الفهرس Only 14 pages are availabe for public view
 |  | from | 94 |  |  |
| | | from | 94 | | |
Abstract
Cast alloy Ni- 50% Cr- 1.6%Nb tube sheet support retired from service in refinery heater after approximately 8 years of service have shown deterioration in mechanical properties of the alloy of interest.
As cast and after service alloy specimens were subjected to different heat treatment regimes and welded at different heat inputs. Analysis of the equilibrium phases, microscopy, Scanning Electron Microscopy (SEM), X-ray diffraction, Dye penetrant, X- ray radiography, Differential Thermal Analysis (DTA) and Hardness tests were run to detect the phase structure evolution during service and welding.
The present study describes the microstructure changes that occurs together with mechanical properties variation of cast as compared with after service alloy. It was found that deterioration of mechanical properties during service is related to precipitation of Cr-rich α-phase inside -phase at low service temperatures (close to 700oC).This precipitation gets slightly enhanced if the alloy is previously heated at higher temperatures (1050oC) due to the high solubility of Cr in the -phase at these temperatures as compared with that at service temperatures (close to 700oC). Also heat input controls the hardness profile across the weld, with higher possibility of formation of local hard zone and hard throat as the cooling rate decreases, especially for the as-cast alloy. Hard throat could be avoided by either increasing or decreasing the heat input beyond a critical value.