Porosity in Steel Sand Castings

You may have to wait until Milo reads this post, but just a guess here, he will need more information.

Regards,
Sapper

Thanks. I will wait. Kumar

I read the Wikipedia definition and frankly am not impressed with many of their entries. I would recommend a more authoritative and accurate book to be the American Foundrymen's Society Third Edition of "Analysis of Casting Defects". The descriptions are re-enforced with clear photographs showing the defects.

Some of the Wikipedia entries miss the mark considerably while some are quite accurate but not well said and somewhat misleading. It does look the part of a "committee drafted document".

I would even question the complete accuracy of their "blowhole in a cast iron casting" photograph. While it is quite possible that the hole is a result of a blow from the green sand cored area that made the elongated slot (excessive moisture especially if the molder had to patch back that area if it ripped up on him) and since it is adjacent to a hub in the center of the casting being feed by spokes that are being reduced in cross sectional area it could be a shrinkage area if no center riser was utilized on that hub. It would be necessary to look into that cavity and see whether the interior is dendritic in nature(typical of shrinkage cavity) or smooth and shinny (typical of a blow). If you notice there appears to be a round black hole just to the right and below where the hub meets the spoke. This could be evidence of an internal connected area not visible here which would indicate that it is a shrinkage cavity caused by inadequate risering of the center hub.

Their reference to "streamlining the mold" to prevent turbulence shows unfamiliarity with the procedure. The reference would be better if it stated that "the gating system should be properly sized to prevent turbulence during pouring, provide the proper amount of time for proper filling keeping the metal velocities low enough to prevent erosion of the mold walls or core materials and provide for the proper risering of the casting utilizing directional solidification." It is the gating system that accomplishes the smooth entry and flow and proper feeding of the casting and not "streamlining the mold".

Nothing like a specialist's opinion to put the wikipedia materials in perspective.

Milo

Wikepedia seems to have a good handle on this.

I'm on the customer's side on this one.

Kumar, what is the source of your stock material. New or salvaged?

If it is new ask your supplier for technical assistance. If it is salvaged consider using new.

Thanks for the suggestions. The foundry had tried various gating patterns to reduce turbulence,digging a trough near the spruce should help reducing turbulence: I will check with the foundry if they have attempted a similar rigging.if not i will advice them to do per your suggestions. How nice it would be to have all this simulated even before pouring? How good is Magmasoft software, in predicting results,provided we give correct inputs? Anybody having experience using this software? Thanks Kumar

Once upon a time that software was called the foundry master's experience.

BAB

Really great post.

I would possibly guess that you put your finger on the problem area as many companies try to reduce the sprue size/weight so that with one pouring they can fill more moulds....and have less waste!!!

Big failure.

Older engineers never make that mistake (again)!

I am with Blue aussie boy's take on this. Couple of other ideas:

1) are you certain that they are porosity rather than voids left from nonmetallic inclusions ripped out during machining? Entrained slag or other materials can cause this.

2) Is Sand the best choice for this part? or just the cheapest?

3) Sprue (s) Size increase will forgive a lot of ills. At the steel mill to increase useable yield on our Ingot castings we would put on "hot tops" on our big end up molds. This allowed the gases and other non metallics that evolved (were divorced out of the solidifying matrix) during solidification to be concentrated in the very top of the ingot/ hot top. This would then be cropped off, leaving a sound ingot body.

4) Deoxidizer grain refiners - is the foundry using sufficient of these? How do they prevent reoxidation / gas pickup of melt? Or do they not? Not knowing batch size, if the porosity tends to be worse on the last ones poured compared to the first ones poured, then gas pick up during your pour operation is implicated.

Obviously a larger sprue adds cost / reduces metal yield, but compared to rejected product...

Lots of great ideas on this thread.

