Low Alloy Steel Til – Austenitic Stainless Steel 347H Bimetallic Tubes for Corrosive Environments in Steam Power Plants

摘要

The results shown in this papermake reference to a research project in which areinvolved different companies from the BasqueCountry (Spain): Tubacex, Babcock WilcoxEspanola (BWE) and Iberdrola, and the researchcentre, Centro de Estudios e Investigaciones Tecncasde Gipuzcoa (CEIT). The objective of this projectwas the fabrication by co-extrusion of bimetallictubes for corrosive environments at high tem-peratures. Although in this project three differentcombinations have been studied, in this paper onlythe results from one of these combinations are takeninto account: Low Alloy Steel Ti 1 inside with anoutside cladding of Austenitic Stainless Steel 347H.The microstructures of the bonding obtained by co-extrusion,have been compared to those obtained in the laboratory bydiffusion bonding using hot plain strain compression and hotisostatic pressing (HIP). In all cases the interdiffusion of thedifferent elements through the interface and the microstructureof both the as-processed and after heat treatment products havebeen analysed by scanning electron microscopy (SEM). On thesteel side, as a result of the diffusion of nickel and chromiumfrom the stainless steel to the steel, a precipitation free austeniticband is observed. While a profuse precipitation, mainly of NbCis found in a region near the interface on the stainless steel sideThe optimum heat treatment performed has taken into accountthe best combination of mechanical strength and corrosionresistance. This heat treatment consists of an austentisation at1050-1100 deg C + water quenching and a stabilisationtreatment at 850-900 deg C.In order to study the behaviour in industrial conditions the tubeshave been installed in the reheater of a supercritical boilermanufactured by BWE of an Electric Power Plant of Iberdrola in Asturias.