Micro Etching Material: Benefits and Uses

[snowJT]

Has anyone here micro etched material? I'm going to start learning the process of micro etching and have the oppertunity to do it to a bolt. I'm wondering what purposes it would serve... I know that from micro etching, you can see how a certain product has been formed from the lines, is this usuful to see if the batch of whatever it is you're producing is good? Thats the only purpose I can see of it, because once the part is eaten away by acid.. you can't really use it.

I'll be doing it on a bolt.. I think I can guess what it would look like, I'll see lines running perpendicular to the head of the bolt, then where the top of the bolt is, the lines will sort of spread out.

 

[snowJT]

Does anyone have information pertaining to seams, porosity and pipe in rolled steel, and differences in machined and cold rolled threads?

(I'm looking at you Astronuc)

 

[Astronuc]

I haven't done microetching myself.

Are you doing electrochemical etching?

Micro-Etch — Micro-etching is used for the examination of a sample under a microscope. Etching solutions tend to reveal structural details because of preferential chemical attack on the polished surface.

from www.mgs-stainless.com/pdf/Glossory.doc[/URL]

This is useful - [url]http://www.mgs-stainless.com/tips.htm[/url]

I presume the term would apply to etching a SEM, TEM or STEM sample?

Maybe Gokul has done this?

 

[Gokul43201]

Mostly on semiconductor samples - and nothing on steel.

There's a lot that is not clear in the OP - for instance, the how, what and why. In the context of the OP (which I think is sample prep for optical miscroscopy), microetching serves one of two primary purposes: (i) removal of surface oxide, organic contamination, etc. and (ii) preferential staining of select phases.

I'm afraid I don't understand the part involving the "lines" in the OP.

 

[snowJT]

I've seen samples before, and when a specimen was dipped in hydrochloic acid for about 30 minutes and pulled out.. there were lines (almost like the grain you see in wood) that showed how the specimen was formed

 

[Astronuc]

I've seen samples before, and when a specimen was dipped in hydrochloic acid for about 30 minutes and pulled out.. there were lines (almost like the grain you see in wood) that showed how the specimen was formed

Yes, the etching can be done such that the reaction takes place at the grain boundaries, which then reveals the grain structure. On worked material, grains elongate in the rolling direction.

This might be of interest -

http://www.tcreng.com/services/corrosion-IGC-tests.shtml

One purpose of corrosion testing is for acceptability - that the material has be properly processed. The Huey test is common for stainless steels.

Nitric Acid, ASTM A262, Practice C, (Huey Test)
The specimens are boiled for five periods, each of 48 hours, in a 65 per cent solution of nitric acid. The corrosion rate during each boiling period is calculated from the decrease in the weight of the specimens. Properly interpreted, the results can reveal whether or not the steel has been heat-treated in the correct manner. The customer must specify the maximum permissible corrosion rate and, in applicable cases, data on sensibilizing heat treatment.

The Huey test environment is strongly oxidizing, and, is only used as a check on whether the material has been correctly heat treated. This test is suitable for the detection of chromium depleted regions as well as intermetallic precipitations, like sigma phase, in the material. The Huey test is also used for materials that come into contact with strongly oxidising agents, e.g. nitric acid. This procedure may also be used to check the effectiveness of stabilizing elements and of reductions in carbon content in reducing susceptibility to inter granular attack in chromium-nickel stainless steels.

In addition to highlighting the grain boundaries, the micro-etching can show secondary phases (which are on the order of nm) within the grains.

 

[PerennialII]

Has anyone here micro etched material? I'm going to start learning the process of micro etching and have the oppertunity to do it to a bolt. I'm wondering what purposes it would serve... I know that from micro etching, you can see how a certain product has been formed from the lines, is this usuful to see if the batch of whatever it is you're producing is good? Thats the only purpose I can see of it, because once the part is eaten away by acid.. you can't really use it.

I'll be doing it on a bolt.. I think I can guess what it would look like, I'll see lines running perpendicular to the head of the bolt, then where the top of the bolt is, the lines will sort of spread out.

We've several lab assistants doing this stuff, nowadays (luckily ) don't have to do the stuff that much myself (grinding nails off, burning skin with the acids and all that ... or the mental strain of preparing the specimen meticulously ... not for me ). Etching itself is a standard part when studying microstructural features of materials. Different etch - material - time (- temperature) combinations reveal different features of the microstructure - whether it be grain boundaries, different phases etc. The key is to know what combination of the above for a certain material leads to the desired result. There are a number of thick reference books available which contain lots of related information (ASM handbooks for one). Bolt ... something "standard" ferritic pretty low carbon ?

