18 2 0.12
lncoloy 825 0.03 22 42 -
Hastelloy G 0.05 22 7 44 1.9 -
9
Hastelloy G-3 0.015 22 13 41 - -
Inconel 625 0.05 22 16 61 - -
Hastelloy C-22 0.015 22 3 - -
Hastelloy C-276 0.02 25 3.5 0.75 -
2205 0.03 22 3 5.5 0.15
Zeron 100 0.03 25 3.5 7 0.75 0.25

n

2.1Nb + Ta,

2.5Co, 1W

5Co, 1.5W

3.65Nb + Ta

3W

1.25Co, 4W

0.75W

0.75W

After Lane and Needham. 12