Branchless Brass SCC

[mrfailure]

I recently evaluated a couple of cracked brass components (from different components and customers) where cracking was transgranular and single cracks, with no branching. Stress corrosion cracking was determined to be the failure mechanism after we realized both components had been exposed to ammonia and/or excess nitrogen. Materials were close to C36000 and Muntz metal.

My real question: I know branchless SCC with single transgranular cracks can happen in brass, but I do not know why. Can anybody point me to some good references or have a good explanation for the phenomenon? I also wonder if there are any tell-tale morphologies from the analytic side that will let me differentiate SCC from fatigue. Thanks!

 

[EdStainless]

The degree of branching is often a result of the loading condition.
If a material is in and SCC environment and you apply a very large load you may hot get any branching as the failure happens too fast.

Try a good failure analysis ref.

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P.E. Metallurgy, Plymouth Tube

 

[metengr]

I would be investigating corrosion fatigue versus SCC.

 

[mrfailure]

I did initially call one of these cracks a fatigue crack (though appropriate corrosion species was not present in service for corrosion fatigue), but subsequent information regarding processing convinces me it was an SCC crack.

In that instance the crack was featureless. The other case was a new part that had never seen service or cyclic loading. The component had been exposed to ammonia (purposely, I think) during cleaning and then sent to me. The crack formed between the time I received it and the time I started my evaluation 10 days later. A previous investigation I had conducted on another sample of the same part number and I had identified branched SCC. The problem goes back to nitrogen exposure during cleaning, a problem which has now been rectified.

So I want to come back to the original question: Why does branchless SCC happen? Is it really SCC or is it an embrittling phenomenon? Again, any thoughts would be appreciated.

 

[EdStainless]

Perhaps the more accurate term is environmentally assisted cracking.
There is not one single mechanism involved.
Do you have any estimate of the levels of residual stress in these parts?
Temperature that they saw?

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P.E. Metallurgy, Plymouth Tube

 

[mrfailure]

The part that never saw service was at ambient indoors temperature. Residual stress was only from machining. It was a valve fitting but the valve had not been installed.

The other part was a valve stem poppet that would have had applied ID residual stress from the valve stem, and may have further seen some cyclic stress during valve operation.

 

[metengr]

mrfailure
I am not an expert in brass alloys. From my experience with brass condenser/heat exchanger tubing in the power generation industry, I have seen corrosion fatigue cracking, SCC, ammonia grooving and dezincification damage mechanisms. The single, circumferential-oriented crack in the brass condenser tube was very obvious and related to conditions where the tube had been flexing in an aqueous environment with ammonia because of shallow grooving on the external surface of the tube. Detailed fractography and metallography confirmed a single, unbranched crack in the tube material from the base of the groove, which confirmed corrosion fatigue.

I have also seen season cracking in brass material, and typically you have branched transgranular cracks. Normally, SCC is identifiable as branched cracks exhibiting transgranular or intergranular appearance. Perhaps you had some type of stress-assisted corrosion that is really unlike SCC, by definition. You can all it what you like, but for SCC you need three conditions to occur simultaneously to propagate SCC in materials, remove any one and the cracking stops.

 

[EdStainless]

I guess that where I was leading is that you should evaluate the residual stress levels, I would expect you to find them much higher than you suspect. Such as significant (~90% of yield) residual in the starting stock that these were machined from.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

C360 brass is free machining, so it seems strange that there would be sufficient residual stress in the part after just machining operations to produce a SCC failure. The data sheets I saw from a couple C360 suppliers recommended a 1.5hr stress relief at 500deg after significant cold working, but milling or turning operations would not produce anywhere near the same level of residual stress.

Here is a reference covering corrosion issues with C360 brass.

http://www.copper.org/resources/properties/db/basic-search.php?alloys-select

[]=C36000&alloy-select-properties[]=corrosion&submit-multiple-alloys=Display

 

[EdStainless]

Yes TB, but we don't know how the stock was made int eh first place.
Bar that has been rotary straightened can have residual stress=yield, if it was done poorly.

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P.E. Metallurgy, Plymouth Tube

 

[weldstan]

I had a similar experience with Admiralty brass. Cracking occurred in new material improperly stored and exposed to animal waste. Tubes had high residual stress induced most likely by straightening or other manufacturing processes.