Investigation on steam oxidation behaviour of TP347H FG Part 1: Exposure at 256 bar

Abstract

The stainless steel TP347H FG is a candidate material for the final stage tubing of superheater and reheater sections of ultra supercritical boilers operated at steam temperatures up to 620 8C in the mild corrosion environments of coal-firing. A series of field tests has been conducted with the aforementioned steel in coal-fired boilers and this paper focuses on the steam oxidation behaviour for specimens tested at various metal temperatures for exposure times of 7700, 23000 and 30000 hours as investigated by light optical and scanning electron microscopy. The oxide present on the specimens is a duplex oxide, where the outer layer consists of two sub-layers, an iron oxide layer and an iron-nickel oxide layer; the inner layer is chromium rich chromium-iron-nickel oxide. Microstructure examination showed that for all these samples the varying grain size of subsurface metal affected the oxide thickness, where the larger the metal grain size, the thicker the oxidation scale. This gave the appearance of uneven inner oxides with a varying pit thickness. Comparison of the pit thickness measurement and oxide composition reveals that the oxidation rate is fast during the initial oxidation stage, but the subsequent growth of oxide from further exposure is slower due to the formation of a healing layer consisting of chromium rich oxide near original alloy grain boundaries. At a temperature region above 600 8C a thin oxide rich in chromium and manganese is sometimes formed. In addition precipitation of secondary carbides in the bulk metal also occurs at this temperature region.

Microstructural Aspect of Long Term Service of the Austenitic TP347HFG Steel / Wpływ Długotrwałej Eksploatacji Na Mikrostrukturę Stali TP347HFG

“…The designed fine-grained microstructure of TP347HFG steel (grain size ≥ 8) ensures high resistance to oxidation and higher plasticity (expressed as the elongation growth in creep tests) with the creep resistance similar to that of coarse-grained steels [2]. Fine-grained microstructure of TP347HFG steel also has an influence on a slower decrease in impact strength during its long-term service [8,9]. Performed identifications of precipitates revealed two types of carbides in the microstructure: NbC carbides and M 23 C 6 carbides (Fig.…”

“…The lack of access to data concerning the microstructure in the as-received condition limits the possibility of analyzing the changes that run in the examined steel during the service. However, considering the applied heat treatment for this grade of steel -hyperquenching in water from the temperature of 1180 ÷ 1250°C [19], and literature data [1,2,8,10,18], it can be assumed that in the as-received condition, the investigated steel was characterized by the microstructure consisting of austenitic matrix and primary NbC precipitations. It shows that the degradation of the microstructure of the examined steels during the service mostly occurred through the precipitation of secondary carbides, i.e.…”

Investigation of steam oxidation behaviour of TP347H FG. Part 2: Exposure at 91 bar

“…Based on the morphology and the thickness of the oxide, it is seen that the oxidation, although similar to that suggested at high temperatures [4], has also some interesting differences. Therefore the following model is proposed for oxidation at low pressures and in the low temperatures ranges from 455 -525 8C (Fig.…”

“…Thus for a USC biomass plant, TP347H FG is a steel that combines the optimal corrosion resistance with the advantageous steamside oxidation features. The steamside oxidation of this steel in long-term field tests at 500 -620 8C and 256 bar has been investigated in the previous paper [4]. This paper will assess the steamside oxidation at a different plant with lower temperature range of 450 -560 8C and lower pressure of 91 bar.…”

“…Henriksen et al [7] indicated that operation parameters, such as steam/ metal temperature, heat flux, operation duration and steam pressure have a very important influence on the steam oxidation rate of superheater tubes, therefore in situ steam oxidation tests have been conducted [8]. The previous paper [4] revealed that increasing temperature results in a better chromium rich healing layer which decreases the oxidation rate. It has also been shown that the final surface treatment of an alloy such as machining can greatly influence the steam oxidation characteristics [9].…”