Effect of aluminizing on surface microstructure of an HH309 stainless steel
Introduction
Heat resistant stainless steels such as HH309 are widely used in petroleum, chemical, nuclear and other applications due to their excellent high temperature strength. Oxidation resistance of these steels is due to the formation of Cr2O3 on the surface. However, this surface layer will be destabilized at high temperature ( above 1000 °C) and will not protect the metal anymore. It has been reported that the addition of Al to stainless steels or nickel base alloys would cause an increase in high temperature oxidation resistance [1]. The beneficial effect of Al on the high temperature oxidation resistance is recognized by providing a protective alumina surface layer during high temperature exposure [2], [3], [4], [5]. The pack cementation technique is most widely used for the deposition of Al to improve the performance of the steels in high temperature corrosive environments [6], [7]. The complex aluminide intermetallic coatings formed during the process exhibit superior resistance to oxidation, carburization and sulfidation [8]. The amount of Al introduced and the associated changes in the microstructure depends upon raw materials, temperature, time and other processing factors [6].
Section snippets
Experimental procedures
In this investigation, an aluminizing process is conducted on an HH309 stainless steel. Chemical composition of this steel is shown in Table 1. For solution treatment, this steel was exposed in argon gas for 3 h at 1060 °C. Then specimens were cut from a square bar with 10 × 10 × 2 mm dimensions. These coupons were ground through 600-grit SiC paper, cleaned and dried. The powder mixture for aluminizing consists of 85 wt.% alumina, 10 wt.% Al and 5 wt.% ammonium chloride. The samples and pack
Results and discussion
By aluminizing process on the HH309 steel, the coating formed on the surface consists of two layers . The cross-section micrograph of the aluminized sample is depicted in Fig. 1. Two distinct layers are observed. Fig. 2 shows the SEM micrograph for the first layer of the coating. Table 2 shows the EDS results of the spectrums shown on this micrograph. Fig. 3 shows the X-ray diffraction pattern of the surface of alumnized sample. The characteristic peaks of FeAl, Fe24Al78, AlFeO3, Fe2O3 and Al2O3
Conclusions
- 1.
The coating obtained by aluminizing the HH309 stainless steel consists of two layers. The outer layer consists of iron aluminde intermetallics (FeAl, Fe24Al78). The interdiffusion layer consists of ferrite that associated with NiAl and Ni3Al precipitates.
- 2.
The aluminum diffusion inside the sample has caused the matrix transformed from austenite to ferrite in the interdiffusion layer of the coating.
- 3.
Aluminizing process increases oxidation resistance at high temperatures by the formation of Al2O3 on
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