I tried to attach two micro photos, but I think only one is being attached. There is no scale bar, but it is taken at 100X magnification with Vilella's etchant. This micro is taken at the mid-thickness of the second layer of a CCO. I've circled two regions in yellow that I am curious about. I am questioning whether they are primary carbides as they are very ordered with a triangular overall structure. I see this morphology somewhat regularly and I don't have a confident explanation for it. I more frequently see this morphology near the interface of layers, so it is presumably affected by cooling rate and therefore I assume it is caused by something along the lines of increased nucleation with lesser growth time. Is anyone able to confirm that these are primary carbides? The implication is that we are doing an image analysis to count the percentage of primary carbides, so this structure gets included in the count. From this image we apply a threshold to determine what gets counted, so the grey eutectic matrix is omitted.
https://files.engineering.com/getfile.aspx?folder=a6a-8bdf-4d94-b1d2-73a332b268f8&file=Second_layer_CCO.jpg
Thanks MagBen,
I've attached another photo (still 100X mag) of the interface between the first and second layer. The ordered phase appears to grow in size as you move from the interface to the surface. This growth is what leads me to think that it may be primary M7C3 (or another composition?). I agree that an EDS scan would solve the question fast, but no one above me is interested to know the answer.
The first layer appears to be all dendritic austenite. There are cracks, but they don't penetrate into the base metal.
https://files.engineering.com/getfile.aspx?folder=ed7--47da-a1d0-a64cc933dd6a&file=1st_2nd_interface_CCO.jpg
This is getting to the root of my question Ed. Our image analysis (via software counting pixels in a two tone image) cannot differentiate this "phase" from M7C3s. A typical image analysis count is 30 - 45% "white" and I estimate that this ordered phase accounts for up to 10% of that white count, so 3 - 4.5% of the total. There is also a wear resistance test that is conducted and, as would be expected, a higher percentage count usually correlates to a better wear resistance test. Client specs are usually cut and dry regarding acceptable criteria, however, it seems the fabricators often have some room to argue the acceptability of their product if they don't meet the client's spec. I am working as a third party for the testing side of things.
there is a big different micro between the first and second layer, second layer shows more typical hypereutectic microstructure!
Strong dendrite may indicate a high cool rate in the first layer.
Are the compositions the same between two layers, any other strong carbide formers added (e.g. Nb, Ti Mo)? the "ordered" carbide does not seem to be primary but rather eutectic carbide, but maybe MC.
I agree. They have very fast cooling rates on their first layer and don't precipitate any primary carbides. But it is the same wire composition for both layers.
I was given another sample to look at today from a different manufacturer. I don't have the chemistry yet, but assume it is still in the hypereutectic region. This is a 3 layer overlay and the attached image is the interface of the 2nd and 3rd layers (larger carbides on the bottom are the top of the 2nd layer). The ordered structure is quite obvious in the third layer and I am still inclined to think that it is a carbide of some composition rather than a eutectic structure. I looked at this up to X and can see the black centre of the carbide rod (in some instances). Below the interface appears more like eutectic matrix between the large carbides.
https://files.engineering.com/getfile.aspx?folder=3df-625a--9cd2-b33fbbc6&file=2nd_3rd_layer_interface.jpg
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CCO, or chromium carbide overlay, only contains chromium-rich carbides. Overlays can also be of the complex carbide type. In these products other alloys such as Nb, Mo, V and W are added to the chromium carbide.
Complex carbides are usually a preferred choice in hot wear situations since they can maintain full wear resistance in temperatures up to 600 °C ( °F). As complex carbides typically give 3-5 times longer service life compared to CCO, they are also suitable if the thickness of the overlay is restricted.
High-performance overlay plates exposed to more than moderate impact forces can use an alloy containing ultra-fine borocarbides. These borocarbides can have a grain size as low as 500 nm, which is approximately 200 times finer than traditional chromium or complex carbides.
In the literature and by different manufacturers, overlay products are also called carbide plate, chrome overlay steel, chrome overlay plate or hardfacing plate.
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When the welds cool, a pattern of fine cracks perpendicular to the welding direction is created. These cracks are intended and don’t affect the plate’s wear resistance. They allow the plate to be roll formed into a curved shape, usually with the hardfacing layer on the inside, although it can also be on the outside. Overlay plates should not be bent parallel to the welding strands.
Chromium or complex carbides can also be deposited with specially formulated welding wires and stick electrodes. This is commonly used for repair and maintenance, or for producing wear parts such as recycling hammers and teeth for excavator buckets.
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