753Alloy C276 1965 to Present

To overcome one of the above serious limitations, the chemical composition of alloy C was modified by a German company, BASF, which basically consisted of reducing both the carbon and silicon levels in this alloy by more than 10-fold to very low levels of typically 50 ppm carbon and 400 ppm silicon. This was only possible due to the invention of a new melting technology, known as the AOD-process and VOD (vacuum oxygen decarburization) process. This low-carbon and silicon content alloy came to be known as alloy C-276, which then was produced in the United States under a license from BASF Company, which was awarded a U.S. patent. The corrosion resistance of both these alloys was essentially similar in many corrosive environments, but without the detrimental effects of continuous grain boundary precipitates in the weld HAZ of alloy C-276. Thus alloy C-276 could be used in most applications in the as-welded condition without suffering severe intergranular attack. The corrosion behavior of both alloy C and alloy C-276 has been adequately covered in the open literature via numerous publications [6, 24]. The grain boundary precipitation kinetics and the time-temperature-transformation (T-T-T) diagram for these alloys are also well documented [25-27]. The industrial applications of alloy C-276 in the process industries are very extensive, diverse, and versatile due to its having excellent resistance in both oxidizing and reducing media even with halogen ion contamination. However, there were certain process conditions, where even alloy C-276 with its low carbon and low silicon was not adequately thermally stable in regard to precipitation of both carbides and intermetallic phases, thus being susceptible to corrosive attack. Within the broad scope of chemical processing, examples exist where serious intergranular corrosion of a sensitized (precipitated) microstructure has occurred. To overcome this, a modification of alloy C-276 was developed in the 1970s, called alloy C-4.

0 0

Post a comment