ABSTRACT

Thin pack and chemical vapor deposited (CVD) aluminide coatings on commercial ferriticmartensitic Fe-9Cr-2W steel are being investigated by creep and corrosion testing at 650°C. Mass gain results indicate that these coatings are protective at 650°C in humid air. Comparison with bare material shows that the coated region makes no contribution to the creep strength. The consumption of metal due to the exposure in water vapor of uncoated specimens is compensated by the growth of a load-bearing oxide scale. Pre-oxidation of the alloy in humid air also affects the alloy microstructure.

INTRODUCTION

The next generation of coal-fired power plants is being designed with higher temperatures and pressures which requires higher performance materials.[1] In order to further increase the maximum use temperature of current alloys, one solution is the use of oxidation-resistant coatings. For example, 9- 12%Cr ferritic-martensitic steels have useful creep strength to >600°C, but their application could be limited because they suffer extensive steam-side corrosion. Recent work has focused on aluminide coatings[2-6] because of the higher stability of alumina scales compared to chromia or silica..[7] Only 1wt%Al has been shown to improve the resistance of Fe-Cr alloys to water vapor environments[8] and several studies have shown good long-term behavior of aluminide coatings on ferritic steels. One concern about coatings is their effect on the mechanical properties of the substrate. This is a particular concern with martensitic steels which have a critical two-stage heat treatment that could be compromised by the coating process. Another concern is interdiffusion with the substrate during service. In order to investigate the effect of aluminide coatings, a program was initiated to examine diffusion aluminide coatings made by pack cementation (PC) or chemical vapor deposition (CVD) on a commercial Fe-9(wt.%)Cr-2W alloy (P92). Initial results were reported previously[9] and this paper provides additional creep and corrosion results for alloy P92 with and without aluminide coatings. In this first stage of the work, humid air was used for the oxidation testing as a lower-cost simulation of a steam environment, and the creep and corrosion testing were done in stages while a combined experimental apparatus is being designed.

EXPERIMENTAL

Dog-bone creep specimens of P92 (Fe-9.1wt%Cr-1.8W-0.5Mo-0.5Mn-0.16Si-0.11C-0.06N- 0.005S) with a 2x2 mm2 gage section were machined from a pipe with a 2.5mm thick wall. The material was supplied by Alstom Power Inc. in the austenitized and tempered condition. All of the specimens were polished to a 600 grit finish and cleaned in acetone and methanol prior to coating or testing. Some of these specimens were aluminized by a PC process for 15 min or by a CVD process for 30 min, both at 1050°C. Details of the processing are provided elsewhere.[6,10] The short times were selected to produce coatings ~40µm thick. Creep tensile testing was performed at 650°C in laboratory air with applied loads ranging from 100 to 120 MPa. Corrosion testing was performed at 650°C in closed tube furnaces with air + 10vol.%H2O flowing at 850 cc/min. Specimens (1.5x10x18mm coupons or creep specimens) were weighed after each 100h cycle at temperature.

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