Microstructure and elevated-temperature erosion-oxidation behaviour of aluminized 9Cr-1Mo Steel

Abstract

Degradation of materials by a combination of erosive wear and atmospheric oxidation at elevated temper-atures constitutes a problem in some power generation processes, such as fluidized-bed combustion. In this work, 9Cr-1Mo steel, a common tube material in combustion chambers, is coated by a pack cemen-tation method from an Alcontaining pack in order to improve the resistance to erosion-oxidation at elevated temperatures. The resulting coating is studied in terms of microstructure and microhardness and tested for its resistance against impacts by sand particles in air at temperatures of 550-700 • C under several conditions, with thickness changes and appearance of the exposed surfaces being studied. The coating was found to contain several phases and layers, the outermost of which was essentially Al-rich and contained e.g., small AlN precipitates. The microhardness values for such coating ranged from 950 to 1100 HV 20g . The coating provided the substrate with increased protection particularly against normal particle impacts, as manifested by smaller thickness losses for coated specimens as compared to uncoated counterparts. However, much of the coating was lost under all test conditions, despite the fact that parti-cle debris formed a homogeneous layer on the surface. These results are described and discussed in this paper.

Steam oxidation behavior of Y-bearing cladding tube with aluminizing coating

“…Secondly, the packing activator of NH 4 Cl would also induce inward diffusion of N in the coatings. Thirdly, AlN exhibits a large negative Gibbs free energy of formation, i.e., −287 kJ mol −1 [24], which facilitates its formation where Al and N are sufficient.…”

Observation of phase transformation and grain growth phenomena in aluminised Super 304H during steam oxidation

“…An SEM/EBSD investigation has also been performed on the slurry aluminised Super 304H samples exposed to steam oxidation conditions at 700 and 750uC, this revealed phase changes occurring because of the columnar growth associated with the Al enrichment of the substrate. In the literature, most of the reported microstructural studies have been carried out on aluminised ferritic (BCC) steels produced by hot dipping in molten aluminium, [11][12][13][14][15][16][17] hence no phase change would be expected to occur as a result of Al enrichment. Limited EBSD work carried out by Danzo et al 13 reported that columnar grains of Fe 2 Al 5 grew on the surface of ferritic steels after aluminising in a molten mixture of Al-1 wt-%Si at 700uC for 20 s.…”

Effect of aluminizing and laser shock peening on high temperature oxidation resistance of AISI 321 stainless steel for solar thermal power generation heat exchanger

“…Aluminizing, including hot-dip aluminizing [8], pack cementation aluminizing [9,10], and reactive air aluminizing (RAA) [11], is often used to hinder the oxidation process by facilitating the formation of a thermodynamically stable thin layer of Al 2 O 3 . Therein, pack cementation is a cost-effective and promising technique to achieve diffusion coatings with a stable structure.…”

“…Therein, pack cementation is a cost-effective and promising technique to achieve diffusion coatings with a stable structure. For instance, Saarivirta et al [9] fabricated the 9Cr-1Mo Steel with Al-rich coating by pack cementation, and demonstrated that the presence of hard AlN precipitates in coating improved the resistance against erosion-oxidation of the steel specimens. Prasetya et al [10] prepared carbon steel by pack cementation with Al and Cr, respectively, and found that the growth of oxide scale was inhibited by Al coating, but not restricted by Cr coating.…”