Milo

We use sand process because it has given us best results for similar castings. We can graduate to other sand process like No Bake which has uniform mold hardness and dimensional stability though at this time, porosity is our problem. Some one mentioned about getting good results using lost wax/Investment process: if they could reason technically how it got solved using the lost wax we can migrate as we have to convince the cost conscious customer to spend more on Investment cast as it is relatively expensive. Again the part weight is around 200 pounds where Investment casting might have limitations. The current yield is 48 % so generous gating is provided: i am checking if they have rigged the gate in such a way turbulence is kept at a minimum. On melting procedures and chemicals they use Foseco and take help from them on treating the metal which i believe are the best in the Industry. It is a low volume requirement and we see porosity on all the castings, more in some and less in others.(we see the pores in the housing bores) The foundry is working on different options in gating plus we are exploring a magmasoft simulation. These will take some time,meanwhile we are welding the pores,radiographing the zone and ship it to the customer. I will post the final results as soon as i receive it. Thanks everyone for the help.

Cooper Alloy switched from green sand and CO2 to practically all Shell molds back in the 80's. They made a lot of stainless valves in CF8M. They found that the shell process was very beneficial in reducing their scrap rate and defects and it was capable of producing some good size castings. A 200# investment casting certainly isn't cheap and won't eliminate the need for additional finish machining so the cost saving benefit would be lost of a near net shape casting.

How much of your own research have you performed so far Kumar?

Start eliminating possible causes.

It is 2012. The information is out there.

There are several ways to deal with this problem. The voids are caused by air in the foldinding of the metal. The old way is to hammer the material until the voids are gone ( by old way I mean the tried and true method of eliminating voids in materials used for deep sea purposes), or you can use an ultrasound or a vibrator such as the do with concrete to eliminate the voids. Your foundry should know these things.

The 'new' way is Hot Isostatic Pressing or 'hipping'. This is used to compress the whole casting to remove porosity. You might find you need to cast a bit oversize if you use this process as it can reduce the overall size of your casting.

I wouldn't recommend trying to peen holes shut with a hammer in a valve application. If the machining doesn't open it up the final hydro-test after machining will and by then the cost of all that machining has been expended. Works OK for decorative stuff especially in aluminum or bronze but NG for pressure vessels which a valve is.

The OP sates that

"The foundry has done everything in controlling the process parameters like temperature,sand control,moisture,pouring speed etc., Finally when it came to the gating they had done many iterations they know to get the best casting".

It sounds like kumar's communication with the foundry is with someone who already has all the answers he needs. Perhaps the answer lies in the economics of the process. It is easy to find "solutions", but they all (as suggested) involve significant costs. Is the customer willing to pay? Is the producer willing to take a reduced profit? Can a new contract be negotiated? Companies have gone out of business with wrong answers to these questions. Good Luck to you, kumar. Sincerely meant.

I too fight the porosity issues. I have found that you get what you pay for. If your standards are high your have to pay more for the quality or have a lot of headaches. I would go to your customer's engineers and find out what the true limits are. Most purchasing people are told in simple terms "no porosity". I would also look at investment casting instead of sand (this is expensive but could solve your problem). I would also look at other foundries. I found one that doesn't tumble the parts, and this solved a lot of my surface defects. If your foundry is unwilling to or unable to solve your issue you need to move on to another foundry. I would keep your customer informed of how you are solving the problem so you don't loose them. They may also have valuable advise that can help.

I deal in pumps for the offshore drilling industry. I had the same problem with 316 SS Pump Bowls. We could not get the foundry to correct the problem on his end. We had to write a procedure for "Repairing Voids in Castings" which described before machining the bowls, filling in the voids with 316 ss welding rods and then do the machining process. Of course hydrostatic testing was allways done before and after each process. All this had to be approved by our client first. This got to be too time consuming, so we switch foundrys to one that use the "Lost Wax Method" for poring the castings. No More problems.

Nice to hear you had a solution with a different process. investment/lost wax cost,is expensive, however a solution without welding is what my customer wants. How the lost wax/Investment process solved the porosity? How is the lostwax/Investment cast process different in cooling characteristics in promoting directional solidification compared to sand? Thanks K

Lost wax method gives usually a high quality finish (in my severely limited experience, only of lower temperature metals mainly silver & gold), which may save on some of the machining costs/machining time/welding repair time taken afterwards.......

