Steels_ Metallurgy and Applications, Third Edition

332 Steels: Metallurgy and Applications

Table 4.9 Prices for high-alloy grades relative to Type 316L

Alloy types Price factor

316L 1

2205 1.1

317L 1.2

317LM 1.3

94L 2

High-nickel alloys 2.3-3

High-molybdenum alloys (>6% Mo) 2.7-3.8

Nickel-base alloys 3.3-9

Titanium 7.6-8.1

contains 25% Ni and will therefore provide better resistance to stress corrosion
than traditional grades such as Type 316.
The last two compositions in Table 4.8 differ from the rest in that the high
chromium and molybdenum contents are not complemented with a large addition
of nickel and this results in a duplex, austenite plus ferrite structure. Such steels
have good resistance to both chloride and sulphide stress corrosion and the duplex
structure also provides high proof stress values, e.g. 480 N/mm 2. This combi-
nation of properties is used to advantage in the petroleum industry, particularly
for the handling of crude oil which is contaminated with hydrogen sulphide.
Typical applications include transfer pipelines, downhole tubing and liners and
also topside process equipment, such as heat exchangers and seawater transport
piping. 13
British Steel 13 has presented the information in Table 4.9 on the prices of some
of the high-alloy grades and nickel superalloys relative to Type 316L.

High.alloy f erritic grades

As indicated earlier, standard austenitic stainless steels such as Type 304
(18% Cr, 9% Ni) are highly susceptible to chloride stress corrosion and the nickel
content must be raised to about 40% in order to provide immunity from this form
of attack. Such materials are expensive and the addition of nickel alone provides
little benefit in terms of resistance to pitting and crevice corrosion. On the other
hand, standard ferdtic grades such as Type 430 (17% Cr) and 446 (25% Cr) are
resistant to stress corrosion but are prone to intergranular corrosion and have poor
toughness, even in moderate plate thicknesses. However, as early as 1951,14 it
was shown that these deficiencies could be overcome by reducing the interstitial
content of ferritic stainless steels to low levels. However, for many years, this
information remained little more than laboratory-based data until the development
of steelmaking processes that enabled the materials to become commercially
viable. In particular, the Airco-Temescal Division of Airco Reduction Co. Inc. in
the United States introduced a vacuum melting process with electron beam stirring
which could achieve total carbon and nitrogen contents of the order of 0.01%. 15
In addition, an LD converter coupled with a Standard-Messo decarburizer was
developed in Germany to achieve total interstitial contents of about 0.025%. 16