To overcome the problems of solution
annealing of Hastelloy C after welding, the chemical composition of alloy C was
modified that reduced carbon and silicon content. This change was made possible
by AOD process – an advanced melting technique. Low carbon and low silicon
alloy was called as Hastelloy C276 that offered similar corrosion resistance as
of alloy C however without negative effects of continuous grain boundary
precipitates in the weld HAZ of alloy C276. This material could be used in many
applications in the as-welded condition without severe intergranular corrosion.
The corrosion behavior of alloy C and alloy C276 has been adequately covered.
The grain boundary precipitation mechanism and time temperature transformation
for these Hastelloy grades are also documented.
Applications of Hastelloy C276 wire in the process industries are wide, different and versatile for its
supreme resistance in both oxidizing and reducing conditions, in fact also with
halogen ion contamination. Although there are specific process conditions where
alloy C276 with its low carbon and silicon content is sensitive to corrosion
because it is not perfectly heat stabilized in correspondence to precipitation
of carbides and intermetallic phases. Within the broad scope of chemical
processing, examples are where extreme intergranular corrosion of a sensitized
microstructure has occurred. To overcome this sensitivity, an enhancement was
made in Hastelloy C276 that was called Hastelloy C4. In highly oxidizing media,
both alloy C276 and C4 containing 16% chromium offer significant resistance.
This limitation has lead to the development of other alloys for example Hastelloy
C22.
Hastelloy C4
Wide reduction in carbon and silicon
content in alloy C, removal of tungsten from its composition and reduction in
iron and addition of titanium, have offered dramatic improvement in the precipitation
mechanism of intermetallic phases when subjected to the sensitizing range of 550oC
to 1090oC for longer periods virtually preventing the intermetallic and grain
boundary precipitation of the mu phase and other phases. These phases affect
ductility, toughness and corrosion resistance. The general corrosion resistance
offered by Hastelloy C276 and Hastelloy C4 are basically same in various
corrosive conditions except in highly reducing media such as HCl, Hastelloy C276
offers better performance however in strong oxidizing conditions, Hastelloy C4
is superior.
Hastelloy C4 offers supreme corrosion
resistance to a wide range of media including organic acids and acid chloride
solution. In contrast to alloy C276 that is widely used in the world, alloy C4
is gradually replaced by alloy 59.
Hastelloy C22
Hastelloy
C22 was developed to hold superior oxidizing properties of Inconel alloy 625
while retaining the localized corrosion resistance properties of Hastelloy
C276. It was developed by eliminating tungsten, decreasing iron content at the
risk of decreased corrosion resistance in oxidizing chloride solutions where
tungsten works good. Additionally, both Hastelloy C276 and C4 are attacked
quickly in oxidizing, nonhalide solutions due to their low chromium concentrations
of about 16%. This alloy composition with about 21% chromium, 13% molybdenum,
3% tungsten, 3% iron with balance nickel offered better corrosion resistance than
alloy C276 and C4.
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