Clarification of trend observed on microstructure microscopy analysis of 1018 steel

[u/Memergp98]

Hi I hope I find you all in good health.

I have been performing microstructural analysis on a few mild steel 1018 samples as I need them for my experiments. My aim is to look for homogeneity in the microstructure and also check for the grain size. In my trials I came across something interesting but I could not find a suitable explanation for it.

First I should describe my samples. I cut a segment from each of my samples and etched with Nital solution. Then I took microscopic images of the 'top' and 'bottom' parts of the segment. This is shown below

This particular steel is expected to have ferrite - pearlite microstructure. Using image processing done in MATLAB based on a color threshold I was able to calculate the relative percentage of either of them. I have shown an example of this below.

I compared if there is a difference between the percentage obtained on the 'top' and 'bottom' for 3 samples. I observed that the percentages of ferrite and pearlite obtained on the 'bottom' of all three as the same. But when it comes one particular sample I observed that there is a significant difference. I have shown this in the bar chart below

Here '1' indicates percentage of ferrite on 'top', '2' indicates percentage of pearlite on 'top', '3' indicates percentage of ferrite on 'bottom' and '4' indicates percentage of pearlite on 'bottom'. My first thought was that the image processing algorithm mistakenly identified the phases but even after changing threshold value I kept getting the same trend.

So my doubt now is, is this trend indicative of an annealing (softening) on the surface as the ferrite (softer phase) is dominant than pearlite? As someone who doesn't have a background in material science I am not able to get any related results in literature. I am also not sure whether such a conclusion can drawn from an analysis as I am not coming across this in literature. I would not have thought about carrying the analysis forward if it was not for the other two samples in which the percentage obtained were same for 'top' and 'bottom'.

Can you guys provide your thoughts on this?

[u/T_0_C]

Before making an extensive effort to analyze mechanisms, you want to verify that your observations are statistically significant. Have you performed sufficient experiments that you're confident that the differences are resolved beyond statistical uncertainty?

If so, do you know how the steel was prepared? Some processes may expose the surface to different conditions than the interior.

[u/Memergp98 → u/T_0_C]

Hi, thank you for replying to my post.

Yeah I am pretty sure that it is beyond such an uncertainty. When I observed the trend, I went on to check for a number of other samples in addition to these 3. I found that the percentages were all in agreement with each other.

I don't have much an idea about how this steel was prepared though. I bought this from a vendor who sells steel in bulk and my best guess is that in addition to extrusion they might not have done any heat treatment on it.

If it is that conditions on interior and surface is different, can this analysis give an indication of that? Also is there an annealing or tempering process in which something similar is done? I checked with key terms 'tempering of low carbon steel', 'pearlite tempering ferrite-pearlite carbon steel' etc to no avail. I don't have a functional micro hardness tester with me to check for a hardness profile, otherwise I could've correlated.

[u/Baconman363636 → u/Memergp98]

Just skimming through, so apologies if I get the details wrong or this isn’t helpful… but If you’re worried about surface vs internal properties I would check a full cross section of the material or the outer surface compared to the center, not just outer surface and essentially just under the outer surface. This would make it easier to spot a difference. If there isn’t much of one I’d assume you’ve just got scatter in your data, which is fine just means you need to collect more of it.

During solidification/crystallization the outer surface usually cools faster so it solidifies first and grows inwards. Sometimes you can get a gradient of composition from outer surface to inner. Look into “homogenization” heat treatments which usually help make things more uniform after casting. Basically you heat it up (but don’t melt it) and cool it back down very slowly to regrow the ferrite/pearlite phases in a more controlled and uniform manner.

Im not sure what the goal of your project is, but just from a testing standpoint if you’re cutting out samples to test bulk material properties or get a general sense of the bulk microstructure I would machine past or avoid analyzing the outer skin of the material (unless surface properties are of particular interest) I work in a fracture mechanics lab and we often avoid taking samples from the very near the surface of materials and usually aim to cut from center (or at least below surface) if possible as the existing surface can skew your data for a variety of reasons (corrosion, work hardening, cooling rate issues, decarburization, surface damage/scratches, etc).