 

[brewnog]

I've done this, whilst working as a skivvy in our materials lab!

The steps for microsample preparation are loosely as follows:

The required section is taken, and mounted in bakelite. The bakelite disc is then polished using increasingly fine sanding, and then polishing discs, until the desired finish (often 1 micron) is achieved. The sample is washed under water and acetone and quickly dried.

The etching process depends on what material you are looking at, and what you're trying to expose, but for most ferrous materials the sample is either bathed in, rinsed with, or swabbed with a solution of nital (I believe it's 2% nitric acid in 98% methanol). The time required for successful etching depends on the material, and what you're trying to show.

The links provided by Astronuc are worth a visit.

 

[snowJT]

thank you all, your knowledge is quite interesting

 

[snowJT]

hmm, can anyone breifly tell me what seams and pipe is in rolled steel? When I try and look it up, I find nothing but water pipes that have been rolled. Can someone just give a brief explanation for me? please?

 

[Astronuc]

I don't know of seams in plate or sheet. What I've seen is a process that takes billet to slab to plate or sheet with successive rolls and annealing. The billet must be free of defects.

Now that said - if one were to weld two billets or slab (side by side) together, then there would be a seam. Or if billets or slab of placed face to face and co-rolled, then there would also be an interface.

Pipe or tubing can be formed seamlessly or from rolling sheet and welding a seam.

Piping can occur in cast billets and normally that should be removed. I've seen piping in bar stock as a result of the extrusion process. The bar stock is ultrasonically tested and the section containing the piping is cut and scrapped for recycle (remelt).

I'll get back to you on bolt - but the difference in rolled threads vs cut is the amount of plastic deformation in the bolt shank and threads. Rolling simply plastically deforms the material without removing, while cutting shears away the material. The internal stresses will be different, as will the surface finish, and perhaps surface imperfections.

 

[snowJT]

This is the eteched bolt and nut.. I would of thought that there would of been lines around the threads that showed the threads being formed. Interesting?

top half

bottom half

nut

 

[PerennialII]

This is bringing back some long lab hours had nearly "erased" . My 1st impression especially from the 2nd and 3rd figs is that your surface preparation doesn't quite "cut it", meaning it's too rough and hides any microstructural detail of orientation (see what brewnog said about sanding and polishing). Also, I'd do the etching in stages not to overdo it (time-wise) and see whether something starts to appear, then etch more if not. Also2 whatever you're going so see is probably not going to appear macroscopically (too well at least), but rather you're going to have to take a pretty close look near the bottom of the threads (since the deformation probably isn't close to what you'd see in for example a forged piece of material).

 

[snowJT]

Well it was originially smooth and clean, but it was amazing how as soon as they were pulled out of the acid, it began to currode so fast that you could infact watch it change colour, and so I dipped it into oil as soon as I relized that I probably should have, then took pictures after.

And what is piping, when I try and look it up.. the first few thousand results are about water pipes... Is it basically an air gap that creats a hole when rolled?

 

[Astronuc]

Piping is internal voiding within a billet, slab or bar.

I've seen in the case of extruding a round billet, and the outer material flows more than the inner material. The end must be cut off to remove the bar with a hollow center.

An Analysis of the Piping Defect at the End of the Stroke in Direct Extrusions.
http://stinet.dtic.mil/oai/oai?&verb=getRecord&metadataPrefix=html&identifier=ADD804125

One can see it in some ingots where during cooling, a pit or cavity forms in the center of the solidifying melt.

It can cause significant problems - " REMARKS- CRANKSHAFT FAILED,SUB-SURFACE INGOT PIPING NOT FOUND DURING MANUFACTURING PROCESSES."
http://www.ntsb.gov/ntsb/brief.asp?ev_id=42560&key=0

Process control is the key to preventing piping. Years ago, ingots would have to be 'cropped' to remove the top associated with piping. Better alloying and melting techniques reduce the piping.

 

[snowJT]

I'm just wondering.. I thought nuts were casted.. however, it appears from the etching that it was pressed or something from a solid piece of steel because the grains kind of curve, but it seems like the inside threads were machined or something because the grain keeps the same direction and doesn't become more dense around the threads.