The oxidation behavior of specimens containing tungsten inert gas welds of UNS NO6025 (NiCrFeAlY) was investigated in air for up to 5,000h at 900°-1000°C and 1,000h at 1100°-1200°C. In general, the microstructure was very homogeneous in the weld with smaller carbides and the Al203 intemal oxidation penetrations were similar or smaller compared to those formed in the base metal. Oxidation rates are low for NO6025 in this temperature range compared to other commercial chromia- forming alloys. As is typical for most alloys above 1000°C, scale spallation becomes more significant and leads to depletion of A1 and Cr in the metal substrate beneath the oxide. The depletion was observed to occur to a similar extent in both the weld and base metal for N06025. The maximum intemal oxidation depth of the base metal at 900° and 1100°C was lower than several other commercial Ni-base alloys.
As application temperatures are increased for various power generation and chemical processes, the need for oxidation resistance narrows the suitable alloy compositions. Above 1 OOO°C, Al-containing alloys become more attractive because of the slow kinetics of alumina scale growth and its relatively higher stability than other protective scales. However, the addition of Al typically causes problems with fabrication and mechanical properties (e.g. lower ductility and creep strength). A recent study compared the oxidation behavior of various commercial alloys at 900°-1100°C for possible use in a black liquor gasification system.' Those alloys with >3wt%Al which formed an exclusive alumina scale performed best at the highest test temperatures. Even the addition of only 2.4%A1 significantly improved the oxidation behavior of alloy 602CA (UNS N06025, also called Nicrofer 6 0 2 5HT*),23 compared to the other commercial chromia-forming alloys in the study.
Alloy NO6025 was selected for further study because the lower A1 content was less likely to affect fabrication, particularly welding. As an initial assessment of the effect of welding on oxidation resistance of this material, a short length of 3mm thick plate was welded and specimens were tested at 900°, 1000° and 1100°C. An additional test was conducted at 1200°C to confirm that this alloy could be used for a containment capsule to protect a refractory metal capsule for a liquid metal compatibility test of Pb-Li of Sic for lOOOh at 1 2 0 0 ° 4-6 After exposure at all four temperatures, the specimens were sectioned to compare the behavior of the weld to the base metal and to determine the extent of internal penetration.
The chemical composition of UNS NO6025 is given in Table 1. Oxidation specimens were cut from a bun joint made at Oak Ridge National Laboratory (ORNL) by joining two pieces of 3mm alloy NO6025 plate supplied by Thyssen Krupp VDM. The plates were cut with a 45°angle at the joint. Gas tungsten arc welding with the recommended filler metal wire (Nicrofer S 6025 - FM 602, same nominal composition as N06025) and multiple passes were used.
