Micrograph Evaluation 11

[Wrenchbender]

Just looking for a few ‘tips’ in an area I am not familiar with. Is there any way to tell if the spots in these unetched micrographs are impurities, or artifacts of sample preparation, or something else? They look very large and numerous to me for what was supposed to be a pretty clean steel. Material is a low carbon alloy steel. It was induction air melted then vacuum ladle degassed. Thanks for your opinions.

 

[DABwilldo]

Bestwrench,

I don't think the spots are artifacts of the sample polishing. And they're not water spots, which was the first thing that popped into my mind after reading your post, but before I looked at the attachement.

They look like some kind of inclusion or structure in the steel. The large black spots remind me of carbides, like titanium or niobium carbides; but without knowing more about the grade of steel in question, this is pure supposition.

The smallest spots look like they might follow grain boundaries, which is again supposition on my part. If my guess is correct they might be chromium carbides or something similar. Again, some more info on the grade of steel would aid in narrowing down the range of explanations.

Even though you describe this as a low carbon steel, you can still find carbides in the structure. It all depends on your definition of 'low'.

The larger spots could also be sulfide inclusions, Manganese Sulfide, for example. This all depends on the melt chemistry of course.

Whether or not I'd describe these as normal constituents of the steel or impurities depends alot on what the steel is supposed to be.

Last thought, given you describe the steel as vacuum degassed, I'm doubtful these are oxide inclusions. As you say, vacuum degassed steel is usually pretty clean of oxide inclusions...

 

[CoryPad]

Probably both inclusions and artifacts. There is some staining, probably from an alcohol or acetone wash. If this isn't done correctly, you can get some redeposited "stuff" that was supposed to be removed by the solvent. Using differential interference contrast (DIC) illumination can help determine if the spots are pits or something on the surface, which helps determine what they are.

 

[redpicker]

It's very difficult to tell by just looking at a posted picture, but they could be mostly artifacts from sample preparation.

What is your sample prep procedure? The reason I ask is that I had been polishing samples for decades before I realized that letting the samples sit with water on them between polishing steps would result in just that type of appearance. I guess why I had not seen it before is I had literally dozens of samples to polish by hand and I would let them sit with clean water on them after 6 micron diamond but before the final alumina polsih. They didn't look to be corroded, but the alumina would polish out the 6 micron scratches and leave the pits. Is other situations where I had that many samples, it was either done by automatic equipment or I processed them in smaller batches, so they never sat with water on them (it apparently only takes a minute or so).

Anyway, take the sample back to grinding and wash and dry with alcohol after each step (alcohol can be corrosiove, too).

rp

 

[Wrenchbender]

Cory, I think you summarized the consensus so far. I’ll ask the original investigator about DIC. They are a college MSE department, so they may have the necessary lab equipment.

Willdo, Yes, there are lots of carbide formering alloys. Nominal composition of the steel is 0.27C – 2.6Cr – 1.0Si – 1.0Ni – 1.0W - 0.65Mn – 0.40 Mo – 0.06V with S < 0.001.

Red, I’ll for standing surface water during sample prep.

Thanks, all.

 

[dbooker630]

The large black spots (oxides) appear to be surrounded by a gray constituent which may be indicative of calcium-treated steel.

 

[deadrange]

Personally, they look like sample prep defects. One question though, is this a transverse or longitudinal cross section relative to cast direction. DABwilldo mentioned MnS inclusions, if this were a longitudinal plane I would have to disagree with his assessment of MnS inclusions. They typically have a soft tail to them, however in a transverse field they could appear completely globular.

I agree oxides surrounding by gray would typically be Calcium treated material, but in this case I think it is a staining issue due to the pit that formed.

I can offer one additional suggestion that hasn't been addressed yet. Whenever I am confused about an artifact being real or related to polish I try and adjust my focus a little bit. By going out of focus to the plan of polish, you can often learn some additional information. I look for depth of the artifact. Normally, having substantial depth to the artifact indicates to me that it is not real and related to polish.

 

[DABwilldo]

Bestwrench,

The prevailing opinions seem to be that most of the spots are polishing artifacts. While some of them may well be, I'll stay with my suggestion that there are several varieties of carbides visible. The nominal chemistry you mentioned is typical of tool steels. Ignoring the water spots and pits for a moment. The vague lines comprised of small black spots still look like grain boundary precipitated chromium carbides to me. The larger spots could be tungsten carbides. If these are in fact pits(and I can't say for certain that they aren't) then they're either gas porosity(in spite of the vacuum degas) or tear-out from the polishing process.

I agree with deadrange's comments about the possibility of MnS inclusions. Given the nominal chemistry this is not possible.

My eyes didn't pick up the grey surrounding some of the spots that others have stated is indicative of a calcium treated steel. I'll defer to them in this, as it's not something I'm familiar with...

Is there a specific reason for not etching this specimen? are you preserving the as-polished surface for micro-hardness testing? Or some other test?

Etching this steel would also serve to help clear up some of this mystery....

Deadrange's suggestion to play with the depth of focus and see what appears makes sense to me as well. If your optical microscope has enough magnification, you might be able to resolve more details about any pits in the surface.

If the university you're working with has an SEM, then the pits could be examined in more detail. Gas porosity would have a very smooth, spherical interior. A rough for faceted interior surface on the pits would indicate something torn out or dissolved during the polishing(although this seems unlikely to me)...

David Benson
Benson's Mobile Welding & Fabrication

http://www.bensonmobilewelding.com

 

[CoryPad]

With the revelation of the composition, I agree with DABwilldo that carbides are likely to be at least some of the dark spots.

 

[unclesyd]

I agree that a light etch might be beneficial. I would first try a very light nital etch to get the lay of the land.

I would also check the history of this sample back to and what was included in the mix, along with a thermal profile of the melt including pressure (vacuum0 vs time and any ladle additions.

 

[redpicker]

You might also mention what the heat treatments were. What is the difference between Heat Treatment 1 and HT2?

rp

 

[Wrenchbender]

Thanks to everyone in the forum for the great feedback! I’d like to share some of the etched micrographs along with the corresponding heat treatments. To my untrained eyeball, the microstructures (lath martensite?) look similar and the particles have little relation to the gb’s. I’d be glad to know if you think I am correct if you have any further comments or suggestions. Much appreciated.

 

[CoryPad]

Definitely lath martensite. Any chance some of the dark spots are pores? They resemble pores.

 

[redpicker]

You don't mention the cooling rate. Based on the chemistry, I'd guess an air cool.

It does appear to be martensitic, but there is a blocky, higher temperature transformation product in the microstructure as well.

When looking at the un-etched photos, there are obviously two types of spots. Large, dark ones and the smaller, lighter ones. On the etched samples, you don't see the small ones as much, so they appear to be small non-metallic inclusions, carbides, or small pits. The large dark spots seem to me to be large pits or voids. I can't tell if they are large voids (porosity), inclusion pull-out from sample prep, or corrosion pits that have been polished. A re-grind and re-polish of the samples might be helpful.

rp

 

[TripleG1983]

In answer to your original question you cannot tell from the unetched pictures what the "dark spots" are. You need to take some high magnification shots and focus in on these features. This way you can interpret whether they are inclusions, dirt on the micro, voids etc. It depends on whether you are focusing in or out on the feature.