Failure Analysis of Steel Rod 3

[mxracer17788]

I am testing a few samples of 1" diameter steel rods and I would like to be able to classify the fractures as either ductile or brittle, or most likely somewhere in-between. Normally, I would send the rods to our contracted materials engineer but we just purchased a nice digital inverted microscope with up to 1000x magnification and I figured I would first try myself since these are just test samples. What is the best way to prepare these samples for viewing under the microscope? They were originally 12" rods before being fractured in half, and I would like to view the fracture surfaces. Does the fractured surface need to be sectioned so that it can be placed on the microscope? Also, do I need to prepare the fractured surface in any way i.e. polishing, etching etc.? In addition, what would be the best magnification to view features that would clue me in as to whether it was brittle or ductile? This isn't going to be an official fractographic analysis, I just wanted to get comfortable with our new equipment and I figured this would be an easy way to start. Any help would be greatly appreciated.

Thanks,
Jeff

 

[metengr]

mxracer17788;
For your application above and with your excitement to run and use your new lab toy, the first step would be to perform a visual examination of the fractured ends to observe/document if any local permanent deformation (reduced cross section) has occurred around the periphery of the failed rod and the texture of the fracture surface itself. This does not require downsizing and sample preparation for an inverted microscope, only a digital camera and lighting.

What you will need to do is to search the internet for information on ductile and brittle fracture appearances in steel. I think if you do this activity, you will be able to characterize the fractures for what you need. Metallographic examination using your new toy would be a follow-up to view the fracture surface in higher magnification and try to observe local ductile tearing features of grains or brittle fracture behavior associated with fracture either within or across grains, etc. You can also characterize the microstructure of the fractured rods.

Actually, after the visual examination step I mentioned above your next step would be observing the fracture surface feature in greater detail using a scanning electron microscope (SEM).
If you really want more information related to the above and other similar topics, I would suggest the book below titled
Metallurgy for the Non-Metallurgist, Second Edition
Editor(s): Arthur C. Reardon

 

[mxracer17788]

Awesome thanks a lot for the help Metengr. When you say "follow-up to view the fracture surface in higher magnification and try to observe local ductile tearing features of grains or brittle fracture behavior associated with fracture either within or across grains, etc. You can also characterize the microstructure of the fractured rods," do I need to prepare the fracture surface in any way i.e. polishing, etching etc? I have a degree in materials engineering so I'm not completely in the dark about most of these concepts, but I dont' have much practical experience with failure analysis and I would like to start somewhere. To look at the fracture surface, would I just cut, say, the last 1/4" of the rod off and place this for viewing on the microscope? That being said, what is the best way to mount the sample on the microscope and with what slides etc?

 

[metengr]

mxracer17788;
Once you completed the visual examination of the fracture surface and something appears to be of interest or you want to evaluate further, you will need to first cut (using a coolant) the end of the fractured rod by some distance (3/8" from the fractured end) and section further into a sample by multiple saw cuts (using a coolant) the plane you want to view on the inverted microscope. The side of the removed sample you want to view (perpendicular view of the fracture surface) is the side that will be mounted down in a plastic mount, polished and eventually etched for viewing.

Again, try to obtain a copy of the referenced book even if you are a materials engineer by degree. This is an excellent reference book.

 

[mrfailure]

A couple of additional comments:

Another book you should consider obtaining is ASM Handbook Volume 12: Fractography. This provides optical and SEM photos of fracture features for a range of materials; this will really help in identifying fracture morphology.

An inverted optical microscope will not generally help in identifying fracture type (Sorry!), though you can sometimes figure out fracture type when looking at etched structure. As Metengr suggested, the SEM is a great way to determine fracture type provided the fracture surface is not too oxidized (sometimes cleaning with a mild agent will help in this regard). In addition, you should evaluate the fracture in a low-mag optical stereo microscope and compare what you see to Volume 12.

Aaron Tanzer

http://www.lehightesting.com

 

[mxracer17788]

Great advice metengr and mrfailure. I've ordered both ASM volume 12 and metallurgy for the non-metallurgist. I will probably have the samples sent out for further analysis, but it definitely wont hurt to see how much I can do on my own.

Thanks,
Jeff

 

[NickE]

My favorite tool for examination of fracture surfaces is a good stereoscope, you want one with separate optical paths for each eye. These provide the depth of field needed to easily see the fracture surface, you should only need around 70x-100x maximum magnification at this point. I find normal inverted metallographs are not much use on fracture surfaces, there's not enough depth to really see more than a planar slice of the surface at one time. If you need more magnification you'll want to go straight to SEM, the extreme parallelness of the beam provides a massive amount of depth of field.



Nick
I love science!

 

[Maui]

mxracer17788, the surface features that characterize ductile and brittle fractures are described in detail in Chapter 16 of the book "Metallurgy for the Non-Metallurgist" starting on page 396. Several examples of each type of fracture surface are provided for reference.

Metengr, a well deserved star for your prior post. Thank you.

Maui

http://www.EngineeringMetallurgy.com

 

[stanweld]

How have the bars been fractured? Impact loading is recommended to be performed at different temperatures assuming the bars are the same grade of ferritic material and all are not now fractured.