Best of luck. Let us know how you get on....

I do not know all the ins & outs of the foundry industry. I do know that when we switched to the lost wax investment casting foundry as a supplier, our hydrostatic rejects for blow holes stopped. We no longer have the long and drawn out process of repairing and repeat testing of the bowls, which saved us time and money. When we receive the bowls now they have the apparence of a machined product, you can see the difference in the two products.

Looking in my reference book "international atlas of casting defects" some of the type of defects that match your description are: Dispersed Shrinkage, Slag-Blowhole, and Sand Inclusions. Dispersed Shrinkage - Narrow cavities resembling tears or fissures. Possible Cause - - Carbon content low. - Nitrogen content too high. - Mold not sufficiently rigid. Remedies - - Reduce nitrogen content. - Reduce proportion of steel in the charge. - Substitute alternate furnace. - Fix the nitrogen in the form of a nitride by use of titanium or aluminum Slag-Blowhole - Non-metalic inclusions which are usually associated with blowholes within the inclusion as well as in the mass of the casting. The inclusions are distributed throughout the casting but occur most often on the cope surface. Possible Cause - - Complex reactions which occur within the liquid alloy, or between its oxides, the atmosphere, ladle linings or the mold and its cast. These reactions often arise as a result of the conditions of melting the metal, and may indeed originate from the quality of the primary materials used or the manner in which they were produced. Remedies - - Use Primary materials containing a minimum of internal oxide inclusions or external oxidation such as rust. - Avoid low Si and high Mn contents, where possible, maintain the ratio Si>Mn+0.5%. - Limit the Al and Ti contents of the melt. - Keep the sulfur below 0.1%. - Melt and tap at high temperature. - Maintain correct mold pouring temperature. - Pour rapidly with a non-turbulent gating system. Sand Inclusions - inclusions of irregular shape, usually compact, in the vicinity of the cope surface of the casting. Often they are visible on the rough casting, but may appear only upon machining. Possible Causes - - Pieces detached from the mold or the cores. - Lack of care in molding. - Erosion or crush. - Crust formation and detachment of the sand due to expansion of the silica. I'm a quality guy and not an engineer, this book has helped me many times in the past. It is published by the American Foundrymen's Society, Inc. Hope this helps.

Thanks, I will buy one.

The porosity in castings could due to:

Aggressive mixing, improper pouring, moisture contamination and excessive exotherm can cause porosity in a casting.

When mixing casting resins and hardeners, avoid inducing air into the mixture by aggressive mixing action. Either stir the material with a spatula or use a mechanical mixer and drill motor. Some mixers, like those of the Jiffy Mixer line, induce less air than others. The best way to remove trapped air due to mixing is to vacuum degas (de-air) the mixture in a vacuum chamber before pouring.

Air can be trapped during the pouring process. If the casting is to be made as an open-faced pour, cast the material in a thin stream into the lowest part of the cavity. Allow the resin to flow out from that point as it pushes the air ahead of the flow. Do not pour the material back and forth causing it to fold over itself. If casting into a closed cavity, make sure the sprue (fill tube) is large enough so the resin can flow down without closing off the opening. Always have the sprue in the lowest part of the casting and provide adequate venting in the highest parts.

Moisture containing materials, like plaster and wood, should be properly sealed and released before castings are made against them. Otherwise, when the epoxy casting resin exotherms during the curing process, the heat will draw out the moisture and create surface porosity in the casting. Using sanding sealer, wax and PVC or PVA film barriers is the best way to protect the casting.

Choosing the proper hardener speed for the size of the casting and the ambient shop temperature will minimize exothermic reactions. If these reactions get hot enough, the casting resin will boil, creating porosity throughout the casting

Other Possible Causes:

Mechanical Gas Entrapment

• A large amount of mold or core gas with insufficient evacuation from the mold cavity
• Entrainment of air due to turbulence in the gating system

Metallurgical Origin

• Excessive gas content in melt
• In the case of steel and irons, formation of carbon monoxide by the reaction of carbon with oxygen present in the melt.

