Sandcast 356 Aluminum Porosity

[avgguyfrom85]

All,

Good day and I hope all is well. I work for a small company who fix and repair low pressure relief devices on above ground storage tanks. We recently procured valves from an OEM and upon recite observed small pinholes on some of the machined surfaces on the units. Managements first response was initially "send them back", however due to lead times and project completion dates, that may not be possible. So I come here with a curious mind and the following pictures for reference to ask the following:


-Are these types of pinholes inherent to sand cast aluminum parts?
-Could these pinholes accelerate potential corrosion? (these machined areas will be affixed to a flange, or other parts, and not directly exposed to the elements)
-How can I measure these pinholes to develop a accept/reject standard for our shop (possibly against a visual reference)?

These units are installed on above ground storage tanks that generally can withstand less than 1.0 PSI of pressure (and usually far less than that). This week we will be pressure testing the bodies/units to 3-5 PSI to see if we can find any leaks.

My apologies for the novice nature of these questions, as material specification is not my area of expertise. Any resources and standards for reading would be greatly appreciated. If I find anything helpful, I'll post it here as well. Thank you.

B.

 

[tbuelna]

Your picture shows a relatively poor quality casting. I would not expect this amount of defects permitted on a sealing surface. Aluminum sand castings will usually have some localized defects, and sometimes they can be repaired by welding. The manufacturing documentation for the casting usually specifies the acceptable size/shape/type of defects in various areas of the casting. MIL-STD-2175 is a standard for casting quality used in aerospace, which you might find helpful.

 

[swall]

The pinholes depicted look like typical microporosity due to hydrogen. Degassing treatments (such as nitrogen flushing) are used at the foundry when high quality (i.e. pressure tight) castings are required. You must specify the degree of soundness required for the application using radiographic standards such as ASTM E-155.

 

[avgguyfrom85]

All,

Thank you for the responses. I'm currently reviewing ASTM B26 and will be using that with ASTM-155 and E2422. B26 identifies "Discontinuity Level" with Grade A through D specifications to be used with the reference radiographs. I'll keep looking into this and post further findings.

 

[arunmrao]

The gas porosities cannot stand leak test and would fail. Consider the potential liability clause before accepting these defective castings. They might be more than what a small company can chew.

"Even,if you are a minority of one, truth is the truth."

Mahatma Gandhi.

 

[WKTaylor]

avgguyfrom85

You are correct: it is really hard to predict corrosion-worthiness of these bare[?] porous low quality castings... especially in an outdoor environment.

I suspect that Your vendor had, or accepted, a low quality standard for these low-stressed ho-hum castings from the sub-tier casting vendor... and may/may-not have intended to compensate for any porosity with impregnation per MIL-STD-276 IMPREGNATION OF POROUS METAL CASTINGS AND POWDERED METAL COMPONENTS... which OBVIOUSLY wasn't done.

IF these castings are in completed assemblies this can get tricky, since a lot machining [coolant/oil, etc] and possibly lubricants have affected the raw castings. IT MAY be possible to strip down and clean the casting... then have it impregnated [sealed] and possibly refinished. IF this is the case then salvage may be practical.

IF this is NOT practical [a casting impregnation vendor should know], then the LOT is probably unreliable/usable for extended outdoor service with internal water and external weather effects.



Regards, Wil Taylor

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[avgguyfrom85]

All,

Again thanks for the replies. Today we will test the bodies with the pressure test and I will relay the results. These particular valves are weight loaded designed to relieve vapor pressure on above ground storage tanks; the particular setting for these units will be 2.0 Oz/in2 (3.4" WC) pressure and 0.5 Oz/in2 (0.9") WC on the vacuum side.

Our main concern is that in some cases these valves can be on tanks up to 5-10 years with out any service based an a tank's PM cycle. Though the area of the valve connecting to the tank flange is substantial (approximately 1" think of aluminum), it would not look good having the AQMD (Air Quality Management District) give our customer a notice of violation due to pinholes allowing VOCs to escape to the atmosphere through a valve body.

I will look into what it would cost money and time wise for impregnation to see if it is feasible. Thanks for all of the avenues of information and solutions.

Regards,


B