Retained Austenite 6

[chr10000]

Does anyone know what the best method would be for detecting retained austenite on 440c? Any images or information is appreciated.

 

[mpoulter]

You can use optical microscopy if the amount of retained austenite is substantial (>15% or so). If it's less than that, x-ray diffraction is the most accurate method, so far as I'm aware.

 

[TVP]

X-Ray Diffraction is always more accurate/precise, but optical methods can be and are frequently used. And by optical, I mean taking a polished cross-section, exposing it to a suitable metallographic etchant, and then observing it under an optical microscope at 250-1000x magnification. The following is an excellent article by the world's foremost expert on metallography and microstructures, George Vander Voort:

http://tss.asminternational.org/sta...oken=5baaf9bcf6fa2066d95c78d7b25165208c6d0051

 

[sodeen]

A decent Q&D test that I have found is to subject the part to a sub-zero cooling cycle (-90 F) for a couple hours, then seeing if the hardness has jumped a couple points from its previous state. If an increase in RHC is seen, chances are there was some retained austenite in the part. (Don't forget to put the part through another temper cycle if you're going to put it into use after the fact.)

 

[chr10000]

sodeen - I agree with you. This is about the only method that I have right now that gives me a clue this is going on. But when looking at the structure I do not see anything that leads me to believe that I am seeing retained austinite.

TVP - The link didnt seem to produce an article on this topic. I am very interested to see what it was. If you know of another working link please let me know. I was hoping to find more information on what types of etchant would produce a good visual.

As of now I have only used Fry's etchant. Is there anything that would be more suitable for use on a polished cross section?

Why is it that when testing hardness on rockwell vs micro, the numbers do not seem to comapare like they do on other types of steel?

 

[redpicker]

Why is it that when testing hardness on rockwell vs micro, the numbers do not seem to comapare like they do on other types of steel?

If what you are asking is why the Rockwell 'C' to Vickers hardness conversions for quench and tempered steels don't seem to be accurate, one reason is these conversions are based on low alloy steels and 440C is really too high in chromium. Another reason could easily be the presence of retained austenite.

Back in the 1970's, I remember seeing some research that attempted to correlate difference between Rockwell 'C' hardness and converted Vickers hardnesses to the amount of retained austenite. I don't know what the results of this research (it was way too long ago and I have not had any need for the past 20 years or son), but I'd bet a bit of research would find some Master's theses or PhD dissertations on the subject.

rp

 

[blacksmith37]

Actually it is the high C in 440C that can lead to large carbides (and retained austenite) and variable hardness test results, especially with small indentataions ,eg 10kg VHN.
If uncertain what you have , you can test some grains with micro hardness; You won't get bulk hardness resulte but you can identify austenite vs carbide grains.

 

[coreman73]

It may or may not help but I have attached an article from George Vander Voort's on Martensite/Retained Austenite.

 

[TVP]

chr10000,

I clicked on the link again, and it works. The article title is "Hints for Imaging Phases in Steels", with the discussion on Analyzing Austenite beginning on page 4 of the .pdf (page 35 of the original article). Figure 10 shows Type 440C stainless etched with Beraha's sulfamic acid no. 4 etch, revealing retained austenite. The article gives the composition of the etchant and how to use it.

 

[metengr]

The above link by TVP only takes you to the ASM web portal, unless you are a member. You can search for the article mentioned, and it is not free unless you are a member of ASM.

 

[TVP]

metengr,

Many thanks for the clarification.


chr1000,

I apologize for the confusion. Apparently I have all of the cookies, etc. turned on in Internet Explorer that allows direct navigation to the articles. Anyway, the following are two links to George Vander Voort's personal website, one for the article I mentioned previously (Imaging of Phases in Steels from Feb 2005 issue of Advanced Materials & Processes) and one for a related article that appeared in the Apr 2009 issue of Industrial Heating:

Imaging of Phases in Steels

http://www.georgevandervoort.com/met_papers/IronandSteel/imaging_phases_in_steels.pdf

Martensite and Retained Austenite

http://www.georgevandervoort.com/met_papers/IronandSteel/Martensite_RetainedAustenite.pdf

 

[armourian78]

I work for a bearing manufacturer, and we are currently doing a lot of work on retained austenite(RA). I confirm that for X-ray diffraction is a reliable method for measuring RA (however the reliably of the technique cannot measure accurately below 1% RA).

It is also possible to do growth calculations before and after an additional temper. Internally we use an additional temper of 220°C for 6 hours, with a growth of 10 microns per 100 mm diameter is equivalent to 1% RA for rings (52100 material). Allow 24-48 hours before re-measurement. As when the austenite is transformed in martensite and volume increase occurs.

Typically 440C has a 6-8% of stable retained austenite due to the high levels of carbon and chromium.

I have a question related to transformation of retained austenite:
If 440C material undergoes an additional temper (150-200°C), after hardening, quenching and tempering cycles (including a deep freeze), why does the material shrink afterwards? Typically having a RA value in region of 6-12%.

If the retained austenite is transformed persumably into martensite, it should grow. Is the martensite temperature not being reached? Therefore changing the austenite into ferrite and cementite (not ever changing to martensite)?

Thanks Ian
Metallurgist, Plymouth.

 

[Maui]

Optical microscopy methods are among the most common for reliably detecting retained austenite in a quenched and tempered microstructure, and a skilled technician can identify retained austenite down to levels of about 5%. When the amount falls below 5%, optical methods will not provide accurate results and this is where X-ray methods must be implemented. 440C may have a substantial amount of retained austenite depending upon the heat treatment cycle that is used. Higher austenitizing temperatures and lower tempering temperatures tend to favor greater amounts of retained austenite, and cold or cryogenic treatments can be used in certain cases to minimize it's presence when high dimensional stability is required.

Amourian, can you provide some data for review to help in the analysis of your problem?

Maui

http://www.EngineeringMetallurgy.com

 

[tbuelna]

"If the retained austenite is transformed persumably into martensite, it should grow."

amourian78,

While I can't answer your question as to why you see shrinkage during heat treatment of your 440C races, I can confirm that the slight, controlled dimensional growth produced by the austenite/martensite transformation during quenching is of great interest with steel bearing races, especially large diameter ones. Large diameter, thin section bearing races are quenched using close fitting internal quench rings to help keep them round. The close fit of the internal quench ring fixture relies on the slight dimensional growth of the race during quenching to ensure that it pops free after quenching.

terry