Remedies

• Include vents in the mold cavity to allow the escape or air
• Review gating design for turbulent areas
• Ensure that the sprue is kept full during pouring
• Reduce pouring height
• For steel, deoxidize the melt adequately
• For iron, avoid using rusty charge material which will introduce oxides into the melt

For non-ferrous alloys, avoid excessive melt temperatures and use care in degassing.

Sridhar.

OP is casting stainless steel not resin...

Your other possible causes and remedies is pertinent though.

Your response is a bit of a composite.....

Thanks for the comments.

would like to add other reasons:

Air Entrapment

During production, workers pour molten metal into the mold. During this process, the they do not always remove all the air from the molten metal. This happens most often during incomplete casting. Air bubbles become entrapped in the skin, forming alloys during the filling. The trapped air expands due to the heat and then contracts when the metal freezes, leading to pores caused by blowholes. When gases are dissolved in liquid metal and become less soluble during solidification, pinholes can form. This comes from small bubbles that form from the less soluble gas. These bubbles are like the bubbles found in carbonated drinks.

Oxides

Some of the porosity in castings comes from oxides. Most of the porosity on the surface of the casting comes from oxide formation. Re-oxidization inclusions can cause much of the porosity in the castings. When blasting removes the inclusion material, the porosity associated with it remains in the casting. This process can leave behind gassy areas in this steel casting.

Shrinkage

During solidification, casting can experience shrinkage. This happens when the liquid metal on the surface of the casting goes under a depression due to exposure to the atmosphere, causing a contraction in the metal on the inside. Then, atmospheric pressure pushes the metal downwards or inwards. We see only the depression on the surface of the casting. Shrinkage porosity can also occur when solid substance surrounds a liquid and solid material is strong enough to resist the depression of the contracting liquid. This leads to pores that are usually greater than 3mm. The pores are mostly on the inside of the castings.

Sridhar, Thanks for the comprehensive reply. Can i have your contact# and email. Kumar

you can check up my profile.

You don't say what process you are using to make molds. Such as Green sand, No-bake, etc. Each one of these processes takes on a different meaning in terms of gating, venting , sprue size so, on and so forth. I would also need to know the pouring temperatures, the hight of pour etc. If your a member of the Foundry Mens Society they have an excellent library on these subjects.

The process is manual sodium silicate process. This is a mixture of sea sand with sodium silicate oil hardened by Co2. I could see this sand does not retain moisture versus others which is good for the mold. Thanks Kumar

Sea sand means nothing to me. Are the particles shape round or irregular. Sodium Silicate may not be the best binder for your process. It may have a tendency to produce gasses once heated. If your pouring steel I might assume your pouring at or around 2,850 deg F. This is in the upper areas of what silicate may like. I might suggest contacting a supplier of iso-cure or iso-cyanate processes as they handle the temperatures better.

You also might look into zircon sands. These work well with the above processes and will not wash away, especially with thin casting sections.

Gas porosity is the formation of bubbles within the casting after it has cooled.casting as porosity or the pore may be trapped inside the metal, which from the casting sand, or other gasses from the melt or mold.

Source-http://en.wikipedia.org/wiki/Casting_defect

Sorry to mbe picky, but I personally don't agree with the wording you used from Wiki.

But even the Wiki article corrects itself later in the same paragraph...look here (a link that functions fully by the way with just a click!):-

http://en.wikipedia.org/wiki/Casting_defect#Gas_porosity

Gas porosity is the formation of bubbles within the casting after it has cooled. This occurs because most liquid materials can hold a large amount of dissolved gas, but the solid form of the same material cannot, so the gas forms bubbles within the material as it cools.

The first comment you used from Wiki is misleading and wrong......the second is fully correct.