Stainless steels 369for secondary plant. Early work had indicated that the niobium-stabilized steel
corroded at less than half the rate of Type 321 in boiling nitric acid and this was
ascribed to the fact that the former had a completely austenitic structure whereas
Type 321 contained some delta ferrite. However, the above authors state that it
was shown subsequently that delta ferrite had no significant effect on the corro-
sion behaviour and that the inferior performance of Type 321 was due to the
presence of TiC particles which are readily dissolved in hot nitric acid.
The use of 18% Cr 13% Ni-Nb and Type 321 remained in force until the
mid-1970s, when several factors brought about a re-appraisal of steel selection.
It is stated that these included the fact that some forgings in 18% Cr 13% Ni-Nb
were badly corroded and that Type 321 was prone to knife-line attack in the heat-
affected zone of welds. Welding problems had been experienced with the fully
austenitic, niobium-stabilized steel and this material was also prone to end grain
attack due to outcropping stringers of coarse NbC carbides. However, perhaps the
greatest impetus for the re-appraisal of material selection stemmed from the fact
that the nitric acid manufacturing industry had ceased to use the stabilized grades
of stainless steels, having experienced improved manufacturing and operating
performance with the low-carbon grades such as 304L and 310L.
Following extensive testing in nitric acid-based liquors and vapours, BNFL
elected to replace 18% Cr 13% Ni-Nb by a low-carbon grade of 18% Cr 10%
Ni steel. Initially, this material was called Nitric Acid Grade 304L but was
subsequently designated by BNFL as NAG 18/lOL. The carbon content of this
grade is restricted to 0.025% max. compared with 0.03% max. in 304L and
restriction on the phosphorus content (0.018% max.) also ensures very low levels
of sensitization and intergranular attack. In addition, the steelmaking practice for
this grade of steel has to be tightly controlled in order to produce a very clean
steel so as to minimize the inclusion content and the tendency for end grain
corrosion. Large quantities of NAG 18/lOL have been used in the construction
of the Thermal Oxide Reprocessing Plant (THORP) at Sellafield, and Shaw and
Elliott report that the steel has given excellent welding performance. Whereas
9.3% of welds showed signs of cracking in 18% Cr 13% Ni-Nb, this has been
reduced to 0.2% in NAG 18/IOL.
Stainless steels are also used in the nuclear power and fuel reprocessing
industries for the storage and transportation of spent fuel elements. For such
applications, the materials must provide substantial neutron absorption, and this
characteristic is provided by the inclusion of up to 1% B in a base steel containing
18% Cr and 10% Ni. The production and properties of such material, designated
Hybor 304L, have been described by King and Wilkinson. 37 The addition of
large amounts of boron to a stainless steel results in the formation of an
austenite-(FeCr)2 B eutectic which reduces the hot workability of the material.
However, steels containing up to 1% B have been successfully rolled to plate
and welded satisfactorily by the TIG, MIG and MMA processes. As illustrated
in Figure 4.41, the addition of boron also produces a substantial dispersion
strengthening effect, coupled with significant loss of toughness and ductility. The
above authors therefore conclude that the conflicting interests of the steelmaker,
fabricator and the nuclear industry are best served by steels with boron contents
in the range 0.5-1%.