title: Dissimilar metal welds
Creators: name: Dawson, Karl
Publisher: name: University of Liverpool Repository
Year: 2013

description: This dissertation details the findings of experimental investigations of welds made between ferritic creep resistant steels that differ in chromium content. Analysis of the microstructural evolution during the application of post weld heat treatments is reported. Particular attention was paid to the key alloy strengthening mechanisms and the manner in which they were affected by carbon redistribution which takes place when these welds are exposed to high temperatures. The fusion interface regions of transition joints, made between P91 parent alloy and P22, P23 and P24 type weld consumables, were analysed in as received and post weld heat treated conditions. Carbon redistribution from the low to higher alloyed material, which resulted in its depletion from weld alloy adjacent to the fusion line, was confirmed in all weld systems subsequent to post weld heat treatment (PWHT). The effect of tempering treatments, carried out at 730°C for two and eight hour durations, on carbide populations in partially decarburised weld alloy was explored. The consequential microstructural changes, which were affected by the dissolution of M23C6 and M7C3 carbides, were compared to those observed in regions of weld alloy unaffected by carbon depletion. High resolution transmission electron microscopy (HRTEM) and field emission scanning electron microscopy (FESEM) were used extensively in the analysis of weld metal and heat affected zone (HAZ) microstructures. Electron diffraction and x-ray energy dispersive spectroscopy were exploited in the crystallographic and chemical characterisation of precipitates. Their evolution as a function of thermal exposure is presented for each alloy. Chemical signatures for each precipitate species, which enabled their identification, were determined for carbides in the different alloys. However, due to variations in the compositions of fusion interface M23C6 carbides, some permutations of which overlapped with compositions of M7C3, satisfactory identification demanded classification of their crystal structure. A significant difference between the microstructures of P23 and P24 alloys, in the weld specimens tested, was observed. Although vanadium and niobium carbonitrides (MX) were identified in both alloys, their distributions were not the same. Retention of carbonitride particles within partially decarburised P23 and P24 weld materials, subsequent to 8 hours post weld heat treatment, has been substantiated. Diffraction intensity distributions in Debye-Scherrer ring patterns, which were generated from MX precipitation, indicated lattice parameters varied. Microanalysis revealed that MX precipitates were present over a wide range of compositions. A combination of the composition analysis and diffraction studies indicated that MX precipitation was stable over a range of compositions in the carbon depleted regions of P24 alloy. Recrystallisation of the bainitic P22 weld alloy adjacent to the fusion line, which was accompanied by a loss of material hardness, was observed in 2 and 8 hour PWHT P91/P22 welds. It has been shown that the microstructural stabilisation of carbon depleted T/P23 and T/P24 alloys was conferred by a dispersion of MX precipitates. Retention of these stable particles, which in many cases are less than 10 nm in diameter, in carbon depleted material, resulted in the complete avoidance of any recrystallisation in 2 hour post weld heat treated T/P23 and T/P24 welds and only isolated occurrences in 8 hour tempered specimens. Subgrain size distributions were determined from electron channeling contrast images of various regions of the dissimilar metal welds. Results showed that, although recrystallisation of MX forming alloys did not occur, destabilisation of lath boundaries, due to the dissolution of M23C6 and M7C3 carbides, results in a coarser subgrain microstructure in carbon depleted P24 weld alloy. The loss of resistance to plastic deformation as a result of recrystallisation, which has been shown to take place in decarburised P22 alloy, was not observed in the alloys which precipitated the MX phase.