Porosity in CA6NM castings 3

[lkrzyzan]

thread330-257321

I am having a similar problem with porosity in CA6NM on impellers and casings. Porosity appears on sections less than 0.5" and naturally in the risers.

Melt process is air induction furnace with Ar plug. Charge includes revert, 1010, ferrochrome (0.05% N according to the vendor) and electrolytic nickel. I've maxed out Ni, minimized Cr. Using Mn and FeSi for initial deoxidation in furnace and Al and Graphidox (54%Si, 10% Ti, 6% Ca) in final procedure before pouring to laddle.

Last two times we poured CA6NM were rainy overcast days with high humidity, both times castings were scrapped. We've had issues in the past but I would have to look up weather on those days to say anything definitive about moisture. However, the porosity observed did appear on the drag side possibly indicating off gassing from the no bake phenolic mold. I've sent samples out to confirm N content, should get them back in a week.

Can anyone recommend laddle deoxidation procedure? I'm curious about using either more Al, or trying FeTi, or Zirconium (read that 0.1% should be enough). If Al, can anyone specify max to avoid AlN embrittlement? Any experience would be helpful.

lkrzyzan

 

[arunmrao]

I am not familiar with your charge mix. What was the Carbon content in the casting? I would like to use electrolytic grade iron instead of 1010 scrap,and for Ni/Cr addition use 304 material ,with L.C FeCr .My carbon was always in the 0.03-0.04%. Hope you had a wash heat of CF8 ,before you started CA6NM heat.

Now to the gas porosity problem. I hope you are not super heating the metal . Please control the pouring temperature. Your observation of wet days is correct,and either I would avoid melting those days or take extra precautions,of drying the molds,ladles,charge materials,alloys etc.

You can add FeSiZr,in the melt before pouring along with CaSi. It should help. As you mention about impellers,are these the open type or closed ones (shroud). Please check the core ,and ensure proper escape of core gases. Perhaps,you might pour one in another alloy,to eliminate the possibility of porosity due to mold/core materials.

Hope it helps.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[metengr]

Experienced advice.

 

[lkrzyzan]

Thank you arunmrao.

C is kept below the max of 0.06. Impellers are closed. Prior heat is usually a stainless 18-8 or variant of it like CF8C. I instruct the melter to keep material below pouring temp until its ready to pour. Last tapping temps were below 2940 F. Laddle is heated, molds are not but the wash is dried before closing. I'll have to talk with the chief foundry engineer regarding any gating mod to reduce turbulence.

I'm curious is anyone can shed light on why CA6NM seems to have a worse problem with porosity than other stainlesses. Solidification to a martensitic structure? Does the alloying change the interfacial energy appreciably enough to foster bubbles or is it the an issue with changing solubility on solidification?

Thanks.

 

[arunmrao]

No ,I do not agree about CA6Nm being more prone to gas porosity. It is the impeller casting that is causing the problem. How large is the core diameter and what is the wall thickness? Also the shroud opening ? You may need to look at the core material,coating and need to remove core gas effectively.

Try pouring one similar casting in carbon steel and look for porosity?

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[mk1979]

Sure, the changing solubility on solidification almost is the problem.

 

[arunmrao]

mk1979,what is meant by changing solubility during solidification please?

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[EdStainless]

An austenitic alloy has higher nitrogen and hydrogen solubility. In marginal cases CF will come out fine and CA will not.
Add vents for gases from mold and cores.
And we used to change the melt schedule based on the weather.
It is a pain but it works.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[arunmrao]

Ed,thanks for the confirmation,,it is always a pain,to process CA15 grade castings,CA6NM is more friendly alloy to process!!However CF8,CF8M are a cake walk, but then CF3 is always a problem in air melting,with respect to carbon control.

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[lkrzyzan]

Agree with the relative cakewalk of CF8 et al. This foundry used to have little trouble pouring CA6NM in the past I was just told. Mold is vented as much as it can be by passive methods. Use the same with HC250 and we don't have many issues.

Talked to the foundry engineer but he said in the past Zr and Ti additions haven't made much of a difference. Pinholes do not worm so I don't think its a hydrogen issue. Vanes are under half inch.

We'll try some vent rope, have the mold heated prior to closing and pour it on a sunny non-humid day. I took some of the modl and will do an LOI to see if there is a significant difference from norm. I'll look for the Zr and add it in the laddle after deoxidation procedure to lock up nitrogen on austenite to martensite tranformation and look into slowing down solidification to allow gas time to diffuse.

Thanks.

 

[Guest102023]

CA-6NM is indeed easier to cast, because of its much narrower freezing range. Too narrow a range presents its own problems for castability. I expect you are casting quite a wide range of section thicknesses.

It's the same reason E410NiMo is easier to weld, but if you really want to improve weldability for the end user, keep the C (and N) as low as technically possible.

 

[Metalhead97]

lkrzyzan,

What is the shape of the porosity. Is gas (round) or pipe (irregular)? Is there any slag connected to or inside the porosity? Calcium Boride is another deoxidizer you could try, but I would think the Mo in the alloy would be an effective deoxidizer. However, this statement is very telling:

"Porosity appears on sections less than 0.5" and naturally in the risers."

To me, it sounds like a feeding issue.


brimstoner,

You are correct. Too narrow a freezing range can lead to fusion problems and folds, however in this case I think I believe it's a feeding issue.

Metalhead

 

[mk1979]

i meant that For multicomponent alloys with three or more components, the process of microstructure formation during solidification is less understood, especially for cases where multiphase reactions occur along the solidification path of the alloy. Such cases are common in industrial practice, therefore,the problem may be in microsegregation associated with the formation of detrimental minority phases and the amount, the morphology, and distribution of the beneficial minority phases.

 

[Metalhead97]

mk1979,

What you said is true, but the fact is that the porosity is only seen in sections of thicknesses less-than 0.5" thick. If it were a micro-porosity issue you would see it throughout the casting- especially in large grain areas. I'm just going on the info that has been provided by lkrzyzan. We still need more info about the shape and size of the porosity.

Metalhead

 

[lkrzyzan]

Porosity was generally round. In a few places the welders excavated up to an inch into the casting.

We poured another casing and impeller. Molds were reheated for longer time prior to closing. It wasn't raining but the day was muggy. Extra vents were added. On surface inspection I have not seen any porosity. I'm waiting for the cleaning room to process the castings further to see if any other issues are found.

 

[Metalhead97]

lkrzyzan,

If you are confident that the porosity is round then it sounds like a gas problem. The extra vents will help you release any trapped air, which can cause gas porosity in sections that freeze too fast. I hope that solves your problem for your welders sake!

MH

 

[lkrzyzan]

After raising Ar flowrate through porous plug, preheating mold prior to closing, picking a non-rainy day for tapping and using Al and Graphidox for deox as before, porosity on repour was less but still existed. It was largely found on the OD. The thinness of the vanes and shround make more venting difficult. Some minor cold folds are noticable to we will need to increase tap temperature.

LK

 

[arunmrao]

Ikrzyan, you are on the right path. However, what is the core binder? Have you tried using Ecolotec ( from Foseco) as a binder. It helps. Alternately,if you could consider ceramic cores ,they would be ideal. The opening I suppose is 5 mm and the O.D of the impeller 500mm.

Fortunately you have not encountered core breakages! .

_____________________________________
"It's better to die standing than live your whole life on the knees" by Peter Mayle in his book A Good Year

 

[Metalhead97]

lkrzyzan,

Is the porosity still in the thin-walled locations?

MH

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