Bin Azahari, Ahmad Azli
(2020)
Characterisation of weldability and metallurgical stability of dissimilar metal welds.
PhD thesis, University of Nottingham.
Abstract
Many industrial sectors require the joining of dissimilar metals, including the power plant industry where the different parts of a power plant are made of different materials due to differing requirements. The locations where the dissimilar steels or alloys are joined are termed dissimilar metal welds (DMWs). It is generally known that a significant percentage of DMWs can fail prematurely in service as a result of both primary system loads and constraints introduced due to differences in the coefficient of thermal expansion across the joint, as well as metallurgical compatibility issues.
Somers Forge Ltd and TWI introduced a novel TransForge fabrication technique which incorporates single-pass EBW and forging, followed by post-forge heat treatment, to produce a DMW joint between two thick sections of metallurgically dissimilar materials. This technique needs to be evaluated in order to assess the process that potentially allows welding with much lower distortion than achievable with conventional arc welding techniques and offers the potential to be of high integrity. Two DMW joints (A508/304L and A508/IN600) produced using this fabrication technique were evaluated using microhardness testing and tensile testing and the microstructure characterised.
Microstructural studies revealed that there are many microstructural zones in the vicinity of the interface, running parallel to the interface. In each of the joints examined, there are different microstructural features at the interface in different parts of the joints. Tensile testing at room temperature indicated that the tensile properties across the weld, at different parts of the joints exhibiting different microstructures, are comparably strong and ductile (or more so) compared to the bulk alloys, showing the viability of the fabrication process. In addition, A508 and 304L steels—which previously have only been joined with the use of electrodes/fillers in fusion DMW—have been directly joined successfully, demonstrating good weldability of this pair of dissimilar alloys when utilising the TransForge fabrication technique.
The most common type of DMW is, however, still fusion welding, particularly in the energy sector. Increasingly, newer 9Cr-1Mo steels such as P92 are used in power plants in order to allow the use of higher steam pressure and pressure, and thus the behaviour of P92 in DMW joints are increasingly studied. However, there tends to be some discrepancy between the failure modes of in-service components and those of accelerated creep-tested joints in labs. It is hypothesised that this may be due to the difference in the magnitudes and states of the applied stress and the durations of exposure to high temperature between the two situations. Precipitate growth is a potentially important factor that is affected by stress and high temperature that may in turn influence the failure modes of DMW, so the behaviour of precipitate growth under different stresses and durations of exposure to high temperature may have a role in DMW failure. To test this, the three DMW specimens from P92/IN182 joint were subjected to 154, 129, and 116 MPa of stress, at temperature of 630°C, which resulted in times-to-fracture of 109, 1453, and 5862 hours, respectively. Separately, additional joint specimens were subjected to thermal ageing for the same durations at the same temperature. The difference between these two sets of specimens are that the crept samples experienced mechanical stress during the exposure to the high temperature, whereas the thermally aged specimens did not.
It was found that while both the Laves phase and the Type I interfacial carbides (which occurs near the interface between a ferritic steel and a nickel alloy filler in fusion DMW) were affected by the applied stress, they are affected differently. Applied stress in crept samples likely accelerated the growth of Type I carbides in the minor axis direction, while the major axis dimensions of the Type I carbides appear to not be affected. High mechanical stresses in creep (as observed in the short duration creep sample) increased the number density of Laves particles, indicating greater Laves nucleation and formation, but may have decreased the coarsening rate. Type IV failure in the longer duration creep may be explained by this Laves behaviour in the fine-grained heat affected zone (FGHAZ).
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