PL, based in Plymouth, design and manufacture an instrument containing various
sensors, that is used deep in oil and gas wells for monitoring purposes and
with the aim of extending the life of the wells. With market opportunities in
the US, Saudi Arabia and Indonesia, Scarlet PL is keen to perfect the product,
including understanding the causes of failure at the end of the product life.
In addition, it is key to understand the metallurgy of the weld used in the
product, so that when production reaches high numbers, the best techniques and
equipment are in use.
What did they want?
Currently Scarlet PL outsource their laser welding and would like to begin laser welding in-house. They want to have their welding analysed to check the quality of the weld and to understand the metallurgy of the weld in order to ensure it supports high production numbers for the new product.
Agreed analysis plan
The welded part had two parts of the same weld in view and it was agreed that both welds would be analysed using electron backscatter diffraction (EBSD), which is a technique used to analyse grain structure. If there was anything of interest in either/both of the welds, then further analysis will be carried out at a higher resolution.
The right-hand weld was analysed first and it was observed to be of good quality with no visible defects, as shown in the top EBSD map. It was observed that there was good interface between the weld and the parent material with the grains in the parent material ‘blending’ into the weld. The centre of the weld had more equiaxed grains which would indicate recrystallisation due to the centre being exposed to higher temperatures.
The left-hand weld was observed to have two cracks at the interface either side of the weld, as shown in the bottom of the EBSD map. Much like with the right-hand weld the grains of the parent material at the interface have ‘blended’ into the weld with recrystallisation occurring in the centre of the weld, although it would appear that a higher degree of recrystallisation may have occurred. Subsequently a new EBSD map was acquired over the area of both cracks, using a higher pixel resolution.
It was observed that the right-hand crack seems to be connected to a void at the interface that then split and propagated on either side of the void, whereas the left-hand crack was not associated with a void and purely cracked along the interface. It was also observed that along both cracks there was a high number of very fine grains which would have been due to recrystallisation under high stress; this was also observed at the bottom of the void. The grains that show a mottling effect (multiple colours) are grains that are under a high amount stress and colour comes from a misorientation in the crystal structure. These mottled grains could split and form much smaller grains, which most likely was what happened along the crack to form the fine grain structure.
TKD and STEM-EDS analysis
Transmission kikuchi diffraction (TKD) and scanning tranmission electron microscopy (STEM) EDS were carried out on a thin lamella section taken from parallel to the left crack on the side of the parent material; this was to analyse the nano-grain structure at the crack. A large grain was observed in the TKD map (top right corner) which was shown to have a large amount of stress. This grain may be the original grain that 'split' to form the much smaller grains shown in the rest of the map. This may suggest that the large grains at the crack that were highly stressed, recrystallised to form this nano-grain structure. The STEM-EDS showed an area with low concentrations of nickel (Ni) and chromium (Cr) and high concentrations of niobium (Nb) and titanium (Ti). This may be the formation of a niobium-titanium-carbide, this formation of carbides may cause cracking as carbides are inherently brittle.