Oxidation behaviour of cast Ni–Cr alloys in steam at 800°C

Abstract

To evaluate the steam oxidation resistance of cast Ni base alloy candidates for advanced steam turbine casings, laboratory experiments were conducted at 800°C. Alloys ranged from weaker, solid solution strengthened alloys 230 and 625 to stronger, precipitation strengthened alloys 105, 263 and 740, which are more difficult to fabricate and join. In general, these Ni–Cr based alloys exhibit low mass gains and form thin, protective Cr rich external oxides in 17 bar steam or 1 bar air. However, Al and Ti in these alloys internally oxidise in all cases. After 5000 h exposures, the average and maximum internal oxide penetration depths were measured, and the values were ranked based on the alloy Al+Ti contents. The middle range of Al+Ti compositions investigated, such as for alloys 617, 263, 282 and 740, showed the deepest penetrations. Further characterisation of the reaction products by electron microprobe showed a complex behaviour including significant Ti incorporation into the scale formed in both steam and air, and Ti rich oxide at both the gas and metal interfaces. Based on the Al and Ti contents, the internal oxidation observed in these alloys in steam was atypical.

Microstructural Analysis of IN617 and IN625 Oxidised in the Presence of Steam for use in Ultra-Supercritical Power Plant

“…The scales were also enriched in titanium near the alloy/scale boundary for both alloys. Small regions of high titanium concentration were also observed on the surface of the scales which is consistent with other studies and suggests the formation of TiO 2 [9,10]. IN625 air exposed samples exhibited similar diffuse titanium enrichment in the scale near the alloy/scale boundary, however, in IN617, titanium in the scale was mostly concentrated in small volumes on the surface.…”

“…IN625 air exposed samples exhibited similar diffuse titanium enrichment in the scale near the alloy/scale boundary, however, in IN617, titanium in the scale was mostly concentrated in small volumes on the surface. The darkly contrasted phase, shown in Figure 1, which grows internally in both alloys within the fine grained region and forms more extensively in IN617 than IN625, comprises of aluminium and oxygen and was confirmed by both EDX and selected area electron diffraction to be alumina, consistent with the observation of other studies [10,11,12]. Figure 2 and Figure 3 show the extent of alumina formation in IN617 and IN625 respectively.…”

Steam Oxidation Resistance of Advanced Steels and Ni-Based Alloys at 700 °C for 1000 h

“…After a longer exposure, more chromium is lost by repeated cracking and healing, and the decreased chromium content in the steel near the outer surface cannot sustain the healing of protective chromia scale. The iron in the steel therefore oxidises rapidly and forms the crater structure [3]. Scale failure at high temperatures is also caused due to relatively rapid rates of diffusion of manganese across the Cr and O scale and the formation of a Mn-Cr spinel on its outer surface The scale consists of a layer of Cr 2 O 3 , containing a small concentration of manganese, with an outer layer of the spinel oxide.…”

“…Currently, attention is paid to austenitic steels and Ni-based alloys with a high Cr content ([ 20 wt%) in order to sustain development of continuous, thin, adherent Cr 2 O 3 oxides for the extended lifetime of super heaters (SH) and reheaters (RH) systems. Recently, Essuman et al [3] has found that Ni-based alloys rich in Cr and develops rich Cr scales; however, the Ni-based alloys 105, 263 and 740 exposed at 800°C underwent an additional deep internal oxidation process. The problem of internal corrosion by oxidation of Al and Ti in grain boundaries is often met in gamma-prime (c 0 ) strengthened alloys [4], and the internal oxidation process is described by Young [5].…”

Parsons Turbine Conference

“…Essuman et al 10 evaluate the steam oxidation resistance at 800uC of cast candidate nickel base alloys for advanced steam turbine casings. In general, these Ni-Cr base alloys form thin, protective Cr rich external oxides in 17 bar steam or 1 bar air.…”