Yes, you can expect similar microstructure for this category of steel. The microstructure you observe will depend a lot on the etching technique and parameters employed. For insight you can refer ASM Handbook Volume 9: Metallography and Microstructures.

Oxide layers can form on the surface, but the microstructure looks more like a ferritic steel to me, because there is no twinning. I would double check if it is really 316l. 

Is it a surface of surgical instrument ? If it is it can be surface after electrolytic polishing of the instrument surface.

Hi, Sebastian,

This could be the microstructure of a solution annealed 316L sample etched electrolytically. The same sample etched using the Kalling reagent may reveal twins. You can also check the ASM Micrograph Database published by ASM International at http://products.asminternational.org/mgd/index.aspx.

I think this sample was overetched and, as far as I can see, there are some artifacts, possibly due to incorrect drying after etching (all those multi-colored small areas look to me like alcohol or water stains). If the sample is re-prepared, the grain boundaries should be better defined, the contrast with the matrix much improved and the stains removed.

As for the oxide layer that can form on the surface, it is not observable by optical microscopy.

I hope it helps!

Thanks for all of you for useful explanations.

The micrograph presents cut piece of 316L stainless steel sheet. It represents of microstructure of as-received steel, thus I do not if this was etched or treated by any ways.

Hello Sebastian.

It is the common austenite microstructure with some quantity of delta ferrite (normal situation). Because the low carbon content, the sample seems to be free of detrimental phases like sigma and Cr carbides. The grain size is big, so maybe the steel was solution annealed.

The microstructure was definitely etched, otherwise you would not have seen the grain boundaries.

this microstructure shows the your sample was on the annealed condition, and this is common for annealed austentic stainless steel. the passive layer can not effect on the microstructure.

The shown micorstructre seems to be overetched, and I miss austenitic twin grains.

All FeNiCrMo steels form a layer of CrxOy oxide that actually prevents the steels form being stained. When this chromium oxyde is somehow lost due to the precipitation of Carbides, the steell will corrode locally 

Depending on the etching recipe used it can be sensitive to the twinning or not. In this case the etching employed is not sensitive to the annealing twins in the austenite structure. I also believe that the employed etching has not properly revealed all the grain boundaries in the austenite and hence the gran size appears slightly larger than the actual grain size.

It looks like equiaxed grains. To look for oxides try edx analysis.

Yes there exist an oxide film that cannot be detected by metallographic methods. Thanks to that oxide film that the steel is "stainless". As Martinez-Madrid also pointed out if the chromium is depleted due to carbide precipitation or exposure to temperatures at which chromium oxide is volatile then heavy cortosion can take place, making the oxide readily detectable. In normal circumstances the surface oxide film requires advanced surface analysis techniques to detect, such as Auger spectroscopy or SIMS.

Hi, all the comments are already here. Just one more independent opinion. 

Specimen was etched to reveal the grain size and then probably heated (so you see thick oxide film on the surface) or imaged under poor illumination. Otherwise normal structure.

Using linear intercept method you can also try to evaluate grain size. ASTM E112 

Dear Mykola, the dull apperance on the surface is certainly due to etching. However, it apperas not to have been overetched in my opinion. I am also not sure if the sample was thermally etched. And I suspect the question of Sebastian is related to general behaviour of this type of steel, rather than on this particular sample/micrograph. Best regards.

I wonder what one would use to get it over etched to see surface brown.  Usually on over etched surface you would see pitting, which is absent on this micrograph.

Honestly, IMHO it is poor illumination and not set WB, a bit too broad grain boundaries, but otherwise quite ordinary micrograph for grain size evaluation, just of not very good quality.   

Thanks All of you for comments.

Yes, it is truth that I just had general doubts regarding the specimen microstructure. In fact I am not familiar much with this type of metallic bulks and I do not know the etching/treatment processing of the specimens.

Thanks again for collaboration.